CBC · California Building Code

What seismic hazards (liquefaction, lateral spreading) must the geotech report address?

If your project is in Seismic Design Category C–F the California Building Code requires a geotechnical investigation that evaluates liquefaction and seismically‑induced lateral spreading (§ 1803.5.11), and for SDC D–F the report must evaluate liquefaction using the applicable peak ground acceleration and quantify consequences (settlement, lateral movement, earth pressure changes, downdrag, flotation) and mitigation options (§ 1803.5.12) .

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Last reviewed: July 5, 2026

What the code requires — 2–4 sentences

The California Building Code requires that a geotechnical investigation for structures in Seismic Design Categories C through F explicitly evaluate seismic hazards including liquefaction and seismically‑induced lateral spreading (among other items) (§ 1803.5.11) . For structures in Seismic Design Categories D through F the investigation must, as applicable, evaluate the potential for liquefaction and soil strength loss for site peak ground acceleration and the resulting consequences (settlement, lateral movement, earth‑pressure changes, downdrag, flotation, etc.) and discuss mitigation measures (§ 1803.5.12) .

Requirements in detail

Which projects need these evaluations

  • Any project assigned to Seismic Design Category C, D, E or F must have a geotechnical investigation that evaluates liquefaction and seismically‑induced lateral spreading or lateral flow (§ 1803.5.11) .
  • For projects in Seismic Design Category D, E or F the geotechnical investigation from § 1803.5.11 must additionally address liquefaction potential using site peak ground acceleration and must assess consequences and mitigations (§ 1803.5.12) .

What the geotechnical report must evaluate (decision‑relevant items)

The critical analysis and deliverables the CBC requires are summarized in the table below.

Decision item / dimension What to produce or check Code reference
Applicability by seismic design category Evaluate liquefaction and lateral spreading for SDC C–F; for SDC D–F perform the enhanced liquefaction/ consequences evaluation (§ 1803.5.12) § 1803.5.11; § 1803.5.12
Peak ground acceleration (PGA) to use Evaluate liquefaction and strength loss using site PGA and earthquake magnitude/source consistent with the maximum considered earthquake; PGA may be determined by a site‑specific study (ASCE 7 Ch. 21) or per ASCE 7 §11.8.3 § 1803.5.12 (Item 2 and 2.1/2.2)
Retaining / foundation walls Determine dynamic seismic lateral earth pressures when supporting more than 6 ft (1.83 m) of backfill § 1803.5.12 (Item 1)
Consequences to evaluate if liquefaction likely Estimate total & differential settlement; lateral soil movement; lateral soil loads on foundations; reduction in bearing capacity; downdrag on piles; increased lateral pressures on retaining walls; flotation of buried structures § 1803.5.12 (Item 3.1–3.7)
Mitigation discussion Provide discussion of mitigation: foundation selection/depth, structural systems to accommodate displacements, ground stabilization, combinations of measures and how they affect design § 1803.5.12 (Item 4.1–4.4)
Reporting contents Include soil borings, soil profiles, groundwater elevations, expected settlements, foundation recommendations and mitigation measures in the geotechnical report § 1803.6 (reporting requirements)

Notes:

  • The CBC (in other parts of the code and appendices) references recognized guidance for liquefaction and lateral spread analyses (e.g., NCEER/You d & Idriss, CGS Special Publication 117A, NCHRP Report 611) which are commonly used methods for the analyses and for estimating lateral displacements and induced pressures; see related provisions below for where that guidance is referenced in the CBC .

Minimum content the geotechnical report should include when liquefaction/lateral spreading are involved

  • Site borings/CPTs and logs showing any potentially liquefiable layers and their thicknesses (§ 1803.6) .
  • Historic high groundwater elevation where relevant (to evaluate liquefaction) (§ 1803A.7 / reporting guidance) .
  • Assessment of liquefaction potential using PGA consistent with the MCE and earthquake magnitude/source parameters (§ 1803.5.12) .
  • Estimates of settlement (total and differential), lateral displacements (lateral spread), and forces/pressure changes to foundations and retaining walls (§ 1803.5.12) .
  • Discussion of mitigation alternatives and their impact on structural design and foundations (§ 1803.5.12) .

Exceptions & special cases

  • Appendix J requires a liquefaction study for sites where the maximum considered earthquake short‑period spectral response Ss > 0.5g as determined by ASCE 7, unless the building official has established local data showing low potential (§ J104.4) .
  • Geohazard reports and some geotechnical reporting requirements have limited exceptions for small, one‑story wood/light‑steel buildings under certain area limits and not located in mapped Earthquake Fault or Seismic Hazard Zones; check § 1803.7 and the geohazard provisions for project‑specific exceptions (§ 1803.7 and related paragraphs) .
  • The CBC requires site‑specific PGA determination options (site study per ASCE 7 Chapter 21 or ASCE 7 §11.8.3) — the geotechnical engineer should pick the method consistent with the project’s Seismic Design Category and the building official’s expectations (§ 1803.5.12) .

Common mistakes

  • Treating liquefaction and lateral spread analysis as optional when the project is in SDC C–F. (§ 1803.5.11 requires evaluation for C–F) .
  • Using a non‑MCE (inappropriate PGA or magnitude) ground motion input for liquefaction screening or consequence analysis; § 1803.5.12 explicitly requires evaluation consistent with the maximum considered earthquake ground motions (§ 1803.5.12) .
  • Omitting consequences: reporting liquefaction potential but failing to quantify settlement, lateral movement, downdrag, loss of bearing capacity, or flotation as required by § 1803.5.12 (Item 3) .
  • Failing to show liquefiable strata and thicknesses on site profiles when liquefaction is indicated (the CBC expects the strata and thickness to be clearly shown) — see the CBC guidance referenced in related sections for formatting expectations .
  • Forgetting to evaluate dynamic seismic lateral earth pressures for walls with >6 ft (1.83 m) of backfill when the project is SDC D–F (§ 1803.5.12 Item 1) .

