CBC · California Building Code

What ground improvement provisions (vibro stone columns, VSC) must be followed?

If your project uses vibro stone columns in California, the CBC requires the geotechnical report to specify diameter (≥2 ft), spacing (≤8 ft), depth and lateral extent (½ depth, min 10 ft), testing (CPT/SPT), installation by vibratory probe, special inspection and a Final Verified Report before foundation work can start; VSCs cannot be used for uplift or as deep foundations.

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Last reviewed: July 5, 2026

What the code requires — 2–4 sentences

The California Building Code requires that vibro stone columns (VSCs) used for ground improvement be designed, installed, tested and inspected so they provide uniform bearing, settlement performance and liquefaction mitigation for the entire building footprint — not just beneath isolated footings (see § 1813). The geotechnical report must specify depth, diameter, spacing, area‑replacement ratio, testing/acceptance criteria and a Final Verified Report (FVR); installation must use vibratory probes with specified aggregate gradation; and construction documents must show size, layout, CPT/SPT verification and a TIO program (see §§ 1813.2, 1813.4, 1813.5).

Requirements in detail

Scope and limitations (what VSCs are and are not)

VSCs in the CBC apply only to vibro stone columns constructed of unbounded aggregate for improving bearing, reducing settlement and mitigating liquefaction for shallow foundations. They are not permitted for grouted/bonded stone columns, for ground improvement intended to change site class, or as deep foundation elements (§ 1813.1).
Ground improvement must be installed under the entire building/structure footprint, not under isolated foundation elements only (§ 1813.1).

Geotechnical report: mandatory contents

The geotechnical report must specify the VSC system details needed to produce uniform total and differential settlement and to address earthquake performance. At minimum it must address: compliance with CGS SP‑117A for compaction/liquefaction mitigation; area replacement ratio and factor of safety; depth and lateral extent beyond foundations; minimum diameter and maximum spacing; modulus of subgrade reaction adjustments; expected long‑term and post‑earthquake settlements; acceptance criteria using CPT (ASTM D5778) and SPT (ASTM D1586) where applicable; special inspection/observation requirements; and preparation/submittal of a Final Verified Report (FVR) when installation is complete (§ 1813.2).

Layout and structural interaction with shallow foundations

VSCs under shallow foundations must be located symmetrically about the centroid of the footing or loading condition (§ 1813.3).
Minimum number: there shall be at least four stone columns under each isolated or continuous/combined footing (or approved equivalent) (§ 1813.3(1)).
VSCs shall not be used to resist tension or overturning uplift from shallow foundations (they are not to be treated as tension‑resisting deep elements) (§ 1813.3(2)).
Foundation analysis should account for increased vertical stiffness of VSCs as point supports unless the geotechnical evidence shows uniform improvement of surrounding soils (so use caution when modeling) (§ 1813.3(3)).

Installation and material requirements

Installation must use vibratory probes (vibro‑probes) to form vertical columns of compacted unbounded aggregate by progressive introduction of gravel at the vibrator tip and repeated penetration/compaction (§ 1813.4).
Aggregate: VSC gravel shall be well‑graded with a maximum particle size of 6 inches and the code also specifies a post‑compaction sizing limitation (text specifies “not more than 10 percent smaller than 3/8 inch after compaction” in the available text excerpt) — see § 1813.4 for the exact grading language and confirm with the geotechnical engineer.

Construction documents and acceptance

Construction documents must, at a minimum, include:

Size, depth and location of VSCs; extent of improvement relative to the building/foundation outlines; field verification and acceptance criteria using CPT/SPT; locations where CPT/SPT will be performed; the Testing, Inspection and Observation (TIO) program; and a note that foundation construction cannot begin until the Final Verified Report (FVR) required by § 1813.2(9) is submitted and approved (§ 1813.5).

Field testing, inspection and final documentation

Acceptance criteria and field verification shall include CPT/Piezocone (ASTM D5778) and, where necessary, SPT (ASTM D1586) to confirm improvement (§ 1813.2(7)).
The geotechnical engineer must specify special inspection/observation requirements and prepare a Final Verified Report documenting installation and demonstrating that acceptance criteria were met; that FVR must be submitted to the enforcement agency (§ 1813.2(8)–(9)).
Special inspections/tests for VSCs are referenced in the special inspection chapter — see required special inspections for Vibro Stone Columns in § 1705A.6.3 and the VSC construction‑document requirements of § 1813.5.

