SLOPE MOVEMENTS AND THEIR EFFECTS ON A HYDROELECTRIC PLANT IN THE CORDILLERA BLANCA REGION, PERU

Aerial photograph interpretation, geologic mapping, borings made for exploration and instrument installation, and seismic refraction lines identified five modes of slope movement and an additional potential slope movement near the Canon del Pato Hydroelectric Project in the Cordillera Blanca Region of Peru. The bedrock in the area is comprised of shale with interbedded fine-grained sandstone that has undergone contact metamorphism (related to the intrusion of the Cordillera Blanca Batholith) to hornfels and quartzite. Mode 1 movement of colluvium above the bedrock is discontinuous in space and time. Some areas of colluvium are presently moving as indicated by inclinometer data at rates of 1.79 to 4.67 mm/month. Mode 2 movement consists of non-translational, chaotic rock falls along foliation joints in the hornfels in four areas of the study area. The rock fall deposits are composed of large angular blocks that have come down the slope violently. Undercutting of the toe of the slope by erosion, or earlier, similar down slope movement below can trigger the rock falls. Mode 3 movement consists of translational sliding without major rotation along foliation joints. Relict bedding planes, foliation joints, and two near vertical joint sets delineate the blocks. Openings up to 3 meters occur along the various discontinuities and down drops along the foliation planes are up to 20 meters. Mode 4 movements are minor slips on relict bedding planes with some associated movement along the foliation joints. Mode 5 movement is sliding along zones of shearing and brecciation related to the batholith intrusion. This movement, approximately 1 mm/month along the lower most failure plane, has been documented by cracking of shotcrete along the main failure plane in the excavation of the complementary intake structure, cracking of the intake structure concrete, and instrumentation data from inclinometers, multi-anchor extensometers, and load cells on rock anchors. The identified potential slope stability issue is alluvial deposits along the sides of the canyon. Movement related to any of these modes can be experienced as a result of gravity, aluviones, rainfall events, high water levels in the river at the base of the slope, and seismic events.