Worked example — applying the rule with numbers

Scenario: A proposed 2‑story building is assigned Seismic Design Category D. Subsurface exploration finds a 10‑ft thick loose, saturated sand layer from 8 to 18 ft below grade. A site‑level MCE PGA from a site‑specific study is 0.6 g.

What the geotechnical report must do under the CBC:

  1. Because the building is SDC D, the geotechnical investigation required by § 1803.5.11 must include liquefaction and lateral‑spread evaluation and, per § 1803.5.12, the liquefaction potential must be evaluated for the site PGA and earthquake magnitude/source consistent with the MCE (§ 1803.5.11; § 1803.5.12) .
  2. The report should show the liquefiable layer and its thickness (10 ft) on the site profile and document groundwater elevation (historic/high if known) (§ 1803.6 / reporting guidance) .
  3. The geotechnical analysis should compute:
    • Factor of safety for liquefaction (e.g., using SPT/CPT to calculate cyclic resistance ratio vs. cyclic demand at PGA = 0.6 g).
    • If factor of safety < 1 for the layer, estimate expected post‑liquefaction total and differential settlement (e.g., compute settlements of 6–12 in total with differential settlement of 2–4 in — example numbers produced by accepted settlement procedures).
    • Estimate free‑field lateral spread displacement for the expected PGA (e.g., using simplified Newmark/Newmark‑type or other accepted methods referenced by the CBC guidance) and determine expected lateral displacement at the foundation line (e.g., 1.0–2.5 ft; the geotechnical engineer’s analysis yields the project‑specific number). The CBC expects lateral movement to be assessed and consequences quantified (§ 1803.5.12) .
  4. Consequence assessment per § 1803.5.12: quantify lateral soil loads on foundations, potential reduction in bearing capacity, downdrag on piles if any, and possible flotation of buried utilities, and state how each will be addressed in design (§ 1803.5.12 Items 3.1–3.7) .
  5. Provide mitigation options: for example, (a) remove and replace the liquefiable layer, (b) densify in‑place by vibro compaction, (c) perform deep soil improvement (stone columns), or (d) select deep foundations founded below the liquefiable zone — and state how each option affects foundation selection and structural design (§ 1803.5.12 Item 4) .

(Concrete numerical settlement or displacement outputs in a permit submittal must come from the project geotechnical engineer’s calculations based on the field data and accepted procedures; the CBC requires that these analyses be performed and reported — § 1803.5.12 and § 1803.6) .

Related provisions

  • Geotechnical reporting requirements: § 1803.6 (geotechnical report contents) .
  • Geohazard reports and preparer qualifications: § 1803.7 (geohazard reports; cert. engineering geologist + geotechnical engineer) .
  • Appendix J liquefaction study trigger: § J104.4 (Ss > 0.5g triggers liquefaction study unless local data show low potential) .
  • Methods and guidance referenced elsewhere in the CBC (useful for lateral spread and liquefaction analyses): Section 3106F.4–3106F.5 (liquefaction potential and lateral spreading guidance, Newmark displacement approach; referenced resources) .
  • PGA determination options tied to ASCE 7: see § 1803.5.12 Item 2 and the ASCE 7 references called out therein (§ 1803.5.12) .

Code references

Grounded in the retrieved California Building Code — click a citation to read the verbatim passage:

  • CBC § 1803.5.9 High relevance — show source text

    1. Field test method for determining the in-place dry density of the compacted fill.

    2. Minimum acceptable in-place dry density expressed as a percentage of the maximum dry density determined in accordance with Item 3.

    3. Number and frequency of field tests required to determine compliance with Item 6.

    1803.5.9 Controlled low-strength material (CLSM). Where shallow foundations will bear on controlled low-strength material (CLSM), a geotechnical investigation shall be conducted and shall include all of the following:

    1. Specifications for the preparation of the site prior to placement of the CLSM.
    2. Specifications for the CLSM.
    3. Laboratory or field test method(s) to be used to determine the compressive strength or bearing capacity of the CLSM .
    4. Test methods for determining the acceptance of the CLSM in the field.
    5. Number and frequency of field tests required to determine compliance with Item 4.

    1803.5.10 Alternate setback and clearance. Where setbacks or clearances other than those required in Section 1808.7 are desired, the building official shall be permitted to require a geotechnical investigation by a registered design professional to demonstrate that the intent of Section 1808.7 would be satisfied. Such an investigation shall include consideration of material, height of slope, slope gradient, load intensity and erosion characteristics of slope material.

    1803.5.11 Seismic Design Categories C through F. For structures assigned to Seismic Design Category C, D, E or F, a geotechnical investigation shall be conducted, and shall include an evaluation of all of the following potential geologic and seismic hazards:

    1. Slope instability.

    2. Liquefaction.

    3. Total and differential settlement.

    4. Surface displacement due to faulting or seismically induced lateral spreading or lateral flow.

    1803.5.12 Seismic Design Categories D through F. For structures assigned to Seismic Design Category D, E or F, the geotechnical investigation required by Section 1803.5.11 shall include all of the following as applicable:

    1. The determination of dynamic seismic lateral earth pressures on foundation walls and retaining walls supporting more than 6 feet (1.83 m) of backfill height due to design earthquake ground motions.
    2. The potential for liquefaction and soil strength loss evaluated for site peak ground acceleration, earthquake magnitude and source characteristics consistent with the maximum considered earthquake ground motions. Peak ground acceleration shall be determined based on one of the following: 2.1. A site-specific study in accordance with Chapter 21 of ASCE 7.

