Answer: D - The impact of expansive clay swelling pressure that may be present behind a retaining wall may be reduced by treating the expansive clay soil with lime, using granular filter material between the clay backfill and the back of the wall, or excavating and replacing soils with acceptable borrow material.
The use of a granular filter material will allow groundwater to be drained from the soil and act as a buffer between the soil and the wall. Lime can be used to reduce the swelling potential of expansive soils only if the soil does not contain gypsum. If the soil contains gypsum, the addition of lime may lead to the formation of ettringite - a material with a much greater welling potential than untreated expansive soils.
