Deep stabilisation of slopes using lime piles
A comprehensive review of the literature provided much evidence of the success of lime piles in treating both soft ground and slopes. The mechanisms of stabilisation postulated by researchers is often contradictory or misleading. The use of the literature for the basis of a definitive experimental programme was not possible. An iterative approach was adopted for the laboratory programme whereby the results from one series of tests were used in the design of the next. This resulted in a range of tests including full-scale box loading tests in which lime piles were installed in clay samples, model scale lime pile tests and soil element tests. The stabilising mechanisms that have been established by the laboratory study are: generation of negative pore water pressure, overconsolidation of the shear zone, clay dehydration, pile strength and increased strength of stabilised clay due to lime migration. These mechanisms combine to improve any particular clay slope containing one or more shear zones. Three field trials were conducted. A small-scale trial was carried out on a canal cutting and provided useful data regarding pore water pressure changes and installation processes. Quantitative data produced by the laboratory study, were used to design two further trials. One trial treated a 30 m stretch of failing slope using a single 'Minuteman' rig (small and lightweight plant). Quicklime was 'poured' into open holes and compacted by the drill operators. Work was complete within two weeks. The third trial, again sited on a canal cutting, was carried out using a much larger rig. One hundred and fifty 200 mm diameter piles were constructed to a depth of 3 metres within a two week period. Monitoring of pore water pressures on both sites is still occurring on a regular basis. Excavation of sections of both trials at some future date will provide additional data on stabilising mechanisms. The research has considerably extended the understanding of the mechanisms controlling lime pile stabilisation, particularly when applied to failing slopes in British soils. Areas where further research would improve this understanding have been highlighted and in some cases work is already underway.