Morgan County Unincorporated
City Zoning Code
City Zoning Code
GEOLOGIC HAZARDS
§ 155.220 PURPOSE.
This subchapter and the regulations and restrictions contained herein are adopted and enacted for the following purposes:
(A) To protect the health, safety and welfare of the citizens of the county, protect the county’s infrastructure and financial health, and minimize adverse effects of geologic hazards to public health, safety, and property by encouraging wise land use;
(B) This subchapter and its appendices, on file with the County Clerk, address surface fault rupture, slope stability, liquefaction, debris flow, rockfall and avalanche hazards and present minimum standards and methods for investigating and reporting on geologic hazards;
(C) The applicant shall present the results of geologic hazard investigations in compliance with this subchapter and its appendices, on file with the County Clerk. The standards set forth in this subchapter and the appendices are minimum requirements. More complex projects may require more detailed and in-depth investigations than outlined herein. In addition, the appendices shall not supersede other more stringent requirements that may be required by other regulatory agencies; and
(D) Geologic hazard study areas are defined in § 155.223 of this code. Site-specific geologic hazard investigations performed by qualified, state-licensed engineering geologists shall be required prior to developing projects located within a geologic hazards study area. In the event known or readily apparent geologic hazards exist in an area subject to a development application, and which area is not included in a geologic hazard study area, the applicant shall nevertheless follow the process outlined in this subchapter and submit the applicable investigation report.
(Prior Code, § 8-5I-1) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.221 DEFINITIONS.
For the purpose of this subchapter, the following definitions shall apply unless the context clearly indicates or requires a different meaning.
ACCEPTABLE AND REASONABLE RISK. No loss or significant injury to occupants, no release of hazardous or toxic substances and minimal structural damage to buildings or infrastructure during a hazard event allowing occupants egress outside.
ACCESSORY BUILDING. Any structure not designed for human occupancy, which may include tool or storage sheds, gazebos and swimming pools. Accessory dwelling units and businesses located in ACCESSORY BUILDINGS must comply with all requirements of main buildings.
ACTIVITY CLASS OF FAULTS. The activity level of a fault is based on the latest Western States Seismic Policy Council policy recommendation defining “surface faulting,” which can be found at: (https://www.wsspc.org/publicpolicy/adoptedrecommendations/). Currently, policy recommendation 18-3 states that based on the time of most recent movement: Latest Pleistocene-Holocene faults are defined as movement in the past 15,000 years, late Quaternary faults are defined as movement in the past 130,000 years and Quaternary faults are defined as movement in the past 2,600,000 years.
ALLUVIAL FAN. A fan-shaped deposit where a fast-flowing stream flattens, slows and spreads, typically at the exit of a canyon onto a flatter plain.
AVALANCHE. A large mass of predominantly snow and ice, but may also include a mixture of soil, rock and/or organic debris that falls, slides and/or flows rapidly downslope under the force of gravity.
BUILDABLE AREA. Based on an accepted geologic hazard investigation report, the portion of a site not impacted by geologic hazards or the portion of a site where it is concluded the identified geologic hazards can be mitigated to an acceptable and reasonable risk. Buildable areas must be clearly marked on approved site plans and/or final approved plats, as appropriate.
COUNTY COMMISSION. The County Commission of Morgan County, Utah.
CRITICAL FACILITIES. Essential, hazardous, special occupancy and all risk categories III and IV structures, as defined in the currently adopted International Building Code (IBC), and lifelines, such as major utility, transportation, communication facilities and their connections to critical facilities.
DEBRIS FLOW. A slurry of rock, soil, organic material and water transported in an extremely fast and destructive manner that flows down channels and onto and across alluvial fans; includes a continuum of sedimentation events and processes, including DEBRIS FLOWS, debris floods, mudflows, sheet flooding and alluvial fan flooding.
DEVELOPMENT. All critical facilities, subdivisions, single-family dwellings, duplexes and multi-family dwellings, commercial and industrial buildings; also includes additions to or intensification of existing buildings, storage facilities, pipelines and utility conveyances and other land uses.
ENGINEERING GEOLOGIST. A state-licensed professional geologist, who, through education, training and experience, practices in the field of engineering geology and geologic hazards meeting the requirements of § 155.225 of the code.
ENGINEERING GEOLOGY. Geologic work that is relevant to engineering and environmental concerns and the health, safety and welfare of the public. ENGINEERING GEOLOGY is the application of geological data, principles and interpretation affecting the planning, design, construction and maintenance of engineered works, land use planning and groundwater issues.
ESSENTIAL FACILITY. Buildings and other structures intended to remain operational in the event of an adverse geologic event, including all structures with an occupancy greater than 1,000 shall also be considered IBC Risk Category III when not meeting the criteria for IBC Risk Category IV; and IBC Risk Category IV buildings and other structures are designated as ESSENTIAL (CRITICAL) FACILITIES.
FAULT. A fracture in the Earth’s crust forming a boundary between rock and/or soil masses that have moved relative to each other, due to tectonic forces. When the fracture extends to the Earth’s surface, it is known as surface fault rupture or a fault trace.
FAULT SCARP. A steep slope or cliff formed by movement along a fault.
FAULT SETBACK. A specified distance on either side of a fault within which structures for human occupancy or critical facilities and their structural supports are not permitted.
FAULT TRACE. The intersection of a fault plane with the ground surface, often present as a fault scarp, or detected as a lineament on aerial photographs or other imagery.
FAULT ZONE. A corridor of variable width along one or more fault traces, within which ground deformation has occurred as a result of fault movement.
GEOLOGIC HAZARD. A geologic condition that presents a risk to life, of substantial loss of real property, or of substantial damage to real property (UCA § 17-27a-103) and includes, but not limited to surface fault rupture, liquefaction, landslides, slope stability, debris flows, rockfalls, avalanches, radon gas and other hazards.
GEOLOGIC HAZARD STUDY AREA. A potentially hazardous area as defined in § 155.223 of this code within which geologic hazard investigations are required prior to development.
GEOTECHNICAL ENGINEER. A state-licensed professional engineer who, through education, training and experience, is competent in the field of geotechnical or geological engineering meeting the requirements of § 155.226 of this code.
GEOTECHNICAL ENGINEERING. The investigation and engineering evaluation of earth materials, including soil, rock and human-made materials and their interaction with earth retention systems, foundations and other civil engineering works. The practice involves the fields of soil and rock mechanics and the earth sciences and requires the knowledge of engineering laws, formulas, construction techniques and performance evaluation.
GOVERNING BODY. The County Commission, or a designee of the Commission.
INFRASTRUCTURE. The improvements which are required to be installed and guaranteed in conjunction with an approved subdivision or other land use approval. INFRASTRUCTURE may be public or private, on-site or off-site, depending on development design and may include streets, curb, gutter, sidewalk, water and sanitary sewer lines, storm sewers, flood control facilities and other similar facilities.
INTERNATIONAL BUILDING CODE (IBC). The latest, statewide adopted International Code Council International Building Code (UCA Title 15A, https://le.utah.gov/xcode/Title15A/15A.html).
LANDSLIDE. The downslope movement of a mass of soil, surficial deposits and/or bedrock, including a continuum of processes between LANDSLIDES, earth flows, debris flows, debris avalanches and rockfalls.
LEGAL LOT OF RECORD. A parcel of land which meets all zoning requirements to be eligible for the development of a dwelling, habitable structure or other facility or structure, pursuant to all county requirements.
LIQUEFACTION. A sudden, large decrease in shear strength of a saturated, cohesionless soil (generally sand and silt) caused by a collapse of soil structure and temporary increase in pore water pressure during earthquake ground shaking. May lead to ground failure, including lateral spreads and flow-type landslides.
LOT. A parcel or tract of land within a subdivision.
MORGAN COUNTY. The County Public Works Director, Engineer, Planning and Development Services Director, Zoning Administrator, Building Official, Commission Administrator, County Commission, land use authority or another county employee or designee.
NON-BUILDABLE AREA. The portion of a site which a geologic hazard investigation report has concluded is impacted by geologic hazards that present an unreasonable and unacceptable risk, and where the siting of habitable structures, accessory structures which house an accessory dwelling unit or business, or critical facilities, are not permitted.
PARCEL. A piece of land created by a partition, subdivision, deed or other instrument recorded with the appropriate recorder. This includes a lot, a lot of record or a piece of land created through other methods.
ROCKFALL. A rock or mass of rock, newly detached from a cliff or other steep slope which moves downslope by falling, rolling, toppling and/or bouncing; includes rockslides, rockfall avalanches and talus.
SETBACK. An area subject to risk from a geologic hazard within which habitable structures or critical facilities and their supports are not permitted.
SLOPE STABILITY. The resistance of a natural or constructed slope to failure by landsliding and assessed under both static and dynamic (earthquake-induced) conditions.
SNOW AVALANCHE. See definition of “avalanche.”
STRUCTURE DESIGNED FOR HUMAN OCCUPANCY. Any residential dwelling or any other structure used or intended for supporting or sheltering any use or occupancy by humans or businesses, includes all Risk Category II structures as defined in the currently adopted International Building Code, but does not include an accessory building which houses no accessory dwelling unit or business.
TALUS. Rock fragments lying at the base of a cliff or a very steep rocky slope.
(Prior Code, § 8-5I-2) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.222 APPLICABILITY.
The regulations contained in this subchapter shall apply to all lands in the unincorporated county.
(A) Every legal lot of record and lot in a proposed land subdivision, within a geologic hazard study area as defined by this subchapter, must have a buildable area safe for the intended use. Each buildable area must also have access from the nearest existing public or private street which is free of unreasonable and unacceptable geologic hazards. Any geologic hazards which must be mitigated in order to provide a buildable area with acceptable and reasonable access must be mitigated prior to issuance of the final plat recordation.
The following are exempt from the provisions of this subchapter:
(1) Detached accessory buildings that are not designed for human occupancy;
(2) Remodeling of existing structures designed for human occupancy if no use change or expansion of the existing structure footprint is proposed; and
(3) Expansion of an existing structure where the structure and proposed addition:
(a) Are not in IBC Risk Category III or IV;
(b) Are not in an area where a site-specific surface fault rupture investigation is required per § 155.236(D);
(c) Are not in an area where slope stability analyses are required per § 155.236(E); and
(d) The area of the addition does not exceed 1,000 square feet or the original structure footprint, whichever is left.
(e) Demolition and replacement of a home or structure made for habitation shall be exempt from the requirements of this section. This shall include demolition due to acts of God as well.
(4) Other properties may also be exempt from certain study requirements, as identified in § 155.236 of this code.
(C) As defined in the currently statewide adopted 2018 International Building Code (IBC), Table 1604.5, the county considers IBC Risk Category III buildings and other structures to represent a substantial hazard to human life in the event of failure, except that any structure with an occupancy greater than 1,000 shall also be considered IBC Risk Category III when not meeting the criteria for IBC Risk Category IV; and IBC Risk Category IV buildings and other structures are designated as essential (critical) facilities.
