Lighthouse Road landslide, Akaroa- monitoring during remedial work

The Lighthouse Road landslide, to the south of Akaroa t wnship, was triggered during heavy rainfall (275 mm) over 3 days 25–27 July 1994. A long-term remedial programme was planned to stabilise it by unloading material from the landslide. A cost-effective and reliable programme to monitor pore-water pressure development and landslide movement during and following the two phases of remedial work acted as an early warning system and provided data to aid in the design of the later phase of remedial works. Data showing the effect of unloading on pore-water pressure development are presented. Part of the reduction of pore-water pressure could be due to swelling caused by unloading the slope. The inconsistency of pore-water pressure differences relative to piezometer locations in the landslide highlights the significance of the disturbance to soil hydrology caused by landslide movement. This raises a question of how accurate methods of instrumentation used to determine the phreatic surface in an active landslide are and the subsequent use of those data in stability analyses. Decisions related to the planning of remedial work based on pore-water pressure data from these instruments must be taken cautiously