CBC · California Building Code

What are the code requirements for foundation capacity and settlement?

The CBC requires foundations to be sized so the site’s allowable bearing pressure is not exceeded and differential settlement is minimized (see § 1808.2). Foundations must be designed for the most unfavorable ASCE 7 or CBC load combinations (§ 1808.3), and if ASCE 7 seismic analysis is used the foundation proportioning for seismic overturning must follow ASCE 7 § 12.13.4 as required by § 1808.3.1. Geotechnical reports supply the settlement predictions and acceptance criteria that implement these code requirements.

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Last reviewed: July 5, 2026

What the code requires — 2-4 sentences

Foundations must be designed so the soil’s allowable bearing capacity is not exceeded and differential settlement is minimized; the rule is stated directly in § 1808.2 of the CBC. Foundations must also be designed for the most unfavorable load combinations (ASCE 7 load combinations or the CBC allowable‑stress alternative) and may include foundation weight and overlying fill in the dead load. If ASCE 7 load combinations and equivalent lateral‑force or modal analysis are used, foundation proportioning for seismic overturning must follow ASCE 7 Section 12.13.4 as required by § 1808.3 and § 1808.3.1 of the CBC.

Requirements in detail

1) Design for capacity and settlement (what the CBC requires)

  • The CBC requires that "the allowable bearing capacity of the soil is not exceeded" and that "differential settlement is minimized." This is the direct mandate in § 1808.2. The section does not give a numeric settlement limit; it sets the performance requirement (avoid exceeding bearing capacity and minimize differential settlement) and relies on project design and geotechnical input to meet it.

2) Design loads to use for foundation sizing (§ 1808.3)

  • Foundations must be designed for the most unfavorable effects of the load combinations in ASCE 7 (Sections 2.3 or 2.4) or for the CBC’s allowable‑stress design combinations (Section 1605.2). Dead load is permitted to include the weight of foundations and overlying fill; reduced live loads (per Sections 1607.13 and 1607.14) may be used where allowed. These requirements are in § 1808.3.

3) Seismic overturning (how to proportion foundations when using ASCE 7 analysis)

  • If you proportion foundations using ASCE 7 load combinations and compute seismic overturning by equivalent lateral‑force analysis or modal analysis, the CBC requires proportioning in accordance with ASCE 7 Section 12.13.4; this requirement is stated in § 1808.3.1. In short: when ASCE 7 seismic combinations are used, follow the ASCE provision for overturning checks.

4) Where settlement numbers and predictions come from

  • The CBC directs designers to use geotechnical information and engineering analysis to predict settlement (the code itself does not supply site numeric limits in § 1808.2). The project geotechnical report is the place to find expected total and differential settlements and acceptance criteria; the geotechnical report requirements and the expectation that expected total and differential settlements be provided are spelled out elsewhere in the code (see geotechnical report requirements).

Decision‑relevant table

Decision / value you need What to do (minimum) Code Reference
Avoid exceeding soil allowable bearing capacity Use site allowable bearing pressure (from geotechnical report) and size footings/pads so applied pressure ≤ allowable § 1808.2
Control differential settlement Obtain predicted total & differential settlements from geotechnical report; design foundation to minimize differential movement that would damage serviceability § 1808.2; geotechnical report expectations
Load combinations for design Use most unfavorable ASCE 7 (§2.3 or §2.4) or CBC allowable‑stress combos (§1605.2); include foundation weight in dead load if appropriate; reduced live loads per §1607.13/§1607.14 allowed § 1808.3
Seismic overturning checks If using ASCE 7 seismic combos and equivalent/modal analysis, proportion foundations per ASCE 7 §12.13.4 § 1808.3.1

Exceptions & special cases

  • The CBC text in § 1808.2 and § 1808.3 sets performance and load‑combination rules but does not provide numeric settlement limits. Numeric settlement criteria and remediation measures (for example, for expansive soils, liquefaction mitigation, or ground improvement) are covered in other CBC sections and in the project geotechnical report; designers must follow those provisions where applicable.
  • Surcharge or new fills adjacent to a structure: § 1808.3.2 requires that fills or surcharges not be placed adjacent to a building unless the building can withstand the additional loads and existing footings affected by excavation are underpinned or otherwise protected. (This is a special case of how loads and settlement can be affected by nearby construction.)

If you need numeric settlement limits (mm/inches), the CBC sections requested do not provide them — those limits come from project geotechnical recommendations, accepted engineering practice, or other CBC sections that address specific hazards (e.g., expansive soils, ground improvement).