(D) Individual property owners of a lot or parcel or subdivision application may use the county’s hazard webmap to determine if there are any known hazards for the subject property. If the map shows known hazards then geohazard ordinance reports are required as outlined herein. However, if there are no known hazards as defined below then the property owner may sign an affidavit for assumption of liability waiver. If an affidavit is signed and notarized waiving all county liability then no geohazards report as defined herein shall be required. If the property is developed, or split with hazards on a portion of the lot or parcel, or within the subdivision boundary, the hazard map may still be used if no structure shall be located within the hazard area and if that area will not be disturbed by construction of other infrastructure or structures.
(Prior Code, § 8-5I-3) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021; Ord. 24-12, passed 5-21-2024)
§ 155.223 GEOLOGIC HAZARDS STUDY AREAS.
Geologic hazard study areas are defined as, but not necessarily be limited to:
(A) Areas designated as Qc, Qm, Qmrf, Qms, Qms1, Qmsb, Qmsh, Qmsy, Qmso, Qmt, Qmc, Qmg, Qac, Qg, Qga, Qgy, Qgmy, Qgo, Qgao, Qgm, Qgmo, Qmdf, Qaf, Qafy, Qafo, Qaf1-5, Qafb, Qafp, Qafoe, Qgr, Qmtr, Qmy, Qct and Tn on the most recent geologic maps published by the state’s Geological Survey (UGS, https://geology.utah.gov/). Most maps are available in the UGS interactive geologic map portal (https://geology.utah.gov/apps/intgeomap/), but contact the UGS for interim, progress update and other non-final maps that may be available, but not online; unit Qal, in and of itself, does not require a geologic hazard investigation unless other qualifiers exist, as identified in § 155.236 of this code;
(B) Landslide areas identified in the UGS Utah Landslide Database, available online at: (https://gis.utah.gov/data/geoscience/landslides/);
(C) Areas requiring slope stability analyses as defined in § 155.236 of this code below;
(D) All properties subject to debris flows; and
(E) Site-specific surface fault rupture investigations are required for all critical facilities and structures for human occupancy (International Building Code (IBC) Risk Category II, III and IV) along latest Pleistocene-Holocene faults, and for critical facilities (IBC Risk Category IV) along late Quaternary and Quaternary faults. For noncritical facilities for human occupancy (IBC Risk Category II and III) along late Quaternary and Quaternary faults, investigations are recommended, but not required. See the UGS Utah Quaternary Fault and Fold Database to locate Quaternary age faults within the county and to determine their activity class (https://geology.utah.gov/apps/qfaults/index.html).
(Prior Code, § 8-5I-4) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021; Ord. 24-12, passed 5-21-2024)
§ 155.224 RESPONSIBILITY FOR GEOLOGIC HAZARD INVESTIGATIONS.
(A) If a property owner (individual lots, parcels, or subdivision applicants) uses the county owned and maintained hazard webmap and that map shows that there are no known hazards or relevant soils, slopes, etc. as defined above then the property may sign an affidavit of liability waiver and no geohazards report shall be required. If the property is split with hazards on a portion of the lot or parcel, or within the subdivision boundary, the hazard map may still be used if no structure shall be located within the hazard area and if that area will not be disturbed by construction of other infrastructure or structures.
(B) Geologic hazard investigations often involve both engineering geology and geotechnical engineering.
(C) Engineering geology investigations shall be performed under the direct supervision of a state-licensed professional geologist specializing in engineering geology as defined in § 155.225 of this code.
(D) Geotechnical engineering investigations shall be performed under the direct supervision of a state-licensed professional engineer specializing in geotechnical engineering as defined in § 155.226 of this code. Licenses may be verified with the state’s Division of Occupational and Professional Licensing
(https://secure.utah.gov/llv/search/index.html)
.
(E) If the property owner wishes to contest the map and believes that there are no hazards due to slope, soil, etc., then they may opt for a geologic hazards reconnaissance where they shall be required to hire a licensed geologist to do preliminary site work to make a determination if the map is incorrect. A report will be required to be submitted by the applicant’s geologist with stamp and signature. This shall be reviewed by the county geologist who will make the final determination if a full report will be required.
(F) The digging of test pits in the area of the foundation shall not be permitted to prevent compromising the stability and integrity of the site.
(Prior Code, § 8-5I-5) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021; Ord. 24-12, passed 5-21-2024)
§ 155.225 MINIMUM QUALIFICATIONS OF THE ENGINEERING GEOLOGIST.
(A) Engineering geology and the evaluation of geologic hazards is a specialized discipline within the practice of geology requiring the technical expertise and knowledge of techniques not commonly used in other geologic investigations. Therefore, geologic hazard investigations involving engineering geology and geologic hazard investigations shall be conducted, signed and sealed by a state-licensed professional geologist specializing in engineering geology and geologic hazards. Proof of qualifications shall be provided to the county upon request.
(B) The minimum qualifications required by the county for an engineering geologist, include:
(1) An undergraduate or graduate degree in geology, engineering geology or geological engineering or closely related field, from an accredited college or university; and
(2) Five full-time years of experience in a responsible position in the field of engineering geology and geologic hazards in the state, or in a state with similar geologic hazards and regulatory environment and experience demonstrating the geologist’s knowledge and application of appropriate techniques in geologic hazard investigations.
(Prior Code, § 8-5I-6) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.226 MINIMUM QUALIFICATIONS OF THE GEOTECHNICAL ENGINEER.
(A) Evaluation and mitigation of geologic hazards often require contributions from a qualified geotechnical engineer, particularly in the design of mitigation measures. Geotechnical engineering is a specialized discipline within the practice of civil engineering requiring the technical expertise and knowledge of techniques not commonly used in civil engineering. Therefore, geologic hazard investigations that include engineering design and related tasks shall be conducted, signed and sealed by a state-licensed professional engineer, specializing in geotechnical engineering and geologic hazards. Proof of qualifications shall be provided to the county upon request.
(B) The minimum qualifications required by the county for a geotechnical engineer, include:
(1) A graduate degree in civil or geological engineering, with an emphasis in geotechnical engineering; or a B.S. degree in civil or geological engineering with 12 semester hours of post B.S. credit in geotechnical engineering, or course content closely related to evaluation of geologic hazards, from an ABET accredited college or university; and
(2) Five full-time years of experience in a responsible position in the field of geotechnical engineering and geologic hazards in the state, or in a state with similar geologic hazards and regulatory environment, and experience demonstrating the engineer’s knowledge and application of appropriate techniques in geologic hazard investigations.
(Prior Code, § 8-5I-7) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.227 PRELIMINARY ACTIVITIES.
This section shall apply to any geologic hazard investigation for the purpose of determining the feasibility of land use approval for a concept plan; preliminary or final plat approval; a commercial, institutional or one-, two- or multi-family dwelling conditional use permit; site plan approval; or for the purpose of exploring, evaluating or establishing locations for permanent improvements. A geologic hazard report shall be submitted to the county as part of the land use application, pursuant to the requirements of this subchapter, for any proposed development in a geologic hazard study area.
(A) (1) Prior to a land use application, the applicant shall schedule a scoping meeting with the county to evaluate the engineering geologist’s or geotechnical engineer’s investigative approach. The investigation approach shall allow for flexibility due to unexpected site conditions. Field findings may require modifications to the work plan. Upon completion of a scoping meeting, an application for an excavation or grading permit, as necessary, may be submitted to the county. At this meeting, the applicant shall present a work plan that includes the locations of anticipated geologic hazards and proposed subsurface exploratory excavations, such as test pits, trenches, borings and cone penetrometer test (CPT) soundings, which meet the minimum acceptable regional standards of practice, this subchapter, the adopted appendices on file with the County Clerk, and includes:
(a) A property location map;
(b) A geologic map;
(c) A topographic map with contours;
(d) A slope map or lidar imagery, if available;
(e) A map showing the location of the proposed development, including structures, roads, depths of basements and foundations and the like; locations of proposed subsurface exploration, such as trenches, borings, test pits and the like; with a description of the proposed development; and
(f) A map showing the slope stability analysis cross-section locations.
(2) The applicant’s professional consultants will find geologic information available from the UGS and other sources useful in planning site development, preparing for the scoping meeting and in performing geologic hazard investigations. Available UGS information includes:
(3) Lidar elevation data are available at (https://gis.utah.gov/data/elevation-and-terrain/) and (http://opentopography.org).
(B) The county will arrange for a state-licensed professional geologist, specializing in engineering geology, to attend the scoping meeting on behalf of the county, at the applicant’s expense. The county’s geologist will provide verbal feedback to the applicant and consultants regarding the proposed work plan and the requirements of this subchapter. Reimbursement to the county for the direct costs of any outsourced staff shall be paid by the applicant prior to the acceptance of a land use or building permit application.
(C) As required by this code and except as otherwise noted herein, no person shall commence or perform any land disturbance, grading, excavation, relocation of earth or any other land disturbance activity, without first obtaining an excavation or grading permit. Application for an excavation or grading permit shall be filed with the county on forms furnished by the county for such purposes only after a scoping meeting has taken place.
(D) The applicant shall specify a primary contact responsible for coordination with the county during the land disturbance activities.
(Prior Code, § 8-5I-8) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.229 GEOLOGIC HAZARD INVESTIGATIONS AND REPORTS REQUIRED.
(A) Any applicant requesting land use approval for a concept plan; preliminary or final plats; a commercial, institutional or one-, two- and multi-family dwelling conditional use permit; or site plan approval on a parcel or parcels of land within a geologic hazard study area or where there are known or readily apparent geologic hazards and the area is not included within a current geologic hazard study area, shall submit to the county three wet-stamped paper copies and one unlocked, digital PDF copy of a site-specific geologic hazard investigation report that specifically relates to the geologic hazards present and affecting on the site.
(B) A field review by the county is required during subsurface exploration activities conducted for geologic investigations (test pits, trenches, drilling and the like) to allow the county to evaluate the subsurface conditions, such as the age and type of deposits encountered and the presence or absence of faulting, with the applicant. Discussions about questionable features or appropriate setback distances are appropriate, but the county will not assist with field logging, explaining stratigraphy or give verbal approval of the proposed development during the field review. Exploratory trenches, when excavated, shall be open, safe and in compliance with applicable federal Occupational Safety and Health Administration, the state and other excavation safety regulations, have the walls appropriately cleaned, and a field log completed by the time of the review. The applicant must provide a minimum notice of 48 hours to schedule the field review with the county.
(Prior Code, § 8-5I-10) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.230 SUBMITTAL AND CERTIFICATION OF GEOLOGIC HAZARD REPORTS.
