What type of slope failure would you expect to find in a well-cemented conglomerate overlying a shale in an area of high groundwater levels?

• A. Debris flow
• B. Rockfall
• C. Rotational slump
• D. Translational slide

Answer: (D)

A translational slide is most likely because the slope consists of a strong, competent layer (the well-cemented conglomerate) resting on a much weaker, fissile layer (the shale) with high groundwater. Groundwater boosts pore pressure, reducing shear strength along the boundary between the two rocks. This creates a relatively flat, planar slip surface at that interface, so the mass of conglomerate and shale above the plane can slide downslope as a unit without significant rotation.

Debris flows require a supply of loose, water-saturated material that behaves like a flowing mass, which isn’t the typical case for a layered rock slope with a discrete planar interface. Rockfall involves blocks detaching and free-falling from cliff faces, not a controlled slide along a plane. A rotational slump features a curved, spoon-shaped slip surface and noticeable rotation of the moving mass, which is more common in cohesive soils or weathered rock rather than a rigid, well-cemented layer over a weak member with high pore pressure.