Ground improvement by vertical drains

The consolidation settlement of soft clay subsoil creates a lot of problems in foundation engineering. Because of the very low clay permeability, the primary consolidation takes a long time to complete. To shorten this consolidation time, vertical drains are installed together with preloading by surcharge embankment or vacuum pressure. Vertical drains are artificially-created drainage paths which can be installed by one of several methods and which can have a variety of physical characteristics. Figure 2.1 illustrates a typical vertical drain installation for highway embankments. In this method, pore water squeezed out during the consolidation of the clay due to the hydraulic gradients created by the preloading, can flow a lot faster in the horizontal direction towards the drain and then flow freely along the drains vertically towards the permeable drainage layers. Thus, the installation of the vertical drains in the clay reduces the length of the drainage paths and, thereby, reduces the time to complete the consolidation process. Consequently, the higher horizontal permeability of the clay is also taken advantage of. Therefore, the purpose of vertical drain installation is twofold. Firstly, to accelerate the consolidation process of the clay subsoil, and, secondly, to gain rapid strength increase to improve the stability of structures on weak clay foundations. Vertical drains can be classified into 3 general types, namely: sand drains, fabric encased sand drains, and prefabricated sand drains. Table 2.1 shows the general types and subtypes of vertical drains.