CBC · California Building Code

When is a geotechnical investigation required for my project?

If your site has high groundwater (within 5 ft), compacted fill deeper than 12 in, deep foundations, expansive soils, or the project is in Seismic Design Category C–F (especially D–F), the California Building Code requires a geotechnical investigation prepared by a qualified engineer. The CBC lists the specific triggers and minimum scope (borings, tests, and report contents) in § 1803 and the sub‑sections § 1803.5.11 and § 1803.5.12, and the building official can waive or require additional work based on available data.

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Last reviewed: July 5, 2026

What the code requires

Under the CBC, geotechnical investigations are required when site or design conditions could affect foundation performance or create geologic/seismic hazards. The general rule is set out in § 1803: investigations must be performed where required by § 1803 and the detailed triggers are listed in § 1803.5 (for example, expansive soils, groundwater within 5 ft of lowest floor, compacted fills, deep foundations, and seismic-design-category triggers) . Investigations that include in‑situ or laboratory testing, engineering calculations, or that are required by the building official must be performed by a registered design professional (engineer) per § 1803.1 .

Requirements in detail

The controlling sections

  • General requirement and who may perform the work: § 1803.1
  • Which investigations are required and where the building official may waive them: § 1803.2
  • Specific investigation triggers and scope: § 1803.3, § 1803.3.1, and § 1803.5 (items 1–12)
  • Additional seismic/geologic requirements for SDC C–F and D–F: § 1803.5.11 and § 1803.5.12

Decision-relevant dimensions and thresholds

Condition / trigger Decision‑relevant value or test Geotechnical action required? Code Reference
Seismic Design Category C, D, E or F Any structure assigned SDC C–F Geotechnical investigation to evaluate slope instability, liquefaction, settlement, surface displacement, etc. § 1803.5.11
Seismic Design Category D, E or F SDC D–F Investigation required to include liquefaction potential for site PGA/MCE, dynamic seismic lateral earth pressures for walls >6 ft backfill, and consequence assessments/mitigation § 1803.5.12
Groundwater relative to lowest floor Groundwater above or within 5 ft (1524 mm) of lowest floor elevation Geotechnical investigation required to determine effects on design/construction § 1803.5.4
Compacted fill under shallow foundations Compacted fill > 12 in (305 mm) depth Geotechnical investigation required; report must address compaction specs, test methods, lift thickness, field density acceptance, testing frequency § 1803.5.8
Controlled low‑strength material (CLSM) under foundations Any use of CLSM as bearing medium Geotechnical investigation required addressing specifications and testing/acceptance § 1803.5.9
Deep foundations Any use of deep foundations Geotechnical investigation required; must include recommended foundation types and capacities, spacing, installation and testing requirements § 1803.5.5; § 1810.1.1
Borings to characterize site Not less than one boring per 5,000 sq ft (465 m2) of building area, minimum two for any one building (subject to enforcement agency approval) Scope and number determined by RDP; collect borings sufficient for visual exam / cores and depth to characterize subsurface § 1803.3.1
Expansive soils PI ≥ 15; >10% passing No.200 sieve; >10% <5 µm OR expansion index >20 Building official shall require soil tests; if conditions met, treat soils as expansive and investigate/mitigate § 1803.5.3

(Each of the rows above summarizes the CBC trigger and the section to consult; the code text contains the required detail and testing standards referenced.)

Scope and who must prepare it

  • The geotechnical investigation’s scope (number/type of borings, in‑situ tests, lab program) is determined by a registered design professional and minimum boring frequency/requirements are stated in § 1803.3.1 and § 1803.3 § 1803.4 (qualified representative requirement) .
  • Where investigations involve in‑situ testing, lab testing, or engineering calculations the work must be performed by a registered design professional and soils classified under ASTM standards as required in § 1803.1 and § 1803.5.1 .

Exceptions & special cases

  • The building official may waive the geotechnical investigation requirement if satisfactory data from adjacent areas demonstrates an investigation is unnecessary for the conditions listed in § 1803.5.2–.6 and § 1803.5.10–.11 (§ 1803.2 exception) .
  • CBC contains administrative exceptions (e.g., limited exemptions for certain small one‑story Type V/II buildings under 4,000 ft² not in mapped Earthquake Fault or Seismic Hazard Zones) in the Chapter 18A (where applicable to DSA/OSHPD-adopted provisions); consult the exception language in § 1803A.2 and related notes before assuming exemption .
  • For projects in mapped Alquist‑Priolo or Seismic Hazard Zones, CGS guidance and site‑specific geohazard considerations apply and must be considered in the geotechnical/geohazard report per the code advisory notes referenced in the chapter .