(A) All applicants for land use approval within a geologic hazard study area shall prepare and submit a geologic hazard report (may be combined with a geotechnical and/or other geologic reports) pursuant to the requirements of this subchapter prior to any consideration for a concept plan; preliminary or final plat; commercial, institutional or one-, two- and multi-family dwelling; or any conditional use permit which requires site plan approval. Additionally, the applicant is required to submit the following additional information:
(1) A written, stamped certification from a state-licensed professional geologist that the geologic hazard report has been prepared pursuant to the requirements of this subchapter;
(2) A written, stamped certification from a state-licensed professional geologist and a professional engineer that every proposed development lot, building pad and parcel does not present an unreasonable or unacceptable risk to the health, safety and welfare of persons or property, including buildings, storm drains, public streets, culinary water facilities, utilities or critical facilities, whether off-site, on adjacent properties or on-site, because of the presence of geologic hazards or because of modifications to the site due to the proposed land use;
(3) A written, stamped certification from a state-licensed professional geologist and a professional engineer that every proposed development lot, building site and parcel layout demonstrates that, consistent with regional standards of practice, the identified geologic hazards can be mitigated to a level where the risk to human life and damage to property are reduced to an acceptable and reasonable level in a manner which will not violate applicable federal, state and local statutes, ordinances and regulations. Mitigation measures shall consider in their design, the intended aesthetic functions of other governing ordinances of the county;
(4) A written, stamped certification from a state-licensed professional geologist and a professional engineer along with a mitigation plan, if necessary, that demonstrates that the identified hazards or limitations will be addressed without impacting or adversely affecting off-site areas, including adjacent properties. Mitigation measures must be reasonable and practical to implement and shall not require ongoing maintenance by property owners; and
(5) Written verification from the issuer of professional errors and omissions liability insurance, in the amount of $2,000,000 each, which covers the state-licensed professional geologist and professional engineer, and which are in effect on the date of preparation and submittal of all required reports and certifications.
(B) The county may set other requirements as are necessary to mitigate any geologic hazards and to ensure that the purposes of this subchapter are met. These requirements may include, but are not limited to:
(1) Additional or more detailed investigations and professional certifications to understand or quantify the hazards and/or determine whether mitigation measures recommended in the report are adequate;
(2) Specific mitigation requirements, establishing buildable and non-buildable areas, limitations on slope grading, controls on grading and/or revegetation;
(3) Prior to receiving a grading, excavation or building permit, final grading plans, when required, shall be prepared, signed and sealed by the state-licensed professional engineer and the engineering geologist (professional geologist) and geotechnical engineer (professional engineer) that prepared the geologic hazard and geotechnical report(s) to verify that their recommendations have been appropriately incorporated in the final grading plan and that building locations are approved;
(4) As built grading plans, when required, shall be prepared, signed and sealed by the state-licensed professional geologist and professional engineer that prepared the geologic hazard and geotechnical report(s) to verify that their recommendations have been appropriately incorporated and that building locations are approved, prior to the issuance of a building permit;
(5) Grading plans, when required, shall include, at a minimum, the following:
(a) Maps of existing and proposed contours and the source and accuracy of topographic data used;
(b) Present and proposed slopes for each graded area;
(c) Existing and proposed drainage patterns;
(d) Location and depth of all proposed cuts and fills;
(e) Description of the methods to be employed to achieve soil and/or rock stabilization and compaction, as appropriate;
(f) Location and capacities of proposed structures, and drainage and erosion control measures based on maximum runoff for a 100-year storm or greater;
(g) Location of existing buildings, structures, roads, wells, retention and other basins, and on-site sewage disposal systems on or within 100 feet of the site that may be affected by the proposed grading and construction; and
(h) A plan for construction monitoring and documentation of testing, field inspection during grading and reporting to the county.
(6) Installation of monitoring equipment for surface and subsurface geologic conditions, including determining groundwater levels; and
(7) Other requirements, such as time schedules for completion of the mitigation and phasing of development.
(C) The county may also set requirements necessary to protect the health, safety and welfare of the citizens of the county, protect the county’s infrastructure and financial health and minimize potential adverse effects of geologic hazards to the public health, safety and property as a condition of approval of any development which requires a geologic hazard report.
(D) The county may require the engineering geologist and geotechnical engineer that prepared the geologic hazard and/or geotechnical report(s) to be on-site, at the cost of the applicant, during certain phases of construction, particularly during grading phases and the construction of retaining walls in excess of four feet in exposed height and geologic hazard mitigation.
(E) The county shall review any proposed land use that requires preparation of a geologic report under this subchapter to determine the possible risks to the health, safety and welfare of persons, property and county infrastructure from geologic hazards.
(1) Prior to consideration of any request for preliminary plat approval or site plan approval, the geologic report, if required, shall be submitted to the county for review.
(2) The county will complete each review in a reasonable time frame, not to exceed 45 days.
(3) All direct costs associated with the review of the geologic report shall be paid by the applicant.
(4) The county shall determine whether the report complies with the following standards:
(a) A suitable geologic hazards report has been prepared by qualified, state-licensed professionals;
(b) The proposed land use does not present an unreasonable risk to the health, safety and welfare of persons or property, including buildings, storm drains, public streets, culinary and other water facilities, utilities or critical facilities, whether off-site or on-site, or to the aesthetics and natural functions of the landscape, such as slopes, streams or other waterways, drainage or wildlife habitat, whether off-site or on-site, because of the presence of geologic hazards or because of modifications to the site due to the proposed land use;
(c) The proposed land use demonstrates that, consistent with the current, regional state of practice, the identified geologic hazards can be mitigated to a level where the risk to human life and damage to property are reduced to an acceptable and reasonable level in a manner which will not violate applicable federal, state and local statutes, ordinances and regulations. Mitigation measures should consider in their design, the intended aesthetic functions of other governing ordinances, such as the county’s Sensitive Lands Overlay Zone. The applicant must include with the geologic report, a mitigation plan that defines how the identified hazards or limitations will be addressed without impacting or adversely affecting off-site areas. Mitigation measures must be reasonable and practical to implement, especially if such measures require on-going maintenance by property owners; and
(d) Should a geologic report be found deficient with respect to this subchapter and/or the current, regional state of practice, a letter will be provided to the applicant summarizing the specific deficiencies. If a report is found deficient three times or a report was excessively deficient, the county may notify the state’s Division of Occupational and Professional Licensing about the licensed professional(s) deficient reports that were submitted to a public entity that were not in compliance with Utah Admin. Code R156-76-502 (professional geologists) and/or R156-22-502 (professional engineers).
(F) For any real property with respect to which development has proceeded on the basis of a geologic hazard and/or geotechnical report which has been accepted by the county, no final inspection shall be completed, certificate of occupancy issued, or performance bond released until the engineering geologist and geotechnical engineer who signed, stamped and approved the report(s), certifies in writing, that the completed development, improvements and structures conform to the descriptions and requirements contained within the report, and that all the required inspections were made and approved by the engineering geologist and geotechnical engineer that prepared said report(s). If the preparing engineering geologist and geotechnical engineer are unavailable, an engineering geologist and geotechnical engineer, similarly qualified and licensed in the state, shall provide the certifications.
(G) An applicant may appeal any decision made under the provisions of this subchapter only after the land use authority has issued a final decision and shall set forth the specific grounds or issues upon which the appeal is based. The appeal shall be submitted in writing to the county in accordance with the appeals provisions of this chapter and current state code.
(H) The county shall retain a copy of each geologic hazard and/or geotechnical report in the Planning and Development Services Department project file, which will be available for public inspection and will provide a copy to the UGS for archiving. Currently, the UGS archives these reports in the GeoData Archive System (https://geodata.geology.utah.gov/pages/search.php?search=!collection476).
(Prior Code, § 8-5I-11) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.231 DISCLOSURE WHEN A GEOLOGIC HAZARD REPORT IS REQUIRED.
(A) Whenever a geologic hazard report is required under this subchapter; the owner of the lot or parcel shall record a notice running with the land on a form provided by the county prior to the approval of any development or subdivision of such parcel or commencement of construction activity. The notice shall be recorded against the property in the County Recorder’s office as per the recording standards.
(B) Disclosure shall include signing a disclosure and acknowledgment form provided by the county, which includes:
(1) Notice that the parcel is located within a geologic hazard study area or as otherwise defined in this subchapter; and
(2) Notice that a geologic hazard report was prepared and is available for public inspection in the county’s files.
(C) Where geologic hazards, related setbacks and non-buildable areas are delineated in a subdivision, the owner shall also place additional notification on the plat stating the above information, prior to final approval of the plat by the county.
(Prior Code, § 8-5I-12) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.232 WARNING AND DISCLAIMER.
(A) The geologic hazard study areas designated herein represent only those potentially geologic hazardous areas known to the county and should not be construed to include all possible potential hazard areas. The geologic hazard ordinance and the geologic hazard study areas may be amended as new information becomes available, pursuant to procedures set forth in § 155.223 of this code.
(B) The provisions of this subchapter do not in any way ensure or imply that areas outside the geologic hazard study areas are free from the possible adverse effects of geologic hazards. This subchapter shall not create any liability on the part of the county, its officers, reviewers or employees thereof, for any damages from geologic hazards that result from reliance on this subchapter or any administrative requirement or decision lawfully made hereunder.
(Prior Code, § 8-5I-13) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.233 CHANGE OF USE.
No change in use which results in the conversion of a building or structure from one not used for human occupancy to one that is so used shall be permitted unless the building or structure complies with the provisions of this subchapter.
(Prior Code, § 8-5I-14) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.234 CONFLICTING REGULATIONS.
In cases of conflict between the provisions of existing zoning classifications, Building Codes, the subdivision ordinance or any other ordinance of the county and the geologic hazard ordinance codified in this subchapter, the most restrictive provision shall apply.
(Prior Code, § 8-5I-15) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.235 COUNTY HOLD HARMLESS AGREEMENT.
(A) Applicants receiving any permit or approval within a geologic hazard study area shall be required to sign and record on the property a hold harmless agreement.
(B) An example of such an agreement is presented in Appendix A, on file with the County Clerk.
(Prior Code, § 8-5I-16) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)
§ 155.236 GEOLOGIC HAZARD INVESTIGATIONS AND REPORTS.
