Comparison of Performance of Various Vertical Drains

Abstract

India has a vast extend of coastal area and has got large deposits of soft marine clays extending to a depth of 15–60 m. Construction on marine clays is a big challenge because of its high compressibility, low shear strength and excessive settlement. Ground Improvement techniques such as sand drain, gravel drains, PVD, stone columns etc. are widely used to improve the engineering properties of sub soil. It was found from field studies that in case of very soft clay, if sand or coarse aggregate alone is provided, diameter of drain gets reduced and continuity of drain get broken, due to necking action of very soft clay. In order to reduce these effects and to increases strength and continuity characteristics of drainage media a hybrid mix comprising of aggregate, sand and synthetic fibre is developed through experimental model studies. The major limitation of displacement vertical drains is the remoulding of soil surrounding each drain caused by installation which reduces permeability and efficiency of drain. The problems of smear zone can be reduced by multiple driving of vertical drains in the same spot which will enhance efficiency of drains by forming new flow paths, as there is increase in diameter and drainage media get penetrated into smear zone. In this paper efficiency comparison of various vertical drains such as sand drain (single and double driving), PVD, and hybrid mix drain (single and double driving) is studied by assessing the behavior in laboratory on large scale model bed tanks. The efficiency is evaluated by comparing settlement with time for different loadings for each types of vertical drain. From the results it is noted that double driven hybrid mix shows much better performance compared with other vertical drains and can be effectively used for very soft clays.