Geotechnical Reports
Foundations and Soils Investigations
Purpose and Background
The 2025 California Building Code Section 1803.2 and the 2025 California Residential Code Section R401.4 establish requirements for foundations and soil investigations. Such geotechnical investigations are required for most structures in the City of Mountain View. This document provides further information about the requirements for such investigations and the types of sites/scopes of work where they are required.
Benefits of Geotechnical Investigations
Geotechnical investigations can assist property owners in identifying specific geologic and seismic hazards on their site without which investigations they have no quantitative means to assess. The owner can then prepare an appropriate design for their specific site, such as where foundation design is required to mitigate the impact of liquefiable soils or differential settlement. Geotechnical investigations can also result in the evaluation of higher soil strengths on-site than the California Building Code’s minimum criteria in Chapter 18, potentially leading to more economical foundation design.
General Requirements
Scope of Investigations: Geotechnical investigations shall assess the subject site for expansive soils, compressible soils, or shifting soils. They shall also evaluate the site for the following potential hazards from earthquake motions: slope instability, liquefaction and soil strength loss, differential settlement, reduction in soil-bearing capacity, and surface rupture due to faulting or lateral spreading.
Qualifications for Persons Preparing Reports: Classification, testing, and investigation of the soil shall be made under the responsible charge of a California registered geotechnical engineer. All reports shall be prepared and signed by a registered geotechnical engineer, certified engineering geologist, and, where applicable, a registered geophysicist. The registered design professional shall have a fully qualified representative on-site during all boring or sampling operations.
Contents of the Report: The geotechnical investigation reports shall be comprehensive and shall include the following items, if applicable to the project’s scope of work:
- A site plan plot showing the location of soil investigations;
- A complete record of the soil boring/penetration test logs and soil samples;
- A record of the soil profile;
- Site class based on soil properties, in accordance with Chapter 20 of ASCE 7;
- Evaluation of the water table;
- Recommendations for foundation type and design criteria, included, but not limited to: bearing capacity of natural or compacted soil; compaction criteria for backfill soil; passive pressure of soil for resisting sliding; coefficient of friction for sliding resistance; modulus of subgrade reaction (soil spring stiffness); provisions to mitigate the effect of expansive soils and/or liquefaction; expected total and differential settlement; varying soil strength across site; impact of loads from adjacent construction (e.g., vehicle and building surcharge), geologic features (e.g., falling rocks), or standing/flowing water (e.g., surcharge, buoyant forces);
- Where required, information for deep foundations in accordance with California Building Code (CBC) Section 1803.5.5;
- Special design and construction provisions for foundations of structures founded on expansive or liquefiable soils, as necessary;
- Special design and construction provisions for foundations of structures situated on hillside or sloping sites, as necessary;
- Special design and construction provisions, including subsurface preparation (e.g., aggregate size, layer depth, vapor barrier, etc.), for foundations of structures on sites with vapor intrusion mitigation systems, as necessary;
- Properties and testing criteria for compacted fill material, in accordance with CBC Section 1803.5.8;
- Properties and testing criteria for controlled low-strength material, in accordance with CBC Section 1803.5.9; and
- Recommendations for nonbuilding structures (e.g., flagpoles, statues, canopies).
Examples of Work that Requires a Geotechnical Investigation
- New, detached structures, including, but not limited to:
- Single-family and two-family dwellings; and
- Detached accessory dwelling units (ADUs).
- Floor additions to an existing structure (e.g., a new second story added to an existing single-story structure).
- Residential additions and remodels greater than or equal to 1,000 square feet.
- Basement additions or excavations.
- Retaining walls retaining more than 4' of level backfill (including swimming pools) or any sloped backfill.
- Any work on sites where expansive soils, compressible soils, shifting soils, or soils with other questionable characteristics are known to be present.
Examples of Work that DOES NOT Require a Geotechnical Investigation
- Single-story residential additions and remodels less than 1,000 square feet.
- On residential lots, detached structures with occupancy classifications S (Storage Group) or U (Utility and Miscellaneous Group) as defined by CBC Sections 311 and 312. Examples include detached carports, storage sheds, barns, gazebos, trellises, and stables.
- Nonhabitable accessory structures, such as ground-supported cantilevered fences not exceeding 8’ in height, decks no greater than 4’ above grade at any point, retaining walls which retain no more than 4’ of level backfill, ground-mounted residential photovoltaic systems, and other similar structures.
- Simple foundation underpinning and floor leveling of existing residential structures.
Code Minimum Criteria for Work that does not Require a Geotechnical Investigation
For projects that do not require a geotechnical investigation but nevertheless specify new shallow foundations, minimum criteria for foundations available in the California Building Code and California Residential Code may be used. These include, but are not limited to:
- The maximum allowable vertical bearing pressure for shallow foundations specified without a geotechnical investigation shall not exceed 1,500 pounds per square foot (PSF), in accordance with 2025 CBC Table 1806.2.
- The maximum allowable lateral bearing pressure for shallow foundations specified without a geotechnical investigation shall not exceed 100 pounds per square foot per foot below natural grade (PSF/ft), in accordance with 2025 CBC Table 1806.2.
- Allowable sliding resistance shall be 130 PSF multiplied by the contact area between the base of the footing and the soil but shall not exceed one-half of the dead load, in accordance with 2025 CBC Table 1806.2 and CBC Section 1806.3.2.
- The minimum depth of new footings below the undisturbed grade surface shall be 12”, and the minimum width of new footings shall be 12”, in accordance with 2025 CBC Section 1809.4.
- The minimum specified compressive strength of concrete, f’c, shall be in accordance with 2025 CBC Table 1808.8.1 and shall in all conditions be no less than 2,500 pounds per square inch (PSI).
Work that requires the installation of deep foundations shall require a geotechnical investigation.
Free viewers are required for some of the attached documents.
They can be downloaded by clicking on the icons below.
