3 m Deep Sump Excavation Next to a Cracked Brick House
A 3.0 m deep excavation is needed for a 6 m × 4 m underground sump inside a built-up colony in Ranchi. The subsoil is a medium-stiff silty clay with SPT N = 6 at 1.5 m depth and N = 8 at 3.0 m; the water table is at 2.2 m below ground level. The nearest boundary is 1.2 m from the proposed pit face, and a 40-year-old single-storey brick house stands on the far side of that boundary, founded about 0.6 m below ground level, with hairline cracks already visible in its walls. The contractor's method statement proposes a 1:1 open-cut slope with 0.9 m clear working space at the base, and dewatering with one 5 HP sump pump kept in a corner of the pit and switched on only during the day shift, arguing that the clay is 'hard' and the cut is shallow. The base slab of the sump must be cast in dry conditions and the empty sump will be handed over before the monsoon.
1. Check whether the proposed 1:1 open cut physically fits the site, and state what it would do to the adjacent house.
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Geometry check. At 1:1, a 3.0 m deep cut needs a 3.0 m horizontal offset. Add the 0.9 m working space at the base, and the slope crest stands 3.9 m clear of the pit face.
Available distance from the tank wall to the boundary is 1.2 m, so the excavation overruns the boundary by 3.9 − 1.2 = 2.7 m - impossible without acquiring land.
Effect on the house. The footing base is only 0.6 m below ground, i.e. 2.4 m above the pit base. A 1:1 slope at that level is already 2.4 m out from the base edge, so it would daylight behind the footing: the slope removes the soil that provides the footing's passive resistance and bearing stratum. On a medium-stiff clay with N = 6–8 this is a classic cause of differential settlement and diagonal cracking in an already cracked, 40-year-old brick house.
Verdict. Reject the method statement. Use a vertical cut with proper shoring (sheet piles driven ahead of the cut, or soldier piles with struts on the open side), keep the excavation inside the 1.2 m, and fix crack gauges on the house before work starts.
2. The contractor says bearing is the only thing that matters because 'clay is strong'. Check the bearing pressure and identify the check that actually governs the design of the sump.
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Bearing. For a clay, cu ≈ 6N ≈ 6 × 8 = 48 kPa. Undrained bearing capacity qu ≈ 5.14 cu ≈ 247 kPa; with a factor of safety of 3, net safe bearing ≈ 82 kPa.
Loads. 2.0 m of water ≈ 19.6 kPa, a 300 mm base slab ≈ 7.5 kPa, walls and roof ≈ 10 kPa → about 37 kPa, well below 82 kPa. Bearing is not the governing problem, and settlement at 37 kPa in this clay should be a few millimetres.
The real check is flotation. The base sits at 3.0 m while the water table is at 2.2 m, so the empty sump sees 0.8 m of head = 7.85 kPa uplift. A 300 mm slab weighs 0.30 × 25 = 7.5 kPa - margin ≈ 0.35 kPa, i.e. effectively none once construction tolerance is allowed. The failure mode is the empty tank heaving and cracking when dewatering is stopped.
Action. Increase the base slab to about 400 mm (10 kPa), or provide anchorage/relief, and specify that dewatering continues until the sump is ballasted. Report bearing as satisfactory but do not sign off the 300 mm slab.
3. Evaluate the dewatering proposal (one 5 HP pump, day shift only) and state what should replace it.
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Why the proposal fails. With pumping only on day shift, water recovers to its static level of 2.2 m overnight - that is 0.8 m of water over the whole base. Concrete cannot be placed on a flooded, softened base, and every morning's first hours are spent pumping out again. Repeated drawdown also softens the clay exactly where the sump must later hold water.
What to specify.
- Continuous pumping with a standby pump of the same capacity, holding the water level at least 0.5 m below the excavation base.
- A graded filter around the sump and a perimeter drain, so pumping does not draw fines from under the base or from beneath the boundary wall.
- In this silty clay the inflow will be small (low permeability), so a sump is hydraulically adequate - the risk is basal disturbance, not large flow, and wellpoints would be over-design.
Monitoring. With shoring in place instead of the rejected slope, install crack gauges on the house and a level point on the boundary wall, read daily; any movement means stop and re-strut. Lowering the water table beside a house founded at only 0.6 m is the second reason to keep the cut tight and strutted.