    2.2. In accordance with Section 11.8.3 of ASCE 7.

    1. An assessment of potential consequences of liquefaction and soil strength loss including, but not limited to, the following:

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    3.1. Estimation of total and differential settlement.

    3.2. Lateral soil movement.

    3.3. Lateral soil loads on foundations.

    3.4. Reduction in foundation soil-bearing capacity and lateral soil reaction. 3.5. Soil downdrag and reduction in axial and lateral soil reaction for pile foundations. 3.6. Increases in soil lateral pressures on retaining walls.

    3.7. Flotation of buried structures.

    1. Discussion of mitigation measures such as, but not limited to, the following: 4.1. Selection of appropriate foundation type and depths. 4.2. Selection of appropriate structural systems to accommodate anticipated displacements and forces.
  • CBC § 3101F.8.2 High relevance — show source text

    If liquefaction is shown to be initiated in the above evaluations, the particular liquefiable strata and their thicknesses shall be clearly shown on site profiles. Resulting hazards associated with liquefaction shall be addressed including translational or rotational deformations of slopes or embankment systems and post liquefaction settlement of slopes or embankment systems and underlying foundation soils, as noted below. If such analyses indicate the potential for partial or gross (flow) failure of a slope or embankment, adequate evaluations shall be performed to confirm such a condition exists, together with analyses to evaluate potential slope displacements (lateral spreads). In these situations and for projects where more detailed numerical analyses are performed, a peer review (see Section 3101F.8.2) may be required.

    3106F.5 Slope or embankment stability and seismically induced lateral spreading. Slope or embankment stability related to the MOT facility, shall be evaluated for static and seismic loading conditions.

    3106F.5.1 Static slope stability. Static stability analysis using conventional limit equilibrium methods shall be performed for site related slope or embankment systems. Live load surcharge shall be considered in analyses based on project-specific information. The long-term static factor of safety of the slope or embankment shall not be less than 1.5.

    3106F.5.2 Pseudo-static seismic slope stability. Pseudo-static seismic slope or embankment stability analyses shall be performed to estimate the horizontal yield acceleration for the slope for the Level 1 and Level 2 earthquakes. During the seismic event, appropri- ate live load surcharge shall be considered.

    If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analyses, as appropriate: 1. Residual strength of liquefied soils 2. Strengths compatible with the pore-pressure generation of potentially liquefiable soils 3. Potential strength reduction of clays

    The residual strength of liquefied soils shall be estimated using guidelines outlined in SCEC [6.4] or other appropriate documents as noted in CGS Special Publication 117A [6.5].

    Pseudo-static analysis shall be performed without considering the presence of the foundation system. Using a horizontal seismic coefficient of one-half of the PGA, if the estimated factor of safety is greater than or equal to 1.1, then no further evaluation of lateral spreading or kinematic loading from lateral spreading is required.

    3106F.5.3 Post-earthquake static slope stability. The static factor of safety immediately following a design earthquake event shall not be less than 1.1 when any of the following are used in static stability analysis: 1. Post-earthquake residual strength of liquefied soils 2. Strengths compatible with the pore-pressure generation of potentially liquefiable soils 3. Potential strength reduction of clays

    3106F.5.4 Lateral spreading – Free field. The earthquake–induced lateral deformations of the slope or embankment and associ- ated foundations soils shall be determined for the Level 1 and Level 2 earthquakes using the associated PGA at the ground surface (not modified for liquefaction). If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analyses, as appropriate: 1. Residual strength of liquefied soils 2. Strengths compatible with the pore-pressure generation of potentially liquefiable soils 3. Potential strength reduction of clays

  • CBC § 4.3. High relevance — show source text

    4.3. Ground stabilization.

    4.4. Any combination of these measures and how they shall be considered in the design of the structure.

    1803.6 Reporting. Where geotechnical investigations are required, a written report of the investigations shall be submitted to the building official by the permit applicant at the time of permit application. This geotechnical report shall include, but need not be limited to, the following information:

    1. A plot showing the location of the soil investigations.

    2. A complete record of the soil boring and penetration test logs and soil samples.

    3. A record of the soil profile.

    4. Elevation of the water table, if encountered.

    5. Recommendations for foundation type and design criteria, including but not limited to: bearing capacity of natural or compacted soil; provisions to mitigate the effects of expansive soils; mitigation of the effects of liquefaction, differential settlement and varying soil strength; and the effects of adjacent loads.

    6. Expected total and differential settlement.

    7. Deep foundation information in accordance with Section 1803.5.5.

    8. Special design and construction provisions for foundations of structures founded on expansive soils, as necessary.

    9. Compacted fill material properties and testing in accordance with Section 1803.5.8.

    10. Controlled low-strength material properties and testing in accordance with Section 1803.5.9. 11. [OSHPD 1R, 2 & 5] The report shall consider the effects of seismic hazard in accordance with Section 1803.7.