Common mistakes

  • Treating § 1808.2 as a numeric standard rather than a performance rule — the code requires that bearing capacity not be exceeded and differential settlement be minimized, but it does not give a one‑size‑fits‑all settlement number. Always rely on the geotechnical report for predicted settlement.
  • Using incorrect load combinations — designers sometimes apply only gravity loads and omit the “most unfavorable” seismic/wind combinations required by § 1808.3.
  • Forgetting to include foundation self‑weight or overlying fill when permitted and relevant — § 1808.3 explicitly allows dead load to include those weights.
  • Ignoring ASCE 7 overturning provisions when ASCE combinations are used — if you use ASCE 7 seismic combos and equivalent/modal analysis, you must comply with ASCE 7 §12.13.4 as required by § 1808.3.1.

Worked example — sizing a footing and checking the code items

Scenario (simple illustrative column footing):

  • Given: column factored axial demand (dead + live from superstructure) = 50 kips total (use unfactored values for allowable‑stress check per project procedure); geotechnical report gives an allowable bearing pressure = 3000 psf (3 ksf). (The CBC requires use of the site allowable — § 1808.2.)
  • Footing area required (simple bearing check): A = Applied vertical load / allowable = 50 kips / (3 ksf) = 16.67 ft². A square footing would be ≈ 4.08 ft × 4.08 ft. This sizing assures the soil bearing capacity is not exceeded, satisfying § 1808.2.
  • Load combinations: Design the foundation for the most unfavorable ASCE 7 or CBC allowable‑stress combinations per § 1808.3; include footing self‑weight and any overlying fill in the dead load where appropriate. If the project uses reduced live loads allowed by § 1607.13/1607.14, document the basis in the design.
  • Settlement check: Request the geotechnical engineer’s predicted total and differential settlements for this footing location (the CBC anticipates the geotechnical report will provide such values and acceptance criteria). If predicted differential settlement could be damaging, change footing geometry, use a more rigid mat, or employ ground improvement as recommended by the geotechnical engineer. The CBC’s § 1808.2 requires minimizing differential settlement but does not give a numeric threshold; rely on the geotechnical report for allowable settlement limits.
  • Seismic overturning check: If you use ASCE 7 seismic load combinations and compute overturning by equivalent lateral‑force or modal analysis, proportioning must follow ASCE 7 § 12.13.4 per § 1808.3.1. If overturning moment requires additional section modulus or anchorage, detail those per the ASCE requirement.

Note: this example shows how to apply the performance and load‑combination rules in §§ 1808.2 and 1808.3; settlement acceptance and mitigation measures must come from the geotechnical report and site‑specific analysis.

Related provisions (CBC sections you will commonly use with § 1808.2 / § 1808.3 / § 1808.3.1)

  • § 1808.6 — Design provisions for expansive soils (useful when settlement is driven by volume change).
  • § 1808.3.2 — Surcharge: restrictions and underpinning requirements for fill placed adjacent to buildings.
  • § 1803 (geotechnical investigation / report requirements) — the geotechnical report must include expected total and differential settlements and foundation recommendations that implement § 1808.2 performance requirements.
  • § 1810.2.3 — For deep foundations, predicted settlement must not cause harmful distortion or instability (useful when deep foundations are considered for settlement control).

Code references

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

  • CBC § 1810.2.1 High relevance — show source text

    Deep foundation elements supporting walls shall be placed alternately in lines spaced not less than 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the foundation elements are adequately braced to provide for lateral stability.

    Exceptions:

    1. Isolated cast-in-place deep foundation elements without lateral bracing shall be permitted where the least horizontal dimension is not less than 2 feet (610 mm), adequate lateral support in accordance with Section 1810.2.1 is provided for the entire height and analysis demonstrates that the element can support the required loads, including mislocations required by Section 1810.3.1.3, with neither harmful distortion nor instability in the structure.
    2. A single row of deep foundation elements without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories above grade plane or 35 feet (10 668 mm) in building height, provided that the centers of the elements are located within the width of the supported wall.

    1810.2.3 Settlement. The settlement of a single deep foundation element or group thereof shall be estimated based on approved methods of analysis. The predicted settlement shall cause neither harmful distortion of, nor instability in, the structure, nor cause any element to be loaded beyond its capacity.

    1810.2.4 Lateral loads. The moments, shears and lateral deflections used for design of deep foundation elements shall be established considering the nonlinear interaction of the shaft and soil, as determined by a registered design professional. Where the ratio of the depth of embedment of the element to its least horizontal dimension is less than or equal to six, it shall be permitted to assume the element is rigid.

    1810.2.4.1 Seismic Design Categories D through F. For structures assigned to Seismic Design Category D, E or F, deep foundation elements on Site Class E or F sites, as determined in Section 1613.2.2, shall be designed and constructed to withstand maximum imposed curvatures from earthquake ground motions and structure response. Curvatures shall include free-field soil strains modified for soil-foundation-structure interaction coupled with foundation element deformations associated with earthquake loads imparted to the foundation by the structure.

