Case Study 2: Road-Cut Stability near Bafoussam
A 6 m high road cut near Bafoussam exposes 2–4 m of red laterite over weathered gneiss. The cut face trends 120°–300° and is benched at 55°. Gneiss foliation is measured at 210°/30°, and a joint set strikes 150° and dips 80° SW. The wet season brings 1,800 mm of rain, mostly June–September. You observe tension cracks 2–5 m behind the crest, 5–15 cm wide; seepage 2 m above the base; and bulging over a 20 m length. About 40 heavy trucks use the road daily. Four options are proposed: regrade the face to 35°, build a 4 m high retaining wall at the toe, install horizontal drains, or realign the road 30 m. Only one can be funded this year. The regional engineer asks you to assess the failure mode and choose the intervention.
1. Assess whether planar sliding on the gneiss foliation is geometrically feasible. State the measurements you use and the assumptions, and explain why geometry alone does not prove sliding.
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Use the slope face dip direction. The cut trends 120°–300°, so the face on the uphill side dips about 210° SW. Foliation dips 210°/30°, essentially parallel to the face. Since face angle 55° is greater than foliation dip 30°, the foliation daylights in the face: planar sliding is geometrically feasible. The 150°/80° joint could act as a lateral release surface, and the laterite cap as a top release. Assumptions: the face is the SW-facing cut; foliation is persistent; the slope is not much thicker than the cut. Geometry is necessary but not sufficient. Sliding also requires a continuous weak surface, a friction angle below 30°, and water pressure reducing normal stress. Seepage and tension cracks suggest water is present, but no shear-strength test is given. The bulging may be toe heave or a laterite slump rather than sliding on gneiss.
2. The tension cracks and toe bulging show active movement. Rank the four options and justify your first choice. Include one monitoring action.
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Ranking: 1 realignment, 2 horizontal drains, 3 regrade to 35°, 4 retaining wall.
- A 35° regrade is not a complete fix: 35° still exceeds the 30° foliation dip, so the adverse plane still daylights. It reduces but does not remove risk.
- A toe wall can be undermined if the sliding surface is deeper than the wall; it also does not address seepage.
- Horizontal drains attack the trigger (pore pressure) and are relatively cheap, but do not change the geometry; with active tension cracks they may only slow movement.
- Realignment is best only if a 30 m shift changes the cut orientation or avoids the 210°/30° foliation. Verify the new cut before committing. Monitoring: install crack gauges across the tension cracks, read weekly and after rain; add a piezometer.
3. What additional data would you collect before final design? Explain how each item could change your choice.
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Collect:
- Shear strength of foliation and laterite by direct shear. If friction is greater than 30°, planar sliding is less likely and drains may suffice.
- Discontinuity persistence and roughness along the 210°/30° surface; a persistent smooth surface favours sliding.
- Groundwater level and permeability from piezometers and packer tests, to design drains and detect perched water.
- Laterite thickness and profile; a thick clay-rich cap makes a circular slump more likely than a rockslide.
- Joint-set persistence to confirm lateral release. Each changes the choice: high friction weakens the case for realignment; deep water strengthens drains; a thick weak laterite makes regrading with drainage more important.