CBC · California Building Code

How must foundations be designed for expansive soils?

If your site has expansive soil, the CBC requires either (1) foundations designed to resist differential swelling and uplift (§ 1808.6.1), or (2) removal of expansive soil to a depth that yields constant moisture content (§ 1808.6.3) or approved stabilization (§ 1808.6.4). The exact depth, design details and whether stabilization is acceptable are determined by geotechnical engineering and the building official; replace removed soil with non‑expansive, properly compacted fill per the code.

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Last reviewed: July 5, 2026

What the code requires — 2-4 sentences

The California Building Code requires foundations on expansive soils to be designed either to resist the differential volume changes of the active zone (foundation design) or (for slabs) designed per the accepted slab-on-ground provisions; alternatively the expansive soil may be removed or the active zone stabilized if those approaches are used and accepted by the building official. See § 1808.6 and its design alternative § 1808.6.1; the removal and stabilization exceptions are in § 1808.6.3 and § 1808.6.4.

Requirements in detail

Core obligation

  • Foundations placed on or within the active zone of expansive soils must be designed to resist differential volume changes and to prevent structural damage. Design must limit deflection and racking so the structure remains usable and serviceable (see § 1808.6.1).

Specific design provisions

  • If foundations extend into or penetrate expansive soils, they must be designed to prevent uplift of the supported structure (§ 1808.6.1.1).
  • Foundations that penetrate expansive soils must either (a) be designed to resist forces from soil volume change, or (b) be isolated from the expansive soil (§ 1808.6.1.2).
  • Where the slab-on-ground (mat/raft) approach is used, moments, shears, and deflections for design are to be determined by the referenced industry guidance (WRI/CRSI or PTI DC 10.5) and the slab designed accordingly (§ 1808.6.2 — related).

Alternatives allowed by exception

  • The requirement to use § 1808.6.1 (or slab rules) may be waived if:
    1. Expansive soil is removed to a depth sufficient to ensure a constant moisture content in the remaining soil (removal option, § 1808.6.3).
    2. The building official accepts stabilization of the active zone by chemical treatment, dewatering, presaturation or equivalent techniques (stabilization option, § 1808.6.4).

Material and fill requirements when removal is used

  • Fill replacing removed expansive soil shall not contain expansive soils and must comply with the fill/compaction provisions referenced in § 1804.5 or § 1804.6 (see § 1808.6.3).

Decision‑relevant table

Decision factor Required value / criterion Code reference
Choose design route Design per § 1808.6.1 (foundations) or slab provisions (§ 1808.6.2) unless a permitted exception is used § 1808.6
Design objective Resist differential volume change; prevent structural damage; limit deflection/racking to maintain serviceability § 1808.6.1
Uplift control Foundations extending into expansive soils must prevent uplift § 1808.6.1.1
Isolation or resistance Foundations penetrating expansive soils must resist volume-change forces or be isolated from soil § 1808.6.1.2
Removal depth Remove to depth sufficient to ensure constant moisture content in remaining soil (or meet confining-pressure exception) § 1808.6.3
Replacement fill No expansive soils; comply with § 1804.5/§ 1804.6 compaction/fill rules § 1808.6.3 (references § 1804.5/1804.6)
Stabilization methods Chemical, dewatering, presaturation or equivalent, subject to acceptance by building official § 1808.6.4

Exceptions & special cases

  • Removal exception: You do not have to use the design provisions if expansive soil is removed to a depth that ensures a constant moisture content in the remaining soil; the replaced fill may not contain expansive soils and must meet the referenced fill sections (§ 1808.6.3).
  • Confining-pressure exception: The code allows not removing to the depth of constant moisture if the confining pressure from overlying fill and structure exceeds the soil’s swell pressure — this is an explicit exception in § 1808.6.3.
  • Stabilization exception: The building official may approve stabilization instead of designing the foundation per § 1808.6.1; accepted stabilization methods include chemical treatment, dewatering, presaturation or equivalent (§ 1808.6.4). Documentation and proof of effectiveness are implied by the requirement for building official approval.

