Cut-slope failure above a coastal access road
Along a 4.2 km stretch of coastal road, a 14 m high cut slope was excavated in 2019 at 41° into a granitic hillside. The saprolite is 6–9 m thick and overlies weathered granite. In June, 236 mm of rain fell in 9 hours; the surface drain was blocked by debris and water ponded at the toe. Residents reported new tension cracks 25–40 mm wide at the crest, bulging at the toe and muddy seepage at mid-slope. A row of 12 low-rise houses sits 18 m from the toe. The road is the only route to a fishing village of 900 people. A previous failure in 2013 removed 1,800 m³ and closed the road for five days. The local authority must choose between immediate evacuation, emergency drainage and toe loading, or a permanent engineered solution, each with different cost and time constraints.
1. Identify the most likely mass movement process and explain, using the site data, the factors that have brought the slope close to failure.
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Most likely process: a deep-seated rotational slump, possibly with a translational component, in the saprolite.
The evidence points to a slip surface several metres below the surface rather than a shallow soil slip:
- Tension cracks at the crest and bulging at the toe are classic signs of rotational movement, where the upper mass moves down and back while the toe is pushed out.
- Muddy seepage at mid-slope shows that water is moving through the saprolite and building a perched water table above the less permeable weathered granite.
- The 6–9 m saprolite thickness is enough to contain a deep slip surface; a shallow failure would not produce such large crest cracks.
Why failure is close:
- The cut removed toe support. A 14 m cut at 41° steepens the slope beyond the angle that the saturated saprolite can sustain. The resisting mass at the toe has been removed, so the driving force is closer to the resisting force.
- Rainfall raised pore water pressure. 236 mm in 9 hours is an extreme input. Water entering the tension cracks and the exposed saprolite reduces effective stress, so frictional resistance falls. The blocked drain and ponding at the toe made this worse by keeping water in the slope.
- The cracks create a feedback loop. Once tension cracks open, they admit water directly to the slip surface, further reducing stability.
- The 2013 failure shows the slope was already preconditioned. The same hillside has failed before, so the material is vulnerable and the drainage system has a history of being overwhelmed.
Judgement: rainfall is the trigger, but the human cut and failed drainage are the underlying causes. The slope is not simply a natural landslide; it is a cut-slope stability problem.
2. Evaluate the two emergency options: (i) evacuate residents and install horizontal drains; (ii) place 2,000 m³ of rockfill at the toe as a counterweight. Which is more defensible in the next 48 hours?
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Option (i) is more defensible in the next 48 hours.
Case for evacuation and drainage:
- Evacuation removes the immediate risk to the 12 houses. With crest cracks of 25–40 mm and toe bulging, failure could occur without further warning, especially if more rain falls.
- Horizontal drains target the actual trigger: high pore water pressure. Lowering the water table increases effective stress and restores some shear strength. This is technically appropriate for a deep slump in saprolite.
- Drains can be installed from the road without adding load to the unstable mass, and monitoring (crack gauges, inclinometers) can run at the same time.
Problems with toe loading:
- Rockfill does increase resisting force if placed beyond the slip circle, but 2,000 m³ on a saturated, bulging toe could cause bearing failure or localised shearing.
- It requires heavy plant on the only access road, which may be unsafe and may block evacuation.
- It treats the symptom (toe movement) without reducing pore water pressure; the deep slip surface could still fail.
Judgement: evacuate first, install emergency drainage and monitor. Toe loading should not be the primary emergency action unless an engineer confirms a stable foundation and a design that adds resisting moment. The 48-hour priority is to remove exposure and reduce pore pressure, not to add weight to a saturated slope.
3. Explain why a permanent solution should combine vegetation, drainage and retaining structures rather than rely on one measure.
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A permanent solution must address several independent factors, so a single measure is unlikely to be sufficient.
- Drainage controls the main trigger by lowering pore water pressure. It is essential for a 6–9 m saprolite where a perched water table can develop, but drains can silt up and need maintenance.
- Retaining structures provide direct structural support at the toe and can protect the road and houses. They are expensive and need foundations below the slip surface; on their own they do not stop water entering the slope.
- Vegetation increases interception and evapotranspiration and adds root cohesion in the upper soil. However, roots are mostly shallow compared with a 6–9 m slip surface, so vegetation alone cannot stabilise a deep slump.
A combined design is more robust because each measure covers another's weakness: drainage reduces the driving force, the structure provides resisting force, and vegetation reduces surface erosion and small-scale shallow movement. It also allows phased maintenance and monitoring. The judgement is not that "more measures are always better" but that the measures must match the failure mechanism: here, deep water pressure plus toe removal. A single measure would leave one of those mechanisms uncontrolled.