Some influences of ice thrusting in geotechnical engineering

Abstract

Abstract Ice-thrust features are widespread on the Interior Plains of Canada and cause geotechnical problems. Case histories here show that shear zones from ice-thrusting have strongly influenced design, construction and Instrumentation of coal mine slopes and earth and tailings dams. The identification of ice-thrust terrain requires aerial photograph analyses, geological field mapping and study of borehole logs and topographic maps. Three geomorphological settings which are susceptible to glaciotectonic deformation are described as escarpment, valley and plains settings. Ice-thrust features can be expected where local slopes are inclined toward the former glacier margin and where proglacial water bodies were impounded. These water bodies cause disintegration of proglacial permafrost hence decreasing the resistance of subglacial strata to ice thrusting. A fissured, brecciated Palaeocene mudstone, once heavily overconsolidated, from an ice-thrust shear zone, behaved as normally to lightly overconsolidated in laboratory tests. Shear deformation was non-brittle with a maximum strength close to residual. Fabric elements present included principal displacement shears, Reidel shears, conjugate sets of particle alignments, cutans, lithorelics and aggregations which had a dense core of randomly oriented clay platelets wrapped by an external layer of oriented clay particles. The fabric is similar to shear zones formed by tectonic activity and by laboratory shear tests, suggesting that all these shear zones were formed under similar kinematic restraints. This was confirmed by back analysis of movements of a mine highwall and two earth dams.