ABSTRACT:

A new method of in situ ground improvement has been developed involving the precipitation of calcite around soil particles and their contacts to improve strength and stiffness. The technique offers an innovative approach to the stabilisation of London Underground Ltd ash embankments with several advantages over existing methods including a simple non-intrusive application procedure that could be conducted largely during track operating hours. This paper describes initial trials on a large model embankment demonstrating large and rapid improvements in ash strength and stiffness.

INTRODUCTION

A new method of ground improvement known as the calcite in situ precipitation system (CIPS) has been developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia. This innovative technique involves the crystallisation of calcite cement around particles and their contacts in a manner that mimics the cementation processes occurring in nature where calcareous sediments are transformed into calcarenite and limestone. Earlier research has examined the behaviour of CIPS treated silica and calcareous sands under laboratory conditions (Kucharski et al. 1996; Kucharski et al. 1997). This paper describes the first engineering application undertaken in collaboration with Imperial College and the Geotechnical Consulting Group to a one metre high, ten metre long embankment constructed from well-graded coal ash. The London Underground Ltd (LUL) network comprises some 400 kilometres of track, of which 123km exists as embankments constructed before 1940 (Skempton, 1996). The base of the embankments consists of end-tipped London Clay, taken from excavation, which was topped with ash used as a lightweight fill and, up to 1945, ballast. Initially steam locomotive ash was used, with coal-fired power station hopper ash taking its place as this source ceased. Embankment crests are narrow with steep upper slopes. Deformation of the track with heavy maintenance costs and some speed restrictions was an increasing problem.

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