    Exception: Deep foundation elements that satisfy the following additional detailing requirements shall be deemed to comply with the curvature capacity requirements of this section:

    1. Precast prestressed concrete piles detailed in accordance with Section 1810.3.8.
    2. Cast-in-place deep foundation elements with a minimum longitudinal reinforcement ratio of 0.005 extending the full length of the element and detailed in accordance with Sections 18.7.5.2, 18.7.5.3 and 18.7.5.4 of ACI 318 as required by Section 1810.3.9.4.2.2.

    1810.2.5 Group effects. The analysis shall include group effects on lateral behavior where the center-to-center spacing of deep foundation elements in the direction of lateral force is less than eight times the least horizontal dimension of an element. The analysis shall include group effects on axial behavior where the center-to-center spacing of deep foundation elements is less than three times the least horizontal dimension of an element. Group effects shall be evaluated using a generally accepted method of analysis; the analysis for uplift of grouped elements with center-to-center spacing less than three times the least horizontal dimension of an element shall be evaluated in accordance with Section 1810.3.3.1.6.

  • CBC § 1813.2 High relevance — show source text

    Ground improvement shall be installed under the entire building/structure footprint and not under isolated foundation elements only.

    Design, construction, testing and inspection shall satisfy the requirements of this code except as modified in Sections 1813.2 through 1813.5.

    1813.2 Geotechnical report. The geotechnical report shall specify vibro stone column requirements to ensure uniformity in total and differential immediate settlement, long-term settlement and earthquake-induced settlement. The report shall address the following: 1. Soil compaction shall be sufficient to mitigate potential for liquefaction as described in California Geological Survey (CGS) Special Publication 117A (SP-117A): Guidelines for Evaluating and Mitigating Seismic Hazard in California. 2. The area replacement ratio for the compaction elements and the basis of its determination shall be explained. Minimum factor of safety for soil compaction shall be in accordance with SP-117A. 3. The depth of soil compaction elements and extent beyond the footprint of structures/foundation shall be defined. Extent beyond the foundation shall be half the depth of the VSCs with a minimum of 10’ or an approved alternative. 4. The minimum diameter and maximum spacing of soil compaction elements shall be specified. VSCs shall not be less than 2 feet in diameter and center to center spacing shall not exceed 8 feet. 5. The modulus of subgrade reactions for shallow foundations shall account for the presence of compaction elements.

    2025 CALIFORNIA BUILDING CODE 18-35

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    SOILS AND FOUNDATIONS

    6. The modulus of subgrade reactions, long-term settlement and post-earthquake settlement shall be specified along with expected total and differential settlements for design. 7. The acceptance criteria for friction cone and piezocone penetration testing in accordance with ASTM D5778 complemented by a standard penetration test (SPT) in accordance with ASTM D1586, if necessary, to verify soil improvement shall be specified 8. The requirements for special inspection and observation by the geotechnical engineer shall be specified. 9. A Final Verified Report (FVR) documenting the installation of the ground improvement system and confirming that the ground improvement acceptance criteria have been met shall be prepared by the geotechnical engineer and submitted to the enforce- ment agency for review and approval.

    1813.3 Shallow Foundations. VSCs under the shallow foundation shall be located symmetrically around the centroid of the footing or load, and:

    1. There shall be a minimum of four stone columns under each isolated or continuous/combined footing or an approved equivalent. 2. The VSCs or deep foundation elements shall not be used to resist tension or overturning uplift from the shallow foundations. 3. The foundation design for the shallow foundation shall consider the increased vertical stiffness of the VSCs as point supports for analysis, unless it is substantiated that the installation of the VSCs results in improvement of the surrounding soils such that the modulus of subgrade reaction, long-term settlement and post-earthquake settlement can be considered uniform throughout.

  • CBC § 18-35 High relevance — show source text

    2025 CALIFORNIA BUILDING CODE 18-35

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    SOILS AND FOUNDATIONS

    6. The modulus of subgrade reactions, long-term settlement and post-earthquake settlement shall be specified along with expected total and differential settlements for design. 7. The acceptance criteria for friction cone and piezocone penetration testing in accordance with ASTM D5778 complemented by a standard penetration test (SPT) in accordance with ASTM D1586, if necessary, to verify soil improvement shall be specified 8. The requirements for special inspection and observation by the geotechnical engineer shall be specified. 9. A Final Verified Report (FVR) documenting the installation of the ground improvement system and confirming that the ground improvement acceptance criteria have been met shall be prepared by the geotechnical engineer and submitted to the enforce- ment agency for review and approval.

    1813.3 Shallow Foundations. VSCs under the shallow foundation shall be located symmetrically around the centroid of the footing or load, and:

    1. There shall be a minimum of four stone columns under each isolated or continuous/combined footing or an approved equivalent. 2. The VSCs or deep foundation elements shall not be used to resist tension or overturning uplift from the shallow foundations. 3. The foundation design for the shallow foundation shall consider the increased vertical stiffness of the VSCs as point supports for analysis, unless it is substantiated that the installation of the VSCs results in improvement of the surrounding soils such that the modulus of subgrade reaction, long-term settlement and post-earthquake settlement can be considered uniform throughout.