(A) Each geologic hazard investigation and report shall be site-specific and shall identify all known or suspected potential geologic hazards, whether previously identified or unrecognized, that may affect the subject property, both on and adjacent to the property, a geologic hazard report may be combined with a geotechnical report and/or contain information on multiple hazards;
(B) All geologic hazard reports shall meet the submittal and preparation requirements of this subchapter, its appendices, UGS Circular 122: Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Hazard Ordinances in Utah, Chapter 2 (https://ugspub.nr.utah.gov/publications/circular/c-122.pdf), and the following:
(1) A one to 24,000 scale geologic map with references, showing the general surface geology, including landslides, rockfall, alluvial fans, bedrock geology where exposed, bedding attitudes, faults, other geologic structural features and the location of any other known geologic hazards;
(2) A detailed site geologic map and geologic cross-section(s), at a scale equal to or more detailed than one inch equals 100 feet to illustrate local geologic structure. The site geologic map shall include locations of all subsurface exploratory trenches, test pits, borings and the like, and site-specific geologic mapping performed as part of the geologic hazard investigation, including boundaries and features related to any geologic hazards, topography and drainage. The site geologic map must show the location and boundaries of the property, geologic hazards, delineation of any recommended setback distances from hazards and recommended locations for structures. Buildable and non-buildable areas shall be clearly identified;
(3) Trench and test pit logs, when applicable, prepared in the field with standard geologic nomenclature at a scale equal to or more detailed than one inch equals five feet. Final, drafted logs may also be submitted with the field prepared logs;
(4) Boring logs, when applicable, prepared with standard geologic and engineering nomenclature;
(5) Conclusions and recommendations, clearly supported by adequate data, included in the report, that summarize the characteristics of the geologic hazards and that address the potential effects of the geologic conditions and geologic hazards on the proposed development and occupants thereof, particularly in terms of risk and potential damage;
(6) An evaluation of whether mitigation measures are required, including an evaluation of multiple, viable mitigation options that include specific recommendations for avoidance or mitigation of the effects of the hazards, consistent with the purposes set forth in § 155.220 of this code, including design or performance criteria for engineered mitigation measures and all supporting calculations, analyses, modeling or other methods and assumptions. Final design plans and specifications for engineered mitigation must be signed and stamped by a qualified, state-licensed geotechnical, civil and/or structural engineer, as appropriate;
(7) A statement shall be provided regarding the suitability of the site for the proposed development from a geologic hazard perspective;
(8) All geologic hazard reports shall include the qualifications and original signature and professional seal(s), both in blue ink, of the qualified, state-licensed professional(s); and
(9) When a submitted report does not contain adequate data to support its findings, additional or more detailed investigations shall be required by the county to explain and/or quantify the geologic hazard and/or to describe how mitigation measures recommended in the report are appropriate and adequate.
(C) When a final geologic hazard report indicates that a geologic hazard does not exist within an adopted geologic hazard study area indicated by a map referenced by this subchapter, the county will consider the new geologic information in potentially revising the adopted hazard maps to remove the specific area from the adopted geologic hazard study area. The county will also forward this information to the state’s Geological Survey for potential update of its hazard maps.
(D) Surface fault rupture is a displacement of the ground surface along a tectonic fault during an earthquake. If a fault were to displace the ground surface beneath a building or other structure, significant structural damage or collapse may occur, possibly causing injuries and loss of life. As a result, the county requires site-specific surface fault rupture hazard investigations and submittal of a report for all properties that contain Quaternary faults, depending on the fault activity level and the IBC Risk Category of proposed structures. These investigations and reports shall, as appropriate, conform with the Guidelines for Evaluating Surface Fault Rupture Hazards in Utah (UGS Circular 122 https://ugspub.nr.utah.gov/publications/circular/c122.pdf), and:
(1) The requirement for site-specific investigation of surface faulting depends on fault activity level as defined by the most recent Western States Seismic Policy Council (WSSPC) policy recommendation (https://www.wsspc.org/public-policy/adopted-recommendations/) for faults that cross properties with proposed structures. The current policy recommendation is 18-3: Definitions of Recency of Surface Faulting for the Basin and Range Province and defines latest Pleistocene-Holocene, late Quaternary and Quaternary faults as:
(a) Latest Pleistocene-Holocene fault. A fault whose movement in the past 15 ka (15,000 years) has been large enough to break the ground surface;
(b) Late Quaternary fault. A fault whose movement in the past 130 ka (130,000 years) has been large enough to break the ground surface; and
(c) Quaternary fault. A fault whose movement in the past 2.6 Ma (2.6 million years) has been large enough to break the ground surface. The county requires site-specific investigation on parcels with latest Pleistocene-Holocene faults for all new critical facilities and structures for human occupancy (IBC Risk Category II, III and IV structures), on parcels with latest Pleistocene-Holocene and late Quaternary faults for all new critical facilities (IBC Risk Category III and IV structures) and on parcels with the faults listed in division (D)(2) below.
(2) (a) The state’s Geological Survey (UGS) Utah Quaternary Fault and Fold Database (https://geology.utah.gov/apps/qfaults/index.html) provides the latest information on Quaternary faulting in Utah to determine fault activity levels as defined above. Where data are inadequate to determine the fault activity class, the fault shall be assumed to be latest Pleistocene-Holocene, pending detailed surface fault rupture and/or paleoseismic investigations.
(b) The database currently includes the following mapped Quaternary faults within the county:
1. East Canyon fault: Quaternary;
2. Morgan fault, central section: One trace Holocene and one trace Quaternary;
3. Morgan fault, northern section: Quaternary;
4. Morgan fault, southern section: Quaternary; and
5. Saleratus Creek fault: Quaternary.
(c) The county may require a site-specific investigation if on-site and/or nearby fault-related features not shown in the database are identified during other geologic or geotechnical investigations or during project construction.
(3) Surface fault rupture hazard maps show the locations of fault traces and recommended special study areas. These maps are published by the UGS but are currently not available for the county. Once these maps are available, at that time they will be adopted to become part of this subchapter. As a result, investigations are required within special study areas as defined by:
(a) Areas that horizontally extend 500 feet on the down thrown and 250 feet on the upthrown side of well-defined faults (solid lines) and 1,000 feet on both sides of buried or inferred faults (dotted lines). For traces of buried or inferred faults less than 1,000 feet long that lie between and on-trend with well-defined faults or lie at the tail end of a well-defined faults, the well-defined fault Special Study Area Zone is used (Figure 155.236-1 below);
(b) In areas where a buffer “window” exists, the window is filled in if its width is less than the greater of the two surrounding buffers (Figure 155.236-1 below). In situations where the ground expression of the fault scarp is larger than the Special Study Zone, in which case the zone does not cover the entire fault scarp, the 1,000-foot buffer is used; and
(c) Well-defined faults are those fault traces that are clearly identifiable by a geologist qualified to conduct surface fault rupture hazard investigations as a physical feature at or just below the ground surface (typically shown as a solid line on geologic maps), and buried or inferred faults are those fault traces that are not evident at or just below the ground surface by a qualified geologist (typically shown as a dotted line for buried faults and a dotted line for inferred faults on geologic maps). Investigations are required for all critical facilities, whether near a mapped Quaternary fault or not, to ensure that previously unknown faults are not present. If evidence for a Quaternary fault is found, subsurface investigations are required and trenching to locate a suitable buildable area may be necessary (IBC §§ 1704.6.1 and 1803.5.11).
(4) When an alternative subsurface exploration plan is proposed in lieu of paleoseismic trenching, a map and written description and plan shall be submitted to the county for review, prior to the scoping meeting and exploration implementation. The plan must include, at a minimum, a map of suitable scale showing the site limits, surface geologic conditions within 2,000 feet of the site boundary, the location and type of the proposed exploration and the anticipated subsurface geologic conditions.
(5) Small-displacement faults or those faults with less than four inches of displacement are not exempted from structure setback requirements. However, if structural risk-reduction measures are proposed for these faults, the following criteria must be met:
(a) Reasonable geologic data indicating that future surface displacement along the faults will not exceed four inches (see UGS Circular 122); and
(b) Specific structural mitigation to minimize structural damage and ensure safe occupant egress designed by a state-licensed structural engineer with plans and specifications reviewed and approved by the county.
(E) Landslides are the downslope movement of earth (soil, rock and/or debris) materials and can cause significant property damage, injury and/or death. The evaluation of landslides generally requires quantitative slope stability analyses, involving engineering geologists and geotechnical engineers experienced in landslide investigation, analysis and mitigation. Considering the complexity inherent in performing slope stability analyses, additional effort beyond the minimum standards presented herein may be required at some sites to adequately address slope stability. Slope stability and landslide hazard investigations and reports shall conform with the Guidelines for Evaluating Landslide Hazards in Utah (UGS Circular 122, https://ugspub.nr.utah.gov/publications/circular/c-122.pdf), as appropriate.
(1) Landslide hazard maps show the location of previous landsliding, areas of potential landsliding and recommended special study areas. These maps are published by the UGS but are not currently available for the county. Once these maps are available, at that time they will be adopted to become part of this subchapter. As a result, investigations are required within geologic hazard study areas as defined by:
(a) Cut and fill slopes steeper than, or equal to, 2H:1V (horizontal (H): vertical (V));
(b) Natural slopes steeper than or equal to 15% or 6.67H:1V;
(c) Natural and cut slopes with geologic conditions, such as bedding, foliation or other structural features, that are potentially averse to slope stability;
(d) Natural and cut slopes that include a geologic hazard, such as an existing landslide, irrespective of the slope height or gradient;
(e) Buttresses and stability fills;
(f) Cut, fill, and natural slopes of water-retention basins or flood-control channels;
(g) Units Qm, Qms, Qms1, Qmsy, Qmso, Qmc, Qmg, Qac, Qg, Qga, Qgy, Qgmy, Qgay, Qgo, Qgao, Qgm, Qgmo and Tn on the most recent geologic maps published by the UGS (c). Most maps are available in the UGS Interactive Geologic Map Portal (https://geology.utah.gov/apps/intgeomap/), but contact the UGS for interim, progress update and other non-final maps that may be available, but not online; and
(h) Mapped landslide areas in the Utah Landslide Database, available at: (https://gis.utah.gov/data/geoscience/landslides/). Development of properties within these areas require submittal and review of a site-specific geologic hazard report discussing landslide hazards, prior to receiving a land use or building permit from the county. It is the responsibility of the applicant to retain a qualified engineering geologist and geotechnical engineer to perform the slope stability analysis.
(2) When evaluating site conditions to determine the need for slope stability analyses, off-property conditions shall be considered (both up-slope to the tops of adjacent, ascending slopes and down-slope to and beyond the toes of adjacent, descending slopes). Also, the professionals shall demonstrate that the proposed hillside development will not affect adjacent sites or limit adjacent property owners’ ability to develop their sites.
(3) Investigations shall also address the potential for surficial instability, rock slope instability, debris/mudflows, rockfalls and soil creep on all slopes that may affect the proposed development, be affected by the proposed development, and along access roads. Intermediate geomaterials (IGM), those earth materials with properties between soil and rock, if present, shall be appropriately investigated, sampled and tested.
(4) An engineering geologist shall provide appropriate input to the geotechnical engineer with respect to the potential impact of the geology, stratigraphy and hydrologic conditions on slope stability. The shear strength and other geotechnical properties shall be evaluated by the geotechnical engineer. Qualified engineering geologists may assess and quantitatively evaluate slope stability; however, the geotechnical engineer shall perform all design stability calculations. Ground motion parameters for use in seismic stability analysis may be provided by either the engineering geologist or the geotechnical engineer.
(5) Except for the derivation of the input ground motions for pseudostatic and seismic deformation analyses described below, slope stability analyses and evaluations shall be performed in general accordance with the latest version of Recommended Procedures for Implementation of DMG Special Publication 117: Guidelines for Analyzing and Mitigating Landslide Hazards in California (Blake and others, 2002). Procedures for developing input ground motions to be used in the county are described below. If on-site sewage and/or stormwater disposal exists or is proposed, the slope stability analyses shall include the effects of the effluent plume on slope stability.
