© 2019 JETIR June 2019, Volume 6, Issue 6 www.jetir.org (ISSN-2349-5162) JETIRDH06023 Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org 148 Application of vertical drains and vacuum preloading in soft soil stabilization *Sabahat Ali Khan, ** Mohd Ahmadullah Farooqi *Assistant Professor, **Associate Professor, Department of Civil Engineering, Zakir Hussain College of Engineering and Technology, AMU, Aligarh, India. ABSTRACT Infrastructural development effort on soft clay deposits that exhibit excessively low bearing capacity and potential for large settlements poses a challenge for geotechnical engineers. An adequate ground improvement program to achieve proper consolidation is thus a prerequisite for both long- and short-term stability. Pre-construction consolidation using surcharge load alone not only takes too long but also practically quite cumbersome and costly to achieve 90% consolidation. A surcharge preloading along with an arrangement of vertical drains when combined with vacuum pressure makes an attractive ground improvement alternative. This reduces the cost of stabilization and increases the effectiveness of the combined system. The provision of negative pressure in the vertical drains intensifies radial flow of porewater that expedites consolidation. This paper discusses nuances of this system and identifies factors to improve its effectiveness. Keywords: Consolidation, PVDs, Soft Soil Stabilization, Vacuum Preloading. 1 Introduction Demand for development of infrastructure on soft compressible soils is continuously on a rise with increase in population, especially in the coastal regions of many countries. These soft soil deposits have a low bearing capacity and exhibit large settlements when subjected to loading. It is therefore inevitable to treat soft soil deposits prior to construction activities in order to prevent differential settlements and subsequently potential damages to structures for both long- and short-term stability. Even though there are a variety of ground modification techniques available, the application of preloading with prefabricated vertical drains (PVDs) is still regarded as one of the preferred methods of practice. Traditionally, preloading has been the prevalent method of consolidation to increase shear strength and control post- construction settlement. Since permeability is low and the length of drainage path is equal to the thickness of soil deposit, preloading alone takes too long to achieve the desired degree of consolidation [1]. In preloading method, the inherent objective is to reduce the amount of preload to avoid two consequences: 1) reduction in the cost of project 2) collapse of soil mass due to the fact that most of the soils are weak enough to withstand higher preloads. Preloading is the application of surcharge load on the site prior to the construction of the permanent structure, until most of the primary consolidation has occurred and is commonly used to improve soft clayey soil deposits. The effective surcharge pressure for preloading can arise from either the weight of imposed fill material e.g., an embankment and/or the application of a vacuum pressure applied to a saturated soil. Using a vacuum pressure has several advantages over embankment loading, e.g., no fill material is required, construction periods are generally shorter, and there is no need for heavy machinery. In addition, the vacuum pressure method does not put any chemical admixtures into the ground and, consequently, it is an environmentally friendly ground improvement method. Several applications of the use of the vacuum consolidation method to improve soft clayey deposits have been reported in [2 - 4, and 15]. 2 Preloading techniques Preloading generally refers to the process of compressing the soil under applied surcharge load prior to construction and placement of the final construction load. The two common preloading techniques are conventional preloading, e. g. by means of an embankment, and vacuum assisted preloading.