Building Construction on Bay Mud
Many structures in the San Francisco Bay Area are constructed on bay mud. The San Francisco Bay Area is one of many areas in the United States which has structures supported on bay mud. Bay mud is the result of alluvial, sand deposited by streams flowing out to the bay. The material deposited is composed of clays, silts, sands, organic and inorganic material which are very dense and typically saturated with water. Bay muds tend to have low strength, can compress quite a bit when loads such as buildings are constructed on them. In high seismic regions, such as the San Francisco Bay Area, bay muds are prone to additional compression, settlement, and liquefaction (making the soil more like a liquid than a solid in a short period) depending on the location of the sand layers within the soil strata. These factors all combine to make it a challenge to design and construct structures on bay mud.
For light-weight structures such as one- and two-story wood residential structures, the foundation design will utilize concrete mat foundations which spread out the weight of the structure over the building’s footprint and tend to “float” on the soil. For heavier structures, the foundation system typically consists of concrete piers or piles which are installed deep into the soil or bear directly on bedrock. Because bay mud is susceptible to consolidation, special consideration needs to be addressed where sidewalks and similar structures connect to the building as the relative movement of the sidewalk compared to the building structure will be different. Typically, a section of the sidewalk is connected to the foundation with a hinge at the foundation, called a hinge slab. This will allow for the relative movement of the slab and foundation while maintaining the proper operation of doors, minimizing tripping hazards. Additionally, the design and installation of underground utilities need to consider the consolidation of the soil and the relative movement of the foundation. Though bay muds provide challenges to the design of building foundations, sidewalks, and underground utilities, these challenges are not insurmountable. Fully understanding the soil conditions and geotechnical requirements allows engineers to design structures supported on bay mud.
EDT is quite familiar with the challenges of designing structures supported on bay mud and can determine if an existing structure was designed to meet the requirements of the project geotechnical report or whether retrofits are required.
About the Author
Michael R. Keaster, P.E., S.E. is a consulting engineer with our Oakland Office. Mr. Keaster provides technical consultation related to construction evaluation of residential and commercial structures, failure analysis, evaluation of scope and cause of damage to structures, as well as water intrusion investigation. You may contact Michael for your forensic engineering needs at firstname.lastname@example.org or (925) 674-8010.