Common mistakes

  • Assuming a small footprint automatically avoids geotechnical work — even a small building must meet the minimum borings rule (minimum two borings per building) unless the enforcement agency approves otherwise (§ 1803.3.1) .
  • Overlooking the 12‑inch compacted‑fill threshold: foundations bearing on compacted fill deeper than 12 in require a geotechnical investigation (§ 1803.5.8) .
  • Forgetting groundwater effects: if groundwater is within 5 ft of the lowest floor elevation, the code requires a geotechnical investigation to study its effects (§ 1803.5.4) .
  • Not addressing SDC triggers: projects in SDC C–F (and especially D–F) have mandatory geotechnical content (liquefaction, settlement, lateral spreading, dynamic earth pressures) per § 1803.5.11 and § 1803.5.12 .
  • Failing to engage the correct licensed professional when in-situ or lab testing or engineering calculations are required — the code requires a registered design professional (and registered geotechnical engineer for soil classification/testing) (§ 1803.1) .

Worked example

Scenario: You plan a new two‑story, wood‑frame house with 3,500 ft² footprint on a site assigned Seismic Design Category D. Part of the lot will be re‑graded with compacted fill 18 in deep under the planned shallow foundations; there is also a localized cut with a 7‑ft backfill height behind a foundation wall; the historic high groundwater is reported about 4 ft below the proposed lowest floor.

Code application:

  • SDC D triggers the requirement for a geotechnical investigation under § 1803.5.11 (SDC C–F) and the expanded requirements under § 1803.5.12 (SDC D–F) — so you must perform an investigation that evaluates liquefaction, settlement, lateral spreading, and includes assessments of dynamic seismic lateral earth pressures for walls/retaining walls with >6 ft backfill height (§ 1803.5.11, § 1803.5.12) .
  • Compacted fill under foundations is 18 in (>12 in), so § 1803.5.8 requires a geotechnical investigation with compaction specifications, test methods, lift thickness limits, in‑place density test methods and testing frequency .
  • Groundwater at 4 ft below lowest floor is within the 5‑ft threshold; § 1803.5.4 requires a geotechnical investigation to determine how groundwater affects design and construction .
  • Borings: the RDP determines the scope, but CBC minimums apply — at least two borings for the building and typically one boring per 5,000 ft² of building area if relevant; the registered design professional will specify additional borings because of SDC D, compacted fill, groundwater and the >6‑ft retaining/backfill condition (§ 1803.3.1) .
    Outcome: A full geotechnical report is required, performed by a qualified registered geotechnical engineer, addressing liquefaction analysis (for MCE/PGA per § 1803.5.12), compaction and testing programs for the fill, groundwater evaluation, expected settlements and mitigation recommendations (foundations, structural selections, drainage, or ground improvement) as required by the cited sections .

Related provisions (quick list)

  • § 1803.1 — General geotechnical investigations and who must perform them
  • § 1803.2 — Investigations required and building‑official waiver provisions; exceptions for limited cases in Chapter 18A
  • § 1803.3 / § 1803.3.1 — Basis and scope of investigation; minimum borings (1 per 5,000 ft², minimum 2)
  • § 1803.5.3 — Expansive soils criteria (PI, sieve, expansion index)
  • § 1803.5.4 — Groundwater threshold (within 5 ft of lowest floor) requiring investigation
  • § 1803.5.8 — Compacted fill >12 in requires investigation and testing program
  • § 1803.5.11 — SDC C–F: geotechnical investigation for seismic/geologic hazards
  • § 1803.5.12 — SDC D–F: additional liquefaction, dynamic earth pressure, consequence assessment requirements
  • § 1810.1.1 — Deep foundations must be based on a geotechnical investigation (see § 1803)
  • § 1803A.7 (geotechnical reporting) — Content and submittal timing for geotechnical reports where Chapter 18A applies (DSA/OSHPD)

