Dewatering Next to a 1930s Masonry Building in Alexandria
A five-basement tower is under construction in Alexandria, 40 m from a three-storey 1930s masonry building on shallow strip footings. The excavation is 18 m deep and supported by an 800 mm diaphragm wall taken 30 m below ground. The profile is 4 m of fill, 12 m of soft silty clay (cu = 25–35 kPa, Cc/(1+e0) ≈ 0.15) and then dense sand; groundwater is at 2.0 m and brackish, with a 0.4 m tidal fluctuation. The contractor proposes 12 dewatering wells at 250 m³/h total, discharging to the storm sewer, with no recharge. After three weeks, survey points on the masonry building show 8 mm of settlement, an angular distortion of about 1/450 and new cracks above the lintels. The contractor says the movement is 'within tolerance' and wants to continue. Inclinometer readings from the wall are not yet available because the tubes were damaged during excavation.
1. Diagnose the likely mechanisms of movement and explain what measurements would distinguish between them.
Read the worked answer
Three mechanisms can produce this settlement: (a) lateral wall deflection and ground loss behind the wall during excavation; (b) consolidation of the 12 m clay layer as pumping lowers the piezometric head in the sand and raises effective stress; (c) cyclic movement driven by the 0.4 m tidal fluctuation, plus construction traffic.
The critical gap is that no inclinometer data exist, so the cause cannot be attributed. I would install replacement inclinometers in the wall (or use a slope-indicator survey from a nearby stable point), nested piezometers at about 5, 15 and 25 m both inside and outside the wall, precise levelling on the building to ±1 mm, crack gauges across the lintel cracks and vibration monitoring.
Interpretation: if outside piezometric levels fall by more than about 1 m and settlement lags pumping, consolidation dominates; if settlement tracks excavation stages while outside head is stable, wall deflection and ground loss dominate; if settlement correlates with the tide, there is a hydraulic connection through the sand. The 8 mm and 1/450 distortion are already at the limit for a masonry building, so 'within tolerance' is not a defensible statement without these records.
2. Check base stability and hydraulic uplift for the proposed wall toe and pumping. Are the wall embedment and the dewatering scheme adequate?
Read the worked answer
Uplift of the plug. With the water table at 2 m and the wall toe at 30 m, the head at the toe is about 28 m, giving a water pressure of roughly 28 × 9.81 ≈ 275 kPa. The submerged weight of the sand plug between the base at 18 m and the toe is about (19 − 9.81) × 12 ≈ 110 kPa, and the upward pressure difference across that plug is about 275 − 157 ≈ 118 kPa. The margin is at or below unity, so piping or base instability is credible; the usual factor of 1.5–2.0 is not met. Relief wells or a deeper cut-off into a confirmed aquitard are required.
Base heave in clay. The undrained check γH/cu gives about 17 × 18/30 = 10.2 against a bearing factor of roughly 5.7–7.5, so the clay would be unstable without the wall; the 12 m toe embedment provides resistance, but the margin must be calculated with the actual wall friction and the tidal range.
Dewatering. 250 m³/h is plausible for the sand, but pumping without recharge guarantees drawdown outside the wall. The scheme is not adequate as submitted: it needs recharge wells, head-control limits and a standby power supply.
3. Propose a monitoring and mitigation plan with trigger values and an agreed response for each trigger.
Read the worked answer
Monitoring. Twice-weekly precise levelling on 12 points on the masonry building and on the adjacent street; four inclinometers in the wall; nested piezometers inside and outside; crack gauges; tide and pumping records; vibration monitoring during excavation and haulage.
Triggers. Green: settlement below 5 mm and distortion below 1/1000, no new cracks. Amber: 5–10 mm, distortion 1/1000 to 1/500, or outside drawdown over 1 m - increase reading frequency, reduce pumping, start recharge. Red: 10 mm, distortion worse than 1/500, crack width over 1 mm or a rising trend over two readings - stop excavation, full recharge, compensation grouting, independent structural assessment of the building and evacuation if required.
Mitigation. Recharge wells between the wall and the building, controlled pumping with variable-frequency drives, a cut-off or additional wall panel depth if the toe check fails, and a pre-construction condition survey with photographs. The response must be written into the contractor's method statement and costed, not left as an intention, and the building's owner must receive the records.