ADOPTS WITH AMENDMENTS:

International Building Code 2018 (IBC 2018)

Heads up: There are no amended sections in this chapter.
This guideline is intended to address the requirements of NRS 278.580 6(b) which mandates governing bodies to amend their building codes to include standards for the investigation of hazards relating to seismic activity including liquefaction.
When a geotechnical investigation report is required by 1803.2 then this guideline specifies the minimum requirements for evaluation of liquefaction hazards. The liquefaction evaluation must be performed by a registered design professional.

The following words and terms shall, for the purposes of this appendix, have the meanings shown herein. Refer to Chapter 2 of this code for general definitions.

CPT. Cone Penetration Test (ASTM D3441).

CSR. Cyclic stress ratio — a normalized measure of cyclic stress severity, expressed as equivalent uniform cyclic shear stress divided by some measure of initial effective overburden or confining stress.

CSReq. The equivalent uniform cyclic stress ratio representative of the dynamic loading imposed by an earthquake.

CSRliq. The equivalent uniform cyclic stress ratio required to induce liquefaction within a given number of loading cycles [that number of cycles considered representative of the earthquake under consideration].

Ground Loss. Localized ground subsidence.

Land Subsidence: The gradual downward settling or sinking of the earth's surface.

Liquefaction. Significant loss of soil strength due to pore pressure increase.

N. Penetration resistance measured in SPT tests (blows/ft).

N1 Normalized SPT N-value (blows/ft). Corrected for overburden stress effects to the N-value which would occur if the effective overburden stress was 1.0 tons/ft2.

(N1)60 Standardized, normalized SPT-value. Corrected for both overburden stress effects and equipment and procedural effects (blows/ft).

qc. Tip resistance measured by CPT probe (force/length2).

qc,1. Normalized CPT tip resistance (force/length2); corrected for overburden stress effects to the qc value which would occur if the effective overburden stress was 1.0 tons/ft2.

SPT. Standard Penetration Test (ASTM D1586).