    1813.4 Installation. VSCs shall be installed with vibratory probes. Vertical columns of compacted unbounded aggregate shall be formed through the soils to be improved by adding gravel near the tip of the vibrator and progressively raising and re-penetrating the vibrator, which will results in the gravel being pushed into the surrounding soil. Gravel aggregate for VSCs shall be well graded with a maximum size of 6 inches and not more than 10 percent smaller than [3] / 8 inch after compaction.

    1813.5 Construction Documents. Construction documents for VSCs, at a minimum, shall include the following: 1. Size, depth and location of VSCs. 2. The extent of soil improvements along with building/structure foundation outlines. 3. Field verification requirements and acceptance criteria using CPT/SPT. 4. The locations where CPT/SPT shall be performed. 5. A Testing, Inspection and Observation (TIO) program indicating the inspection and observation required for the VSCs.

    18-36 2025 CALIFORNIA BUILDING CODE

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    CALIFORNIA BUILDING CODE – MATRIX ADOPTION TABLE

    CHAPTER 18A – SOILS AND FOUNDATIONS

    (Matrix Adoption Tables are nonregulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)

  • CBC § 1810.1.4 High relevance — show source text

    1810.1.4 Special types of deep foundations. The use of types of deep foundation elements not specifically mentioned herein is permitted, subject to the approval of the building official, upon the submission of acceptable test data, calculations and other information relating to the structural properties and load capacity of such elements. The allowable stresses for materials shall not in any case exceed the limitations specified herein.

    1810.2 Analysis. The analysis of deep foundations for design shall be in accordance with Sections 1810.2.1 through 1810.2.5.

    1810.2.1 Lateral support. Any soil other than fluid soil shall be deemed to afford sufficient lateral support to prevent buckling of deep foundation elements and to permit the design of the elements in accordance with accepted engineering practice and the applicable provisions of this code.

    Where deep foundation elements stand unbraced in air, water or fluid soils, it shall be permitted to consider them laterally supported at a point 5 feet (1524 mm) into stiff soil or 10 feet (3048 mm) into soft soil unless otherwise approved by the building official on the basis of a geotechnical investigation by a registered design professional.

    1810.2.2 Stability. Deep foundation elements shall be braced to provide lateral stability in all directions. Three or more elements connected by a rigid cap shall be considered to be braced, provided that the elements are located in radial directions from the centroid of the group not less than 60 degrees (1 rad) apart. A two-element group in a rigid cap shall be considered to be braced along the axis connecting the two elements. Methods used to brace deep foundation elements shall be subject to the approval of the building official.

    Deep foundation elements supporting walls shall be placed alternately in lines spaced not less than 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the foundation elements are adequately braced to provide for lateral stability.

    Exceptions:

    1. Isolated cast-in-place deep foundation elements without lateral bracing shall be permitted where the least horizontal dimension is not less than 2 feet (610 mm), adequate lateral support in accordance with Section 1810.2.1 is provided for the entire height and analysis demonstrates that the element can support the required loads, including mislocations required by Section 1810.3.1.3, with neither harmful distortion nor instability in the structure.
    2. A single row of deep foundation elements without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories above grade plane or 35 feet (10 668 mm) in building height, provided that the centers of the elements are located within the width of the supported wall.

    1810.2.3 Settlement. The settlement of a single deep foundation element or group thereof shall be estimated based on approved methods of analysis. The predicted settlement shall cause neither harmful distortion of, nor instability in, the structure, nor cause any element to be loaded beyond its capacity.

    1810.2.4 Lateral loads. The moments, shears and lateral deflections used for design of deep foundation elements shall be established considering the nonlinear interaction of the shaft and soil, as determined by a registered design professional. Where the ratio of the depth of embedment of the element to its least horizontal dimension is less than or equal to six, it shall be permitted to assume the element is rigid.

  • CBC § 3.4. High relevance — show source text

    _ 4. Site data report in accordance with the CBC 1980 shall establish that seismically induced differential settlement does not exceed 1 ″ in 40 ′ .

    5. Adjacent buildings shall satisfy the SPC building separation requirements in accordance with the California Administrative Code, Chapter 6 Section 3.4. 6. The addition of new structural elements or strengthening of existing structural elements for retrofit of nonconforming build- ings to SPC-4D shall comply with the following: a) The seismic demand (forces or displacements) shall be in accordance with the CBC 1980; b) Capacity, detailing and connections for new structural elements shall satisfy the requirements in the CBC 2025 for new construction; and c) The strengthening of existing structural elements shall use capacities determined in accordance with the CBC 2025 for new construction consistent with the detailing and connections used in the strengthened member. 7. All construction, quality assurance and quality control shall be in accordance with the new construction provisions of CBC 2025.

