Why Does Concrete Sink?
Fluctuating Moisture Content
Most soils have clay content in them and the more clay found in the soil the more expansive they can get. So, when it rains, the clay soaks up as much water as possible and begins to swell. Then, as the soil dries out, it begins to shrink and can no longer support the pressure from the heavy concrete slab on top of it, causing your concrete slabs to sink irregularly.
In Idaho and Oregon where the temperature fluctuates between freezing and thawing, expansive soils can lead to problems. Expansive soils are those with clay properties that swell and contract when exposed to water and dryness. When these expansive soils are beneath your concrete, the saturated earth can push up the concrete when it freezes. Then, after it thaws and the water evaporates, the ground can shrink up. This constant change in water can lead to settlement and cracks in your concrete.
If your home was built on sandy soil, you may be dealing with soil washout. Soil washout is when rainwater flows beneath the slab, the sandy soil is easily disrupted and can start to wash out with the water. This erosion leaves voids beneath the slab when the soil can no longer support the heavy concrete slab and it will start to crack and sink.