Observations and predictions of the behaviour of large, slow-moving landslides in schist, Clyde Dam reservoir, New Zealand

Abstract

Major remedial works involving toe buttressing, pumped drainage, gravity drainage and/or infiltration protection were undertaken at nine of the seventeen large landslides along the Clyde dam reservoir margins prior to lake filling in 1992–1993. All 17 landslides continue to be routinely monitored and inspected as part of the Dam Safety Assurance Programme undertaken by Contact Energy. Movement of the landslides is monitored by a combination of inclinometers, extensometers and survey, and is correlated with controlling events including lake filling, storms, floods, prolonged wet periods and earthquakes. Most monitoring records extend over periods of 15 years, with some histories as long as 25 years. Over 100 years of rainfall records are available for nearby locations. Very small deformation responses attributable to lake filling occurred at four of the stabilized slides. Slightly accelerated movement occurred at one slide that had not been stabilized, and minor reactivation was detected at three others. Two landslides have demonstrated movement rate sensitivity to rainfall events. Neither slide was significantly affected by lake filling. Monitoring records show that movement of a known ‘active’ part of the Brewery Creek landslide is triggered when a key piezometer reaches or exceeds a critical threshold level. Movement continues until the piezometer drops below that level. Movement of part of Ripponvale landslide increases following prolonged rainfall events. The slide was barely affected by lake filling and has not responded to subsequent transient increases in toe inundation levels (floods). Data from Zone A of the slide mass (which is entirely above the lake) shows the slide to be very sensitive to the effects of prolonged wet periods. Movement rates increase over a period of months and then gradually reduce. Based on the records to date, it can be predicted that the slide will exhibit an increased movement rate when the cumulative rainfall over a period of 3 to 4 months exceeds about 300 mm.