Decision‑relevant dimensions & values (quick reference table)

Item	Value / Requirement	Code Reference
Minimum VSC diameter	2 ft	§ 1813.2(4)
Maximum center‑to‑center spacing	8 ft	§ 1813.2(4)
Lateral extent beyond foundation	½ the VSC depth, minimum 10 ft (or approved alternative)	§ 1813.2(3)
Minimum number of columns per isolated/combined footing	4 columns (minimum)	§ 1813.3(1)
Installation method	Vibratory probes forming compacted unbounded aggregate columns	§ 1813.4
Aggregate gradation	Well‑graded; maximum size 6 in.; post‑compaction sizing limitation noted in § 1813.4	§ 1813.4
Field acceptance testing	CPT/piezocone (ASTM D5778); SPT (ASTM D1586) as required	§ 1813.2(7)
Final documentation required	Final Verified Report (FVR) by geotechnical engineer before foundation work starts	§ 1813.2(9); § 1813.5(6)
TIO / Special inspection reference	Per § 1813.5 and special inspection requirements in § 1705A.6.3	§ 1813.5; § 1705A.6.3

Exceptions & special cases

The CBC expressly excludes grouted/bonded stone columns and use of VSC requirements for deep foundations or for changing seismic site class; VSCs are not to be treated as deep foundations (§ 1813.1). Use an alternative design path (and appropriate code sections) for those applications.
If the geotechnical report demonstrates that the VSC installation produces uniform modulus and settlement behavior, the engineer may model the system without treating VSCs as discrete point supports; otherwise the increased vertical stiffness should be modeled as point supports (§ 1813.3(3)).
The code allows approved alternatives where the geotechnical or structural evidence supports them, but the building official must approve these in accordance with general CBC alternative material/method provisions and local enforcement practices (§ 1.2.3 references). When in doubt, document the rationale and obtain approval.

Common mistakes

Treating VSCs as deep foundations (or assuming they can carry uplift/tension) — the code prohibits VSCs from being used to resist tension or overturning uplift (§ 1813.3(2)).
Installing improvement only under isolated footings instead of the entire footprint — the code requires full‑footprint improvement (§ 1813.1).
Omitting required documentation and the Final Verified Report before starting foundation construction — § 1813.5 and § 1813.2(9) make the FVR and the TIO program mandatory items of the construction documents.
Not specifying or following acceptance criteria using CPT/SPT (ASTM D5778 and D1586) described in the geotechnical report (§ 1813.2(7)).
Failing to account for minimum diameter and spacing (2 ft diameter, max 8 ft spacing) or the required lateral extent beyond foundations in the design (§ 1813.2(3)–(4)).

Worked example — applying the rule with numbers

Scenario: A one‑story light industrial building with a 10,000 ft² rectangular footprint will be supported on shallow footings. The geotechnical engineer recommends VSCs to mitigate liquefaction and control settlements; the engineer specifies VSC depth = 30 ft.

How the CBC rules apply:

Lateral extent beyond foundation: the improvement must extend laterally one‑half the VSC depth beyond the foundation perimeter = 0.5 × 30 ft = 15 ft (minimum requirement is 10 ft, so 15 ft satisfies § 1813.2(3)).
Geometry: use the CBC minimum VSC diameter 2 ft and maximum center‑to‑center spacing 8 ft (§ 1813.2(4)). A regular 8 ft × 8 ft grid gives one column per 64 ft². For 10,000 ft² the approximate number of columns = 10,000 / 64 ≈ 156 columns (round and arrange to meet symmetry and edge conditions; ensure a minimum of four columns under individual footings per § 1813.3(1)).
Installation and acceptance: specify vibratory probe installation and well‑graded aggregate (max 6 in.) per § 1813.4, and identify CPT locations and acceptance criteria (ASTM D5778 and ASTM D1586 as needed) in the geotechnical report/contract documents per § 1813.2(7) and § 1813.5. Prepare for special inspection per the TIO program and submit the FVR after installation and acceptance testing before foundations proceed (§ 1813.5; § 1813.2(9)).

Related provisions

§ 1813 — Vibro Stone Columns for Ground Improvement (general scope and limitations)
§ 1813.2 — Geotechnical report requirements for VSCs (depth, spacing, area replacement ratio, CPT/SPT acceptance, FVR)
§ 1813.3 — Shallow foundation interaction, minimum number of columns, uplift prohibition and stiffness considerations
§ 1813.4 — Installation method and aggregate requirements (vibratory probes; aggregate grading)
§ 1813.5 — Construction documents requirements, CPT/SPT locations, TIO program and FVR note
§ 1705A.6.3 — Special inspections and tests of vibro stone columns (see special inspection chapter for TIO requirements)

Code references

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

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 § 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.

