1、Landslides and Deformation of Rock SlopesLandslide is the downslope movement of rock,sediment,and soil under the influence of gravity without the aid of the other agents of erosion such as fiowing water,wide,or ice. Landslide debris are transported off a slope within a matter of seconds or minutes to the imperceptible slow creep of individual particles down a gradual slope.these types of movements may be triggered by the vibrations from earthquakes or the mechanical pushing or heaving of particl
2、es downslope,or they may occur spontaneously if the gravitational forces exceed the forces holding the marerial to the slope.Five basic types of landslide are definded, based on the type of movement involved: falls, siumps , and creep. Falls are created by the free fall of rocks or coherent masses of sediment from steep cliffs.The coarse debris that breaks loose from the cliff tumbles over the slope and accumulates to form talus or scree deposits at the base of the slope.Slides are slope failure
3、s in which a large mass of rock and debris slips downslope along a zone of weakness,usually a bedding plane or structural surface(a fault or joint).The debris removed from the slope may come to rest anywhere from a meter to a kilometer downslope.A scar is left on the slope delineating the area in which the debris originated. Slumps are generated when a block of sediment breaks loose from its bed and slides downward and outward as a coherent unit along a curved faiure plane.As the failed block ro
4、tates from its original position,it produces a scarp or concave scar on the slope.Sliding and slumping may occur suddenly in one great landslide or in a series of small displacements that take place over months or years.Flows are created by the downslope movement of water saturated debris.The material being transported may remain semi coherent or may become jumbled and mixed.These soggy masses may move rapidly downslope in a few centimeters (mudflows and debris flows on alluvial fans) or at a ra
5、te of only a few centimeters or meters per hour or day for a short period (solifuction and gelifraction);creep is the extremely slow,almost undetectable downslope movement of soil particles in the upper meter of the soil caused by a variety of mechanisms. Particles may simply roll down the hillside,burrowing animals may push sediment down the slope,or particles may slowly move or freezing and thawing. The term colluvium is used to describe sediments that are eroded,trans-ported, and deposited on
6、 and at the base of slopes by gravity.Colluvial deposits range from accumulations of coarse rock fragments to clay-size particles but are commonly a poorly sorted mixture of both coarse and fine-grained particles. In a slop in which the rock is jointed but where there are no significant discontinuities dipping out of the slope which could cause sliding. In an extreme case,where the rock mass consists of near vertical joints separating columns of massive rock,toppling movement and failure may occ
7、ur. For example aWahleach project is located 120 km east of Vancouver and power is generated from 620m of head between Wahleach Lake and a surface powerhouseb located to the Fraser River.Water flows through a 3500m long three meter diameter concrete encased steel lined shaft inclined at 48 to the horizontal, a 300m long lower tunnel and a 485m long surface penstock to the powerhouse. The tunnels were excavated mainly in granodiorite which varies from highly fractured and moderately weathered in
8、the upper portions of the slope to moderately fractured and fresh in both the lower portions of the slope and below the highly fractured mass. Two main joint sets occur in the rock mass,one set striking parallel to the slope and other perpendicular to it.Both dip very steeply .Average joint spacings range from 0.5 to 1.0m.A few joints occur sub-parallel to the ground surface and these joints are most welldeveloped in the ground surface adjacent to the inclined shaft.Thorough investigations faile
9、d to reveal any significant shear zones or faults oriented in a direction conducive to sliding. The top of the slope is buried beneath colluvial and fan deposits from two creeks which have incised the Fraser Valley slope to from the prominence in which the inclined shaft was excavated.This prominence is crossed by several linear troughs which trend along the ground surface contours and are evidence of previous down-slope movement of the prominence.Mature trees growing in these troughs indicate a history of movement of at least several hundred years. The water conduit operated without incident between the initial filling in 1952 and May 1981 when leakage was first noted from the upper access adit located near the intersection of the inclined shaft and the upper tunnel.This leakage stopped when two drain pipes embedded in the concrete backfill beneath the steel lining were plugged at their upsteam ends.Large holes had been eroded in these drainage pipes where the
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