Note: The code does not prescribe specific chemicals, treatment concentrations, or exact presaturation procedures — it requires stabilization by those techniques and acceptance by the building official (§ 1808.6.4).

Common mistakes

  • Assuming a standard shallow foundation without evaluating the active zone: § 1808.6.1 requires design to resist differential volume change when foundations sit in the active zone. Failure to evaluate the active zone depth can omit required uplift or isolation measures.
  • Removing soil but failing to reach constant moisture content or to replace with non‑expansive, properly compacted fill (violates § 1808.6.3 and referenced § 1804 fill rules).
  • Relying on an unapproved stabilization method or failing to secure building official approval for stabilization (must comply with § 1808.6.4 and be approved).
  • Ignoring uplift forces: when footings penetrate expansive layers, designers sometimes omit uplift checks or isolation details required by § 1808.6.1.1.

Worked example — applying the rule with numbers (illustrative)

Scenario: A single‑family dwelling is to be supported on shallow strip footings. A geotechnical report identifies an active zone of expansive clay to 5 ft below finished grade, with a measured swell pressure of 3,000 psf. The owner and engineer consider the removal option.

Steps consistent with the CBC:

  1. Because the active zone extends to 5 ft, the design options are (a) design the footings to resist differential volume change per § 1808.6.1 (including uplift resistance/isolation) or (b) remove expansive soil per § 1808.6.3.
  2. If using removal: remove the expansive soil to a depth sufficient to ensure a constant moisture content in the remaining soil — in practice that means remove to at least the full 5 ft active zone so remaining soil below that depth shows stable moisture in lab/field tests. Replace with non‑expansive compacted fill meeting § 1804.5/1804.6 compaction requirements (as referenced in § 1808.6.3). Document compaction tests per accepted practice.
  3. If choosing stabilization instead: propose a stabilization program — for example, lime‑treatment to a depth of the active zone with testing demonstrating reduction of swell, or presaturation with monitoring — and submit methodology and test evidence to the building official for approval under § 1808.6.4. The code does not mandate the method; it requires an accepted stabilization technique.

Important: the code language in § 1808.6.3 uses “sufficient to ensure a constant moisture content” — it does not prescribe a fixed depth in feet; the depth is determined by the geotechnical evaluation.

Related provisions

  • § 1808.6 — Design for expansive soils (overview and alternatives).
  • § 1808.6.1 — Foundations placed on or within active zone: resist differential volume changes; uplift/isolation requirements.
  • § 1808.6.2 — Slab‑on‑ground design references for expansive soils (WRI/CRSI or PTI DC 10.5).
  • § 1808.6.3 — Removal of expansive soil; constant moisture depth; fill requirements (references § 1804.5/1804.6).
  • § 1808.6.4 — Stabilization techniques that the building official may accept (chemical, dewatering, presaturation).
  • § 1804.5 / § 1804.6 — Fill and compaction requirements referenced by the removal provision (see § 1808.6.3).

Code references

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

  • CBC § 6.1 High relevance — show source text

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

    SOILS AND FOUNDATIONS

    1808 A .6 Design for expansive soils. Foundations for buildings and structures founded on expansive soils shall be designed in accordance with Section 1808 A .6.1 or 1808 A .6.2.

    Exceptions: Foundation design need not comply with Section 1808 A .6.1 or 1808 A .6.2 where one of the following conditions is satisfied:

    1. The soil is removed in accordance with Section 1808 A .6.3.

    2. The building official approves stabilization of the soil in accordance with Section 1808 A .6.4.

    1808 A .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 A .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. Using the moments, shears and deflections determined above, nonprestressed slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with WRI/CRSI and post-tensioned slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with PTI DC 10.5. It shall be permitted to analyze and design such slabs by other methods that account for soil-structure interaction, the deformed shape of the soil support, the plate or stiffened plate action of the slab as well as both center lift and edge lift conditions. Such alternative methods shall be rational and the basis for all aspects and parameters of the method shall be available for peer review.

    1808 A .6.3 Removal of expansive soil. Where expansive soil is removed in lieu of designing foundations in accordance with Section 1808 A .6.1 or 1808 A .6.2, the soil shall be removed to a depth sufficient to ensure a constant moisture content in the remaining soil. Fill material shall not contain expansive soils and shall comply with Section 1804 A .5 or 1804 A .6.

