Most people don’t give much thought to dirt. So long as soil stays outside, many homeowners don’t ever give it another thought. If their soils are behaving properly, the biggest soils concern might be making sure that dirty footprints don’t get tracked across the living room floor.
But California and Bay Area soils are known for behaving poorly when they get wet. If a hillside property is not properly drained and supported, then it can behave poorly when the soils get soaked with water. Some soils can lose their strength when they get wet. When this happens, these soils can lose their holding power, and there can be a “slump” or slide.
Evidence of these kinds of slides are visible on hillsides around the Bay Area. I’ve seen several of them on hillsides adjoining Bay Area freeways. When you know what to look for, you can see green hillsides where slumps of soil have pulled away from the hill. There’s often a sharp dropoff at the top of the slump, and a small hill or pile of soil at the bottom. You can tell that the soil has partially slipped down the hillside.
When these “slumps” happen on vacant land hillsides, there might not be much of a problem. But there can be disastrous results when slumps or soils movement occurs in a subdivision with homes. Modern engineering and construction methods can be used to address some issues with soils movement. Retaining walls can be built to hold back a hillslope that would otherwise be unconfined. And deep concrete piers can be drilled to address other concerns with soils movement. Drains can be installed to carry away water that would otherwise penetrate the soil. But some soil movements can be so severe that even modern design and construction methods may not be able to withstand them. When this happens, a house can experience movement that can either be minor or significant.