(6) The minimum acceptable static factor of safety (FS) is one and one-half for both overall and surficial slope stability and one for a calibrated pseudostatic analysis using Stewart and others (2003) or other method preapproved by the county.
(7) Soil and/or rock sampling shall be based on current ASTM International or American Association of Highway Officials (AASHTO) standards, as appropriate.
(8) Soil and/or rock properties, including unit weight and shear strength parameters (cohesion and friction angle), shall be based on conventional laboratory tests on appropriate samples. Laboratory tests shall be performed using current ASTM International or AASHTO standards, as appropriate, in a laboratory accredited by the AASHTO Materials Reference Laboratory and/or the U.S. Army Corps of Engineers to ensure compliance with current laboratory testing standards and quality control procedures. Where appropriate, such as for landslide slip surfaces, along bedding planes, for surficial stability analyses and the like, laboratory tests for saturated, residual shear strengths must be performed. Estimation of the shear resistance along bedding or landslide planes normally requires an evaluation of saturated, residual, along-bedding strength values of the weakest interbedded or slide plane material encountered during the subsurface exploration, or in the absence of enough exploration, the weakest material that may be present, consistent with site geologic conditions. Soil strength parameters derived solely from CPT data are most often not appropriate for slope-stability analysis in many cases, particularly for strengths along existing slip surfaces, where residual strengths have developed. Additional guidance on the selection of strength parameters for slope stability analyses is contained in Blake and others (2002).
(9) Residual strength parameters may be determined using direct or ring shear testing equipment; however, ring shear tests are preferred. If performed properly, direct shear test results may approach ring shear test results. The specimen must be subjected to enough deformation (such as, a significant number of shearing cycles in the direct shear test or a significant amount of rotation in the ring shear test) to ensure that residual strength has been developed. In the direct and ring shear tests, stress-deformation curves can be used to determine when an enough shearing cycles have been performed by showing that no further significant drop in shear strength results with the addition of more cycles or rotation. The stress-deformation curves obtained during the shear tests must be submitted with the other pertinent laboratory test results. It shall be recognized that for most clayey soils, the residual shear strength envelope is curved and passes through the origin (for example, at zero normal stress there is zero shear strength). Any apparent shear strength increases resulting from a non-horizontal shear surface, such as ramping or bulldozing in residual direct shear tests, shall be discounted in the interpretation of the strength parameters.
(10) Inherent in the analyses, the geotechnical engineer will need to use judgment in the selection of appropriate shear test methods and in the interpretation of the results to develop shear strength parameters commensurate with the slope stability conditions to be evaluated. Scatter plots of shear strength data may need to be presented to allow for assessment of idealized parameters. The report shall summarize shear strength parameters used for slope stability analyses and describe the methodology used to interpret test results and estimate those parameters.
(a) Peak shear strengths may be used to represent across-bedding failure surfaces or compacted fill, in situations where strength degradations are not expected to occur (see Blake and others, 2002). Where peak strengths cannot be relied upon, fully softened or lower strengths shall be used.
(b) Ultimate shear strength parameters shall be used in static slope stability analyses when there has not been past deformation. Residual shear strength parameters shall be used in static slope stability analyses when there has been past deformation.
(c) Averaged strength parameters may be appropriate for some across-bedding conditions, if enough representative samples have been carefully tested. Analyses for along bedding or along existing landslide slip surfaces shall be based on the lower-bound interpretations of residual shear strength parameters and comparison of those results to correlations, such as those of Stark and others (2005).
(11) In the county, failure surfaces for known landslides commonly occur within the Norwood Formation. In cases when the failure surface has been sampled, tested and back-calculations performed of historic landslides, relatively low residual-shear-strength values of cohesion equal to zero pounds per square foot (psf) and friction angles equal to seven to nine degrees have been determined. To assist in understanding shear strengths of these materials, a cohesion equal to zero psf and a friction angle equal to seven degrees, shall be used for landslide failure surfaces and along weak layers within the Norwood Formation, unless otherwise determined. If site-specific testing produces lower residual shear strength than these values, the site-specific test results shall be used. If site-specific testing produces higher values, documentation must be provided to clearly demonstrate that the weakest materials were sampled, properly tested and that the materials sampled truly represent the basal landslide slip surface.
(12) The potential effects of soil creep shall be addressed where any proposed structure is planned near an existing fill or natural slope. The potential effects on the proposed development shall be evaluated and mitigation measures proposed, including appropriate setback recommendations that consider the potential effects of creep forces.
(13) Gross stability includes rotational and translational deep-seated slope failures or portions of slopes existing within or outside of, but potentially affecting the proposed development. The following guidelines, in addition to those in Blake and others (2002), shall be followed when evaluating slope stability:
(a) Stability shall be analyzed along cross-sections depicting the most adverse conditions, such as the highest slope, most adverse bedding planes, shallowest likely groundwater table, steepest slope and the like. Often, analyses are required for different conditions and for more than one cross-section to demonstrate which condition is the most adverse. When evaluating the stability of an existing landslide, analyses must also address the potential for partial reactivation. Inclinometers may be used to help determine critical failure surfaces, and along with high-precision GPS/GNSS, the activity state of existing landslides. The critical failure surfaces on each cross-section shall be identified, evaluated and plotted on the large-scale cross-section;
(b) Rock slope stability shall be based on current rock mechanics practice, using the methods of Wyllie and Mah (2004), based on Hoek and Bray (1981); Practical Rock Engineering: (https://www.rocscience.com/assets/resources/learning/hoek/Practical-Rock-Engineering-FullText.pdf); Federal Highway Administration (1989); and similar references, such as the following: (https://www.rocscience.com/learning/hoeks-corner/publications);
(c) If the long-term static FS is < l.5, mitigation measures shall be required to bring the factor of safety up to the required level or the project may be redesigned to achieve a minimum FS of > 1.5;
(d) The temporary stability of excavations shall be evaluated, and mitigation measures shall be recommended as necessary to obtain a minimum FS of > 1.3;
(e) Long-term slope stability shall be analyzed using the highest known and anticipated groundwater level based upon a groundwater assessment as described in UGS Circular 122: Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Hazard Ordinances in Utah, Chapter 2, along with groundwater sensitivity analyses: (https://ugspub.nr.utah.gov/publications/circular/c-122.pdf);
(f) Slope stability cannot be contingent on uncontrollable factors, such as limiting landscape irrigation and the like;
(g) Where back-calculation is appropriate, shear strengths utilized for design shall be no higher than the lowest strength computed using back calculation. If a professional proposes to use shear strengths higher than the lowest back-calculated value, justification shall be required. Assumptions used in back-calculations regarding pre-sliding topography and groundwater conditions at failure must be discussed and justified;
(h) Reports shall describe how the shear strength testing methods used are appropriate in modeling field conditions and the long-term performance of the analyzed slope. The utilized design shear strength values shall be justified with laboratory test data and geologic descriptions and history, along with past performance history, if known, of similar materials;
(i) Reports shall include shear strength test plots consisting of normal stress versus shear resistance (failure envelope). Plots of shear resistance versus displacement shall be provided for all residual and fully softened (ultimate) shear tests;
(j) The degree of saturation for all test specimens shall be reported. Direct shear tests on partially saturated samples may grossly overestimate the cohesion that can be mobilized when the material becomes saturated in the field. This potential shall be considered when selecting shear strength parameters. If the rate of shear displacement exceeds five thousandths of one inch per minute, the professional shall provide data to demonstrate that the rate is sufficiently slow for drained conditions;
(k) Shear strength values higher than those obtained through site-specific laboratory tests will generally not be accepted;
(l) If direct shear or triaxial shear testing is not appropriate to model the strength of highly jointed and fractured rock masses, the design strengths shall be evaluated in a manner that considers overall rock mass quality and be consistent with current rock mechanics practice;
(m) Shear strengths used in slope stability analyses shall be evaluated considering the natural variability of engineering characteristics inherent in earth materials. Multiple shear tests on each site material are likely to be required;
(n) Direct shear tests do not always provide realistic strength values (Watry and Lade, 2000). Correlations between liquid limit, percent clay fraction and strength (fully softened and residual) with published data (e.g., Stark and others, 2005) shall be performed to verify tested shear strength parameters. Strength values used in analyses that exceed those obtained by the correlation must be appropriately justified;
(o) Shear strengths for proposed fill slopes shall be evaluated using samples mixed and remolded to represent anticipated field conditions. Tests to confirm strengths may be required during grading;
(p) Where bedding planes and/or discontinuities are laterally unsupported in slopes, potential failures along the unsupported bedding planes and/or discontinuities shall be analyzed. Similarly, stability analyses shall be performed where bedding planes and/or discontinuities form a dip-slope or near dip-slope using composite, potential failure surfaces that consist of potential slip surfaces along bedding planes and/or discontinuities in the upper portions of the slope, in combination with slip surfaces across bedding planes and/or discontinuities in the lower portions of the slope;
(q) The stability analysis shall include the effect of expected maximum moisture conditions on unit weight;
(r) For effective stress analyses, measured groundwater conditions adjusted to consider likely unfavorable conditions with respect to anticipated future groundwater levels, seepage and pore pressure shall be included in the slope stability analyses;
(s) Tension crack development shall be considered in the analyses of potential failure surfaces. The height and location of the tension crack shall be determined by modeling;
(t) Anticipated surcharge loads, as well as external boundary pressures from groundwater, shall be included in the slope stability evaluations, as deemed appropriate;
(u) Analytical chart solutions may be used, provided they were developed for conditions like those being analyzed. Generally, computer-aided modeling techniques shall be used, so that the potential failure surface with the lowest factor of safety can be located. Examples of typical modeling techniques are illustrated on Figures 9.1a to 9.1f in Blake and others (2002). However, verification of the reasonableness of the analytical results is the responsibility of the geotechnical engineer and/or engineering geologist; and
(v) The critical potential failure surface used in the analysis may be composed of circles, wedges, planes or other shapes considered to yield the minimum FS most appropriate for the geologic site conditions. The critical potential failure surface having the lowest factor of safety with respect to shearing resistance must be sought. Both the lowest FS and the critical failure surface shall be documented.
(14) Surficial slope stability refers to slumping and sliding of near-surface materials and is most critical during the snowmelt and rainy season or when excessive landscape water is applied. The assessment of surficial slope stability shall be based on analysis procedures for stability of an infinite slope with seepage parallel to the slope surface or an alternate failure mode that would produce the minimum factor of safety. The minimum acceptable saturation depth for surficial stability evaluation shall be four feet.
(a) Residual shear strengths comparable to actual field conditions shall be used in surficial stability analyses. Surficial stability analyses shall be performed under rapid draw-down conditions, where appropriate, such as for debris and detention basins.
