Preloading

Preloading is a soil improvement method in which a temporary load (surcharge), usually an earth fill, is placed on soft compressible soil before construction, so that most of the settlement takes place before the structure is built.

Method

1. The site is cleared and instrumented (settlement plates, piezometers).
2. A fill equal to or greater than the future structure load (a surcharge, often 1.2 to 1.5 times) is placed over the area.
3. The load is left in place until the required consolidation (generally 90%) is reached. Settlement is monitored, and the surcharge is removed, after which the structure is built with small additional settlement.
4. In thick clay layers the drainage path is long, so vertical drains (sand drains or prefabricated band drains) are installed to shorten it; the time of consolidation is $t = \dfrac{T_vH^2}{c_v}$ and reduces with the square of the drainage path.

   Surcharge fill
   /////////////////
  =================  sand blanket
   | | | | | | |     vertical drains
  ~ soft clay ~~~~

Advantages: simple, cheap, and increases the strength of the clay. Limitation: takes a long time, and needs space and a fill material.