    Exception: Expansive soil need not be removed to the depth of constant moisture, provided that the confining pressure in the expansive soil created by the fill and supported structure exceeds the swell pressure.

    1808 A .6.4 Stabilization. Where the active zone of expansive soils is stabilized in lieu of designing foundations in accordance with Section 1808 A .6.1 or 1808 A .6.2, the soil shall be stabilized by chemical, dewatering, presaturation or equivalent techniques.

  • 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 § 1808.5 High relevance — show source text

    ** 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. Using the moments, shears and deflections determined above, nonprestressed slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with WRI/CRSI and post-tensioned slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with PTI DC 10.5. It shall be permitted to analyze and design such slabs by other methods that account for soil-structure interaction, the deformed shape of the soil support, the plate or stiffened plate action of the slab as well as both center lift and edge lift conditions. Such alternative methods shall be rational and the basis for all aspects and parameters of the method shall be available for peer review.

    1808.6.3 Removal of expansive soil. Where expansive soil is removed in lieu of designing foundations in accordance with Section 1808.6.1 or 1808.6.2, the soil shall be removed to a depth sufficient to ensure a constant moisture content in the remaining soil. Fill material shall not contain expansive soils and shall comply with Section 1804.5 or 1804.6.

    Exception: Expansive soil need not be removed to the depth of constant moisture, provided that the confining pressure in the expansive soil created by the fill and supported structure exceeds the swell pressure.

    18-16 2025 CALIFORNIA BUILDING CODE

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

    SOILS AND FOUNDATIONS

    1808.6.4 Stabilization. Where the active zone of expansive soils is stabilized in lieu of designing foundations in accordance with Section 1808.6.1 or 1808.6.2, the soil shall be stabilized by chemical, dewatering, presaturation or equivalent techniques.

  • CBC § 10.5. High relevance — show source text

    Using the moments, shears and deflections determined above, nonprestressed slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with WRI/CRSI and post-tensioned slabs-on-ground, mat or raft foundations on expansive soils shall be designed in accordance with PTI DC 10.5. It shall be permitted to analyze and design such slabs by other methods that account for soil-structure interaction, the deformed shape of the soil support, the plate or stiffened plate action of the slab as well as both center lift and edge lift conditions. Such alternative methods shall be rational and the basis for all aspects and parameters of the method shall be available for peer review.

    1808 A .6.3 Removal of expansive soil. Where expansive soil is removed in lieu of designing foundations in accordance with Section 1808 A .6.1 or 1808 A .6.2, the soil shall be removed to a depth sufficient to ensure a constant moisture content in the remaining soil. Fill material shall not contain expansive soils and shall comply with Section 1804 A .5 or 1804 A .6.

    Exception: Expansive soil need not be removed to the depth of constant moisture, provided that the confining pressure in the expansive soil created by the fill and supported structure exceeds the swell pressure.

    1808 A .6.4 Stabilization. Where the active zone of expansive soils is stabilized in lieu of designing foundations in accordance with Section 1808 A .6.1 or 1808 A .6.2, the soil shall be stabilized by chemical, dewatering, presaturation or equivalent techniques.

    1808 A .7 Foundations on or adjacent to slopes. The placement of buildings and structures on or adjacent to slopes steeper than one unit vertical in three units horizontal (33.3-percent slope) shall comply with Sections 1808 A .7.1 through 1808 A .7.5.

    1808 A .7.1 Building clearance 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 1808 A .7.5 and Figure 1808 A .7.1, the following criteria will be assumed to provide this protection. Where the existing slope is steeper than one unit vertical in one 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.

    FIGURE 1808 A .7.1—FOUNDATION CLEARANCES FROM SLOPES

    For SI: 1 foot = 304.8 mm.

  • 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 § 3.1 High relevance — show source text

    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.

    1808 A .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.

    2025 CALIFORNIA BUILDING CODE 18A-11

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

    SOILS AND FOUNDATIONS

    1808 A .6 Design for expansive soils. Foundations for buildings and structures founded on expansive soils shall be designed in accordance with Section 1808 A .6.1 or 1808 A .6.2.