Code references

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

  • CBC § 1803.1 High relevance — show source text

    SECTION 1803—GEOTECHNICAL INVESTIGATIONS

    1803.1 General. Geotechnical investigations shall be conducted in accordance with Section 1803.2 and reported in accordance with Section 1803.6. Where required by the building official or where geotechnical investigations involve in-situ testing, laboratory testing or engineering calculations, such investigations shall be conducted by a registered design professional. [OSHPD 1R, 2 & 5] The classi- fication, testing and investigation of the soil shall be made under the responsible charge of a California registered geotechnical engineer. All recommendations contained in geotechnical and geohazard reports shall be subject to the approval of the enforcement agency. All reports shall be prepared and signed by a registered geotechnical engineer, certified engineering geologist and a registered geophysicist, where applicable.

    1803.1.1 General and where required for applications listed in Section 1.8.2.1.1 regulated by the Department of Housing and Community Development. [HCD 1] Foundation and soils investigations shall be conducted in conformance with Health and Safety Code Sections 17953 through 17957 as summarized below.

    1803.1.1.1 Preliminary soil report. Each city, county, or city and county shall enact an ordinance which requires a preliminary soil report, prepared by a civil engineer who is registered by the state. The report shall be based upon adequate test borings or excavations, of every subdivision, where a tentative and final map is required pursuant to Section 66426 of the Government Code.

    The preliminary soil report may be waived if the building department of the city, county, or city and county, or other enforce- ment agency charged with the administration and enforcement of the provisions of Section 1803.1.1, shall determine that, due to the knowledge such department has as to the soil qualities of the soil of the subdivision or lot, no preliminary analysis is necessary.

    1803.1.1.2 Soil investigation by lot, necessity, preparation and recommendations. If the preliminary soil report indicates the presence of critically expansive soils or other soil problems which, if not corrected, would lead to structural defects, such ordi- nance shall require a soil investigation of each lot in the subdivision.

    The soil investigation shall be prepared by a civil engineer who is registered in this state. It shall recommend corrective action which is likely to prevent structural damage to each dwelling proposed to be constructed on the expansive soil.

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    1803.1.1.3 Approval, building permit conditions, appeal. The building department of each city, county, or city and county, or other enforcement agency charged with the administration and enforcement of the provisions of Section 1803.1.1, shall approve the soil investigation if it determines that the recommended action is likely to prevent structural damage to each dwelling to be constructed. As a condition to the building permit, the ordinance shall require that the approved recommended action be incorpo- rated in the construction of each dwelling. Appeal from such determination shall be to the local appeals board.

  • CBC § 1803.3.1 High relevance — show source text

    1803.3.1 Scope of investigation. The scope of the geotechnical investigation including the number and types of borings or soundings, the equipment used to drill or sample, the in-situ testing equipment and the laboratory testing program shall be determined by a registered design professional.

    [OSHPD 1R, 2 & 5] There shall not be less than one boring or exploration shaft for each 5,000 square feet (465 m [2] ) of building area at the foundation level with a minimum of two provided for any one building. A boring may be considered to reflect subsurface conditions relevant to more than one building, subject to the approval of the enforcement agency.

    Borings shall be of sufficient size to permit visual examination of the soil in place or, in lieu thereof, cores shall be taken.

    Borings shall be of sufficient depth and size to adequately characterize subsurface conditions.

    Exception: Single-story Type V skilled nursing or intermediate care facilities utilizing wood-frame or light-steel frame construction.

    1803.4 Qualified representative. The investigation procedure and apparatus shall be in accordance with generally accepted engineering practice. The registered design professional shall have a fully qualified representative on site during all boring or sampling operations.

    1803.5 Investigated conditions. Geotechnical investigations shall be conducted as indicated in Sections 1803.5.1 through 1803.5.12.

    1803.5.1 Classification. Soil materials shall be classified in accordance with ASTM D2487. Rock shall be classified in accordance

    with ASTM D5878.

    1803.5.2 Questionable soil and rock. Where the classification, strength, moisture sensitivity or compressibility of the soil or rock is in doubt or where a load-bearing value superior to that specified in this code is claimed, the building official shall be permitted to require that a geotechnical investigation be conducted.

    1803.5.3 Expansive soil. In areas likely to have expansive soil, the building official shall require soil tests to determine where such soils do exist.

