The soil, as in most of the Bay Area, is predominately highly expansive clay except for some areas (e.g. Oakley) that are sand. The expansive clays swell in the winter due to an increase in moisture and shrink in the summer upon drying. This annual cyclic process of shrinking and swelling is a likely candidate for foundation movement. The cyclic moisture variation could be caused by rainfall, poor drainage, or possibly a leaking water or sewer pipe. There are other possible sources of issues creating this problem including a cut-fill lot grading condition, poor soil fill during lot preparation, or possibly tree roots.
The foundation flexibility combined with cyclic movement in the support soil telegraph the movement to the building causing brittle wall finish cracking and sticking doors. Often this movement is not a structural issue but a nuisance requiring the occupant to do continual repairs.
Many contractors, to resolve this concern, recommend remedial repairs using perimeter micropiles (such as push piers etc.) and possibly floor releveling with costs exceeding $20K. These types of repairs often do not solve the problem because they only address the perimeter foundation and not the interior.
Effective foundation repair can be complex and expensive and should be considered only when structural safety or expensive structures are involved. The cause of the foundation movements should be investigated and designed by a qualified Geotechnical Engineer and Structural Engineer. Costs for such extensive repairs can be expensive and possibly as high as $100K to $200K. Your success will be highly dependent on your budget and skill of your design professionals.
We recommend, as a first step by the homeowner, to look carefully at the lot drainage and make sure that the water drains away from the house. The crawl space (if you have one) should always be dry even in winter. No crawl space or no vented condition would indicate you have a concrete slab foundation. The water should drain from the back yard to the street quickly preferably through a closed conduit system and maybe French drains. You also need to look and your landscape irrigation and make sure that follows the same drainage pattern. I would strongly suggest placing a concrete walk or patio around the entire house and remove any planting beds next to the foundation. If planting beds are built, place them in raised planters over the slab. This is the cheapest and easiest way to mitigate a foundation movement problem associated with expansive soils and often can resolve the problem.
If the problems persist causing continual expensive repairs and/or structural safety concerns, please give us a call for a preliminary investigation.