EFFECT OF S/D RATIO OF PREFABRICATED VERTICAL DRAINS ON CONSOLIDATION BEHAVIOR OF SOIL

Abstract

Soil now a days is generally not safe for supporting modern day infrastructures like highways, residential towers, ports, dams etc. Ongoing infrastructure development in several countries has forced engineers to suggest measure to construct modern and high class infrastructure facilities including highways , residential towers, ports, railways etc over soft marine and alluvial deposits. These soft soils have very low bearing capacity and undergo large detrimental settlements when acted upon by heavy loads. Therefore it is required to improve these soil properties before construction work is started on them in order to avoid damages to life and property. When load is applied on soft clayey soil, addition of load leads to development of excess pore water pressure in the soil. This excess pore water pressure is initially taken up by water present within the soft layer of soil. This pore pressure developed in soil is equal to externally applied load on the soil. As time passes, water slowly moves out from the voids of soil depending upon the permeability of soil and drainage path in soil, therefore this excess pore water pressure reduces and the pressure gets transferred to soil grains. As more and more time passes, excess pore water pressure keeps on decreasing and the effective stresses in the layer keeps on increasing. After a very long time the excess hydrostatic pore pressure becomes equal to zero and the entire consolidation pressure becomes an effective stress transmitted acting on grains of the soil. Now total externally applied is acting on soil grains therefore soil grains gets consolidated due to this effective stress and settlements continues for longer duration unless remedial measures are taken to help pore water pressure dissipate quickly. Therefore as long as consolidation continues, foundation of the structure will keep on settling. By this time, irreparable damage to structure would have occurred. Due to low permeability and poor drainage properties of soft soil, during construction load will keep on increasing water will keep on draining out until ultimate settlement is reached. Moreover, due to high compressibility of these soils, the consolidation settlements are of a very high magnitude and from structural safety point of view, it is important that major portion of this consolidation settlement takes place before/during construction phase itself. Out of the different methods available for ground improvement, pre-loading is the most successful one. The main disadvantage of this method is that, consolidation still requires longer time as drainage path is still not reduced and also external load required is of higher magnitude. Therefore to increase the speed of drainage before construction pre-loading alone is not always a good solution In these cases, the presence of prefabricated vertical drain with loading greatly reduces the load required and time for It is a well known fact that coefficient of consolidation in the horizontal direction is much higher than that in the vertical direction, installation of vertical drains also shortens the drainage path effectively , therefore the effectiveness of preloading with vertical drains is very high in achieving desired consolidation and that too in reduced time. Therefore, to accelerate the consolidation process, and reduce consolidation time prefabricated vertical drains are installed with preloading.