Walking of Flatwork on Expansive Soils

Expansive soils are characterized by their moisture sensitiveness. They will expand when given access to water and shrink when they dry out. This can cause cycles of movement, causing flatwork to separate away from the structure. This progressive movement of flatwork away from the structure is known as “walking.” In many cases, there will be appurtenant structures (such as a patio shade cover) that are attached to the house and also derive support from the flatwork. As the flatwork walks away from the house, these appurtenant structures are pulled laterally and frequently damaged. The results of a field experiment indicated that most lateral movement of flatwork on expansive soils occurs during the wet periods. With each additional wet period, the flatwork continues to experience vertical and horizontal movement, but the magnitude of movement decreases. One important factor in the amount of lateral movement of flatwork on expansive soils is the moisture condition of the clay prior to construction of the flatwork. If the clay is dry, then more lateral movement can be expected than if the clay is initally wet and soaked.