Year

2018

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Natural prefabricated vertical drains (NPVDs) 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 environmentally friendly natural fibre drains were first introduced almost 30 years ago, they have not been used widely due to their limited supply and our limited understanding of their complex flow 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 discussed. The results from the current analytical solutions are then compared to a numerical approach where a subroutine captures the degrading permeability of drains with time. The proposed solution is then verified with previous studies that demonstrate how the reduced discharge capacity of drains affects consolidation. 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 and genomic and micro-analyses are carried out on decayed fibres to properly understand the biochemical activities of a soil-drain system. This study indicates that soil consolidation can be seriously hampered when natural fibre drains decay rapidly in adverse environments.

A series of experimental investigations into the hydraulic behaviour of fibre drains is carried out, followed by post-processing of the fibre drains in order to understand how microcharacteristics can affect their hydraulic properties. These results are then used to validate the Carman-Kozeny geohydraulic theoretical method in relation to the hydraulic conductivity of fibres. A novel numerical approach is then proposed in which fibres are modelled by the Discrete Element Method (DEM) and the corresponding fluid flow is described by Computational Fluid Dynamics (CFD), 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 also proposed. The results of this numerical work are then compared to the results from the experimental data and previous studies where significantly different solutions were used bonded on other concepts. This study indicates that CFD-DEM coupling is a powerful and cost-effective approach to model natural fibre drains with respect to fluid-particle interaction.

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