    Soils meeting all four of the following provisions shall be considered to be expansive, except that tests to show compliance with Items 1, 2 and 3 shall not be required if the test prescribed in Item 4 is conducted:

    1. Plasticity index (PI) of 15 or greater, determined in accordance with ASTM D4318.

    2. More than 10 percent of the soil particles pass a No.200 sieve (75 µm), determined in accordance with ASTM D6913.

    3. More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D6913.

    4. Expansion index greater than 20, determined in accordance with ASTM D4829.

    1803.5.4 Groundwater. A geotechnical investigation shall be performed to determine if:

    1. Groundwater is above or within 5 feet (1524 mm) below the elevation of the lowest floor level where such floor is located below the finished ground level adjacent to the foundation.
    2. The groundwater depth will affect the design and construction of buildings and structures.

    1803.5.5 Deep foundations. Where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data on which to base the design and installation is otherwise available:

    1. Recommended deep foundation types and installed capacities.

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  • CBC § 5.1 High relevance — show source text

    Borings shall be of sufficient size to permit visual examination of the soil in place or, in lieu thereof, cores shall be taken.

    Borings shall be of sufficient depth and size to adequately characterize sub-surface conditions.

    1803 A .4 Qualified representative. The investigation procedure and apparatus shall be in accordance with generally accepted engineering practice. The registered design professional shall have a fully qualified representative on site during all boring or sampling operations.

    1803 A .5 Investigated conditions. Geotechnical investigations shall be conducted as indicated in Sections 1803 A .5.1 through 1803 A .5.12.

    1803 A .5.1 Classification. Soil materials shall be classified in accordance with ASTM D2487. Rock shall be classified in accordance

    with ASTM D5878.

    1803 A .5.2 Questionable soil and rock. Where the classification, strength, moisture sensitivity or compressibility of the soil or rock is in doubt or where a load-bearing value superior to that specified in this code is claimed, the building official shall be permitted to require that a geotechnical investigation be conducted.

    1803 A .5.3 Expansive soil. In areas likely to have expansive soil, the building official shall require soil tests to determine where such soils do exist.

    Soils meeting all four of the following provisions shall be considered to be expansive, except that tests to show compliance with Items 1, 2 and 3 shall not be required if the test prescribed in Item 4 is conducted:

    1. Plasticity index (PI) of 15 or greater, determined in accordance with ASTM D4318.

    2. More than 10 percent of the soil particles pass a No.200 sieve (75 µm), determined in accordance with ASTM D6913.

    3. More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D6913.

    4. Expansion index greater than 20, determined in accordance with ASTM D4829.

    1803 A .5.4 Groundwater. A geotechnical investigation shall be performed to determine if:

    1. Groundwater is above or within 5 feet (1524 mm) below the elevation of the lowest floor level where such floor is located below the finished ground level adjacent to the foundation.
    2. The groundwater depth will affect the design and construction of buildings and structures.

    1803 A .5.5 Deep foundations. Where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data on which to base the design and installation is otherwise available:

    1. Recommended deep foundation types and installed capacities.
    2. Recommended center-to-center spacing of deep foundation elements.
    3. Driving criteria.
    4. Installation procedures.
    5. Field inspection and reporting procedures (to include procedures for verification of the installed bearing capacity where required).
    6. Load test requirements.
    7. Suitability of deep foundation materials for the intended environment.
    8. Designation of bearing stratum or strata.
    9. Reductions for group action, where necessary.

    1803 A .5.6 Rock strata. Where foundations are to be constructed on or in rock, the geotechnical investigation shall assess variations in rock strata depth, competency and load-bearing capacity.

  • 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 § 1803.5.3 High relevance — show source text

    1803.5.3 Expansive soil. In areas likely to have expansive soil, the building official shall require soil tests to determine where such soils do exist.

    Soils meeting all four of the following provisions shall be considered to be expansive, except that tests to show compliance with Items 1, 2 and 3 shall not be required if the test prescribed in Item 4 is conducted:

    1. Plasticity index (PI) of 15 or greater, determined in accordance with ASTM D4318.

    2. More than 10 percent of the soil particles pass a No.200 sieve (75 µm), determined in accordance with ASTM D6913.

    3. More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D6913.

    4. Expansion index greater than 20, determined in accordance with ASTM D4829.

    1803.5.4 Groundwater. A geotechnical investigation shall be performed to determine if:

    1. Groundwater is above or within 5 feet (1524 mm) below the elevation of the lowest floor level where such floor is located below the finished ground level adjacent to the foundation.
    2. The groundwater depth will affect the design and construction of buildings and structures.

