Soils that shrink or swell are found throughout Los Angeles and surrounding cities in Southern California. Soils with shrink/swell potential can lead to shifting soil conditions and may result stability problems for buildings constructed on it.
As ground surface water content increases, soils swell and expand upward. As water content decreases, soils shrinks,the ground surface recedes and pulls away from the foundation. In particular these conditions create problems in homes with shallow foundations. Read about Foundation Basics.
There are no perfect soil conditions for building houses. When a developer searches for land to build on, he selects land for various reasons, which may include availability, price, proximity to schools, and proximity to industrial areas. The main reason for selecting a parcel to develop is that there is a high demand of people who want to purchase homes in that area and a profit can be made from building and selling those homes.
The best way to find out if the soil beneath your house is expansive is to have a report done by a geo-technical engineer. In many housing developments a Soils Report is prepared, but this requirement varies depending on the region or country.
A geo-technical engineer will drill soil brings on your site and take samples so these soil samples can be tested for expansiveness. These samples will show how expansive the soil is and at what depths. The geo-technical engineer provides a written report on his findings.
What is the “active zone?”
From the ground surface downward, there is a depth over which expansive soils experience a change in moisture conditions as the climate (or seasons) change. This results in the soils shrinking or heaving. This zone is 18’ deep on average.
Expansive Clay Soils
Expansive clay soil will swell/ expand when wet, and contract/recede when dry. If the foundation system is in the active zone (a shallow foundation), the foundation will expand/contract as moisture conditions change in the active zone.
Select Fill/ Loam
Select fill is normally defined as a sandy loam that is minimally effected by moisture variations. A building pad that is properly built with select fill/loam will support the foundation. Problems may arise if erosion occurs that effects the bearing capacity of the soil.
Sand will not change as moisture conditions change. However, sand can erode if drainage around the lot allows water to filter down under the foundation. Sand can also recede into a crack created by drying sub-soils and cause the foundation to drift (move horizontally).
Rock will only erode and expand slightly if it is a low density of shale. In some slope conditions, fractures/ faults in the rock lead to sliding and failure if not properly secured with tie back anchors.
When a structure is supported by various soil conditions, the house may move in accordance with each soil type. If one half of the foundation sits upon expansive clay and the other half bears on select fill and/or rock, the amount of seasonal movement will vary from one half of the house to the other. If the foundation system is not properly designed, the differential movement may cause damage to the foundation and structure.
Frequently, building pads will be cut and/or filled so the bearing soil is all of the same type.