The special engineering properties of expansive soil—swelling upon contact with water, shrinking and cracking upon water loss, and repeated deformation—cause significant harm to engineering projects. Accurately understanding the characteristics of expansive soil and the conditions under which it changes allows one to predict how the foundation will deform, and thus to adopt appropriate foundation treatment measures. There are five commonly used treatment methods for expansive soil foundations:
　　(1) Soil replacement
　　Non-expansive materials or lime-soil may be used, with the replacement thickness determined through deformation calculations. For foundation treatment of Grade I and II expansive soil on flat sites, sand or gravel cushion layers should preferably be used, with a cushion thickness ≥300mm; the cushion width should exceed the base width, both sides should be backfilled with the same material as the cushion, and waterproofing measures should be properly implemented. The soil replacement method can achieve a greater foundation bearing capacity than other treatment methods, fundamentally changes the properties of the foundation soil, and has a relatively short construction period.
　　(2) Soil improvement
　　Non-expansive materials such as lime and cement, or chemical additives that cause expansive soil to lose its expansiveness, are added to the expansive soil. Mixing a certain amount of lime or cement into expansive soil can reduce or eliminate its expansiveness. Meanwhile, organic and inorganic chemical additives have also been applied in expansive soil improvement, which can reduce the plasticity index and swelling potential of expansive soil.
　　(3) Use of pile foundations
　　When the expansive soil layer is relatively thick, pile foundations should be used, with pile tips bearing on the non-expansive soil layer or on a stable layer below the atmospheric influence depth.
　　(4) Pre-wetting for expansion
　　By adding water to the soil before construction to make it wet and expand, and maintaining a high moisture content in the soil, the soil will essentially maintain a constant volume and thus will not cause structural damage.
　　(5) Waterproofing method
　　According to the characteristics of expansive soil, changes in the soil's moisture content are the fundamental condition for the harm caused by expansive soil. By adopting comprehensive measures to cut off external water seepage conditions beneath the foundation, the stability of the foundation can be ensured. Various treatment measures are sometimes used individually and sometimes in combination.