    1803.7 Geohazard reports. [OSHPD 1R, 2 & 5] Geohazard reports shall be required for all proposed construction.

    Exceptions: 1. Reports are not required for one-story, wood-frame and light-steel-frame buildings of Type V skilled nursing or intermediate care facilities construction and 4,000 square feet (371 m [2] ) or less in floor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS); 2. Reports are not required for the following scopes of work in existing buildings:

    Nonstructural alterations, voluntary structural alterations without foundation work, and incidental structural additions or alterations, and structural repairs for other than earthquake damage. 3. A previous report for a specific site may be resubmitted, provided that a reevaluation is made and the report is found to be currently appropriate.

    The purpose of the geohazard report shall be to identify geologic and seismic conditions that may require project mitigations. The reports shall contain data which provide an assessment of the nature of the site and potential for earthquake damage based on appropriate investigations of the regional and site geology, project foundation conditions and the potential seismic shaking at the site. The report shall be prepared by a California-certified engineering geologist in consultation with a California-registered geotech- nical engineer.

    The preparation of the geohazard report shall consider the most recent CGS Note 48; Checklist for the Review of Engineering Geol- ogy and Seismology Reports for California Public School, Hospitals and Essential Services Buildings. In addition, the most recent version of CGS Special Publication 42: Earthquake Fault Zones, A Guide for Government Agencies, Property Owners / Developers, and Geoscience Practitioners for Assessing Fault Rupture Hazards in California, shall be considered for project sites proposed within an _Alquist-Priolo Earthquake Fault Zone.

  • CBC § 6.1 High relevance — show source text

    Site Class E also includes any soil profile with more than 10 feet of soft clay (defined as a soil with a plasticity index, PI > 20, water content > 40 percent and Su < 500 psf)._
    2. The plasticity index, P1, and the moisture content shall be determined in accordance with ASTM D4318 [6.1] and ASTM D2216 [6.2], respectively.
    3. Conversion of CPT data to estimate equivalent Vs, SPT blow count, or Su is allowed.|

    3106F.3 Seismic loads for geotechnical evaluations. Section 3103F.4 defines the earthquake loads to be used for structural and geotechnical evaluations in terms of design Peak Ground Accelerations (PGA), spectral accelerations and design earthquake magnitude. Values used for analyses are based on Probabilistic Seismic Hazard Analyses (PSHA) using two levels of seismic performance criteria (Section 3104F.2.1 and Table 31F-4-1).

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    MARINE OIL TERMINALS

    3106F.4 Liquefaction potential. The liquefaction potential of the soils in the immediate vicinity of or beneath each MOT, and associated slopes, embankments or rock dikes shall be evaluated for the PGAs associated with seismic performance Levels 1 and 2. Liquefaction potential evaluation should follow the procedures outlined in NCEER report [6.3], SCEC [6.4] and CGS Special Publication 117A [6.5].

    If liquefaction is shown to be initiated in the above evaluations, the particular liquefiable strata and their thicknesses shall be clearly shown on site profiles. Resulting hazards associated with liquefaction shall be addressed including translational or rotational deformations of slopes or embankment systems and post liquefaction settlement of slopes or embankment systems and underlying foundation soils, as noted below. If such analyses indicate the potential for partial or gross (flow) failure of a slope or embankment, adequate evaluations shall be performed to confirm such a condition exists, together with analyses to evaluate potential slope displacements (lateral spreads). In these situations and for projects where more detailed numerical analyses are performed, a peer review (see Section 3101F.8.2) may be required.

    3106F.5 Slope or embankment stability and seismically induced lateral spreading. Slope or embankment stability related to the MOT facility, shall be evaluated for static and seismic loading conditions.

    3106F.5.1 Static slope stability. Static stability analysis using conventional limit equilibrium methods shall be performed for site related slope or embankment systems. Live load surcharge shall be considered in analyses based on project-specific information. The long-term static factor of safety of the slope or embankment shall not be less than 1.5.

    3106F.5.2 Pseudo-static seismic slope stability. Pseudo-static seismic slope or embankment stability analyses shall be performed to estimate the horizontal yield acceleration for the slope for the Level 1 and Level 2 earthquakes. During the seismic event, appropri- ate live load surcharge shall be considered.

  • CBC § 6.4 High relevance — show source text

    The residual strength of liquefied soils shall be estimated using guidelines outlined in SCEC [6.4] or other appropriate documents as noted in CGS Special Publication 117A [6.5].

    Pseudo-static analysis shall be performed without considering the presence of the foundation system. Using a horizontal seismic coefficient of one-half of the PGA, if the estimated factor of safety is greater than or equal to 1.1, then no further evaluation of lateral spreading or kinematic loading from lateral spreading is required.

    3106F.5.3 Post-earthquake static slope stability. The static factor of safety immediately following a design earthquake event shall not be less than 1.1 when any of the following are used in static stability analysis: 1. Post-earthquake residual strength of liquefied soils 2. Strengths compatible with the pore-pressure generation of potentially liquefiable soils 3. Potential strength reduction of clays

    3106F.5.4 Lateral spreading – Free field. The earthquake–induced lateral deformations of the slope or embankment and associ- ated foundations soils shall be determined for the Level 1 and Level 2 earthquakes using the associated PGA at the ground surface (not modified for liquefaction). If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analyses, as appropriate: 1. Residual strength of liquefied soils 2. Strengths compatible with the pore-pressure generation of potentially liquefiable soils 3. Potential strength reduction of clays

    The presence of the foundation system shall not be included in the “free field” evaluations.

    Initial lateral spread estimates shall be made using the Newmark displacement approach documented in NCHRP Report 611 [6.6] or other appropriate but similar procedures.

