Choosing a Hill-Road Alignment in the Aravalli Phyllite Belt
The Public Works Department is widening a 9 km hill road between Udaipur and Gogunda. Two alignments are on the table. Alignment A cuts a 14 m high slope on the valley side; the rock is phyllite with foliation dipping 38° in the same direction as the slope, and the cut will expose the foliation plane. Alignment B is 1.4 km longer, costs about ₹18 crore more, and runs on the ridge side with a 6 m cut in massive quartzite. The region receives about 850 mm of rain, almost all between June and September, and the road crosses a first-order stream at chainage 4.2 km. A debris slide blocked the existing road for 11 days in 2022. The department must choose an alignment before the next monsoon and has a fixed budget that cannot cover both the longer alignment and full slope protection.
1. Which alignment should the department adopt, and what structural and geomorphic evidence supports that judgement?
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Recommendation: Alignment B, with conditions attached.
The decisive evidence is the relationship between foliation and slope. In Alignment A the phyllite foliation dips 38° into the cut, in the same direction as the hillside. That creates a dip slope in which the discontinuity planes daylight in the excavation. Rainwater entering these planes reduces effective friction, and phyllite is already a low-strength, cleavable rock. A 14 m cut simply removes the support at the toe and converts a stable hillside into a potential plane failure. The 2022 debris slide is consistent with this mechanism and shows that the slope already responds to monsoon infiltration.
Alignment B is not risk-free: quartzite is stronger and massive, but a 6 m cut still needs drainage and controlled blasting. However, the extra 1.4 km and ₹18 crore buy a much lower probability of failure. The budget constraint matters: if the department insists on Alignment A, the remaining money will not cover both adequate rock reinforcement and drainage, so it would be buying a high-risk road.
Conditions for B: provide a properly sized culvert or causeway at the stream crossing, intercept surface runoff above the cut, avoid side-casting spoil into the stream, and budget for bioengineering on the cut face. The department should also instrument the existing slide zone during construction, because traffic will continue on the old road until the new one opens.
2. Suppose the department selects Alignment A for cost reasons and proposes rock bolts plus a retaining wall instead of realignment. Assess whether this is defensible.
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It is defensible only as a heavily qualified engineering solution, not as a cost-saving equivalent to realignment.
Rock bolts work by transferring load into competent rock behind the failure surface. In phyllite with closely spaced foliation parallel to the slope, the anchor zone itself may be anisotropic and weak; grout may not develop reliable bond, and bolts installed at a shallow angle to the foliation can simply clamp a slab that still slides. The design must therefore specify bolts oriented across the foliation, with lengths extending well beyond the potential failure plane, and verified by pull-out tests. A retaining wall adds weight at the toe, which can increase driving force if it is founded on the same foliated rock; it also needs a deep, drained foundation.
Water is the controlling variable. With 850 mm of monsoon rain, a wall without a drainage blanket and weep holes will act as a dam. The 2022 slide suggests that infiltration, not lack of bolts, was the trigger.
Therefore the proposal is defensible only if it includes: (i) surface and subsurface drainage first, (ii) bolts designed for the foliation geometry, (iii) a retaining structure founded below the weathered zone, (iv) slope monitoring and a closure protocol during heavy rain, and (v) a contingency budget. Without these, the department is underestimating residual risk and should revisit realignment.