    8. Elements not part of the Seismic Force-Resisting System (SFRS), including those identified in the California Administrative Code Chapter 6, Article 10, shall be evaluated using seismic forces and the requirements of the CBC 1980. 9. Any column or wall that forms part of two or more intersecting SFRS and is subjected to axial load due to seismic forces acting along either principal plan axis equaling or exceeding 20 percent of the axial design strength of the column or wall shall be evaluated for the most critical load effect due to application of seismic force in any direction. The most critical load effect may be deemed to be satisfied if members and their foundations are evaluated for 100 percent of the forces for one direction plus 30 percent of the forces for the perpendicular direction, whereby the combination produces the maximum effect. Exceptions: The following buildings (with structural irregularities or unusual configuration/system) shall not be eligible for the SPC-4D upgrade using the prescriptive provisions in this section: 1. Buildings with prohibited irregularities in accordance with California Building Code 2022 Section 1617A.1.10.

    2025 CALIFORNIA EXISTING BUILDING CODE 5A-3

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    �

    PRESCRIPTIVE COMPLIANCE METHOD

    2. Buildings taller than 5 stories or 65' height above the base having horizontal or vertical irregularities in accordance with ASCE 7-10 Tables 12.3-1 Items #1a, 1b and 3 or 12.3-2 Items #1a, 1b, 5a and 5b.

    3. Buildings with unusual configuration or structural system, as determined by the building official.

    501A.3.2 Prescriptive compliance provisions for SPC-4D using the new building design requirements of this code. Structures satisfying the requirements of the California Building Code for new general acute care hospital buildings design shall be deemed to satisfy the SPC-4D requirements of Table 2.5.3, Chapter 6 of the California Administrative Code.

  • CBC § 1806.2 High relevance — show source text

    M g = Moment in the post at grade, in foot-pounds (kN-m). S 3 = Allowable lateral soil-bearing pressure as set forth in Section 1806.2 based on a depth equal to the depth of embedment in pounds per square foot (kPa).

    2025 CALIFORNIA BUILDING CODE 18-15

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    SOILS AND FOUNDATIONS

    1807.3.2.3 Vertical load. The resistance to vertical loads shall be determined using the vertical foundation pressure set forth in Table 1806.2.

    1807.3.3 Backfill. The backfill in the annular space around columns not embedded in poured footings shall be by one of the following methods:

    1. Backfill shall be of concrete with a specified compressive strength of not less than 2,000 psi (13.8 MPa). The hole shall be not less than 4 inches (102 mm) larger than the diameter of the column at its bottom or 4 inches (102 mm) larger than the diagonal dimension of a square or rectangular column.
    2. Backfill shall be of clean sand. The sand shall be thoroughly compacted by tamping in layers not more than 8 inches (203 mm) in depth.
    3. Backfill shall be of controlled low-strength material ( CLSM ).

    SECTION 1808—FOUNDATIONS

    1808.1 General. Foundations shall be designed and constructed in accordance with Sections 1808.2 through 1808.9. Shallow foundations shall satisfy the requirements of Section 1809. Deep foundations shall satisfy the requirements of Section 1810. 1808.2 Design for capacity and settlement. Foundations shall be so designed that the allowable bearing capacity of the soil is not exceeded, and that differential settlement is minimized. Foundations in areas with expansive soils shall be designed in accordance with the provisions of Section 1808.6. 1808.3 Design loads. Foundations shall be designed for the most unfavorable effects due to the combinations of loads specified in Section 2.3 or 2.4 of ASCE 7 or the alternative allowable stress design load combinations of Section 1605.2. The dead load is permitted to include the weight of foundations and overlying fill. Reduced live loads, as specified in Sections 1607.13 and 1607.14, shall be permitted to be used in the design of foundations. 1808.3.1 Seismic overturning. Where foundations are proportioned using the load combinations of Section 2.3 or 2.4 of ASCE 7 and the computation of seismic over-turning effects is by equivalent lateral force analysis or modal analysis, the proportioning shall be in accordance with Section 12.13.4 of ASCE 7.

    1808.3.2 Surcharge. Fill or other surcharge loads shall not be placed adjacent to any building or structure unless such building or structure is capable of withstanding the additional loads caused by the fill or the surcharge. Existing footings or foundations that will be affected by any excavation shall be underpinned or otherwise protected against settlement and shall be protected against detrimental lateral or vertical movement or both.

    Exception: Minor grading for landscaping purposes shall be permitted where done with walk-behind equipment, where the grade is not increased more than 1 foot (305 mm) from original design grade or where approved by the building official. **1808.4 Vibratory loads.

  • CBC § 2.1 High relevance — show source text

    1810 A .2 Analysis. The analysis of deep foundations for design shall be in accordance with Sections 1810 A .2.1 through 1810 A .2.5.

    1810 A .2.1 Lateral support. Any soil other than fluid soil shall be deemed to afford sufficient lateral support to prevent buckling of deep foundation elements and to permit the design of the elements in accordance with accepted engineering practice and the applicable provisions of this code.

