which is always present and the consolidation settlement will be present only when a fully saturated compressible clayey type of medium is present within the significant zone. The consolidation process is a time dependent process that may result in large magnitudes of settlement over longer periods of time during service of the structure resulting distress to the structure or may cause it un-serviceable. In view of this, in all important projects, it is preferred to enable the consolidating medium to undergo consolidation even before the intended structure is built under the application of a pre-load. This process is called precompression. 1.2 Consolidation acceleration methods In application of pre-load, the intensity of surcharge pressure due to pre-load plays an important role. If the preload exerts a pressure same as that of the intended structure, pre-compression may take place, but the time of consolidation may still by few years. It may not be practically feasible. Therefore it becomes necessary to accelerate the consolidation process which can be done by the following methods. 1.2.1 Application of additional preload In this process, the intensity of pressure due to the preload is provided more than that due to the intended structure such that, the required degree of consolidation takes place within a shorter period than what it could have been due to a preload of pressure equivalent to the intended structure. The effect of application of extra surcharge is illustrated in Fig. 1.
Fig. 1: The effect of preload (after Das, 1990)
However, fixing the magnitude of extra surcharge is governed by the ability of the consolidating medium. In soft clays with low bearing capacities, the preload and the extra preload may be applied in stages. If The Bridge and Structural Engineer
required, the bearing capacity may be improved by application of geosynthetics. 1.2.2 Application of vertical drains The application of extra surcharge may not be feasible in all the site conditions. However, the application of vertical drains may be a viable solution in all types of soils. When consolidation under the preload is aided with vertical drains, the excess pore water is provided drainage access in radial direction in addition to the already existing vertical drainage, by this, the drainage path is drastically decreased and hence the consolidation process is accelerated. The traditional vertical drains include the sand drains, which have now been replaced by the pre-fabricated vertical drains (PVDs). 1.3 Historical development of prefabricated vertical drains Prefabricated vertical drains were first used in Sweden in 1937. These drains were manufactured cardboard, the so-called cardboard wick (Kjellman, 1948). Approximately 10 years earlier sand drains were developed in California to expedite consolidation. Especially in the Netherlands sand drains were applied on a large scale since 1950. Dutch soil mainly consists of clay and peat layers which sometimes present to greater depths. A sandy surcharge was often placed on the top of compressible sub soil in those places where an industrial or residential estate or infrastructure had to be developed. Settlements in the subsoil were expedited by using sand drains. The synthetic drain was introduced in 1972 for a building pit at the Hemweg power station in Amsterdam. Its development was then accelerated. Synthetic drains are superior to sand drains because of their flexibility and better filtration, and they became a formidable competitor. Now-a-days, sand drains are hardly ever used. This chapter covers many aspects of the synthetic drain, from application to quality control, from design methodology to laboratory tests.
2. Operation 2.1 Principle Soil stabilization with vertical drains is used on compressible, saturated soils, like clays and peat. Volume 45 Number 4 December 2015  55