Paraglacial Rock Slope Landsystems

Glaciation and deglaciation affect rock slope stability in two ways. First, glacial erosion may steepen and lengthen rock slopes, thereby increasing the self-weight (overburden) shear stresses acting within the rock mass. Second, during glacial periods the weight of overlying or adjacent glacier ice increases stress levels within rock masses. Part of the resulting ice-load deformation is elastic and stored within the rock as strain energy. During deglaciation and consequent unloading of glacially stressed rock, release of strain energy causes 'rebound' or stress release within the rock. Stress release causes extension of the internal joint network, together with reduced cohesion along joint planes and a reduction of internal locking stresses (Wyrwoll, 1977). These changes may initiate three types of response, namely catastrophic rock-slope failure during or after deglaciation, rock-slope readjustment through slow rock-mass deformation, or progressive adjustment through numerous small-scale rockfalls. The type of response is determined primarily by joint density and the orientation and inclination of fractures relative to the newly exposed rock face (Augustinus, 1995a).

17.3.1 Catastrophic Rock-Slope Failure

Examples of catastrophic rockslides or rock avalanches occurring during or shortly after recent deglaciation are legion (Evans and Clague, 1994). Such failures have been widely attributed to a combination of stress release and debuttressing of glacially steepened rockwalls by the thinning of glacier ice (Fig. 17.4). There is also evidence for widespread paraglacial rock-slope failure following Late Pleistocene deglaciation. Caine (1982), for example, has shown that scarp-edge toppling in Tasmania represents a paraglacial response to Late Pleistocene deglaciation, and in Great Britain paraglacial rock-slope failure was widespread in mountain areas in Lateglacial and Early Holocene times (Ballantyne, 1986, 1997; Shakesby and Matthews, 1996), though some failures did not occur until several millennia after deglaciation (Ballantyne et al., 1998).

Cruden and Hu (1993) have proposed that the temporal pattern of paraglacial rock-slope failure following Late Pleistocene deglaciation can be approximated by an exhaustion model similar to that outlined above (see equation). Their model assumes that there are a finite number of potential failure sites following deglaciation, that each site fails only once, and that the probability of occurrence of individual rockslides remains constant. Under these

2500

2500

Mount Fletcher (2450 m)

"a

1500-

Mount Fletcher (2450 m)

Former glacier

Rockslide mass; 1992

Downwasting surface of Maud Glacier

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