(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)