Lime has been used for many years to stabilize road beds and air fields, but now lime is also being used to stabilize buildings sites. Lime stabilization is used primarily to upgrade poor quality clay soils in order to provide adequate subgrade support- a prime requisite for good concrete slab performance. Lime's use is especially valuable when expansive clays are encountered. Expansive clays have been known to crack concrete slabs or to create rough joints as the slabs heaved and then settled during wet and dry cycles. Lime alleviates this problem first by reducing the soils's expansive properties and second by forming a moisture barrier which helps prevent water from reaching the underlying expansive subsoil. Lime stabilization is essentially a type of chemical stabilization process involving hydrated lime. The technique is applicable to heavy clay and silty clay soils, and to plastic aggregates such as pit-run gravel. The reaction between the lime and the clay is essentially two fold: first, during mixing, the clay particles are brought together into one mass due to base exchange, forming coarser silt sizes. This reduces the plasticity and swell and increases the friability of the soil. There is also a pronounced drying action. Secondly, after compaction, the lime reacts with the clay to form a type of cement which binds the soil particles firmly and greatly increases the strength and stability of the soil. It also renders the soil relatively impervious to water. Lime stabilization of base soil can be accomplished in three ways: (1) conventional stabilization- This involves spreading lime, mixing and compacting it and then curing the resulting lime-soil. The end result is a well-cemented, stable layer, generally 6 inches thick. (2) Soil modification- this is similar to the above, except less lime is used. The soil will still be upgraded but to a lesser degree. (3) Post-treatment- here the virgin soil in place is impregnated with lime to a shallow depth. However, the soil is not mixed nor compacted. Instead, the lime is introduced by drilling, trench irrigation, or pressure injection.