    Where deep foundation elements stand unbraced in air, water or fluid soils, it shall be permitted to consider them laterally supported at a point 5 feet (1524 mm) into stiff soil or 10 feet (3048 mm) into soft soil unless otherwise approved by the building official on the basis of a geotechnical investigation by a registered design professional.

    1810 A .2.2 Stability. Deep foundation elements shall be braced to provide lateral stability in all directions. Three or more elements connected by a rigid cap shall be considered to be braced, provided that the elements are located in radial directions from the centroid of the group not less than 60 degrees (1 rad) apart. A two-element group in a rigid cap shall be considered to be braced along the axis connecting the two elements. Methods used to brace deep foundation elements shall be subject to the approval of the building official.

    Deep foundation elements supporting walls shall be placed alternately in lines spaced not less than 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the foundation elements are adequately braced to provide for lateral stability.

    Exceptions:

    1. Isolated cast-in-place deep foundation elements without lateral bracing shall be permitted where the least horizontal dimension is not less than 2 feet (610 mm), adequate lateral support in accordance with Section 1810 A .2.1 is provided for the entire height and analysis demonstrates that the element can support the required loads, including mislocations required by Section 1810 A .3.1.3, with neither harmful distortion nor instability in the structure.
    2. A single row of deep foundation elements without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories above grade plane or 35 feet (10 668 mm) in building height, provided that the centers of the elements are located within the width of the supported wall.

    2025 CALIFORNIA BUILDING CODE 18A-15

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    SOILS AND FOUNDATIONS

    1810 A .2.3 Settlement. The settlement of a single deep foundation element or group thereof shall be estimated based on approved methods of analysis. The predicted settlement shall cause neither harmful distortion of, nor instability in, the structure, nor cause any element to be loaded beyond its capacity.

    1810 A .2.4 Lateral loads. The moments, shears and lateral deflections used for design of deep foundation elements shall be established considering the nonlinear interaction of the shaft and soil, as determined by a registered design professional. Where the ratio of the depth of embedment of the element to its least horizontal dimension is less than or equal to six, it shall be permitted to assume the element is rigid.

  • CBC § 3.3 High relevance — show source text

    1807 A .3.3 Backfill. The backfill in the annular space around columns not embedded in poured footings shall be by one of the following methods:

    1. Backfill shall be of concrete with a specified compressive strength of not less than 2,000 psi (13.8 MPa). The hole shall be not less than 4 inches (102 mm) larger than the diameter of the column at its bottom or 4 inches (102 mm) larger than the diagonal dimension of a square or rectangular column.
    2. Backfill shall be of clean sand. The sand shall be thoroughly compacted by tamping in layers not more than 8 inches (203 mm) in depth.
    3. Backfill shall be of controlled low-strength material (CLSM).

    SECTION 1808 A —FOUNDATIONS

    1808 A .1 General. Foundations shall be designed and constructed in accordance with Sections 1808 A .2 through 1808 A .9. Shallow foundations shall satisfy the requirements of Section 1809 A . Deep foundations shall satisfy the requirements of Section 1810 A .

    1808 A .2 Design for capacity and settlement. Foundations shall be so designed that the allowable bearing capacity of the soil is not exceeded, and that differential settlement is minimized. Foundations in areas with expansive soils shall be designed in accordance with the provisions of Section 1808 A .6.

    The enforcing agency may require an analysis of foundation elements to determine subgrade deformations in order to evaluate their effect on the superstructure, including story drift.

    1808 A .3 Design loads. Foundations shall be designed for the most unfavorable effects due to the combinations of loads specified in Section 2.3 or 2.4 of ASCE 7 or the alternative allowable stress design load combinations of Section 1605 A .2. The dead load is permitted to include the weight of foundations and overlying fill. Reduced live loads, as specified in Sections 1607 A .13 and 1607 A .14, shall be permitted to be used in the design of foundations.

    1808 A .3.1 Seismic overturning. Where foundations are proportioned using the load combinations of Section 2.3 or 2.4 of ASCE 7 and the computation of seismic over-turning effects is by equivalent lateral force analysis or modal analysis, the proportioning shall be in accordance with Section 12.13.4 of ASCE 7.

    1808 A .3.2 Surcharge. Fill or other surcharge loads shall not be placed adjacent to any building or structure unless such building or structure is capable of withstanding the additional loads caused by the fill or the surcharge. Existing footings or foundations that will be affected by any excavation shall be underpinned or otherwise protected against settlement and shall be protected against detrimental lateral or vertical movement or both.

    Exception: Minor grading for landscaping purposes shall be permitted where done with walk-behind equipment, where the grade is not increased more than 1 foot (305 mm) from original design grade or where approved by the building official.

    1808 A .4 Vibratory loads. Where machinery operations or other vibrations are transmitted through the foundation, consideration shall be given in the foundation design to prevent detrimental disturbances of the soil.