    1803.5.5 Deep foundations. Where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data on which to base the design and installation is otherwise available:

    1. Recommended deep foundation types and installed capacities.

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    1. Recommended center-to-center spacing of deep foundation elements.
    2. Driving criteria.
    3. Installation procedures.
    4. Field inspection and reporting procedures (to include procedures for verification of the installed bearing capacity where required).
    5. Load test requirements.
    6. Suitability of deep foundation materials for the intended environment.
    7. Designation of bearing stratum or strata.
    8. Reductions for group action, where necessary.

    1803.5.6 Rock strata. Where foundations are to be constructed on or in rock, the geotechnical investigation shall assess variations in rock strata depth, competency and load-bearing capacity.

    1803.5.7 Excavation near foundations. Where excavation will reduce support from any foundation, a registered design professional shall prepare an assessment of the structure as determined from examination of the structure, available design documents, available subsurface data, and, if necessary, excavation of test pits. The registered design professional shall determine the requirements for support and protection of any existing foundation and prepare site-specific plans, details and sequence of work for submission. Such support shall be provided by under-pinning, bracing, excavation retention systems or by other means acceptable to the building official.

    1803.5.8 Compacted fill material. Where shallow foundations will bear on compacted fill material more than 12 inches (305 mm) in depth, 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 compacted fill material.
    2. Specifications for material to be used as compacted fill.
    3. Test methods to be used to determine the maximum dry density and optimum moisture content of the material to be used as compacted fill.
    4. Maximum allowable thickness of each lift of compacted fill material.
    5. Field test method for determining the in-place dry density of the compacted fill.
  • CBC § 2.4 High relevance — show source text

    SECTION 1802 A —DESIGN BASIS

    1802 A .1 General. Allowable bearing pressures, allowable stresses and design formulas provided in this chapter shall be used with the allowable stress design load combinations specified in ASCE 7, Section 2.4 or the alternative allowable stress design load combinations of Section 1605 A .2. The quality and design of materials used structurally in excavations and foundations shall comply with the requirements specified in Chapters 16, 19, 21, 22 and 23. Excavations and fills shall comply with Chapter 33.

    SECTION 1803 A —GEOTECHNICAL INVESTIGATIONS

    1803 A .1 General. Geotechnical investigations shall be conducted in accordance with Section 1803 A .2 and reported in accordance with Section 1803A.7. The classification and investigation of the soil shall be made under the responsible charge of a California regis- tered geotechnical engineer. All recommendations contained in geotechnical and geohazard reports shall be subject to the approval of the enforcement agency. All reports shall be prepared and signed by a registered geotechnical engineer, a certified engineering geolo- gist and a registered geophysicist, where applicable.

    1803 A .2 Investigations required. Geotechnical investigations shall be conducted in accordance with Sections 1803 A .3 through 1803A.6 .

    Exception s : 1. Geotechnical 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. Allowable foundation and lateral soil pressure values may be determined from Table 1806A.2.

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    2. 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.

    1803 A .3 Basis of investigation. Soil classification shall be based on observation and any necessary tests of the materials disclosed by borings, test pits or other subsurface exploration made in appropriate locations. Additional studies shall be made as necessary to evaluate slope stability, soil strength, position and adequacy of load-bearing soils, the effect of moisture variation on soil-bearing capacity, compressibility, liquefaction and expansiveness.

  • 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 § 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 § 1801.1.2 High relevance — show source text

    1801.1.2 Amendments in this chapter. [OSHPD 1R, 2 & 5] OSHPD adopts this chapter as amended.

    1801.1.3 Identification of amendments. [OSHPD 1R, 2 & 5] Office of Statewide Hospital Planning and Development (OSHPD) amendments appear in this chapter preceded with the appropriate acronym, as follows:

    [OSHPD 1R ] – For applications listed in Section 1.10.1.

    [OSHPD 2] – For applications listed in Section 1.10.2.

    [OSHPD 5] – For applications listed in Section 1.10.5.