(b) Where 2H:1V or steeper slopes have soil conditions that can result in the development of an infinite slope with parallel seepage, calculations shall be performed to demonstrate that the slope has a minimum static FS of one and one-half, assuming a fully saturated four-foot thickness. If conditions will not allow the development of a slope with parallel seepage, surficial slope stability analyses may not be required if approved by the county.
(c) Surficial slope stability analyses shall be performed for fill, cut and natural slopes assuming an infinite slope with seepage parallel to the slope surface or other failure mode that would yield the minimum FS against failure. A suggested procedure for evaluating surficial slope stability is presented in Blake and others (2002).
(d) Soil properties used in surficial stability analyses shall be determined as noted for residual strengths above. Residual shear strength parameters for surficial slope stability analyses shall be developed for a stress range that is consistent with the near-surface conditions being modeled. It shall be recognized that for most clayey soils, the residual shear strength envelope is curved and passes through the origin (for example, at zero normal stress, there is zero shear strength). For sites with deep slip surfaces, the guidelines given by Blake and others (2002) should be followed.
(e) The minimum acceptable vertical depth for which seepage parallel to the slope shall be applied is four feet for cut or fill slopes. Greater depths may be necessary when analyzing natural slopes that have significant thicknesses of loose surficial material.
(15) In addition to static slope stability analyses, slopes shall be evaluated for seismic slope stability as well. Acceptable methods for evaluating seismic slope stability include using calibrated pseudostatic limit-equilibrium procedures and simplified methods (such as, those based on Newmark (1965)) to estimate permanent seismic slope movements and are summarized in Blake and others (2002). Nonlinear, dynamic finite element/finite difference numerical methods also may be used to evaluate slope movements resulting from seismic events, if the procedures, input data and results are thoroughly documented and deemed acceptable by the county.
(a) Regarding design ground accelerations for seismic slope-stability analyses, the county prefers a probabilistic approach to determining the likelihood that different levels of ground motion will be exceeded at a site within a given time period. In order to more closely represent the seismic characteristics of the region, design ground motion parameters for seismic slope stability analyses shall be based on the peak accelerations with a 2.5% probability in 50 years (2,500-year return period).
(b) Peak ground accelerations (PGA) shall be used from the most recent USGS National Seismic Hazard Maps (https://earthquake.usgs.gov/hazards/hazmaps/) and adjusted for effects of soil/rock (site-class) conditions in accordance with Seed and others (2001) or other appropriate methods that consider the site-specific soil conditions and their potential for amplification or de-amplification of the high-frequency strong motion. Site-specific response analysis may also be used to develop PGA values if the procedures, input data and results are thoroughly documented and deemed acceptable by the county.
(c) Pseudostatic methods for evaluating seismic slope stability are acceptable if minimum factors of safety are satisfied and due consideration is given in the selection of the seismic coefficient (k) reduction in material shear strengths and the factor of safety for pseudostatic conditions.
(d) Pseudostatic seismic slope stability analyses can be performed using the “screening analysis” procedure described in Blake and others (2002). For that procedure, a k-value is selected from seismic source characteristics (modal magnitude and distance, and firm rock PGA) and < two inches (five cm) of deformation is specified. For that procedure, a factor of safety of > one is considered acceptable; otherwise, an analysis of permanent seismic slope deformation shall be performed.
(16) For seismic slope stability analyses, estimates of permanent seismic displacement are preferred and may be performed using the procedures outlined in Blake and others (2002). It should be noted that Bray and Rathje (1998), referenced in Blake and others (2002), has been updated and superseded by Bray and Travasarou (2007), which is the county’s currently preferred method. For those analyses, calculated seismic displacements shall be < four inches (ten cm), or mitigation measures shall be proposed to limit calculated displacements to < four inches (ten cm). For specific projects, different levels of tolerable displacement may be possible, but site-specific conditions, which shall include the following, must be considered:
(a) The extent to which the displacements are localized or broadly distributed; broadly distributed shear deformations would generally be less damaging, and more displacement could be allowed;
(b) The displacement tolerance of the foundation system: Stiff, well-reinforced foundations with lateral continuity of vertical support elements would be more resistant to damage and could potentially tolerate larger displacements than typical slabs-on-grade or foundation systems with individual spread footings; and
(c) The potential of the foundation soils to experience strain softening: Slopes composed of soils likely to experience strain softening should be designed for relatively low displacements if peak strengths are used in the evaluation of the yield coefficient (ky) due to the potential for progressive failure, which could involve very large displacements following strain softening. In order to consider a threshold larger than two inches, the project professional shall provide prior, acceptable justification to the county and obtain the county’s approval. Such justification shall demonstrate, to the satisfaction of the county, that the proposed project will achieve acceptable performance.
(17) Slope stability analyses shall be performed for cut, fill and natural slopes of water-retention basins or flood-control channels. In addition to analyzing typical static and seismic slope stability, those analyses shall consider the effects of rapid drawdown, if such a condition could occur.
(18) When slope stability hazards are determined to exist on a project, measures to mitigate impacts from those hazards shall be implemented. Some guidance regarding mitigation measures is provided in Blake and others (2002) and methods include:
(a) Hazard avoidance;
(b) Grading to improve slope stability;
(c) Reinforcement of the slope and/or improvement of the soil within the slope; and
(d) Reinforcement of the structures built on the slope to tolerate anticipated slope displacements. Where mitigation measures that are intended to add stabilizing forces to the slope are to be implemented, consideration may need to be given to strain compatibility. For example, if a compacted fill buttress is proposed to stabilize laterally unsupported bedding or a landslide, the amount of deformation needed to mobilize the recommended shear strength in the buttress shall be considered to confirm that it will not result in adverse movements of the upslope bedding or landslide deposits. Similarly, if a series of drilled piers is to be used to support a potentially unstable slope and a structure will be built on the piers, pier deformations resulting from movements needed to mobilize the soil’s shear strength shall be compared to tolerable deflections in the supported structure.
(19) Full mitigation of slope stability hazards shall be performed for developments in the county. Remedial measures that produce static FS > one and one-half and acceptable seismic displacement estimates shall be implemented as needed.
(20) On some projects or portions of, such as small structural additions, residential infill projects, nonhabitable structures and non-structural natural-slope areas, full mitigation of seismic slope displacements may not be possible, due to physical and/or economic constraints. In those cases, partial mitigation, to the extent that it prevents structural collapse, injury and loss of life, may be possible if consistent with IBC design criteria, and if it is approved by the county. The applicability of partial mitigation to specific projects shall be evaluated on a case-by-case basis.
(21) For developments when full mitigation of seismic slope displacements is not implemented, a notice of geologic hazard shall be recorded with the proposed development describing the displacement hazard at issue and the partial mitigation employed. The notice shall clearly state that the seismic displacement hazard at the site has been reduced by the partial mitigation, but not eliminated. In addition, the owner shall assume all risks, waive all claims against the county and its consultants and indemnify and hold the county and its consultants harmless from any and all claims arising from the partial mitigation of the seismic displacement hazard.
(F) Liquefaction is a process by which strong shaking during an earthquake causes the ground to temporarily lose its strength and to behave like a viscous liquid rather than a solid material. Liquefaction can cause buildings to tip and settle; roads to crack, deform and flood; buried storage tanks to rise towards the surface; and other types of damage to buildings and infrastructure. Liquefaction hazard investigation reports shall conform with the requirements described below and be prepared by a qualified geotechnical engineer as defined above.
(1) Liquefaction hazard maps show the location and relative anticipated severity of liquefaction during an earthquake. These maps are published by the UGS but are not currently available for the county. Once these maps are available, at that time they will be adopted to become part of this subchapter. As a result, investigations are required, prior to approval of any land use for facilities identified in the table provided in division (F)(5) below and within geologic hazard study areas defined by:
(a) Units Qac, Qay, Qaf, Qafy, Qafo, Qaf1, Qaf2, Qaf3, Qaf4, Qaf5, Qafb, Qafp and Qafoe on the most recent geologic maps published by the UGS (https://geology.utah.gov/). Most maps are available in the UGS Interactive Geologic Map Portal (https://geology.utah.gov/apps/intgeomap/), but contact the UGS for interim, progress update and other non-final maps that may be available, but not online; and
(b) For all critical and essential facilities, regardless of whether the site lies within a designated geologic hazard study area or not.
(2) A liquefaction-hazard investigation shall be performed in conjunction with any geotechnical and/or geologic hazards investigation prepared within the county.
(3) For all structures where liquefaction-hazard analyses indicates that ground settlement and/or lateral spread may be anticipated, the project structural engineer must provide documentation that the building is designed to accommodate the predicted ground settlements and displacements in such a manner as to be protective of life (collapse prevention) during and after the design seismic event.
(4) The investigation of liquefaction hazard is an interdisciplinary practice. The site investigation report must be prepared by a qualified geotechnical engineer, who must have competence in the field of seismic hazard evaluation and mitigation. Because of the differing expertise and abilities of qualified engineering geologists and geotechnical engineers, the scope of the site investigation report for a project may require that both types of professionals prepare and review the report, each practicing in the area of their expertise. Involvement of both a qualified engineering geologist and geotechnical engineer will generally provide greater assurance that the hazard is properly identified, assessed and mitigated. Liquefaction-hazard analyses are the responsibility of the geotechnical engineer, although the engineering geologist should be involved in the application of screening criteria and general geologic site evaluation to map the likely extent of liquefiable deposits and shallow groundwater. Engineering properties of earth materials shall be evaluated by the geotechnical engineer. The performance of quantitative liquefaction-hazard analyses resulting in a numerical factor of safety and quantitative assessment of settlement and liquefaction-induced permanent ground displacement shall be performed by the geotechnical engineer. The geotechnical and civil engineers shall develop all mitigation and design recommendations. Ground motion parameters for use in quantitative liquefaction-hazard analyses may be provided by either the engineering geologist or geotechnical engineer.
(5) Except for the derivation of input ground motion (see below for details), liquefaction-hazard investigations shall be performed in general accordance with the latest version of Recommended Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and Mitigating Liquefaction in California (Martin and Lew, 1999). Additional protocol for liquefaction hazard investigations is provided in Youd and Idriss (1997, 2001), Assessment of the Liquefaction Susceptibility of Fine-Grained Soils (Bray and Sancio, 2006) and SPT-Based Liquefaction Triggering Procedures (Idriss and Boulanger, 2010). Acceptable factors of safety are shown in the following table:
Type of Facility | Minimum Factor of Safety (FS) |
Critical facilities, including essential or hazardous facilities and special occupancy structures | 1.3 |
IBC Category III and IV structures | 1.3 |
Industrial and commercial structures | 1.25 |
(6) Soil liquefaction is caused by strong seismic ground shaking where saturated, cohesionless, granular soil undergoes a significant loss in shear strength that can result in settlement and permanent ground displacement. Surface effects of liquefaction include settlement, bearing capacity failure, ground oscillations, lateral spread and flow failure. It has been well documented that soil liquefaction may occur in clean sands, silty sands, sandy silt, non-plastic silts and gravelly soils. The following conditions must be present for liquefaction to occur:
(a) Soils must be submerged below the water table;
(b) Soils must be loose to moderately dense;
(c) Earthquake ground shaking must be relatively intense; and
(d) The duration of ground shaking must be large enough for the soils to generate seismically induced excess pore water pressure and lose their shearing resistance.