Liquefaction shall be evaluated for all projects that require a geotechnical investigation report. Liquefaction potential is associated with three soil conditions that include: presence of low density silts and/or sands; saturation and non-cohesive soil behavior.
Exception: Detached accessory structures associated with a single family residence that do not have any habitable space and are classified as group S or U occupancies; examples include carports, patio covers (shade structures), detached garages, storage sheds, agricultural buildings, barns, etc.
Liquefaction potential may be considered low when any of the following conditions are identified:
  1. Groundwater conditions have been evaluated to a depth of 50 feet below the ground surface and no saturated soils have been identified within this zone. Groundwater conditions may be evaluated by traditional geotechnical exploration methods or published well data may be referenced. Evaluation of groundwater conditions should take into account seasonal variation in groundwater elevation.
  2. Geotechnical exploration logs indicate that there are no soil strata present in the upper 50 feet that consist of low density silts and/or sands which have standardized blow counts (ASTM D1586) less than 15 blows per foot.
  3. Geotechnical exploration logs indicate that there are soil strata present in the upper 50 feet that consist of low density silts and/or sands which possess cohesive soil properties that reduce the likelihood of liquefaction. Soils having a plasticity index (PI) greater than 12 are generally expected to behave like clays; however, if the PI is greater than 7 and the in-situ moisture content of the soils is less than 85% of the liquid limit, clay-like behavior may also expected. Engineering judgment must be applied when using this criteria.
When liquefaction potential cannot be shown as low per the requirements of O103.1.1, then a detailed liquefaction analysis shall be performed.
The field investigation shall be based on visual observations of the soil and any necessary tests of soil materials disclosed by borings, test pits or other subsurface exploration methods made in appropriate locations. In addition, surficial deposits shall be evaluated and described along with any exposed earth. The surficial deposits or exposed area shall be defined in terms of environment of deposition and the relationship to existing topography. The investigation shall be conducted by a qualified representative approved by the registered design professional. The field investigation shall include the following:
  1. Soil Classification by the Unified Soil Classification System (ASTM D2487). Backup data shall be included, for a minimum of one sample, for every two borings or test pits or other subsurface exploratory method distributed among the prominent horizons in the soil profile. This data shall include particle size distribution, Atterberg Limits, unit weight and in-situ moisture content of the sampled soil.
  2. Correlation and analysis of soil horizons based on in-situ Standard Penetration Test (STP) data and/or Cone Penetration Test (CPT) data.
  3. Flood zones or any know historic areas of liquefaction.
  4. Depth to relative groundwater elevation, reported as Below Ground Surface (bgs). The relative groundwater elevation must be based on boring logs, test pits, monitor well data, geophysical investigations or available groundwater maps.
  5. Evaluation of the geometry of potentially liquefiable soils. Deposits of liquefiable soils shall require lateral investigation for the determination of hazardous weakened plane areas and areas susceptible to sliding that may pose a risk to lateral spreading.
  6. A minimum of 30% of the explorations required by 1803.3.2 or 1 exploration, whichever is greater, shall extend to a depth of 50 feet below the ground surface.
To evaluate for a potential liquefaction hazard a probabilistically derived peak ground acceleration with a 10% probability of exceedance in 50 years (i.e. 475-year return period) shall be used when site-specific analyses are performed. However, special considerations must be made for certain structures as defined by the building official. The factor of safety for level ground liquefaction resistance has been defined as FS = CSRliq / CSReq, where CSReq is the cyclic stress ratio generated by the anticipated earthquake ground motions at the site, and CSRliq is the cyclic stress ratio required to generate liquefaction. The factor of safety shall comply with Table O103.1.2.2. This factor of safety is based on quality, site-specific penetration resistance, laboratory data and appropriate ground-motion data used in the analyses. However, larger factors of safety may be applicable for differing field conditions and types of construction. If lower factors of safety are calculated for some soil zones, an evaluation of the level (or severity) of the hazard associated with potential liquefaction of these soils shall be determined.
TABLE O103.1.2.2
FACTORS OF SAFETY FOR LIQUEFACTION HAZARD ASSESSMENT
CONSEQUENCE OF LIQUEFACTION (N1)60 CLEAN SAND FACTOR OF SAFETY
RISK CATEGORY I AND II RISK CATEGORY III AND IV
Settlement ≤ 15 1.1 1.3
≤ 30 1.0 1.2
Surface Manifestation ≤ 15 1.2 1.4
≤ 30 1.0 1.2
Lateral Spread ≤ 15 1.3 1.5
≤ 30 1.0 1.2
Mitigation method(s) shall provide an acceptable level of protection in both: 1) Translational site instability (sliding, edge failure, lateral spreading, flow failure, etc.) that may potentially affect all or large portions of the site; and 2) Localized hazard(s) at and immediately adjacent to the structures and/or facilities of concern (e.g., bearing failure, settlement, localized lateral movements).
Where an investigation, evaluation and mitigation of liquefaction hazards has been performed/completed, a written report shall be submitted to the building official. This report shall include, but not limited to, the following information:
  1. If any method other than Standard Penetration Test (SPT; ASTM D1586) and Cone Penetration Test (CPT; ASTM 3441) are used, a description of the equipment and procedural details of the field measurements must be summarized.
  2. If SPTs are performed, boring logs must show (unmodified) N-values. If CPTs are conducted, probe logs must show qc-values and plots of sleeve friction.
  3. An explanation of the basis of the methods used to convert SPT, CPT or non-standard data to "corrected" and "standardized" values.
  4. Tabulation and/or plots of corrected values used for analyses.
  5. An explanation of methods used to develop estimates of field loading equivalent uniform cyclic stress ratios (CSReq) used to represent the anticipated field earthquake excitation (cyclic loading).
  6. An explanation of the basis for evaluation of the equivalent uniform cyclic stress ratio necessary to cause liquefaction (CSRliq) at the number of equivalent uniform loading cycles considered representative of the design earthquake.
  7. Factors of safety against liquefaction at various depths and/or within various potentially liquefiable soil units.
  8. Conclusions regarding the potential for liquefaction and estimated deformation and its potential impact on the proposed project.
  9. Proposed mitigation measures that are determined to reduce potential damage caused by liquefaction.
  10. Describe the criteria necessary for SPT-based or CPT-based and/or other types of acceptance testing that will be used to demonstrate satisfactory remediation.
  11. Confirmation of Site Class in accordance with ASCE 7-16 Chapter 20 to identify if Site Class F conditions exist.
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