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Severe subsidence in some parts of California Central Valley

Two piers, with hydraulic jacks installed. The red box is the compressor

Two piers, with hydraulic jacks installed. The red box is the compressor

Subsidence occurs when land drops and doesn’t come back. For example, one side of a house drops an inch or so and the rest stays where it is. This will cause drywall cracks, malfunctioning doors and windows, and cracked cement in driveways. More than a couple of inches and there easily could be serious structural damage.

There has been so much groundwater pumping in the Central Valley of California that some areas are having severe, quite possibly dangerous subsidence. Arbuckle, a small town on I-15 in the Sacramento Valley, has had 4-8 inches of subsidence in eight years. That’s enough to endanger water mains, gas lines, canals for water, electric lines, as well as buildings. Unremediated, a house with areas that drop 8 inches in eight years might well be deemed unsafe to live in.

It is possible to save homes by fastening them to piers that go down 30-40 feet into bedrock. We did this on a rental property we owned in southern Utah. There was no choice. And it saved the house. The photo shows part of the operation. The piers are connected to each other by hydraulic jacks. That little red box connects to a GPS inside the house telling the jacks what to do.

Now imagine subsidence like this happenings on roads, canals, lowering some phone poles but not others, and you get an idea what the problem is!

Inelastic subsidence occurs when the structure of a clay is compromised during compaction, to the point where it is unable to expand to its original thickness even when groundwater levels rise. When that layer of clay, or silt, in the underground compresses, ground levels drop.

Subsidence is not uniform. There are certain areas that can drop due to subsidence while the surrounding area stays the same.

You get a lumpy type of dropping going on,” Hull said. “Slumping on roads, stuff like that.”

Hull said this can occur in California from three different factors: tectonic movement, oil and gas extractions, and groundwater extractions.

The drought and accompanying drilling ever-deeper for agricultural water is almost certainly the cause of the subsidence.

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