    3106F.6 Seismically induced settlement. Seismically induced settlement shall be evaluated. Based on guidelines outlined in SCEC [6.4] or other appropriate documents such as CGS Special Publication 117A [6.5]. If seismically induced settlement is anticipated, the resulting design impacts shall be considered, including the potential development of downdrag loads on piles.

    3106F.7 Earth pressures. Both static and seismic earth pressures acting on MOT structures shall be evaluated.

    3106F.7.1 Earth pressures under static loading. The effect of static active earth pressures on structures resulting from static load- ing of backfill soils shall be considered where appropriate. Backfill sloping configuration, if applicable, and backland loading conditions shall be considered in the evaluations. The loading considerations shall be based on project-specific information. The earth pressures under static loading should be based on guidelines outlined in NAVFAC DM7-02 [6.7] or other appropriate documents.

    3106F.7.2 Earth pressures under seismic loading. The effect of earth pressures on structures resulting from seismic loading of back- fill soils, including the effect of pore-water pressure build-up in the backfill, shall be considered. The seismic coefficients used for this analysis shall be based on the Level 1 and Level 2 earthquake PGA values.

    Evaluation of earth pressures under seismic loading, should be based on NCHRP Report 611 [6.6] or other appropriate methods.

    3106F.8 Pile axial behavior.

  • CBC § 1803A.5.11 High relevance — show source text

    In addition, the most recent version of_ CGS Special Publication 42: Earthquake Fault Zones, A Guide for Government Agencies, Property Owners / Developers, and Geoscience Practitioners for Assessing Fault Rupture Hazards in California, shall be considered for project sites proposed within an Alquist-Priolo Earthquake Fault Zone. The most recent version of CGS Special Publication 117, Guidelines for Evaluating and Mitigating Seismic Hazards in California, shall be considered for project sites proposed within a Seismic Hazard Zone. All conclusions shall be fully supported by satisfactory data and analysis.

    In addition to requirements in Sections 1803A.5.11 and 1803A.5.12, the report shall include, but need not be limited to, the following: 1. Site geology. 2. Evaluation of the known active and potentially active faults, both regional and local. 3. Ground-motion parameters, as required by Sections 1613A and 1617A, and ASCE 7.

    1803A.7 Geotechnical reporting. Where geotechnical investigations are required, a written report of the investigations shall be submitted to the building official by the permit applicant at the time of permit application. The geotechnical report shall provide completed evaluations of the foundation conditions of the site and the potential geologic/seismic hazards affecting the site. The geotechnical report shall include, but shall not be limited to, site-specific evaluations of design criteria related to the nature and extent of foundation materials, groundwater conditions, liquefaction potential, settlement potential and slope stability. The report shall contain the results of the analyses of problem areas identified in the geohazard report. The geotechnical report shall incorporate esti- mates of the characteristics of site ground motion provided in the geohazard report. This geotechnical report shall include, but need not be limited to, the following information:

    1. A plot showing the location of the soil investigations.
    2. A complete record of the soil boring and penetration test logs and soil samples.
    3. A record of the soil profile.
    4. Elevation of the water table, if encountered. Historic high ground water elevations shall be addressed in the report to adequately evaluate liquefaction and settlement potential.
    5. Recommendations for foundation type and design criteria, including but not limited to: bearing capacity of natural or compacted soil; provisions to mitigate the effects of expansive soils; mitigation of the effects of liquefaction, differential settlement and varying soil strength; and the effects of adjacent loads.
    6. Expected total and differential settlement.
    7. Deep foundation information in accordance with Section 1803 A .5.5.
    8. Special design and construction provisions for foundations of structures founded on expansive soils, as necessary.
    9. Compacted fill material properties and testing in accordance with Section 1803 A .5.8.
    10. Controlled low-strength material properties and testing in accordance with Section 1803 A .5.9. 11. The report shall consider the effects of stepped footings addressed in Section 1809A.3. 12. The report shall consider the effects of seismic hazards in accordance with Section 1803A.6 and shall incorporate the associ- ated geohazard report.
  • CBC § 1803.5.11 High relevance — show source text

    Nonstructural alterations, voluntary structural alterations without foundation work, and incidental structural additions or alterations, and structural repairs for other than earthquake damage. 3. A previous report for a specific site may be resubmitted, provided that a reevaluation is made and the report is found to be currently appropriate.

    The purpose of the geohazard report shall be to identify geologic and seismic conditions that may require project mitigations. The reports shall contain data which provide an assessment of the nature of the site and potential for earthquake damage based on appropriate investigations of the regional and site geology, project foundation conditions and the potential seismic shaking at the site. The report shall be prepared by a California-certified engineering geologist in consultation with a California-registered geotech- nical engineer.

    The preparation of the geohazard report shall consider the most recent CGS Note 48; Checklist for the Review of Engineering Geol- ogy and Seismology Reports for California Public School, Hospitals and Essential Services Buildings. In addition, the most recent version of CGS Special Publication 42: Earthquake Fault Zones, A Guide for Government Agencies, Property Owners / Developers, and Geoscience Practitioners for Assessing Fault Rupture Hazards in California, shall be considered for project sites proposed within an Alquist-Priolo Earthquake Fault Zone. The most recent version of CGS Special Publication 117, Guidelines for Evaluating and Mitigat- ing Seismic Hazards in California, shall be considered for project sites proposed within a Seismic Hazard Zone. All conclusions shall be fully supported by satisfactory data and analysis.

    In addition to requirements in Sections 1803.5.11 and 1803.5.12, the report shall include, but need not be limited to, the following: 1. Site geology. 2. Evaluation of the known active and potentially active faults, both regional and local. 3. Ground-motion parameters, as required by Section 1613 and ASCE 7.

