Following a deep-seated failure of a Gault Clay cutting slope on the M25, the technique selected to improve the stability of the slope was that of a single row of spaced bored piles accompanied by extensive drainage works. The opportunity therefore existed to complement earlier desk, model and analytical studies on this piling technique undertaken by TRL with an instrumented case history study. Instrumentation was installed to monitor the subsurface lateral movements of the slope and piles, the bending moment distribution in the piles, and the pore water pressure regime in the remediated slope. This report describes the instrumentation and discusses the monitoring data obtained both during the construction and over the first 26 months in service following the remedial works. The implications of the findings are discussed with a view of improving the design guidance for future construction of a similar type. (A)

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