CBC · California Building Code

Shallow & Deep Foundations, Retaining Walls & Ground Improvement

This hub orients users to the CBC’s soils and foundation rules — geotechnical investigations, design criteria, inspections and ground‑improvement requirements.

Ask a question free See coverage

Last reviewed: July 5, 2026

Overview

This area of the California Building Code (CBC) covers soils and foundation practice — from geotechnical investigations and site preparation to design and construction of shallow and deep foundations, foundation and retaining walls, earth-retaining shoring, and ground‑improvement methods such as vibro stone columns. The CBC organizes these requirements in Chapter 18 (and the state‑adopted Chapter 18A), with key provisions in §1803 (geotechnical investigations), §1807 (foundation and retaining walls), §1809 (shallow foundations), §1810 (deep foundations) and §1813/1813A (vibro stone columns / ground improvement) .

These rules matter because they translate subsurface risk (settlement, liquefaction, slope instability, seismic lateral loads) into mandatory investigations, design criteria and construction controls. The CBC requires geotechnical reporting and site‑specific evaluation for many projects (including seismic considerations for SD C–F), specifies special inspection and testing for soils, foundations and ground improvement, and sets documentation and final verified report expectations for installation of systems such as vibro stone columns .

In this section

Code references

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

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

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

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 § 18A-28 High relevance — show source text
Monitoring shall be performed by a licensed surveyor and shall consist of vertical and lateral_ movement of the existing structures. Prior to starting shoring installation a preconstruction meeting shall take place between the contractor, shoring design engineer, surveyor, geotechnical engineer and building official to identify monitoring locations on existing buildings. 6. If in the opinion of the building official or shoring design engineer, monitoring data indicate excessive movement or other distress, all excavation shall cease until the geotechnical engineer and shoring design engineer investigate the situation and make recommendations for remediation or continuing. 7. All readings and measurements shall be submitted to the building official and shoring design engineer.

18A-28 2025 CALIFORNIA BUILDING CODE

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

SOILS AND FOUNDATIONS

1812A.8 Tolerances. The following tolerances shall be specified on the construction documents. 1. Soldier piles: a. Horizontal and vertical construction tolerances for the soldier pile locations. b. Soldier pile plumbness requirements (angle with vertical line).

2. Tie-back anchors:

a. Allowable deviation of anchor projected angle from specified vertical and horizontal design projected angle. b. Anchor clearance to the existing/new utilities and structures.

SECTION 1813A—VIBRO STONE COLUMNS FOR GROUND IMPROVEMENT

1813A.1 General. This section shall apply to vibro stone columns (VSCs) for ground improvement using unbounded aggregate materials. Vibro stone column provisions in this section are intended to increase bearing capacity, reduce settlements and mitigate liquefaction for shallow foundations. These requirements shall not be used for grouted or bonded stone columns, ground improvement for deep founda- tion elements or changing site class. VSCs shall not be considered a deep foundation element. Ground improvement shall be installed under the entire building/structure footprint and not under isolated foundation elements only. Design, construction, testing and inspec- tion shall satisfy the requirements of this code except as modified in Sections 1813A.2 through 1813A.5.

1813A.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. 1. Soil compaction shall be in accordance with California Geological Survey (CGS) Special Publication 117A (SP-117A): Guidelines for Evaluating and Mitigating Seismic Hazard in California. 2. 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. 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 ft or an approved alternative. 4. 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. 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.
CBC § 18-3 High relevance — show source text
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

[OSHPD 1R, 2 & 5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-35

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

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

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

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

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

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

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

CONTENTS

1807A Foundation Walls, Retaining Walls and Embedded Posts and Poles . . . . . . . . . . . . . . . . . . . 18A-10
CBC § 1813A.3 High relevance — show source text
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.

1813A.3 Shallow foundations. VSCs under the shallow foundation shall be located symmetrically around the centroid of the footing or load.

1. There shall be a minimum of four stone columns under each isolated or continuous/combined footing or approved equivalent. 2. The VSCs 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 result 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.

1813A.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 repenetrating 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.

1813A.5 Construction documents. Construction documents for VSCs, as a minimum, shall include the following: 1. Size, depth and location of VSCs. 2. 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. The testing, inspection and observation (TIO) program shall indicate the inspection and observation required for the VSCs. 6. A note indicating foundation construction shall not commence until the final verified report specified in Section 1813A.2 Item 9 has been submitted to and approved by the enforcement agency.