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    SOILS AND FOUNDATIONS

    SECTION 1804—EXCAVATION, GRADING AND FILL

    1804.1 Excavation near foundations. Excavation for any purpose shall not reduce vertical or lateral support for any foundation or adjacent foundation without first underpinning or protecting the foundation against detrimental lateral or vertical movement, or both, in accordance with Section 1803.5.7.

    1804.2 Underpinning. Where underpinning is chosen to provide the protection or support of adjacent structures, the underpinning system shall be designed and installed in accordance with provisions of this chapter and Chapter 33.

    1804.2.1 Underpinning sequencing. Underpinning shall be installed in a sequential manner that protects the neighboring structure and the working construction site. The sequence of installation shall be identified in the approved construction documents.

    1804.3 Placement of backfill. The excavation outside the foundation shall be backfilled with soil that is free of organic material, construction debris, cobbles and boulders or with a controlled low-strength material ( CLSM ). The backfill shall be placed in lifts and compacted in a manner that does not damage the foundation or the waterproofing or dampproofing material.

    Exception: CLSM need not be compacted.

  • CBC § 5.11 High relevance — show source text

    ** For structures assigned to Seismic Design Category D, E or F, the geotechnical investigation required by Section 1803 A .5.11 shall include all of the following as applicable:

    1. The determination of dynamic seismic lateral earth pressures on foundation walls and retaining walls supporting more than 6 feet (1.83 m) of backfill height due to design earthquake ground motions.
    2. The potential for liquefaction and soil strength loss evaluated for site peak ground acceleration, earthquake magnitude and source characteristics consistent with the maximum considered earthquake ground motions. Peak ground acceleration shall be determined based on one of the following: 2.1. A site-specific study in accordance with Chapter 21 of ASCE 7.

    2.2. In accordance with Section 11.8.3 of ASCE 7.

    1. An assessment of potential consequences of liquefaction and soil strength loss including, but not limited to, the following:

    3.1. Estimation of total and differential settlement.

    3.2. Lateral soil movement.

    3.3. Lateral soil loads on foundations.

    3.4. Reduction in foundation soil-bearing capacity and lateral soil reaction. 3.5. Soil downdrag and reduction in axial and lateral soil reaction for pile foundations. 3.6. Increases in soil lateral pressures on retaining walls.

    3.7. Flotation of buried structures.

    1. Discussion of mitigation measures such as, but not limited to, the following: 4.1. Selection of appropriate foundation type and depths. 4.2. Selection of appropriate structural systems to accommodate anticipated displacements and forces.

    4.3. Ground stabilization.

    4.4. Any combination of these measures and how they shall be considered in the design of the structure.

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    1803A.6 Geohazard reports. Geohazard reports shall be required for all proposed construction.

    Exceptions: 1. Reports are not required for one-story, wood-frame and light-steel-frame buildings of Type II or Type V construction and 4,000 square feet (371 m [2] ) or less in floor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS) or in seismic hazard zones as defined in the Safety Element of the local General Plan. 2. Reports are not required for the following scopes of work in existing buildings: nonstructural alterations, voluntary struc- tural alterations without foundation work, or structural repairs for damage not caused by an earthquake. [OSHPD 1 & 4] Incidental structural additions or alterations.

    3. A previous report for a specific site may be resubmitted, provided that a reevaluation is made and the report is found to be currently appropriate.

    The purpose of the geohazard report shall be to identify geologic and seismic conditions that may require project mitigations. The reports shall contain data which provide an assessment of the nature of the site and potential for earthquake damage based on appro- priate investigations of the regional and site geology, project foundation conditions and the potential seismic shaking at the site. The report shall be prepared by a California-certified engineering geologist in consultation with a California-registered geotechnical engineer.

  • CBC § 18A-5 High relevance — show source text

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    1803A.6 Geohazard reports. Geohazard reports shall be required for all proposed construction.

    Exceptions: 1. Reports are not required for one-story, wood-frame and light-steel-frame buildings of Type II or Type V construction and 4,000 square feet (371 m [2] ) or less in floor area, not located within Earthquake Fault Zones or Seismic Hazard Zones as shown in the most recently published maps from the California Geological Survey (CGS) or in seismic hazard zones as defined in the Safety Element of the local General Plan. 2. Reports are not required for the following scopes of work in existing buildings: nonstructural alterations, voluntary struc- tural alterations without foundation work, or structural repairs for damage not caused by an earthquake. [OSHPD 1 & 4] Incidental structural additions or alterations.

    3. A previous report for a specific site may be resubmitted, provided that a reevaluation is made and the report is found to be currently appropriate.

    The purpose of the geohazard report shall be to identify geologic and seismic conditions that may require project mitigations. The reports shall contain data which provide an assessment of the nature of the site and potential for earthquake damage based on appro- priate investigations of the regional and site geology, project foundation conditions and the potential seismic shaking at the site. The report shall be prepared by a California-certified engineering geologist in consultation with a California-registered geotechnical engineer.

    The preparation of the geohazard report shall consider the most recent CGS Note 48; Checklist for the Review of Engineering Geology and Seismology Reports for California Public School, Hospitals and Essential Services Buildings. In addition, the most recent version of CGS Special Publication 42: Earthquake Fault Zones, A Guide for Government Agencies, Property Owners / Developers, and Geoscience Practitioners for Assessing Fault Rupture Hazards in California, shall be considered for project sites proposed within an Alquist-Priolo Earthquake Fault Zone. The most recent version of CGS Special Publication 117, Guidelines for Evaluating and Mitigating Seismic Hazards in California, shall be considered for project sites proposed within a Seismic Hazard Zone. All conclusions shall be fully supported by satisfactory data and analysis.

