Deep excavation beside an existing brick building
A 5.0 m deep basement is to be excavated for a new office building. The site is underlain by dense sand with a unit weight of 18 kN/m³ and an angle of shearing resistance of 34°. The groundwater table is at 3.0 m depth. An existing three-storey brick building stands 2.5 m from the excavation. Its strip footings are 1.2 m wide, founded 1.8 m below ground, and carry a bearing pressure of about 120 kPa. The new basement is supported by secant piles with temporary ground anchors; the design assumed drained conditions and a wall deflection of 10 mm. Excavation has reached 3.5 m. A 4 mm wide diagonal crack has appeared on the existing building, and survey readings show 12 mm of settlement and 8 mm of outward wall movement. Dewatering has been running for two weeks. The contractor proposes to continue excavation to 5.0 m.
1. Assess the significance of the 4 mm crack and the 12 mm settlement/8 mm wall movement. What is the likely mechanism, and should excavation continue?
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The movements are not cosmetic. The design assumed 10 mm wall deflection, and 8 mm has already occurred at 3.5 m of a 5.0 m excavation, with 12 mm of building settlement and a 4 mm diagonal crack. In dense sand, the excavation reduces lateral support and the wall yields; dewatering lowers the water table, increases effective stress and causes additional settlement. The diagonal crack indicates differential movement or distortion of the brick building, not necessarily a bearing failure. Continuing to 5.0 m will increase the unloaded height, anchor loads and wall deflection, and the movements are likely to accelerate. Excavation should stop until the wall and building are re-assessed. A simple stress-relief check is useful: removing 3.5 m of sand reduces the vertical total stress at footing level by about 3.5×18 = 63 kPa, which is a large fraction of the 120 kPa bearing pressure; the footing is also losing lateral confinement. The governing issue is serviceability and stability of the supported excavation, not just the bearing capacity of the footing.
2. Recommend immediate and permanent measures, and the monitoring that should be in place before restarting.
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Immediate: stop excavation and dewatering adjustments; install additional temporary anchors or struts to reduce wall movement; protect the cracked wall and prop the building if movement is ongoing. Control pore pressures with recharge wells or reduced pumping rather than simply pumping harder, because drawdown in dense sand increases settlement. Permanent: underpin the existing footings or use compensation grouting if the building cannot tolerate further movement; review the wall embedment and anchor capacity for the full 5.0 m depth. Monitoring: crack gauges across the 4 mm crack; prisms and total-station readings on the building; inclinometers in the wall and retained soil; piezometers; anchor-load cells. Set trigger and alarm values, for example 2 mm/week and 15 mm total settlement, with a named response plan. Restart only when the rate of movement has stabilised and the revised design has been checked.
3. What calculations should the designer perform to justify the remaining excavation?
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Use a staged beam-on-spring or finite-element analysis for each excavation stage, with the secant-pile stiffness, anchor prestress and realistic sand parameters. Check wall bending and deflection, anchor loads and bond length, basal heave and overall stability. For basal heave in sand, check seepage and piping rather than the undrained cuNc formula. Calculate the settlement trough behind the wall and compare the building's differential settlement with damage criteria. Include the dewatering drawdown as a load case; lowering the water table from 3.0 m changes effective stresses and increases wall loads. Verify the anchors for the final depth, and check that the wall toe has adequate embedment. If the revised analysis cannot keep movements within the agreed limits, the remaining excavation should not proceed without additional support or a change in construction method.