2025 CALIFORNIA BUILDING CODE 18A-29

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

18A-30 2025 CALIFORNIA BUILDING CODE

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

CALIFORNIA BUILDING CODE – MATRIX ADOPTION TABLE

CHAPTER 19 – CONCRETE

(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 § 1813A.2 High relevance — show source text
1813A.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. 1. Soil compaction shall be in accordance with California Geological Survey (CGS) Special Publication 117A (SP-117A): Guidelines for Evaluating and Mitigating Seismic Hazard in California. 2. 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. 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 ft or an approved alternative. 4. 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. 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 the 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.

1813A.3 Shallow foundations. VSCs under the shallow foundation shall be located symmetrically around the centroid of the footing or load.

1. There shall be a minimum of four stone columns under each isolated or continuous/combined footing or approved equivalent. 2. The VSCs 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 result 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.

1813A.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 repenetrating 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.

CBC § 1705A.6 Medium relevance — show source text

TABLE 1705A.6—REQUIRED SPECIAL INSPECTIONS AND TESTS OF SOILS	Col2	Col3
TYPE	CONTINUOUS SPECIAL INSPECTION	PERIODIC SPECIAL INSPECTION
1. Verify materials below shallow foundations are adequate to achieve the design bearing capacity.	—	X
2. Verify excavations are extended to proper depth and have reached proper material.	—	X
3. Perform classification and testing of compacted fill materials.	—	X
4. During fill placement, verify use of proper materials and procedures in accordance with the provisions of the approved geotechnical report. Verify densities and lift thicknesses during placement and compaction of compacted fill.	X	—
5. Prior to placement of compacted fill, inspect subgrade and verify that site has been prepared properly.	—	X

1705A.6.1 Soil fill. All fills used to support the foundations of any building or structure shall be continuously inspected by the geotechnical engineer or his or her qualified representative. It shall be the responsibility of the geotechnical engineer to verify that fills meet the requirements of the approved construction documents and to coordinate all fill inspection and testing during construc- tion involving such fills.

The duties of the geotechnical engineer or his or her qualified representative shall include, but need not be limited to, the inspec- tion of cleared areas and benches prepared to receive fill; inspection of the removal of all unsuitable soils and other materials; the approval of soils to be used as fill material; the inspection of placement and compaction of fill materials; the testing of the completed fills; and the inspection or review of geotechnical drainage devices, buttress fills or other similar protective measures in accordance with the approved construction documents.

A verified report shall be submitted by the geotechnical engineer as required by the California Administrative Code. The report shall indicate that all tests and inspections required by the approved construction documents were completed and whether the tested materials and/or inspected work meet the requirements of the approved construction documents.

1705A.6.2 Earth-retaining shoring. Special inspections and tests of earth-retaining shoring shall be in accordance with applicable portions of Section 1812A.

1705A.6.3 Vibro stone columns. Special inspections and tests of vibro stone columns for ground improvement shall be in accordance with Section 1813A.5.

1705 A .7 Driven deep foundations. Special inspections and tests shall be performed during installation of driven deep foundation elements as specified in 1810A.3.3.1.2 and Table 1705 A .7. The approved geotechnical report and the construction documents prepared by the registered design professionals shall be used to determine compliance.

TABLE 1705A.7—REQUIRED SPECIAL INSPECTIONS AND TESTS OF DRIVEN DEEP FOUNDATION ELEMENTS	Col2	Col3
TYPE	CONTINUOUS SPECIAL INSPECTION	PERIODIC SPECIAL INSPECTION
1. Verify element materials, sizes and lengths comply with the requirements.	X	—
2.

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

CBC § 1705A.6.2 Medium relevance — show source text

1705A.6.2 Earth-retaining shoring. Special inspections and tests of earth-retaining shoring shall be in accordance with applicable portions of Section 1812A.

1705A.6.3 Vibro stone columns. Special inspections and tests of vibro stone columns for ground improvement shall be in accordance with Section 1813A.5.

TABLE 1705A.7—REQUIRED SPECIAL INSPECTIONS AND TESTS OF DRIVEN DEEP FOUNDATION ELEMENTS	Col2	Col3
TYPE	CONTINUOUS SPECIAL INSPECTION	PERIODIC SPECIAL INSPECTION
1. Verify element materials, sizes and lengths comply with the requirements.	X	—
2. Determine capacities of test elements and conduct additional load tests, as required.	X	—
3. Inspect driving operations and maintain complete and accurate records for each element.	X	—
4. Verify placement locations and plumbness, confirm type and size of hammer, record number of blows per foot of penetration, determine required penetrations to achieve design capacity, record tip and butt elevations and document any damage to foundation element.	X	—
5. For steel elements, perform additional special inspections in accordance with Section 1705_A_.2.	In accordance with Section 1705_A_.2	In accordance with Section 1705_A_.2
6. For concrete elements and concrete-filled elements, perform tests and additional special inspections in accordance with Section 1705_A_.3.	In accordance with Section 1705_A_.3	In accordance with Section 1705_A_.3
7. For specialty elements, perform additional inspections as determined by the registered design professional in responsible charge.	In accordance with Statement of Special Inspections	In accordance with Statement of Special Inspections

