Concrete is heavy! When the soils underneath a concrete slab are unable to support its weight, the slab will sink. If part of the concrete slab is supported by soils and other sections are not, the unsupported pieces may crack away from the other section and result in unlevel concrete.
The soils underneath an outdoor concrete slab can fail to support the weight of the slab for these two primary reasons:
Different types of soil have different load-bearing capacities. When a soil's load-bearing capacity is exceeded, the soil will compact and settle.
It's also possible for the soil beneath a slab to settle and compact of its own accord. Soil that contains large amounts of clay and/or silt will shrink substantially when it gets dry.
As the soil compacts underneath a concrete slab, the unsupported slab will sink as well, usually cracking in one or more spots.
The soil that is underneath your driveway, patio, or concrete steps may not necessarily remain there to provide adequate support for the concrete element.
As water moves underneath your concrete slabs, it can wash away the soil that's supporting the weight of the concrete. As this happens, it creates a void, or empty space, underneath. Over time, with nothing to support it, your concrete slab can begin to sink or cave in.
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