Question 8 of 10: Question 5, Option 1: Factors Influencing Mass Wasting

Question 5, Option 1: Factors Influencing 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.

Every mass-wasting process is governed by the balance between gravity-driven shear stress on a slope and the shear strength resisting it (factor of safety $FS=\tau_{resist}/\tau_{drive}$); the factors below all act by shifting one side of that balance.

Slope angle/gradient. A steeper slope increases the down-slope component of gravity (driving shear stress) directly – the single strongest geometric control on stability.

Water content / pore-water pressure. Saturating a slope adds weight, and – more importantly – raises pore-water pressure $u$, which reduces the effective normal stress ($\sigma'=\sigma-u$) and hence the available frictional shear strength (Mohr–Coulomb $\tau_f=c'+\sigma'\tan\phi'$); most landslides are triggered by heavy rain or snowmelt for exactly this reason.

Material type and cohesion. Cohesive, well-consolidated rock/soil resists shear far better than loose, granular, or weathered material; sensitive (leached, salt-depleted) clays can lose most of their strength on remoulding.

Vegetation cover. Root systems mechanically reinforce shallow soil and remove moisture via transpiration (increasing strength); removing vegetation (fire, logging, clearing) is a well-documented landslide trigger.

Slope undercutting / toe removal. River or wave erosion, or excavation, that removes material from a slope's toe eliminates a buttress that was resisting movement, steepening the effective slope and reducing $FS$.

Overloading. Added weight at the slope crest or mid-slope (fill, construction, stockpiles) increases driving stress without adding strength.

Discontinuities (joints, bedding, faults). Planes of weakness oriented sub-parallel to the slope face (daylighting, as in Question 4.2 above) provide a ready-made failure surface with much lower shear strength than intact rock.

Triggering events. Earthquakes (cyclic loading, liquefaction), intense rainfall, and rapid snowmelt provide the sudden stress increase or strength loss that converts a marginally stable slope ($FS$ just above 1) into an actively failing one.