2025 CALIFORNIA BUILDING CODE 17A-15

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

SPECIAL INSPECTIONS AND TESTS

1705A.7.1 Driven deep foundations observation. The installation of driven deep foundations shall be continuously observed by a qualified representative of the geotechnical engineer responsible for that portion of the project.

The representative of the geotechnical engineer shall make a report of the deep foundation pile-driving operation giving such pertinent data as the physical characteristics of the deep foundation pile-driving equipment, identifying marks for each deep founda- tion pile, total depth of embedment for each deep foundation; and when the allowable deep foundation pile loads are determined by a dynamic load formula, the design formula used, and the permanent penetration under the last 10 blows. One copy of the report shall be sent to the enforcement agency.

CBC § 20.5 Medium relevance — show source text
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.|

�

1808.8.3 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.

1808.8.4 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.

1808.8.5 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.

1808.8.6 Seismic requirements. See Section 1905 for additional requirements for foundations of structures assigned to Seismic Design Category C, D, E or F.

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

Exception: [OSHPD 1R, 2 & 5] Not permitted by OSHPD. Detached one- and two-family dwellings of light-frame construction and two stories or less above grade plane are not required to comply with the provisions of Section 18.13 of ACI 318.

1808.9 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—SHALLOW FOUNDATIONS

1809.1 General. Shallow foundations shall be designed and constructed in accordance with Sections 1809.2 through 1809.13.
CBC § 8.5 Medium relevance — show source text
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.

1809 A .3 Stepped footings. The top surface of footings shall be level. The bottom surface of footings shall be permitted to have a slope not exceeding 1 unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation of the top surface of the footing or where the surface of the ground slopes more than 1 unit vertical in 10 units horizontal (10-percent slope).

Individual steps in continuous footings shall not exceed 18 inches (457 mm) in height and the slope of a series of such steps shall not exceed 1 unit vertical to 2 units horizontal (50 percent slope) unless otherwise recommended by a geotechnical report. The steps shall be detailed on the drawings. The local effects due to the discontinuity of the steps shall be considered in the design of the foundation.

1809 A .4 Depth and width of footings. The minimum depth of footings below the undisturbed ground surface shall be 12 inches (305 mm). Where applicable, the requirements of Section 1809 A .5 shall be satisfied. The minimum width of footings shall be 12 inches (305 mm).

1809 A .5 Frost protection. Except where otherwise protected from frost, foundations and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:
1. Extending below the frost line of the locality.
2. Constructing in accordance with ASCE 32.
3. Erecting on solid rock.
CBC § 18-35 Medium relevance — show source text
2025 CALIFORNIA BUILDING CODE 18-35

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

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.)

Frequently asked questions

When is a geotechnical investigation required?

When the scope or seismic design category and site conditions warrant it — the CBC requires geotechnical investigations and a written geotechnical report per §1803, and mandates expanded evaluation (liquefaction, seismic lateral earth pressures, settlements, mitigation measures) for structures in Seismic Design Categories C–F (see §1803.5.11–.12) .

Are vibro stone columns treated as deep foundations?

No. The code states vibro stone columns (VSCs) are ground‑improvement elements intended for shallow foundations and shall not be considered deep foundations; VSC provisions and installation, testing and reporting requirements appear in §1813/1813A, including the requirement that ground improvement be applied across the structure footprint and be supported by a Final Verified Report from the geotechnical engineer .

What inspections and documentation are expected for foundations and ground improvement?

Special inspections and tests for soils, shallow foundations, deep foundations and VSCs are required per the CBC special‑inspection tables and Sections 1705A–1705 (and referenced 1810/1813A). Construction documents and a Testing, Inspection and Observation (TIO) program plus a Final Verified Report for ground improvement installations are specifically required in §1813A.5 and related special‑inspection sections .

Ask about the CBC

Get cited, plain-English answers on the California Building Code for your project — any code section, any scenario.

Start Free Trial

Related in the CBC

Geotechnical Investigations & Reports

Site Preparation, Excavation, Grading, Fill & Moisture Protection

Foundation Design: Capacity, Settlement & Expansive Soils

Foundations & Soils

California Building Code