Cut-Slope Stability on a Koraput Ghat Road
During widening of a 6 km ghat road near Koraput, a 120 m long cut exposes khondalite under 3 m of laterite. The cut face is 28 m high and stands at 70°. Foliation strikes N20°E and dips 42°SE; the road runs N10°E on the east side of the cut, so the foliation daylights toward the carriageway. Two joint sets are present: J1 strikes N80°E and dips 75°NW, and J2 strikes N10°E and is vertical. A 3 m masonry retaining wall at the toe has opened 8 mm cracks after a 180 mm/24 h storm, and a 0.5 m wide wedge of rock and laterite has already fallen onto the shoulder. The design options are: (a) realign 25 m west into the hill, (b) reduce the slope to 45° with benching and drainage, or (c) keep the slope and add 6 m rock bolts plus mesh. Traffic must continue on a single lane. Rainfall is 1,400 mm/year, and a fault gouge zone is suspected 40 m north.
1. Carry out a kinematic assessment of the cut and identify the likely failure mode.
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Planar sliding is kinematically possible. The foliation dips 42°SE, and the cut face, with the road on the east side, faces approximately east, so the foliation daylights into the excavation. With a friction angle of about 30–35° for foliated gneiss, a 42° dip exceeds the available friction. J1 (N80°E/75°NW) can act as a lateral release; J2 is vertical and roughly parallel to the face, so it may define the back and side of a wedge. The 8 mm wall crack and the fallen 0.5 m wedge are consistent with active sliding rather than isolated rockfall. The 3 m laterite adds weight and infiltration, and the 180 mm storm raised pore pressure. The suspected gouge 40 m north is not the current failure, but it is a weak zone that should not be oversteepened.
2. Evaluate options (a), (b) and (c), and recommend a sequence that keeps one lane open.
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Option (c) alone is inadequate. Rock bolts and mesh control local blocks but do not remove a continuous foliation daylighting through a 28 m face; bolts would need to pass the slip surface and anchor in intact rock, and the toe wall is already cracked. Option (a), realigning 25 m west, removes the daylighting but creates a new cut and is slow under single-lane traffic. Option (b) is the best permanent fix only if the slope is flattened below the foliation dip, not merely to 45°: use 8 m benches at 35–38°, a catch ditch, subhorizontal drains and scaling. Because the foliation is 42°, a 45° face would still daylight. Sequence: close the lane at night, scale the loose wedge, install drainage and a temporary barrier, then bench and flatten; use bolts only for local wedges. Avoid the gouge zone and divert surface water.
3. Design a monitoring scheme and set action thresholds.
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Monitoring should link movement to rainfall. Install three crack gauges across the wall and two wire extensometers on the face; record daily during the monsoon. Install a piezometer in the laterite and one in a borehole behind the face to track pore pressure. Set a displacement alert at 5 mm/day or 15 mm cumulative, and a rainfall threshold based on the 180 mm event; at alert, close the lane and inspect. Walk the face after every storm, map new seepage and fresh rockfall, and photograph the toe wall. If movement continues after drainage and scaling, stop traffic and proceed with realignment.