Drainage of the Cairnmuir Landslide, New Zealand

The 20 million m³ Cairnmuir Landslide is located on the right bank of Lake Dunstan about 15 km upstream of the Clyde Dam. Investigations and remedial works concentrated on the 8 million m³ active segment, a 40-70m thick translational slide composed of schist debris. The groundwater model is a relatively simple stepped watertable below the slide but a highly compartmentalised system of aquifers within the debris. Groundwater below the slide was easily drained by drainage drives and drainholes. However, the thin compartmentalised aquifer systems within the debris was difficult to drain. Prior to remedial works the active segment was moving at rates of up to 100 mm/yr. Since the completion of the underground drainage and surface infiltration protection works the slide velocity has reduced to approximately 5 mm/yr.