    SECTION 1802—DESIGN BASIS

    1802.1 General. Allowable bearing pressures, allowable stresses and design formulas provided in this chapter shall be used with the allowable stress design load combinations specified in ASCE 7, Section 2.4 or the alternative allowable stress design load combinations of Section 1605.2. The quality and design of materials used structurally in excavations and foundations shall comply with the requirements specified in Chapters 16, 19, 21, 22 and 23. Excavations and fills shall comply with Chapter 33.

    [HCD 1] For limited-density owner-built rural dwellings, pier foundations, stone masonry footings and foundations, pressure-treated lumber, poles or equivalent foundation materials or designs may be used, provided that the bearing is sufficient for the purpose intended.

    SECTION 1803—GEOTECHNICAL INVESTIGATIONS

    1803.1 General. Geotechnical investigations shall be conducted in accordance with Section 1803.2 and reported in accordance with Section 1803.6. Where required by the building official or where geotechnical investigations involve in-situ testing, laboratory testing or engineering calculations, such investigations shall be conducted by a registered design professional. [OSHPD 1R, 2 & 5] The classi- fication, testing and investigation of the soil shall be made under the responsible charge of a California registered geotechnical engineer. All recommendations contained in geotechnical and geohazard reports shall be subject to the approval of the enforcement agency. All reports shall be prepared and signed by a registered geotechnical engineer, certified engineering geologist and a registered geophysicist, where applicable.

    1803.1.1 General and where required for applications listed in Section 1.8.2.1.1 regulated by the Department of Housing and Community Development. [HCD 1] Foundation and soils investigations shall be conducted in conformance with Health and Safety Code Sections 17953 through 17957 as summarized below.

    1803.1.1.1 Preliminary soil report. Each city, county, or city and county shall enact an ordinance which requires a preliminary soil report, prepared by a civil engineer who is registered by the state. The report shall be based upon adequate test borings or excavations, of every subdivision, where a tentative and final map is required pursuant to Section 66426 of the Government Code.

  • CBC § 5.7 High relevance — show source text

    1803 A .5.7 Excavation near foundations. Where excavation will reduce support from any foundation, a registered design professional shall prepare an assessment of the structure as determined from examination of the structure, available design documents, available subsurface data, and, if necessary, excavation of test pits. The registered design professional shall determine the requirements for support and protection of any existing foundation and prepare site-specific plans, details and sequence of work

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    for submission. Such support shall be provided by under-pinning, bracing, excavation retention systems or by other means acceptable to the building official.

    1803 A .5.8 Compacted fill material. Where shallow foundations will bear on compacted fill material more than 12 inches (305 mm) in depth, 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 compacted fill material.

    2. Specifications for material to be used as compacted fill.

    3. Test methods to be used to determine the maximum dry density and optimum moisture content of the material to be used as compacted fill.

    4. Maximum allowable thickness of each lift of compacted fill material.

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

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

    7. 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.

  • CBC § 1806A.2. Medium relevance — show source text

    1803 A .2 Investigations required. Geotechnical investigations shall be conducted in accordance with Sections 1803 A .3 through 1803A.6 .

    Exception s : 1. Geotechnical 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. Allowable foundation and lateral soil pressure values may be determined from Table 1806A.2.

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    2. 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.

    1803 A .3 Basis of investigation. Soil classification shall be based on observation and any necessary tests of the materials disclosed by borings, test pits or other subsurface exploration made in appropriate locations. Additional studies shall be made as necessary to evaluate slope stability, soil strength, position and adequacy of load-bearing soils, the effect of moisture variation on soil-bearing capacity, compressibility, liquefaction and expansiveness.

    1803 A .3.1 Scope of investigation. The scope of the geotechnical investigation including the number and types of borings or soundings, the equipment used to drill or sample, the in-situ testing equipment and the laboratory testing program shall be determined by a registered design professional. There shall not be less than one boring or exploration shaft for each 5,000 square feet (465 m [2] ) of building area at the foundation level with a minimum of two provided for any one building. A boring may be considered to reflect subsurface conditions relevant to more than one building, subject to the approval of the enforcement agency.

    Borings shall be of sufficient size to permit visual examination of the soil in place or, in lieu thereof, cores shall be taken.

    Borings shall be of sufficient depth and size to adequately characterize sub-surface conditions.