    Exceptions: Foundation design need not comply with Section 1808 A .6.1 or 1808 A .6.2 where one of the following conditions is satisfied:

    1. The soil is removed in accordance with Section 1808 A .6.3.

    2. The building official approves stabilization of the soil in accordance with Section 1808 A .6.4.

    1808 A .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 A .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 § 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 § 2.5 High relevance — show source text

    Cast-in-place deep foundation elements not enclosed by a steel pipe, tube or permanent
    casing|2.5 inches| |5. Cast-in-place deep foundation elements enclosed by a steel pipe, tube or permanent casing|1 inch| |6. Structural steel core within a steel pipe, tube or permanent casing|2 inches| |7. Cast-in-place drilled shafts enclosed by a stable rock socket|1.5 inches| |For SI:1 inch = 25.4 mm.|For SI:1 inch = 25.4 mm.|

    1808 A .8.3 Placement of concrete. Concrete shall be placed in such a manner as to ensure the exclusion of any foreign matter and to secure a full-size foundation. Concrete shall not be placed through water unless a tremie or other method approved by the building official is used. Where placed under or in the presence of water, the concrete shall be deposited by approved means to ensure minimum segregation of the mix and negligible turbulence of the water. Where depositing concrete from the top of a deep foundation element, the concrete shall be chuted directly into smooth-sided pipes or tubes or placed in a rapid and continuous operation through a funnel hopper centered at the top of the element.

    2025 CALIFORNIA BUILDING CODE 18A-13

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

    SOILS AND FOUNDATIONS

    1808 A .8.4 Protection of concrete. Concrete foundations shall be protected from freezing during depositing and for a period of not less than 5 days thereafter. Water shall not be allowed to flow through the deposited concrete.

    1808 A .8.5 Forming of concrete. Concrete foundations are permitted to be cast against the earth where, in the opinion of the building official, soil conditions do not require formwork. Where formwork is required, it shall be in accordance with Section 26.11 of ACI 318.

    1808 A .8.6 Seismic requirements. See Section 1905 A for additional requirements for foundations of structures assigned to Seismic Design Category D, E or F.

    For structures assigned to Seismic Design Category D, E or F, provisions of Section 18.13 of ACI 318 shall apply where not in conflict with the provisions of Sections 1808 A through 1810 A .

    1808 A .9 Vertical masonry foundation elements. Vertical masonry foundation elements that are not foundation piers as defined in Section 202 shall be designed as piers, walls or columns, as applicable, in accordance with TMS 402.

    SECTION 1809 A —SHALLOW FOUNDATIONS

    1809 A .1 General. Shallow foundations shall be designed and constructed in accordance with Sections 1809 A .2 through 1809 A . 15 .

    1809 A .2 Supporting soils. Shallow foundations shall be built on undisturbed soil, compacted fill material or controlled low-strength material (CLSM). Compacted fill material shall be placed in accordance with Section 1804 A .6. CLSM shall be placed in accordance with Section 1804 A .7.

  • CBC § 7.1 High relevance — show source text

    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.

    FIGURE 1808 A .7.1—FOUNDATION CLEARANCES FROM SLOPES

    For SI: 1 foot = 304.8 mm.

    1808 A .7.2 Foundation setback from descending slope surface. Foundations on or adjacent to slope surfaces shall be founded in firm material with an embedment and set back from the slope surface sufficient to provide vertical and lateral support for the foundation without detrimental settlement. Except as provided for in Section 1808 A .7.5 and Figure 1808 A .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.

    18A-12 2025 CALIFORNIA BUILDING CODE

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

    SOILS AND FOUNDATIONS

    1808 A .7.3 Pools. The setback between pools regulated by this code and slopes shall be equal to one-half the building footing setback distance required by this section. That portion of the pool wall within a horizontal distance of 7 feet (2134 mm) from the top of the slope shall be capable of supporting the water in the pool without soil support.