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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.

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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 § 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.

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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 § 18-35 High relevance — show source text
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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.

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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 § 1705A.6 High 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 § 1812A.7 High relevance — show source text
Supple-_ mental shoring shall be devised to eliminate further movement and the building official shall review and approve the supplemental shoring before excavation continues. 11. Monitoring of tie-back anchor loads: a. Load cells shall be installed at the tie-back heads adjacent to buildings at maximum interval of 50 feet, with a minimum of one load cells per wall. b. Load cell readings shall be taken once a day during excavation and once a week during the remainder of construction. c. Load cell readings shall be submitted to the geotechnical engineer, shoring design engineer, engineer in responsible charge and building official. d. Load cell readings can be terminated once the temporary shoring no longer provides support for the buildings.

1812A.7 Monitoring of existing structures. 1. The contractor shall complete a written and photographic log of all existing structures within 100 ft or three times depth of shoring, prior to construction. A licensed surveyor shall document all existing substantial cracks in adjacent existing structures. 2. The contractor shall document existing condition of wall cracks adjacent to shoring walls prior to start of construction. 3. The contractor shall monitor existing walls for movement or cracking that may result from adjacent shoring. 4. If excessive movement or visible cracking occurs, the contractor shall stop work and shore/reinforce excavation and contact the shoring design engineer and building official. 5. Monitoring of the existing structure shall be at reasonable intervals as required by the registered design professional subject to approval of the building official. 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.

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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
CBC § 2.25 High relevance — show source text
00|2.25|0.70|DR|3.70|1.05|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|33 mil cold-
formed steel
stud|#10 screw|Steel thick-
ness plus 3
threads|16|3.85|1.45|DR|DR|3.40|DR|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|33 mil cold-
formed steel
stud|#10 screw|Steel thick-
ness plus 3
threads|24|3.40|DR|DR|DR|2.70|DR|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|43 mil or
thicker cold-
formed steel
stud|#8 Screw|Steel thick-
ness plus 3
threads|12|3.00|1.80|DR|DR|3.00|0.65|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|43 mil or
thicker cold-
formed steel
stud|#8 Screw|Steel thick-
ness plus 3
threads|16|3.00|1.00|DR|DR|2.85|DR|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|43 mil or
thicker cold-
formed steel
stud|#8 Screw|Steel thick-
ness plus 3
threads|24|2.85|DR|DR|DR|2.20|DR|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|43 mil or
thicker cold-
formed steel
stud|#10 screw|Steel thick-
ness plus 3
threads|12|4.00|3.85|2.80|1.80|4.00|3.05|1.50|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|43 mil or
thicker cold-
formed steel
stud|#10 screw|Steel thick-
ness plus 3
threads|16|4.00|3.30|1.95|0.60|4.00|2.25|DR|DR| |Minimum 33
mil steel
furring or
minimum
1x wood
furringc|43 mil or
thicker cold-
formed steel
stud|#10 screw|Steel thick-
ness plus 3
threads|24|4.00|2.25|DR|DR|4.00|0.65|DR|DR| |For SI: 1 inch = 25.4 mm, 1 pound per square foot (psf) = 0.0479 kPa, 1 pound per square inch = 0.00689 MPa.
DR = Design Required, o.c. = on center.
a. Wood furring shall be spruce-pine-fir or any softwood species with a specific gravity of 0.42 or greater.

CBC § 1705A.6.2 High 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

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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 § 18-34 Medium relevance — show source text
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SOILS AND FOUNDATIONS

10. If the total cumulative horizontal or vertical movement (from start of construction) of the existing buildings reaches [3] / 4 inch or soldier piles movement reaches 1 [1] / 2 inches all excavation activities shall be suspended until the causes, if any, can be deter- mined. Supplemental shoring shall be devised to eliminate further movement and the building official shall review and approve the supplemental shoring before excavation continues. 11. Monitoring of tie-back anchor loads: a. Load cells shall be installed at the tie-back heads adjacent to buildings at maximum interval of 50 feet, with a minimum of one load cell per wall. b. Load cell readings shall be taken once a day during excavation and once a week during the remainder of construction. c. Load cell readings shall be submitted to the geotechnical engineer, shoring design engineer, engineer in responsible charge and the building official. d. Load cell readings can be terminated once the temporary shoring no longer provides support for the buildings.

