Year

2019

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

An experimental investigation of the subsoil is of paramount importance before commencing any construction project to obtain relevant soil properties required in the design. If the properties of the soil are found to be unsuitable, ground improvement solutions need to be adopted for improving its engineering characteristics. These ground improvement techniques typically include dynamic grouting, blasting, surface compaction and preloading. One of the popular and economical ground improvement techniques is to install an array of prefabricated vertical drains (PVDs) in the soft ground which aid in the dissipation of pore water pressure of the soil. These drains are usually made of a plastic core with a longitudinal channel which is encapsulated by a sleeve made out of synthetic fabrics. Since the horizontal permeability of soil (especially in clays) is higher than their vertical permeability, vertical drains reduce the consolidation time of the soil by providing an alternate radial drainage path thus making the ground improvement schedule much shorter.

Owing to environmental issues and low-cost availability of natural fibers in developing countries such as India and Bangladesh, a new class of Natural Prefabricated Vertical Drains (NPVD) has become a viable option. These are made of fibers such as coir or jute which can be a substitute to commercially available drains made of synthetic fibers such as polyester or polyethylene. However, attributed to their degradability, they tend to become inefficient after a certain time thus reducing the speed of the consolidation process. Since jute/coir fibers have a much higher absorption capacity than synthetic fibers, their initial consolidation behavior can also be different.

This study aims to assess the efficiency of jute drains through analytical & experimental investigation and compare their performance with synthetic drains. A number of experiments were designed including consolidation tests, test of physical properties etc. in order to study the consolidation process through PVJD in greater detail. These experiments were conducted to characterize their degradation behavior as well offer an insight into what type of a drain leads to better consolidation efficiency. The consolidation & degradation tests for these experiments were carried out for a period of 24 months and to the best of author’s knowledge such an extensive experimental study has not been conducted on jute drains in the literature.

A generalized analytical model was also developed to simulate the consolidation processes under different soil and loading conditions. A unique boundary condition has been proposed at the soil-drain interface so as to take into account the effect of clogging in vertical drains. This condition has been used in a free-strain model, a modified camclay model and an elasto-viscoplastic model to prove its applicability.

The primary objective of this study was to assess whether jute drains can achieve a similar level of performance as that of synthetic drains. It has been shown that these drains show considerable promise for consolidation in pyritic estuarine clays and if designed properly are a good alternative to synthetic drains.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.