(7) The following screening criteria may be applied to determine if further quantitative evaluation of liquefaction hazard is required:
(a) If the estimated maximum past-, current- and maximum-future-groundwater-levels (i.e., the highest groundwater level applicable for liquefaction-hazard analyses) are determined to be deeper than 50 feet below the existing ground surface or proposed finished grade (whichever is deeper), liquefaction-hazard assessments are not required. For soil materials that are located above the groundwater level, a quantitative assessment of seismically induced settlement is required,
(b) If bedrock underlies the site, those materials need not be considered liquefiable and no analysis of their liquefaction potential is necessary,
(c) If the corrected standard penetration test (SPT) blow count, (N1) 60, is > 33 in all samples with an acceptable number of blow counts recorded, liquefaction-hazard assessments are not required. If CPT soundings are made, the corrected CPT tip resistance, qclN, should be > 180 in all soundings in sandy soils; otherwise, liquefaction-hazard assessments are needed; and
(d) If plastic soils with a plasticity index (PI) > 18 are encountered during site exploration, those materials may be considered non-liquefiable. Additional acceptable screening criteria regarding the effects of plasticity on liquefaction susceptibility are presented in Boulanger and Idriss (2004), Bray and Sancio (2006) and Seed and others (2003). Youd and others (2002) provide additional guidance on analyzing lateral spreads. If the screening investigation clearly demonstrates the absence of liquefaction hazards at a project site and the county concurs, the screening investigation will satisfy the site investigation report requirement for liquefaction hazards. If not, a quantitative evaluation is required to assess the liquefaction hazards.
(8) (a) Geologic research and reconnaissance are important to provide information to define the extent of unconsolidated deposits that may be prone to liquefaction. Such information shall be presented on geologic maps and cross-sections and provide a description of the formations present at the site that includes the nature, thickness and origin of Quaternary deposits with liquefaction potential. There shall also be an analysis of groundwater conditions at the site that includes the highest recorded water level and the highest water level likely to occur under the most adverse foreseeable conditions in the future, including seasonal changes.
(b) During the field investigation, the engineering geologist shall map the limits of unconsolidated deposits with liquefaction potential. Liquefaction typically occurs in cohesionless silt, sand and fine-grained gravel deposits of Holocene to late Pleistocene age, in areas where the groundwater is shallower than about 50 feet, but other soil types are may also be liquefiable. Shallow groundwater may exist for a variety of natural and/or human-made reasons.
(c) Landscape irrigation, on-site sewage disposal and unlined human-made lakes, reservoirs and storm-water detention basins may create a shallow groundwater table in soils that were previously unsaturated.
(9) (a) Subsurface exploration shall consist of drilled borings and/or CPT soundings. The exploration program shall be planned to determine the soil stratigraphy, groundwater level and indices that could be used to evaluate the potential for liquefaction by in-situ testing or laboratory testing of soil samples.
(b) If borings are utilized, the use of mud-rotary drilling methods is highly recommended to achieve minimal disturbance of the in-situ soils. If mud-rotary drilling is not used, a through explanation is required in the submitted report. Borings and CPT soundings must penetrate a minimum of 45 feet below the final ground surface. If during the investigation, the liquefaction evaluation indices the liquefaction potential may extend below 45 feet, the exploration shall be continued for a minimum of ten feet, to the extent possible, until non-liquefiable soils are encountered.
(c) For saturated cohesionless soils where the SPT N 160 values are < 15 or where CPT tip resistances are < 60 tsf, grain-size analyses, hydrometers tests and Atterberg limits tests shall be performed on these soils to further evaluate their potential for permanent ground displacement (Youd et al., 2002) and other forms of liquefaction-induced ground failure and settlement. In addition, it is also recommended that these same tests be performed on saturated cohesionless soils with SPT (NI) 60 values between 15 and 30 to further evaluate the potential for liquefaction-induced settlement.
(d) Where a structure may have below grade construction and/or deep foundations, such as drilled shafts or piles, the investigation depth shall extend to a minimum of 20 feet below the lowest expected foundation level (e.g., drilled shaft or pile tip) or to 45 feet below the existing ground surface or lowest proposed finished grade, whichever is deeper. If during the investigation, the liquefaction evaluation indices indicate that liquefaction potential may extend below that depth, the exploration shall be continued at least ten additional feet, to the extent possible, until non-liquefiable soils are encountered.
(10) (a) For the design ground accelerations used in liquefaction analyses, the county prefers a probabilistic approach to determining the likelihood that different levels of ground motion will be exceeded at a site within a given time period. In order to more closely represent the seismic characteristics of the region, design ground motion parameters for seismic slope stability analyses shall be based on the peak accelerations with a 2% probability in 50 years (2,500-year return period). PGA values shall be obtained from the USGS national seismic hazard maps and site-specific data webpage (https://earthquake.usgs.gov/hazards/hazmaps/) using the latest long-term model.
(b) PGAs obtained from the USGS shall be adjusted for effects of soil/rock (site-class) conditions in accordance with Seed and others (2001) or other appropriate and documented methods that are deemed acceptable by the county that consider the site-specific soil conditions and their potential for amplification or deamplification of the high frequency strong ground motion. Site-specific response analysis may also be used to develop PGA values if the procedures, input data and results are thoroughly documented and deemed acceptable by the county.
(11) Sites, facilities, buildings, structures and utilities that are founded on or traverse liquefiable soils may require further remedial design and/or relocation to avoid liquefaction-induced damage. These shall be investigated and evaluated on a site-specific basis with appropriate geologic and geotechnical investigation to support the remedial design and/or mitigative plan. This design or plan may include changes/modifications to the soil, permanent dewatering, earthquake drains, foundation systems, building structural frame or support and the like. Remedial design and/or mitigation measures shall be reviewed and approved by the county.
(12) (a) Liquefaction hazard reports shall include: Boring logs; geologic cross-sections; laboratory data; a detailed explanation pertaining to how idealized subsurface conditions and parameters used for the analyses were developed; analytical results and software output files; and summaries of the liquefaction-hazard analyses and conclusions regarding liquefaction potential and likely types and magnitudes of ground failure in addition to the other report requirements detailed in this subchapter.
(b) Subsurface geologic and groundwater conditions developed by the engineering geologist must be illustrated on geologic cross-sections and must be utilized by the geotechnical engineer for the liquefaction-hazard analyses. If on-site sewage or storm-water disposal exists or is proposed, the liquefaction-hazard analyses shall include the effects of the effluent plume on liquefaction potential.
(c) The results of any liquefaction-hazard analyses must be submitted with pertinent documentation, including calculations, software output and the like. Documentation of input data, output data, and graphical plots must be submitted for each computer-aided liquefaction-hazard analysis and included as an appendix to the report. Additional information and/or data may be requested to facilitate the county’s review.
(G) (1) Debris flows are fast-moving, flow-type landslides composed of a slurry of rock, mud, organic matter and water that move down drainage basin channels onto alluvial fans. In addition to threatening lives, debris flows can damage structures and infrastructure by sediment burial, erosion, direct impact and associated water flooding.
(2) Debris flow hazard investigations and reports shall conform with the Guidelines for the Geologic Investigation of Debris-Flow Hazards on Alluvial Fans in Utah (UGS Circular 122, https://ugspub.nr.utah.gov/publications/circular/c-122.pdf).
(3) Debris flow hazard maps show the locations of previous debris flows, areas of potential debris flows and recommended special study areas. These maps are published by the UGS but are currently not available for the county. Once these maps are available, at that time they will be adopted to become part of this subchapter. As a result, investigations are required within geologic hazard study areas as defined by:
(a) Units Qmdf, Qaf, Qafy, Qafo, Qaf1, Qaf2, Qaf3, Qaf4, Qaf5, Qafb, Qafp and Qafoe on the most recent geologic maps published by the UGS (https://geology.utah.gov/). Most maps are available in the UGS Interactive Geologic Map Portal (https://geology.utah.gov/apps/intgeomap/), but contact the UGS for interim, progress update and other non-final maps that may be available, but not online;
(b) Other environmentally sensitive areas that the county’s Planning Commission and County Commission find to be of significance to the health, safety and welfare of the citizens of the county;
(c) All properties located on alluvial fans and drainage channels subject to flash flooding and debris flows; and
(d) Additions to existing structures are exempt from the provisions in this section.
(H) Rockfall is a type of landslide and a natural mass-wasting process that involves the dislodging and rapid downslope movement of individual rocks and rock masses. Rockfall hazard investigations and reports shall conform with the Guidelines for Evaluating Rockfall Hazards in Utah (UGS Circular 122, https://ugspub.nr.utah.gov/publications/circular/c-122.pdf).
(1) Rockfall hazard maps show the locations of known rockfall, areas of potential rockfall and recommended special study areas. These maps are published by the UGS but are currently not available for the county. Once these maps are available, at that time they will be adopted to become part of this subchapter. As a result, investigations are required geologic hazard study areas as defined by: Units Qmrf, Qmt, Qmtr, Qm and Qmr on the most recent geologic maps published by the UGS (https://geology.utah.gov/). Most maps are available in the UGS Interactive Geologic Map Portal (https://geology.utah.gov/apps/intgeomap/), but contact the UGS for interim, progress update and other non-final maps that may be available, but not online.
(2) Additions to existing structures are exempt from the provisions in this section.
(I) Avalanches are landslides consisting mainly of snow and ice, but can contain soil, rock and/or debris. These investigations and reports shall be prepared by a qualified avalanche expert, conform with Colorado Geological Survey Bulletin 49: Snow-Avalanche Hazard Analysis for Land Use Planning and Engineering, geologic reports should consider the hazard from avalanches in areas identified in division (I)(1) below and discuss the need for further detailed avalanche analysis or mitigation measures.
(1) Avalanche hazard maps show the locations of previous avalanches, areas of potential avalanches and recommended special study areas. These maps are currently not available for the county. Once these maps are available, at that time they will be adopted to become part of this subchapter. As a result, investigations are required within geologic hazard study areas as defined by those areas within the county above elevations of 6,000 feet with an adequate snow supply to produce snow avalanches and which include slopes greater than 47% (25 degrees).
(2) Avalanche areas shall be delineated on a detailed site avalanche map, at a scale equal to or more detailed than one inch equals 100 feet. The site avalanche map shall include the location and boundaries of the property, locations of avalanche areas, avalanche-source areas, avalanche-runout areas and buildable and non-buildable areas; delineation of recommended setback distances from the hazard; and recommended locations for structures. Avalanche-source areas may be off-site and, in areas of steep terrain, may be at great distances from the site.
