Survey of Residential Foundation Design Practice on Expansive Soils in the San Francisco Bay Area

The San Francisco Bay Area, California has experienced significant population growth over the last 60 years. The design practices for residential foundations have evolved substantially over this period, as a result of improved geologic characterizations, better engineering understandings of foundation performance, building code changes, and project litigation. The majority of residential foundations are constructed on expansive soils and bedrock, with the primary movement as a result of swell due to wetting of materials in an arid environment. A survey of design practice of practicing geotechnical engineers in the bay area was conducted using a written questionnaire and telephone interviews to compile data regarding the most commonly used design procedures, design details, drainage recommendations, and construction monitoring practices. The results of this survey are compiled and presented in this paper. Three primary foundation systems were identified in the survey as being commonly used in the bay area—rigid footing grids, drilled piers, and mats/slabs. To illustrate problems that have occurred with each of these foundation systems, case histories are presented for recent bay area expansive soil projects for each of these three foundation types.