Engineering and Construction

If the expected settlement for a proposed shallow foundation is too large, then other options for foundation support or soil stabilization must be evaluated. Some commonly used alternatives are as follows:

1. Grading. Grading operations can be used to remove the compressible soil layer and replace it
with structural fill. Usually the grading option is only economical if the compressible soil layer
is near the ground surface and the groundwater table is below the compressible soil layer or the
groundwater table can be economically lowered. Grading will be discussed later.

2. Surcharge. If the site contains an underlying compressible cohesive soil layer, the site can be
surcharged with a fill layer placed at the ground surface. Vertical drains (such as wick drains or
sand drains) can be installed in the compressible soil layer to reduce the drainage paths and speed-
up the consolidation process. Once the compressible cohesive soil layer has had sufficient con-
solidation, the fill surcharge layer is removed and the building is constructed.

3. Densification of soil. There are many different methods that can be used to densify loose or soft soil. For example, vibro-flotation and dynamic compaction are often effective at increasing the density of loose sand deposits. Another option is compaction grouting, which consists of intruding a mass of very thick consistency grout into the soil, which both displaces and compacts the loose soil.

4. Floating foundation. A floating foundation is a special type of deep foundation where the weight of the structure is balanced by the removal of soil and construction of an underground basement.