The Sinking Road on the Ridge
A 12 km ridge road in a Caribbean island runs along a slope underlain by volcanic tuff and weathered andesite. Residents report a 40 m stretch that has dropped about 0.8 m in five years, with cracks widening after every wet season (May–November). Rainfall averages 1,800 mm a year, with short intense storms. A new housing estate of 60 homes is planned on the slope above the road. Engineers note that the tuff absorbs water and loses strength when saturated, that the slope is 18°, and that a small spring emerges 30 m below the road. There is no money for a full retaining wall; options are drainage, terracing, or relocating the road.
1. Explain the most likely cause of the road dropping, using the evidence in the setup.
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The evidence points to mass movement on a saturated slope rather than a single dramatic event.
- Geology: tuff is porous and weakens sharply when wet, so it forms a slippery layer beneath the road.
- Water: 1,800 mm of rain, short intense storms and a spring 30 m downslope show water is entering the slope and moving through it.
- Slope: 18° is steep enough for soil and weathered rock to creep or slide once lubrication occurs.
- Timing: cracks widen after each wet season, linking movement to rainfall.
- Size: a slow 0.8 m drop over five years suggests creep or a slow slump, not a rapid landslide.
So the road is subsiding because the saturated tuff is slowly deforming downslope, and the spring shows water is escaping from within the slope.
2. Evaluate drainage versus terracing as responses, given the budget.
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Drainage targets the cause. If water is what weakens the tuff, cutting drains and intercepting the spring reduces pore pressure and slows movement. It is relatively cheap and can be done in stages. Limits: drains can clog, and on a 18° slope water must be led safely away or it may create new failure points.
Terracing reduces slope angle by cutting steps, which lowers driving force. It is effective but needs earthmoving over 40 m plus the housing site, and cut slopes can expose fresh weak tuff. Cost is likely higher and disturbance greater.
Judgement: start with drainage and spring interception, because it addresses the water that the evidence shows is the trigger, and monitor cracks with simple pins. Add terracing only on the steepest section. Relocating the road avoids the problem but is expensive and leaves the 60 homes still at risk, so the housing estate should not proceed until drainage is proven to slow movement.
3. What monitoring would you recommend before approving the housing estate?
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A short, low-cost monitoring plan should test whether the slope is still moving and how water behaves:
- Crack pins across the road cracks, measured monthly, to record rate and direction of movement.
- Simple inclinometer tubes or markers in the slope above and below the road to detect depth of movement.
- Rainfall and spring flow records to link movement to wet-season water levels.
- Ground inspection after storms for new springs, bulges or tilted trees.
- Drainage trial for one wet season, checking whether movement slows.
Approval should depend on movement slowing to near zero. If cracks keep widening, the estate should be refused or moved, because 60 homes on a moving slope create a serious hazard and would add weight and water from septic systems and paved surfaces.