A debris slide on a Western Ghats road cut
The Hulikal ghat road in Uttara Kannada district was widened in 2019. For 1.2 km the alignment was cut into a hillslope of about 32°, leaving an 8 m high, near-vertical cut face. The cut exposes 2.5 m of red loam resting on weathered gneiss that has closely spaced vertical and horizontal joints. Cut material was dumped on the downslope shoulder, raising the ground there by about 3 m. Between 1 and 22 July the area received 2,350 mm of rain, of which 210 mm fell in 24 hours on 25 July. On 26 July about 18,000 cubic metres of soil and weathered rock moved downslope as a single water-saturated mass, blocking the road for nine days and burying 200 m of a coffee estate. The hillside above the cut shows no backward-tilted scar; the failure surface is a joint plane roughly parallel to the cut face. In August, springs appeared 6 m below the cut crest and a road culvert was found choked with spoil.
1. Identify the type of mass movement that occurred and justify your identification by ruling out a slump, a rockfall and an earthflow using the evidence given.
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The event is a debris slide (a slope failure in unconsolidated regolith moving along a planar surface).
Why not a slump: a slump is rotational. It leaves a concave-up slip surface and a scar with the displaced block tilted backward towards the slope, and the moving mass usually stays largely coherent. The report explicitly says there is no backward-tilted scar, and the failure plane is a joint plane parallel to the cut, not a curved one.
Why not a rockfall: a rockfall is free fall of joint-bounded blocks from a steep face, producing a talus of angular debris at the base. Here 18,000 m³ of soil plus weathered rock moved as one saturated mass with a defined basal shear plane, not as falling blocks.
Why not an earthflow: an earthflow is slow, lobate and often seasonal, without a well-marked slip plane; it does not produce a single catastrophic displacement of this volume in one day.
Positive evidence: unconsolidated loam over weathered rock; a planar joint-parallel failure surface; water-saturated material; a single rapid displacement triggered within 24 hours of a 210 mm cloudburst; movement down a 32° slope. All four together indicate a debris slide (a variety of rapid flow-slide in saturated regolith).
2. Using the concept of shear strength and shear stress, explain why a slope that had been stable for forty years failed in July.
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Stability depends on the balance between driving shear stress and resisting shear strength along the potential slip plane, where shear strength is roughly c + (σ − u)tan φ. The failure can be traced to simultaneous changes in both sides of this balance.
Driving stress increased. The 8 m cut steepened the lower slope and removed the toe, which had been providing a resisting buttress. The dumped spoil added a load high on the slope, increasing the weight driving failure along the joint plane.
Resisting strength fell. The 210 mm fall in 24 hours infiltrated the loam and the open joints of the gneiss. Pore-water pressure (u) rose, so effective normal stress (σ − u) dropped and the frictional term collapsed; the loam also lost apparent cohesion on saturation. The joints already provided a continuous, low-strength plane.
Concentration of water. The choked culvert diverted runoff onto the slide area, and the springs at 6 m depth afterwards show that a perched water table had built up behind the cut.
The forty years of stability were therefore conditional, not intrinsic: the factor of safety had been quietly reduced by the cut and the spoil dump, and the cloudburst supplied the final trigger.
3. Recommend three mitigation measures for this specific cut slope, explaining for each which part of the failure mechanism it addresses.
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1. Catch drains above the crest and a benched, drained cut face. An intercepting drain 5–10 m above the crest carries surface runoff away before it can infiltrate, addressing the pore-pressure rise directly. Benching breaks the 8 m face into shorter slopes, reducing driving stress on any single plane.
2. Horizontal subsurface drains (weep holes) in the cut face. Because the failure was driven by a perched water table on jointed gneiss, drainage must lower the water table itself, not just divert surface water. Weep holes at 3–4 m spacing reduce u and restore effective stress, and the August springs show exactly where the seepage zone lies.
3. A toe retaining structure (breast wall or gabion) plus controlled spoil disposal. Restoring toe support replaces the resisting mass lost in widening. Spoil must be carried to a designated fill away from the shoulder, since a 3 m dump on the downslope side both loaded the slide and created a new, loose, failure-prone material.
Supporting measures: bioengineering with deep-rooted grasses on the cut face; restricting blasting and earthwork during the monsoon; closing the road or retreating the alignment if the joint plane daylights across the full cut.