    In addition to requirements in Sections 1803A.5.11 and 1803A.5.12, the report shall include, but need not be limited to, the following: 1. Site geology. 2. Evaluation of the known active and potentially active faults, both regional and local. 3. Ground-motion parameters, as required by Sections 1613A and 1617A, and ASCE 7.

    1803A.7 Geotechnical reporting. Where geotechnical investigations are required, a written report of the investigations shall be submitted to the building official by the permit applicant at the time of permit application. The geotechnical report shall provide completed evaluations of the foundation conditions of the site and the potential geologic/seismic hazards affecting the site. The geotechnical report shall include, but shall not be limited to, site-specific evaluations of design criteria related to the nature and extent _of foundation materials, groundwater conditions, liquefaction potential, settlement potential and slope stability.

  • CBC § 5.9 High relevance — show source text
    1. Number and frequency of field tests required to determine compliance with Item 6.

    1803 A .5.9 Controlled low-strength material (CLSM). Where shallow foundations will bear on controlled low-strength material (CLSM), a geotechnical investigation shall be conducted and shall include all of the following:

    1. Specifications for the preparation of the site prior to placement of the CLSM.
    2. Specifications for the CLSM.
    3. Laboratory or field test method(s) to be used to determine the compressive strength or bearing capacity of the CLSM.
    4. Test methods for determining the acceptance of the CLSM in the field.
    5. Number and frequency of field tests required to determine compliance with Item 4.

    1803 A .5.10 Alternate setback and clearance. Where setbacks or clearances other than those required in Section 1808 A .7 are desired, the building official shall be permitted to require a geotechnical investigation by a registered design professional to demonstrate that the intent of Section 1808 A .7 would be satisfied. Such an investigation shall include consideration of material, height of slope, slope gradient, load intensity and erosion characteristics of slope material.

    1803 A .5.11 Seismic Design Categories C through F. For structures assigned to Seismic Design Category C, D, E or F, a geotechnical investigation shall be conducted, and shall include an evaluation of all of the following potential geologic and seismic hazards:

    1. Slope instability.

    2. Liquefaction.

    3. Total and differential settlement.

    4. Surface displacement due to faulting or seismically induced lateral spreading or lateral flow.

    1803 A .5.12 Seismic Design Categories D through F. For structures assigned to Seismic Design Category D, E or F, the geotechnical investigation required by Section 1803 A .5.11 shall include all of the following as applicable:

    1. The determination of dynamic seismic lateral earth pressures on foundation walls and retaining walls supporting more than 6 feet (1.83 m) of backfill height due to design earthquake ground motions.
    2. The potential for liquefaction and soil strength loss evaluated for site peak ground acceleration, earthquake magnitude and source characteristics consistent with the maximum considered earthquake ground motions. Peak ground acceleration shall be determined based on one of the following: 2.1. A site-specific study in accordance with Chapter 21 of ASCE 7.

    2.2. In accordance with Section 11.8.3 of ASCE 7.

    1. An assessment of potential consequences of liquefaction and soil strength loss including, but not limited to, the following:

    3.1. Estimation of total and differential settlement.

    3.2. Lateral soil movement.

    3.3. Lateral soil loads on foundations.

    3.4. Reduction in foundation soil-bearing capacity and lateral soil reaction. 3.5. Soil downdrag and reduction in axial and lateral soil reaction for pile foundations. 3.6. Increases in soil lateral pressures on retaining walls.

    3.7. Flotation of buried structures.

    1. Discussion of mitigation measures such as, but not limited to, the following: 4.1. Selection of appropriate foundation type and depths. 4.2. Selection of appropriate structural systems to accommodate anticipated displacements and forces.

    4.3. Ground stabilization.

    4.4. Any combination of these measures and how they shall be considered in the design of the structure.

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  • CBC § 5.5. High relevance — show source text

    The report shall_ contain the results of the analyses of problem areas identified in the geohazard report. The geotechnical report shall incorporate esti- mates of the characteristics of site ground motion provided in the geohazard report. This geotechnical report shall include, but need not be limited to, the following information:

    1. A plot showing the location of the soil investigations.
    2. A complete record of the soil boring and penetration test logs and soil samples.
    3. A record of the soil profile.
    4. Elevation of the water table, if encountered. Historic high ground water elevations shall be addressed in the report to adequately evaluate liquefaction and settlement potential.
    5. Recommendations for foundation type and design criteria, including but not limited to: bearing capacity of natural or compacted soil; provisions to mitigate the effects of expansive soils; mitigation of the effects of liquefaction, differential settlement and varying soil strength; and the effects of adjacent loads.
    6. Expected total and differential settlement.
    7. Deep foundation information in accordance with Section 1803 A .5.5.
    8. Special design and construction provisions for foundations of structures founded on expansive soils, as necessary.
    9. Compacted fill material properties and testing in accordance with Section 1803 A .5.8.
    10. Controlled low-strength material properties and testing in accordance with Section 1803 A .5.9. 11. The report shall consider the effects of stepped footings addressed in Section 1809A.3. 12. The report shall consider the effects of seismic hazards in accordance with Section 1803A.6 and shall incorporate the associ- ated geohazard report.

