Cracked Foundations, Adobe Clay Soils and Water in Silicon Valley

In Santa Clara County, as in much of California, we have adobe clay soil and it’s expansive.  That is, when the dirt gets wet, it expands, and when it dries out, it contracts. Hence it’s sometimes referred to as “shrink-swell” soils.  (Every state in the union has areas with this problem – a color-coded map on geology.com shows areas with more and less expansive soils.)

Why is expansive soil an issue for homeowners and would-be homeowners in Silicon Valley?

The trouble is that the expanding and contracting soil is far stronger than concrete and the foundations upon which a home sits. A well written and illustrated six page paper can be found online explaining the mechanics involved for those interested in more detail on the hows and whys of expansive soils. (It states that the ground can life as much as 5,500 pounds per square inch!)

What I’d like to focus on here is mitigating the risks and preventing the problems associated with expansive soils.

The trouble is not so much that the soil is wet or dry.  The problem is in the back and forth, the movement. When the soil is kept at an even amount of moisture, it does not expand and contract.

Obviously, rain is seasonal and we cannot control all moisture on or near the house. We can, though, work to move water away from the house and away from the foundation.

Keep rain away from foundations on adobe clay soil!

Winter storms can bring an enormous amount of water onto a home’s roof, and when it channels down gutters and downspouts, there can be a large amount of water exiting in just a few places.  Where does that water go?

 

downspout no extender near foundation

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How deep is the groundwater?

Valley Water depth to first groundwaterValley Water depth to first groundwater

Valley Water depth to first groundwater in Santa Clara County

In most of Santa Clara County, home owners do not own the right to drill a well and pump groundwater under their property. That will be clarified in the preliminary title report (similarly, oil and mineral rights are usually not sold with residential neighborhood parcels here). The depth of the water may be of interest, though, as a high water table may have possible risks.

High Water Table Risks

In come cases there could be problems with springs under the home. When that happens, the groundwater may percolate up under the house during times of heavy rains, causing water in the crawl space and possibly creating foundation damage later.

Another risk is underground water moving environmental hazards to your property. A plume of water could potentially move toxins released at a leaking underground storage tank (“LUST” site) to your property’s area, where you may be responsible for the cleanup! (For info on the leaking underground storage tanks, you’d want to read the JCP or similar report for natural and environmental hazards. That will tell you if there are any LUST sites within 1 mile of the property.)

Does your home, or the one you want to buy in the Santa Clara Valley, have a high water table? One way to learn is to check out the interactive map on the Valley Water website.  Much or most of Campbell seems to have first groundwater at a depth of 50 to 100 feet. Much of the low-lying areas of San Jose and Santa Clara have water at a mere 0 – 10′!

The interactive map that I saw includes most, but not all, of the valley. Enjoy checking it out!

Santa Clara County Depth to First Groundwater

 

 

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