ABSTRACT:

Dewatering of soils by wells and wellpoints is a cost effective way of controlling pore water pressures and inflows into deep excavations. Traditionally installations have been carried out by drilling from the surface and installing wells with sand filters placed around a wellscreen by gravity. This has restricted their use to essentially vertical wells. The methods of overcoming the filter placement problem means that cost effective dewatering solutions can be applied in certain situations which would traditionally have been overcome by the more expensive and time consuming techniques of grouting, compressed air or freezing. The paper addresses the methods that have been developed and the practical constraints encountered by means of a series of case histories.

INTRODUCTION

The author became interested in alternative ways of improving drainage over twenty years ago whilst studying the oil industry technique of introducing "proppants" into hydraulically fractured wells to provide higher permeability drainage paths. Early experiments in using such techniques in soft soils by injecting sand suspended in a polymer to accelerate consolidation led to an understanding of the problems and in 1991 an opportunity arose to demonstrate that this work could be utilized in a dewatering project where conventional techniques did not apply, i.e. from a tunnel under the sea. Horizontal drains have long been used in rock and competent soils, the Qanats or Karezes of the Middle East probably represent the oldest form of horizontal drainage. In Hong Kong the techniques have been developed for draining rock slopes to improve slope stability and many of the drilling and maintenance problems addressed (Martin et al, 1994). Horizontal controlled drilling techniques have been used in recent years to install relatively simple drains in landfill sites where presumably a slotted pipe is installed within a casing string and the casing removed.

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