Ground improvement by preloading and vertical drain

Abstract

In Malaysia, soft soil deposits ar e widespread all over the country and mostly found in the coastal area. In general, soft soil posses low strength and high compressibil ity and thus having low bearing capacity. The soil needs to be improved to avoid excessive settlement and prevent stability failure that affecting the safety of the infrastructure. Preloading is one of the mo st effective and economical methods to reduce settlement and improve the bearing capacity of the soft soil. The application of preloading does help in applying stress to the existing soil, thus increasing the pore water pressure. Consolidation happens when the wate r in soil is squeezed out from the soil matrix. With the elimination of water from the soil, the soil strength is increased. However, this method does not have the advantages of speeding up the process of pore water pressure dissipation hence settlement might takes years to complete. The preloading method is usually co mbined with vertical drain. The use of prefabricated vertical drain in a soft soil layer can shorten the treatment time of ground by increasing the rate of consolidation. The consolidation time depends on the travelling distance of the water to flow out from the soil matrix. The installation of vertical drain shor tens the drainage path by allowing the water to flow horizontally towards the drain, and drain out to the soil surface through the vertical drain. Generally, the installation of vertic al drain is carried out in an extensive area. The behavior of ve rtical drain can be expressed as axisymmetric unit cell. There are studies carried out by researchers on vertical drain using linear model, e.g. embankment on soft Bangkok clay (Bergado 1993). Beside s, analytical methods are also used by designers assuming that consolidation happens in uniform soil column with linear compressibility characteristics without any lateral movement.