  • CBC § 1808.6. High relevance — show source text

    ** Foundations shall be so designed that the allowable bearing capacity of the soil is not exceeded, and that differential settlement is minimized. Foundations in areas with expansive soils shall be designed in accordance with the provisions of Section 1808.6. 1808.3 Design loads. Foundations shall be designed for the most unfavorable effects due to the combinations of loads specified in Section 2.3 or 2.4 of ASCE 7 or the alternative allowable stress design load combinations of Section 1605.2. The dead load is permitted to include the weight of foundations and overlying fill. Reduced live loads, as specified in Sections 1607.13 and 1607.14, shall be permitted to be used in the design of foundations. 1808.3.1 Seismic overturning. Where foundations are proportioned using the load combinations of Section 2.3 or 2.4 of ASCE 7 and the computation of seismic over-turning effects is by equivalent lateral force analysis or modal analysis, the proportioning shall be in accordance with Section 12.13.4 of ASCE 7.

    1808.3.2 Surcharge. Fill or other surcharge loads shall not be placed adjacent to any building or structure unless such building or structure is capable of withstanding the additional loads caused by the fill or the surcharge. Existing footings or foundations that will be affected by any excavation shall be underpinned or otherwise protected against settlement and shall be protected against detrimental lateral or vertical movement or both.

    Exception: Minor grading for landscaping purposes shall be permitted where done with walk-behind equipment, where the grade is not increased more than 1 foot (305 mm) from original design grade or where approved by the building official. 1808.4 Vibratory loads. Where machinery operations or other vibrations are transmitted through the foundation, consideration shall be given in the foundation design to prevent detrimental disturbances of the soil. 1808.5 Shifting or moving soils. Where it is known that the shallow subsoils are of a shifting or moving character, foundations shall be carried to a sufficient depth to ensure stability. 1808.6 Design for expansive soils. Foundations for buildings and structures founded on expansive soils shall be designed in accordance with Section 1808.6.1 or 1808.6.2.

    Exceptions: Foundation design need not comply with Section 1808.6.1 or 1808.6.2 where one of the following conditions is satisfied:

    1. The soil is removed in accordance with Section 1808.6.3.

    2. The building official approves stabilization of the soil in accordance with Section 1808.6.4. 1808.6.1 Foundations. Foundations placed on or within the active zone of expansive soils shall be designed to resist differential volume changes and to prevent structural damage to the supported structure. Deflection and racking of the supported structure shall be limited to that which will not interfere with the usability and serviceability of the structure.

    Foundations placed below where volume change occurs or below expansive soil shall comply with the following provisions:

    1. Foundations extending into or penetrating expansive soils shall be designed to prevent uplift of the supported structure.
    2. Foundations penetrating expansive soils shall be designed to resist forces exerted on the foundation due to soil volume changes or shall be isolated from the expansive soil.

    1808.6.2 Slab-on-ground foundations. Moments, shears and deflections for use in designing slab-on-ground, mat or raft foundations on expansive soils shall be determined in accordance with WRI/CRSI or PTI DC 10.5.

  • CBC § 1.7 High relevance — show source text

    R403.1.7 Footings on or adjacent to slopes. The placement of buildings and structures on or adjacent to slopes steeper than 1 unit vertical in 3 units horizontal (33.3-percent slope) shall conform to Sections R403.1.7.1 through R403.1.7.4.

    R403.1.7.1 Building clearances from ascending slopes. In general, buildings below slopes shall be set a sufficient distance from the slope to provide protection from slope drainage, erosion and shallow failures. Except as provided in Section R403.1.7.4 and Figure R403.1.7.1, the following criteria will be assumed to provide this protection. Where the existing slope is steeper than 1 unit vertical in 1 unit horizontal (100-percent slope), the toe of the slope shall be assumed to be at the intersection of a horizontal plane drawn from the top of the foundation and a plane drawn tangent to the slope at an angle of 45 degrees (0.79 rad) to the horizontal. Where a retaining wall is constructed at the toe of the slope, the height of the slope shall be measured from the top of the wall to the top of the slope.

    2025 CALIFORNIA RESIDENTIAL CODE 4-15

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    FOUNDATIONS

    For SI: 1 foot = 304.8 mm.

    FIGURE R403.1.7.1—FOUNDATION CLEARANCE FROM SLOPES

    FACE OF

    AT LEAST THE SMALLER OF H/2 AND 15 FEET

    R403.1.7.2 Footing setback from descending slope surfaces. Footings on or adjacent to slope surfaces shall be founded in material with an embedment and setback from the slope surface sufficient to provide vertical and lateral support for the footing without detrimental settlement. Except as provided for in Section R403.1.7.4 and Figure R403.1.7.1, the following setback is deemed adequate to meet the criteria. Where the slope is steeper than 1 unit vertical in 1 unit horizontal (100-percent slope), the required setback shall be measured from an imaginary plane 45 degrees (0.79 rad) to the horizontal, projected upward from the toe of the slope.