    1803A.8 Geotechnical peer review. [DSA-SS and DSA-SS/CC] When alternate foundations designs or ground improvements are employed or where slope stabilization is required, a qualified peer review by a California-licensed geotechnical engineer, in accordance with Section 322 of Part 10, Title 24, CCR, may be required by the enforcement agency. In Section 322 of Part 10, Title 24, CCR, where reference is made to structural or seismic-resisting system, it shall be replaced with geotechnical, foundation or ground improvement, as appropriate.

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    SECTION 1804 A —EXCAVATION, GRADING AND FILL

    1804 A .1 Excavation near foundations. Excavation for any purpose shall not reduce vertical or lateral support for any foundation or adjacent foundation without first underpinning or protecting the foundation against detrimental lateral or vertical movement, or both, in accordance with Section 1803 A .5.7.

    1804 A .2 Underpinning. Where underpinning is chosen to provide the protection or support of adjacent structures, the underpinning system shall be designed and installed in accordance with provisions of this chapter and Chapter 33.

    1804 A .2.1 Underpinning sequencing. Underpinning shall be installed in a sequential manner that protects the neighboring structure and the working construction site. The sequence of installation shall be identified in the approved construction documents.

  • CBC § 1803.5.12 High relevance — show source text

    1803.5.12 Seismic Design Categories D through F. For structures assigned to Seismic Design Category D, E or F, the geotechnical investigation required by Section 1803.5.11 shall include all of the following as applicable:

    1. The determination of dynamic seismic lateral earth pressures on foundation walls and retaining walls supporting more than 6 feet (1.83 m) of backfill height due to design earthquake ground motions.
    2. The potential for liquefaction and soil strength loss evaluated for site peak ground acceleration, earthquake magnitude and source characteristics consistent with the maximum considered earthquake ground motions. Peak ground acceleration shall be determined based on one of the following: 2.1. A site-specific study in accordance with Chapter 21 of ASCE 7.

    2.2. In accordance with Section 11.8.3 of ASCE 7.

    1. An assessment of potential consequences of liquefaction and soil strength loss including, but not limited to, the following:

    2025 CALIFORNIA BUILDING CODE 18-5

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    SOILS AND FOUNDATIONS

    3.1. Estimation of total and differential settlement.

    3.2. Lateral soil movement.

    3.3. Lateral soil loads on foundations.

    3.4. Reduction in foundation soil-bearing capacity and lateral soil reaction. 3.5. Soil downdrag and reduction in axial and lateral soil reaction for pile foundations. 3.6. Increases in soil lateral pressures on retaining walls.

    3.7. Flotation of buried structures.

    1. Discussion of mitigation measures such as, but not limited to, the following: 4.1. Selection of appropriate foundation type and depths. 4.2. Selection of appropriate structural systems to accommodate anticipated displacements and forces.

    4.3. Ground stabilization.

    4.4. Any combination of these measures and how they shall be considered in the design of the structure.

    1803.6 Reporting. Where geotechnical investigations are required, a written report of the investigations shall be submitted to the building official by the permit applicant at the time of permit application. This geotechnical report shall include, but need not be limited to, the following information:

    1. A plot showing the location of the soil investigations.

    2. A complete record of the soil boring and penetration test logs and soil samples.

    3. A record of the soil profile.

    4. Elevation of the water table, if encountered.

    5. Recommendations for foundation type and design criteria, including but not limited to: bearing capacity of natural or compacted soil; provisions to mitigate the effects of expansive soils; mitigation of the effects of liquefaction, differential settlement and varying soil strength; and the effects of adjacent loads.

    6. Expected total and differential settlement.

    7. Deep foundation information in accordance with Section 1803.5.5.

    8. Special design and construction provisions for foundations of structures founded on expansive soils, as necessary.

    9. Compacted fill material properties and testing in accordance with Section 1803.5.8.

    10. Controlled low-strength material properties and testing in accordance with Section 1803.5.9. 11. [OSHPD 1R, 2 & 5] The report shall consider the effects of seismic hazard in accordance with Section 1803.7.

    1803.7 Geohazard reports. [OSHPD 1R, 2 & 5] Geohazard reports shall be required for all proposed construction.

Frequently asked questions

Who must prepare the liquefaction/lateral spread evaluation?

A California‑licensed geotechnical engineer (in coordination with a California‑certified engineering geologist where geohazard reporting is required) should prepare the investigation and report as required by § 1803.5.11/§ 1803.5.12 and the reporting rules in § 1803.6 .

If my project is Seismic Design Category C, do I still need liquefaction analysis?

Yes. § 1803.5.11 requires that geotechnical investigations for SDC C–F include an evaluation of liquefaction and seismically induced lateral spreading or lateral flow .

What PGA should the geotechnical report use for liquefaction evaluation?

For SDC D–F the CBC requires evaluation for site peak ground acceleration consistent with the maximum considered earthquake; PGA may be determined by a site‑specific study per ASCE 7 Chapter 21 or per ASCE 7 §11.8.3 as allowed in § 1803.5.12 Item 2 and 2.1/2.2 .

Do I need to show liquefiable layers on the boring logs and profiles?

Yes. When liquefaction is indicated the strata and their thicknesses should be clearly shown on site profiles and stated in the report, and resulting hazards must be addressed in the analyses and recommendations (§ 1803.6 and related guidance) .

What consequences must be evaluated if liquefaction is possible?

The CBC requires estimation of total and differential settlement; lateral soil movement; lateral soil loads on foundations; reduction in bearing capacity; downdrag on piles; increases in lateral pressures on retaining walls; and flotation of buried structures where applicable (§ 1803.5.12 Item 3) .

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