    1808 A .7.4 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.

    1808 A .7.5 Alternate setback and clearance. Alternate setbacks and clearances are permitted, subject to the approval of the building official. The building official shall be permitted to require a geotechnical investigation as set forth in Section 1803 A .5.10.

    1808 A .8 Concrete foundations. The design, materials and construction of concrete foundations shall comply with Sections 1808 A .8.1 through 1808 A .8.6 and the provisions of Chapter 19 A .

    1808 A .8.1 Concrete or grout strength and mix proportioning. Concrete or grout in foundations shall have a specified compressive strength ( f ′ c ) not less than the largest applicable value indicated in Table 1808 A .8.1.

    Where concrete or grout is to be pumped, the mix design including slump shall be adjusted to produce a pumpable mixture.

  • CBC § 8.2 High relevance — show source text

    1808 A .8.2 Concrete cover. The concrete cover provided for prestressed and nonprestressed reinforcement in foundations shall be not less than the largest applicable value specified in Table 1808 A .8.2. Longitudinal bars spaced less than 1 [1] / 2 inches (38 mm) clear distance apart shall be considered to be bundled bars for which the concrete cover provided shall be not less than that required by Section 20.5.1.3.5 of ACI 318. Concrete cover shall be measured from the concrete surface to the outermost surface of the steel to which the cover requirement applies. Where concrete is placed in a temporary or permanent casing or a mandrel, the inside face of the casing or mandrel shall be considered to be the concrete surface.

    TABLE 1808A.8.2—MINIMUM CONCRETE COVER Col2
    FOUNDATION ELEMENT OR CONDITION MINIMUM COVER
    1. Shallow foundations In accordance with Section 20.5 of ACI 318
    2. Precast nonprestressed deep foundation elements
    Exposed to seawater
    Not manufactured under plant conditions
    Manufactured under plant control conditions    		 3 inches
    2 inches
    In accordance with Section 20.5.1.3.3 of ACI 318
    3. Precast prestressed deep foundation elements
    Exposed to seawater
    Other    		 2.5 inches
    In accordance with Section 20.5.1.3.3 of ACI 318
    4. Cast-in-place deep foundation elements not enclosed by a steel pipe, tube or permanent
    casing    		 2.5 inches
    5. Cast-in-place deep foundation elements enclosed by a steel pipe, tube or permanent casing 1 inch
    6. Structural steel core within a steel pipe, tube or permanent casing 2 inches
    7. Cast-in-place drilled shafts enclosed by a stable rock socket 1.5 inches
    For SI:1 inch = 25.4 mm. For SI:1 inch = 25.4 mm.

    1808 A .8.3 Placement of concrete. Concrete shall be placed in such a manner as to ensure the exclusion of any foreign matter and to secure a full-size foundation. Concrete shall not be placed through water unless a tremie or other method approved by the building official is used. Where placed under or in the presence of water, the concrete shall be deposited by approved means to ensure minimum segregation of the mix and negligible turbulence of the water. Where depositing concrete from the top of a deep foundation element, the concrete shall be chuted directly into smooth-sided pipes or tubes or placed in a rapid and continuous operation through a funnel hopper centered at the top of the element.

    2025 CALIFORNIA BUILDING CODE 18A-13

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

    SOILS AND FOUNDATIONS

    1808 A .8.4 Protection of concrete. Concrete foundations shall be protected from freezing during depositing and for a period of not less than 5 days thereafter. Water shall not be allowed to flow through the deposited concrete.

  • CBC § 6.4 High relevance — show source text

    1808 A .6.4 Stabilization. Where the active zone of expansive soils is stabilized in lieu of designing foundations in accordance with Section 1808 A .6.1 or 1808 A .6.2, the soil shall be stabilized by chemical, dewatering, presaturation or equivalent techniques.

    1808 A .7 Foundations on or adjacent to slopes. The placement of buildings and structures on or adjacent to slopes steeper than one unit vertical in three units horizontal (33.3-percent slope) shall comply with Sections 1808 A .7.1 through 1808 A .7.5.

    1808 A .7.1 Building clearance 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 1808 A .7.5 and Figure 1808 A .7.1, the following criteria will be assumed to provide this protection. Where the existing slope is steeper than one unit vertical in one 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.

