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B-28 When is a Soils Investigation Required?

The purpose of this technical bulletin is to clarify when a soils investigation is required as part of a building permit application. The requirements below are intended as general policy guidelines.

Section 1803 of the California Building Code (CBC) outlines specific instances of when a geotechnical report is required based on soil conditions and construction methods. In addition, Section 1803 allows the Building Official to require a geotechnical investigation for any project. Where a geotechnical investigation is required, the content of the report shall conform to the requirements of Section 1803.

Effective date: 02/2008
Revised: 05/05/2015, 12/27/2018

General

In all cases, if a soils report has been previously performed for the parcel, this report must be included in the application. If the existing report is more than two years old, a soils report review letter from a qualified licensed professional must be included with the application. The recommendations of the report and review letter must be followed.

Technical Details

A soils investigation is required as follows:

  1. New or replacement structures for Occupancy Groups A, B, E, F, H, I, M, R-1, R-2, R-4, and S occupancies unless specified below – Soils investigation required
  2. Specified new or replacement structures accessory to Occupancy Groups A, B, E, F, H, I, M, R-1, R-2, R-4, and S:  trash enclosures and other structures which are not occupied by human beings – not required.
  3. New or replacement structures for Occupancy Group U:  commercial engineered structures.  New or replacement structures for Occupancy Group R-3:  single family dwellings, accessory dwelling units, guest houses, pool houses or miscellaneous residential occupancies such as home office, gym, yoga room, art studio, recreational or media room, etc. – Soils investigation required when any of the following conditions exist, as determined by site review or mapped information:
    1. Area of suspected expansive soil
    2. Area of suspected slides, slumps, or soil creep
    3. Area of previous or proposed fill placement
    4. Area without sufficient setback to slopes, steams, or adjacent foundations
    5. Area subject to possible liquefaction
    6. Area of suspected soft, compressible, or organic soil with low bearing capacity
    7. Area of high moisture content in soil
    8. Area subject to high erosion
    9. Area of soft soil due to past deep ripping or cultivation below minimum foundation depth
    10. Area within 1000 feet of a solid waste disposal site
    11. Design values in structural calculations exceed presumptive load bearing values per Table 1806.2
    12. Project includes deep foundations or shallow footings bearing on compacted fill material more than 12"
    13. There is evidence of structural failure of the existing foundation due to soil conditions
    14. When the structure is R-3 occupancy and engineered (non-prescriptive code standards)
    15. Replacement structures that increase loading to existing foundation
  4. Specified new or replacement Occupancy Group U structures accessory to an Occupancy Group R-3:  private garages, barns, storage buildings, workshops, and water tanks exceeding 10,000 gallons or other Occupancy Group U’s designated by the building official  –  Soils investigation required when any of the following conditions exist, as determined by site review or mapped information:
    1. Area without sufficient slope setback
    2. Area without recommended setback from stream
    3. Proposed building is potentially hazardous to other structures
    4. There is evidence of structural failure of the existing foundation due to soil conditions
    5. Area of previous or proposed fill placement
    6. Replacement structures that increase loading to existing foundation
  5. Foundation upgrades – Soils investigation required if:
    1. There is evidence of structural failure of the existing foundation due to soil conditions
    2. Area without sufficient slope or stream setback
  6. Flood elevation projects to raise structure above the base flood elevation – Soils investigation not required if the existing foundation is found sufficient. If the existing foundation is replaced or retrofitted, a geotechnical report may be required per the other sections of this bulletin.
  7. Remodels with no increase in square footage and no foundation repair or upgrade – Soils investigation not required.
  8. Swimming pools – Soils investigation not required if the design complies with all of the following:
    1. Expansive soils, if present, are assumed in the pool design
    2. The pool is not in any area listed in Section 3, Items b. through j.
    3. Design values in structural calculations do not exceed presumptive load bearing values per Table 1806.2
    4. If a soils investigation is not required by the product listing
  9. A change of occupancy will be treated as a “new” structure for the proposed occupancy.
  10. Retaining/Basement/Foundation Walls with more than 6 feet of backfill – A geotechnical engineer shall determine the dynamic seismic lateral earth pressure due to design earthquake ground motions (CBC 1803.5.12) with specifications for back fill placement, compaction, and material recommendations. Other report requirements listed in CBC 1803 are not required unless the proposed retaining wall design is subject to any of the items listed in Section 3, Items a. through l.
  11. Room additions – Soils investigation not required if:
    1. The proposed foundation is the same type as the existing foundation, and
    2. The existing foundation is a perimeter or pier and grade beam foundation,  and
    3. The existing foundation complied with the code when it was constructed, if a building code was adopted at that time, and
    4. The addition has sufficient slope and stream setback, and
    5. There is no evidence of structural failure of the existing foundation due to soil conditions,  and
    6. The proposed foundation complies with current code, and
    7. If the proposed foundation is a pier and grade beam foundation, the piers must be the same depth as the existing piers.
  12. Room additions – Soils investigation required if:
    1. The proposed work does not comply with number K above, and
    2. Any of the conditions described in number D above exist

References

California Building Code, Chapter 18

Contact Information

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2550 Ventura Avenue
Santa Rosa, CA 95403
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