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)

The key idea here is how pore pressure and a weak plane control how a slope fails. High groundwater raises pore-water pressure, which lowers shear strength along a potential slip surface. If the slope has a relatively strong unit on top (well‑cemented conglomerate) resting on a weaker, clay‑rich layer (shale), the failure tends to occur along that weak plane rather than by bending and rotating as a block.

Under these conditions the mass would slide downslope as a coherent block along a nearly flat, planar surface parallel to the slope—the hallmark of a translational slide. A rotational slump would require a curved surface and significant rotation of the moved mass, which isn’t favored when the slip surface is a roughly planar bedding or interface. A rockfall involves free-falling pieces from an exposed cliff, which isn’t implied here. A debris flow would be a rapid, muddy flow of mixed materials, which would not maintain a coherent sliding block along a planar surface.

So, the combination of a weak shale layer beneath a strong conglomerate and high groundwater promotes a translational slide.