    FIGURE 1808 A .7.1—FOUNDATION CLEARANCES FROM SLOPES

    For SI: 1 foot = 304.8 mm.

    1808 A .7.2 Foundation setback from descending slope surface. Foundations on or adjacent to slope surfaces shall be founded in firm material with an embedment and set back from the slope surface sufficient to provide vertical and lateral support for the foundation without detrimental settlement. Except as provided for in Section 1808 A .7.5 and Figure 1808 A .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.

    18A-12 2025 CALIFORNIA BUILDING CODE

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

    SOILS AND FOUNDATIONS

    1808 A .7.3 Pools. The setback between pools regulated by this code and slopes shall be equal to one-half the building footing setback distance required by this section. That portion of the pool wall within a horizontal distance of 7 feet (2134 mm) from the top of the slope shall be capable of supporting the water in the pool without soil support.

  • CBC § 17A-4 Medium relevance — show source text

    1704A Special Inspections and Tests, Contractor Responsibility and Structural Observation . . . . . . . . . . . . . . . . . . .17A-4

    1705A Required Special Inspections and Tests . . . . . . . . . .17A-6

    1706A Design Strengths of Materials . . . . . . . . . . . . . . . . . .17A-22

    1707A Alternative Test Procedure . . . . . . . . . . . . . . . . . . . . .17A-22

    1708A In-Situ Load Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17A-22

    1709A Preconstruction Load Tests . . . . . . . . . . . . . . . . . . . .17A-22

    1710A Off-Site Construction [OSHPD 1 & 4] . . . . . . . . . . . . .17A-23

    CHAPTER 18 SOILS AND FOUNDATIONS. . . . . . . . . . . . . . 18-1

    1801 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-3

    1802 Design Basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-3

    1803 Geotechnical Investigations. . . . . . . . . . . . . . . . . . . . 18-3

    1804 Excavation, Grading and Fill. . . . . . . . . . . . . . . . . . . . 18-7

    1805 Dampproofing and Waterproofing . . . . . . . . . . . . . . 18-7

    1806 Presumptive Load-Bearing Values of Soils . . . . . . . 18-9

    1807 Foundation Walls, Retaining Walls and Embedded Posts and Poles . . . . . . . . . . . . . . . . . 18-10

    1808 Foundations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-16

    1809 Shallow Foundations . . . . . . . . . . . . . . . . . . . . . . . . . 18-18

    1810 Deep Foundations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-20

    1811 Prestressed Rock and Soil Foundation

    Anchors [OSHPD 1R, 2 & 5]. . . . . . . . . . . . . . . . . . . . . 18-31

    1812 Earth Retaining Shoring

    [OSHPD 1R, 2 & 5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-32

    1813 Vibro Stone Columns for Ground Improvement

Frequently asked questions

Do I always need a geotechnical report to design foundations on expansive soils?

Yes — because the code requires addressing the active zone, swell potential and moisture conditions to choose between design, removal, or stabilization; a geotechnical investigation is the normal way to define those parameters and inform compliance with § 1808.6 and § 1808.6.1.

If I remove expansive soil, how deep must I excavate?

The code requires removal to a depth sufficient to ensure a constant moisture content in the remaining soil — it does not set a universal depth in feet. The geotechnical engineer determines the necessary depth based on site conditions in accordance with § 1808.6.3.

Can I use chemical stabilization instead of removing soil?

Yes, but the stabilization method (chemical, dewatering, presaturation, or equivalent) must be accepted by the building official per § 1808.6.4, and you should provide documentation that the treatment achieves the intended reduction in swell/activity.

What does “isolate the foundation from expansive soil” mean?

Isolation means detailing the foundation so that volume changes in the expansive soil do not transfer damaging uplift or lateral loads to the structure (an option in § 1808.6.1.2). Isolation strategies vary and must be shown in the structural/geotechnical design.

If I use a post‑tensioned slab, which guidance applies?

Moments, shears and deflections for slab-on-ground foundations on expansive soils are to be determined per the referenced guidance (WRI/CRSI or PTI DC 10.5) and the slab designed accordingly; see § 1808.6.2 for the slab pathway.

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