    R403.1.7.3 Foundation elevation. On graded sites, the top of any exterior foundation shall extend above the elevation of the street gutter at point of discharge or the inlet of an approved drainage device not less than 12 inches (305 mm) plus 2 percent. Alternate elevations are permitted subject to the approval of the building official, provided that it can be demonstrated that required drainage to the point of discharge and away from the structure is provided at all locations on the site.

    R403.1.7.4 Alternate setbacks and clearances. Alternate setbacks and clearances are permitted, subject to the approval of the building official. The building official is permitted to require an investigation and recommendation of a qualified engineer to demonstrate that the intent of this section has been satisfied. Such an investigation shall include consideration of material, height of slope, slope gradient, load intensity and erosion characteristics of slope material.

    R403.1.8 Foundations on expansive soils. Foundation and floor slabs for buildings located on expansive soils shall be designed in accordance with Section 1808.6 of the California Building Code .

    Exception: Slab-on-ground and other foundation systems that have performed adequately in soil conditions similar to those encountered at the building site are permitted subject to the approval of the building official.

  • CBC § 2.34 High relevance — show source text

    where:

    A = 2.34 P /( S 1 b ).

    b = Diameter of round post or footing or diagonal dimension of square post or footing, feet (m).

    d = Depth of embedment in earth in feet (m) but not over 12 feet (3658 mm) for purpose of computing lateral pressure.

    h = Distance in feet (m) from ground surface to point of application of “ P .”

    P = Applied lateral force in pounds (kN).

    S 1 = Allowable lateral soil-bearing pressure as set forth in Section 1806 A .2 based on a depth of one-third the depth of embedment in pounds per square foot (psf) (kPa).

    1807 A .3.2.2 Constrained. The following formula shall be used to determine the depth of embedment required to resist lateral loads where lateral constraint is provided at the ground surface, such as by a rigid floor or slab-on-ground.

    Equation 18 A -2

    or alternatively

    Equation 18 A -3 d = ----------------- g - where: 3

    M g = Moment in the post at grade, in foot-pounds (kN-m). S 3 = Allowable lateral soil-bearing pressure as set forth in Section 1806.2 based on a depth equal to the depth of embedment in pounds per square foot (kPa).

    1807 A .3.2.3 Vertical load. The resistance to vertical loads shall be determined using the vertical foundation pressure set forth in Table 1806 A .2.

    1807 A .3.3 Backfill. The backfill in the annular space around columns not embedded in poured footings shall be by one of the following methods:

    1. Backfill shall be of concrete with a specified compressive strength of not less than 2,000 psi (13.8 MPa). The hole shall be not less than 4 inches (102 mm) larger than the diameter of the column at its bottom or 4 inches (102 mm) larger than the diagonal dimension of a square or rectangular column.
    2. Backfill shall be of clean sand. The sand shall be thoroughly compacted by tamping in layers not more than 8 inches (203 mm) in depth.
    3. Backfill shall be of controlled low-strength material (CLSM).

    SECTION 1808 A —FOUNDATIONS

    1808 A .1 General. Foundations shall be designed and constructed in accordance with Sections 1808 A .2 through 1808 A .9. Shallow foundations shall satisfy the requirements of Section 1809 A . Deep foundations shall satisfy the requirements of Section 1810 A .

    1808 A .2 Design for capacity and settlement. Foundations shall be so designed that the allowable bearing capacity of the soil is not exceeded, and that differential settlement is minimized. Foundations in areas with expansive soils shall be designed in accordance with the provisions of Section 1808 A .6.

    The enforcing agency may require an analysis of foundation elements to determine subgrade deformations in order to evaluate their effect on the superstructure, including story drift.

  • 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.

Frequently asked questions

What numeric settlement limit does § 1808.2 specify?

It does not specify a numeric settlement limit. § 1808.2 requires that allowable bearing capacity not be exceeded and that differential settlement be minimized; numeric predictions and acceptance criteria come from the project geotechnical report or other CBC provisions.

Can I count the weight of backfill or foundation when computing dead load for footing design?

Yes — § 1808.3 permits including the weight of foundations and overlying fill in the dead load for foundation design.

If I use ASCE 7 seismic combinations, what does the CBC require for overturning?

If you proportion foundations using ASCE 7 load combinations and compute seismic overturning by equivalent lateral‑force or modal analysis, the CBC requires proportioning in accordance with ASCE 7 § 12.13.4 as stated in § 1808.3.1.

Who provides the expected settlement values and acceptance criteria?

The project geotechnical engineer provides predicted total and differential settlements and acceptance criteria; the CBC anticipates those values in the geotechnical report used to design foundations to meet § 1808.2 performance requirements.

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Foundation Design: Capacity, Settlement & Expansive Soils

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