Question 19 of 21: Question 7, Part 1: Factors Influencing Mass Wasting

Question 7, Part 1: Factors Influencing Mass Wasting (8 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.

Slope angle (steepness). Steeper slopes have a larger down-slope (driving) shear- stress component and a smaller resisting normal-stress component acting on any potential failure surface, so they are inherently closer to failure than gentle slopes for the same material.

Material type and strength (cohesion, friction angle). Unconsolidated regolith, weak clay-rich soils, or closely-fractured/weathered rock have low shear strength and fail at much lower slope angles than intact, well-cemented bedrock.

Water content / pore-water pressure. Infiltrating water adds weight, reduces the effective normal stress (and therefore frictional resistance) by raising pore-water pressure, and can lubricate potential slip surfaces – the single most common trigger of slope failure (e.g. following heavy rain or rapid snowmelt).

Vegetation cover. Root systems mechanically reinforce shallow soil and increase evapotranspiration (drying the soil); removing vegetation (logging, fire, construction) measurably increases slope failure frequency.

Presence of pre-existing weak planes. Bedding planes, joints, foliation, or fault surfaces oriented parallel to (or shallower than) the slope face provide ready-made, low-friction failure surfaces (as in a planar or wedge rock slide).

Seismic shaking / dynamic loading. Earthquake ground motion (or blasting, heavy traffic) applies additional transient shear stress and can momentarily liquefy saturated, loose granular material, triggering sudden failure even on slopes that were previously stable.

Slope undercutting. Natural (river/wave erosion at the slope toe) or human (highway/foundation excavation) removal of a slope's toe support steepens the effective slope and removes buttressing resistance.

Climate (freeze–thaw cycling). Repeated freezing of water in cracks wedges rock apart (frost wedging), progressively weakening slope material and priming it for later failure.