Year

2018

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Biodegradable natural prefabricated vertical drains (NPVDs or BPVDs) made from natural fibres such as jute and coir have some promising engineering properties and they are rapidly emerging as a suitable alternative to conventional polymeric drains. Although these environmentally friendly drains were first introduced almost 30 years ago, they have not been used much due to their limited supply and our limited understanding of their complex biodegradation and micro-hydraulic characteristics. This thesis, therefore, aims to clarify those issues and suggest practical designs and solutions to expedite their manufacturing capacity.

The influence that biodegradation of NPVDs has on soil consolidation is addressed, and an analytical method in which the biodegradation of NPVDs is incorporated in conventional soft soil consolidation is also proposed. The results from the current analytical solutions are then compared to a novel numerical (based on Finite Element Method-FEM) approach where a subroutine is created to capture the degrading permeability of drains with time. The proposed solution is also verified with previous studies where the influence of reduced discharge capacity of drains on soil consolidation is considered. In addition, a laboratory investigation into the biodegradation of NPVDs installed in different saturated soils is also carried out. The degradation of fibre properties is recorded while the genomic DNA and micro-analyses are carried out on decayed fibres to properly understand the biochemical activities of soil-drain systems. This study indicates that soil consolidation can seriously be hampered when natural fibre drains decay rapidly in adverse environments.

A series of experimental investigations into the hydraulic behaviour of fibre drains are carried out, followed by post-processing of the fibre drains in order to understand how micro-characteristics can affect their hydraulic properties. These results are then used to validate the Kozeny- Carman geohydraulic theoretical method in relation to the hydraulic conductivity of fibres. A novel numerical approach in which fibres are modelled by the Discrete Element Method (DEM) and the corresponding fluid flow is described by Computational Fluid Dynamics (CFD) is then proposed. An effort is also made to model natural fibres by bonding individual particles in DEM. Apart from conventional bond models, i.e., Parallel Bond Model, a modified version which can capture the nonlinear stress-strain behaviour of natural fibres is proposed. The results of this numerical work are then compared to the results from the experimental data and previous studies where different concepts were used to predict hydraulic behaviour of fibre drains. This study indicates that CFD-DEM coupling is a powerful and cost-effective approach to capture fluid-particle interactions in a soil-fibre drain system.

Comments

This thesis was originally titled: Modelling natural prefabricated vertical drains

FoR codes (2008)

0905 CIVIL ENGINEERING, 0907 ENVIRONMENTAL ENGINEERING, 0912 MATERIALS ENGINEERING

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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.