Question 3 of 9: Mass Wasting

Question 3: Mass Wasting (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Two types of mass wasting. Rockfall – the rapid, free-fall (or bouncing/rolling) detachment of individual rock blocks from a steep cliff face, usually along pre-existing joints or bedding planes undercut by weathering, with no coherent internal deformation during motion. Slump (rotational slide) – a slow-to-moderate mass movement of soil/rock along a concave-upward (spoon-shaped) failure surface, in which the moving block rotates backward, tilting the original ground surface and often leaving a curved head scarp and a bulging toe; common in thick, cohesive soil/weak rock on moderate slopes, frequently triggered by saturation, toe erosion (river/wave undercutting), or excavation removing lateral support.

b) Sensitive (quick) clays. Sensitive clays are glaciomarine or glaciolacustrine clays (classically the Leda/Champlain Sea clays of eastern Canada) that were originally deposited in a saline marine or brackish environment, where dissolved salts allowed a loose, open, "house-of-cards" (flocculated) fabric of clay platelets to form and remain stable, held together by salt-ion bridging in the pore water. Subsequent post-glacial isostatic rebound raised the land above sea level, and percolating fresh groundwater slowly leached the original salts out of the pore water over thousands of years. This leaching destroys the electrochemical bonding between clay particles without changing the open fabric itself, so the clay retains its in-situ strength until disturbed – but once remoulded (by a slope failure, vibration, or excavation), the fabric collapses catastrophically and the clay can liquefy into a fluid-like slurry, losing most of its strength (a high sensitivity ratio, undisturbed/remoulded shear strength). This is the mechanism behind large retrogressive "flowslides" in sensitive-clay terrain.

c) Common factor in all mass wasting. Every mass-wasting process is fundamentally a competition between the destabilizing force of gravity acting on a slope and the resisting shear strength of the material along a potential failure surface; movement initiates whenever the driving shear stress exceeds the available shear resistance (i.e. the factor of safety $FS = \tau_{resisting}/\tau_{driving}$ drops below 1). Everything else – water content, slope angle, material type, triggering event – acts through this one balance.