1812.7 Monitoring of existing OSHPD 1, 1R, 2, 4 and 5 structures. 1. The contractor shall complete a written and photographic log of all existing OSHPD 1, 1R, 2, 4 & 5 structures within 100 feet or three times depth of shoring, prior to construction. A licensed surveyor shall document all existing substantial cracks in adja- cent existing structures. 2. The contractor shall document the existing condition of wall cracks adjacent to shoring walls prior to start of construction. 3. The contractor shall monitor existing walls for movement or cracking that may result from adjacent shoring. 4. If excessive movement or visible cracking occurs, the contractor shall stop work and shore/reinforce excavation and contact the shoring design engineer and the building official. 5. Monitoring of the existing structure shall be at reasonable intervals as required by the registered design professional, subject to approval of the building official. 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 the building official to identify monitoring loca- tions 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 reading and measurements shall be submitted to the building official and shoring design engineer.

1812.8 Tolerances. The following tolerances shall be specified on the construction documents. 1. Soldier piles: i. Horizontal and vertical construction tolerances for the soldier pile locations. ii. Soldier pile plumbness requirements (angle with vertical line).

2. Tie-back anchors:

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

SECTION 1813—VIBRO STONE COLUMNS FOR GROUND IMPROVEMENT [OSHPD 1R, 2 & 5]
CBC § 18941.8. Medium relevance — show source text
— Enforcing agency State or local agency specified by the applicable provisions of law.

— Authority cited Health and Safety Code Section 18941.8.

Reference — Health and Safety Code Section 18941.8.

1.2.2.1 Adopting agency identification. The provisions of this code applicable to buildings identified in this section will be identified in the Matrix Adoption Tables under the acronym BSC-CG .

1.2.3 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code,

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ADMINISTRATION

provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.

1.2.3.1 Research reports. Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.

1.2.3.2 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code, or evidence that a material or method does not conform to the requirements of this code, or in order to substantiate claims for alternative materials or methods, the building official shall have the authority to require tests as evidence of compliance to be made at no expense to the jurisdiction. Test methods shall be as specified in this code or by other recognized test standards. In the absence of recognized and accepted test meth- ods, the building official shall approve the testing procedures. Tests shall be performed by an approved agency. Reports of such tests shall be retained by the building official for the period required for retention of public records.

SECTION 1.3—BOARD OF STATE AND COMMUNITY CORRECTIONS

1.3.1 Specific scope of application of the agency responsible for enforcement, the enforcement agency and the specific authority to adopt and enforce such provisions of this code, unless otherwise stated.

Application— Local detention facilities.

Enforcing agency— Board of State and Community Corrections.

Authority cited— Penal Code Section 6030; Welfare and Institutions Code Sections 207.1, 210 and 885.

Reference— Penal Code Section 6030; Welfare and Institutions Code Sections 207.1, 210 and 885.

1.3.2 Adopting agency identification. The provisions of this code applicable to buildings identified in this section will be identified in the Matrix Adoption Tables under the acronym BSCC.

SECTION 1.4—DEPARTMENT OF CONSUMER AFFAIRS

Frequently asked questions

Can vibro stone columns be used to carry uplift or overturning forces?

No. The CBC explicitly states VSCs shall not be used to resist tension or overturning uplift from shallow foundations (§ 1813.3(2)) .

Must the entire building footprint be improved with VSCs?

Yes. Ground improvement must be installed under the entire building/structure footprint and not only under isolated foundation elements (§ 1813.1) .

What tests are required to accept VSC installation?

The geotechnical report must specify acceptance criteria using friction cone/piezocone CPT per ASTM D5778 and, if necessary, SPT per ASTM D1586 to verify improvement (§ 1813.2(7)) .

Is a Final Verified Report (FVR) required before foundation work begins?

Yes. The geotechnical engineer must prepare and submit an FVR documenting that the ground improvement meets acceptance criteria; construction documents must note that foundation construction shall not commence until the FVR is submitted and approved (§ 1813.2(9); § 1813.5) .

What are the minimum diameter and maximum spacing for VSCs in the CBC?

Minimum diameter is 2 feet and maximum center‑to‑center spacing is 8 feet unless the geotechnical design provides an approved alternative (§ 1813.2(4)) .

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How are deep foundations (piles, drilled shafts) analyzed, designed and detailed?

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Shallow & Deep Foundations, Retaining Walls & Ground Improvement

California Building Code