(3) If the avalanche analysis indicates that the site may be impacted by avalanches, the report shall delineate the following areas:
(a) A “red zone” of high avalanche potential corresponding to a return period of 25 years or less, and/or impact pressures 2,600 pounds per square foot (psf) within which critical facilities or structures for human occupancy are not permitted; and
(b) A “blue zone” corresponding to a return period between 25 and 300 years, and impact pressures less than 600 psf within which critical facilities or structures for human occupancy shall only be permitted when at least one of the following requirements has been met:
1. The structure is designed to incorporate direct protection measures that address the estimated impact forces of flowing snow/debris and powder blast loading. The estimated impact forces shall be calculated by the avalanche expert and the structure shall be designed by, and the plans stamped by, a qualified, state-licensed professional structural engineer; or
2. Appropriate engineering controls (such as, deflection structures, snow retention nets, dams and the like) are designed and installed to mitigate the avalanche hazard. Design or performance criteria for engineered mitigation measures, including estimated impact forces, flow heights, location and dimensions of the mitigation structures and all supporting modeling or other analyses, calculations and assumptions, shall be calculated by the avalanche expert and included in the report. Final design plans and specifications for engineered mitigation must be signed and stamped by a qualified, state-licensed professional geotechnical or structural engineer, as appropriate.
(4) The report shall include:
(a) The probability of avalanche occurrence, if possible, estimates of avalanche volumes, and the likely effects of avalanches on the proposed development;
(b) A description of the avalanche expert’s qualifications to perform the investigation; and
(c) Engineering design parameters for avalanche mitigation, as appropriate, implications of the risk reduction measures on the development and adjacent properties and the need for long-term maintenance.
(Prior Code, § 8-5I-17) (Ord. 10-02, passed 6-1-2010; Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021; Ord. 24-12, passed 5-21-2024)
§ 155.237 REFERENCES.
Bartlett, S.F. and Youd, T. L., 1995,
Empirical Prediction of Liquefaction-Induced Lateral Spread: Journal of Geotechnical Engineering, v. 121, n. 4 - April, American Society of Civil Engineers, pp. 316-329. |
Beukelman, G.S and Hylland, M.D., 2016, Guidelines for Evaluating Landslide Hazards in Utah in Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Geology Ordinances in Utah: Utah Geological Survey Circular 122, p. 59-73, online: |
Blake, T.F., Hollingsworth, R.A. and Stewart, J.P., editors, 2002, Recommended Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and Mitigating Landslide Hazards in California: Southern California Earthquake Center, p. 110, (http://www-scec.usc.edu/resources/catalog/LandslideProceduresJune02.pdf). |
Boulanger, R.W. and Idriss I.M., 2004, Evaluating the Potential for Liquefaction Resistance or Cyclic Failures of Silts and Clays: University of California, Davis Center for Geotechnical Modeling Report UCD/CGM-04/01, (https://faculty.engineering.ucdavis.edu/boulanger/wp-content/uploads/sites/71/2014/09/Boulanger _Idriss_CGM04-01_2004.pdf). |
Bowman, S.D. and Lund, W.R., editors, 2016, Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Hazard Ordinances in Utah: Utah Geological Survey Circular 122, p. 203, online: (https://ugspub.nr.utah.gov/publications/circular/c-122.pdf). |
Bowman, S.D. and Lund, W.R., 2016, Guidelines for Conducting Engineering-Geology Investigations and Preparing Engineering-Geology Reports in Utah in Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Geology Ordinances in Utah: Utah Geological Survey Circular 122, p. 15-30, online: (https://ugspub.nr.utah.gov/publications/circular/c-122.pdf). |
Bray, J.D. and Rathje, E.M., 1998, Earthquake-Induced Displacements of Solid Waste Landfills: Journal of Geotechnical and Geoenvironmental Engineering, v. 124, no. 3, pp. 242-253. |
Bray J. D. and Sancio R. B., 2006, Assessment of Liquefaction Susceptibility of Fine-Grained Soils: Asce Journal of Geotechnical and Geoenvironmental Engineering, September 2006. |
Bray, J.D. and Travasarou, T, 2007, Simplified Procedure for Estimating Earthquake-Induced Deviatoric Slope Displacements: Journal of Geotechnical and Geoenvironmental Engineering, v. 133, no. 4, April 1, 2007, pp. 381-392. |
Bryant, B., 1988, Geology of the Farmington Canyon Complex, Wasatch Mountains, Utah: U.S. Geological Survey Professional Paper 1476, p. 54, scale 1:50,000, (part of the Bountiful Peak quadrangle and west 1/2 of the Porterville quadrangle, Morgan County, Utah), online: (https://pubs.er.usgs.gov/publication/pp1476). |
Federal Highway Administration, 1989, Rock Slopes-Design, Excavation, Stabilization: Federal Highway Administration Publication, FHWA-TS-89-045, paginated online: (https://geodata.geology.utah.gov/pages/view.php?ref=58219). |
Giraud, R.E., 2016, Guidelines for the Geologic Investigation of Debris-Flow Hazards on Alluvial Fans in Utah in Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Geology Ordinances in Utah: Utah Geological Survey Circular 122, p. 75-91, online: (https://ugspub.nr.utah.gov/publications/circular/c-122.pdf). |
Giraud, R.E. and Shaw, L.M., 2007, Landslide Susceptibility Map of Utah: Utah Geological Survey Map M-228, p. 11, 1 plate, scale 1:500,000, online: (http://ugspub.nr.utah.gov/publications/maps/m-228/m-228.pdf). |
Hoek, E., Bray, J.W., 1981, Rock Slope Engineering, revised third edition: E & FN Spon, London, p. 358. |
Idriss, I.M. and Boulanger, R.W., 2010, SPT-Based Liquefaction Triggering Procedures: University of California, Davis Center for Geotechnical Modeling Report No. UCD/CGM 10/02, p. 259: |
International Code Council, Inc., 2017, 2018 International Building Code: International Code Council, Country Club Hills, Illinois, p. 728, (https://codes.iccsafe.org/content/IBC2018). |
Kaliser, B.N., 1972, Geologic Hazards in Morgan County with Applications to Planning Purposes, Utah Geological and Mineralogical Survey Bulletin 93, p. 56, 2 plates, online: (https://ugspub.nr.utah.gov/publications/bulletins/B-93.pdf). |
Lund, W.R., Christenson, G.E., Batatian, L.D. and Nelson, C.V., 2016, Guidelines for Evaluating Surface-fault-rupture Hazards in Utah in Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Geology Ordinances in Utah: Utah Geological Survey Circular 122, p. 31-58, online: (https://ugspub.nr.utah.gov/publications/circular/c-122.pdf). |
Lund, W.R. and Knudsen, T.R., 2016, Guidelines for Evaluating Rockfall Hazards in Utah in Guidelines for Investigating Geologic Hazards and Preparing Engineering-Geology Reports, with a Suggested Approach to Geologic-Hazard Ordinances in Utah: Utah Geological Survey Circular 122, p. 111-123, online: |
Martin, G.R. and Lew, M., editors., 1999, Recommended Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and Mitigating Liquefaction Potential in California: Southern California Earthquake Center, University of Southern California, p. 63, (http://scecinfo.usc.edu/resources/catalog/LiquefactionproceduresJun99.pdf). |
Mears, A.I., 1992, Snow-Avalanche Hazard Analysis for Land Use Planning and Engineering: Colorado Geological Survey Bulletin B-49, p. 55: |
National Academies of Sciences, Engineering and Medicine, 2016, State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and its Consequences: The National Academies Press, online: |
Newmark, N.M., 1965, Effects of Earthquakes on Dams and Embankments: Geotechnique, v. 25, no. 4. |
Seed, H.B. and Idriss, I.M., 1982, Ground Motion and Soil Liquefaction During Earthquakes: Earthquake Engineering Research Institute, Oakland, California, p. 135. |
Seed, R.B., Cetin, K.O., Moss, R.E.S., Kammerer, A.M., Wu, J., Pestana, J.M. and Riemer, M.F., 2001, Recent Advances in Soil Liquefaction Engineering and Seismic Site Response Evaluation, Fourth International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, University of Missouri-Rolla, Rolla, Missouri, 2001, Paper No. SPL-2, p. 45. |
Seed, R.B., Cetin, K.O., Moss, R.E.S., Kammerer, A.M., Wu, J., Pestana, J.M. and Riemer, M.F., Sancio, R.B., Bray, J.D., Kayen, R.E. and Faris, A., 2003, Recent Advances in Soil Liquefaction Engineering: a Unified and Consistent Framework: Earthquake Engineering Research Institute Report No. EERC 2003-06, p. 71, online: |
Stark, T.D., Choi, H. and McCone, S., 2005, Drained Shear Strength Parameters for Analysis of Landslides: Journal of Geotechnical and Geoenvironmental Engineering, v. 131, no. 5, pp. 575-588. |
Stewart, J.P., Blake, T.M. and Hollingsworth, R.A., 2003, Development of a screen analysis procedure for seismic slope stability: Earthquake Spectra, v. 19, no. 3, pp. 697-712. |
Van Horn, R. and Crittenden, Jr., M.D., 1987, Map Showing Surficial Units and Bedrock Geology of the Fort Douglas Quadrangle and Parts of the Mountain Dell and Salt Lake City North Quadrangles, Davis, Salt Lake and Morgan Counties, Utah: U.S. Geological Survey Miscellaneous Investigation Series Map I-1762, scale 1:24,000, online: (https://ngmdb.usgs.gov/Prodesc/proddesc_9901.htm). |
Youd, T.L. and Idriss, I.M., editors., 1997, Proceedings of the Nceer Workshop on Evaluation of Liquefaction Resistance of Soils: National Center for Earthquake Engineering Research Technical Report NCEER 97-0022, also Youd, T.L. and Idriss, I.M., 2001, Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils: Journal of Geotechnical and Geoenvironmental Engineering, v. 127, no. 4, April 2001, (http://www.ce.memphis.edu/7137/PDFs/Reference2/Youd%20ad%Idriss.pdf). |
Youd, T.L. and Gilstrap, S.D., (1999), Liquefaction and Deformation of Silty and Fine-Grained Soils: Proceedings of the Second International Conference on Earthquake Geotechnical Engineering, Lisboa, Portugal, 21-25 June 1999, Seco e Pinto, pp. 1013-1020. |
Youd, T.L., Hansen, C.M. and Bartlett, S.F., (2002), Revised MLR Equations for Predicting Lateral Spread Displacement, Asce Journal of Geotechnical and Geoenvironmental Engineering, December 2002. |
Watry, S.M. and Lade, P.V., 2000, Residual Shear Strengths of Bentonites on Palos Verdes Peninsula, California: Proceedings of the session of Geo-Denver 2000, American Society of Civil Engineers, pp. 323-342. |
Wyllie, D.C. and Mah, C.W., 2004, Rock Slope Engineering, Civil and Mining, 4th edition: Spon Press, New York, p. 431. |
(Prior Code, § 8-5I-18) (Ord. 19-09, passed 10-15-2019; Ord. 21-07, passed 6-15-2021)