    1803 A .4 Qualified representative. The investigation procedure and apparatus shall be in accordance with generally accepted engineering practice. The registered design professional shall have a fully qualified representative on site during all boring or sampling operations.

    1803 A .5 Investigated conditions. Geotechnical investigations shall be conducted as indicated in Sections 1803 A .5.1 through 1803 A .5.12.

    1803 A .5.1 Classification. Soil materials shall be classified in accordance with ASTM D2487. Rock shall be classified in accordance

    with ASTM D5878.

    1803 A .5.2 Questionable soil and rock. Where the classification, strength, moisture sensitivity or compressibility of the soil or rock is in doubt or where a load-bearing value superior to that specified in this code is claimed, the building official shall be permitted to require that a geotechnical investigation be conducted.

  • CBC § 5.4 Medium relevance — show source text
    1. Expansion index greater than 20, determined in accordance with ASTM D4829.

    1803 A .5.4 Groundwater. A geotechnical investigation shall be performed to determine if:

    1. Groundwater is above or within 5 feet (1524 mm) below the elevation of the lowest floor level where such floor is located below the finished ground level adjacent to the foundation.
    2. The groundwater depth will affect the design and construction of buildings and structures.

    1803 A .5.5 Deep foundations. Where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data on which to base the design and installation is otherwise available:

    1. Recommended deep foundation types and installed capacities.
    2. Recommended center-to-center spacing of deep foundation elements.
    3. Driving criteria.
    4. Installation procedures.
    5. Field inspection and reporting procedures (to include procedures for verification of the installed bearing capacity where required).
    6. Load test requirements.
    7. Suitability of deep foundation materials for the intended environment.
    8. Designation of bearing stratum or strata.
    9. Reductions for group action, where necessary.

    1803 A .5.6 Rock strata. Where foundations are to be constructed on or in rock, the geotechnical investigation shall assess variations in rock strata depth, competency and load-bearing capacity.

    1803 A .5.7 Excavation near foundations. Where excavation will reduce support from any foundation, a registered design professional shall prepare an assessment of the structure as determined from examination of the structure, available design documents, available subsurface data, and, if necessary, excavation of test pits. The registered design professional shall determine the requirements for support and protection of any existing foundation and prepare site-specific plans, details and sequence of work

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    for submission. Such support shall be provided by under-pinning, bracing, excavation retention systems or by other means acceptable to the building official.

    1803 A .5.8 Compacted fill material. Where shallow foundations will bear on compacted fill material more than 12 inches (305 mm) in depth, 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 compacted fill material.

    2. Specifications for material to be used as compacted fill.

    3. Test methods to be used to determine the maximum dry density and optimum moisture content of the material to be used as compacted fill.

    4. Maximum allowable thickness of each lift of compacted fill material.

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

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

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

Frequently asked questions

Do I always need a geotechnical report for a detached single‑family home?

Not always. The code sets triggers (SDC, groundwater, compacted fill, deep foundations, expansive soils, etc.) that require investigation. Also limited exceptions exist for small one‑story Type V/II buildings under certain conditions — but check § 1803 and the Chapter 18A exceptions and consult the local building official as the official can require or waive the investigation (§ 1803, § 1803.2, § 1803A.2) .

What professional must prepare the investigation?

When in‑situ testing, lab testing or engineering calculations are involved, the work must be performed and reported by a registered design professional; soil classification and testing are under the responsible charge of a California‑registered geotechnical engineer as required by § 1803.1 .

If my lot has a prior soils report, can I reuse it?

A previous site report may be resubmitted if reevaluated and found currently appropriate. The building official may also accept satisfactory adjacent data in limited cases (§ 1803.2) .

How many borings do I need?

The registered design professional selects the scope, but the code sets a minimum of one boring per 5,000 sq ft of building area and at least two borings for any one building unless the enforcement agency approves otherwise (§ 1803.3.1) .

My site is in Seismic Design Category D — what extra items must the report address?

For SDC D (and E, F), the geotechnical investigation must address liquefaction potential for site PGA and MCE conditions, dynamic seismic lateral earth pressures for walls/retaining walls with >6 ft backfill, and assess settlements, lateral soil movement and other consequences and mitigations as described in § 1803.5.11 and § 1803.5.12 .

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