Adjacent Building Response to a 9 m Basement Excavation
A 9 m deep basement is being excavated in 8 m of soft alluvium over sandstone. The temporary works comprise 600 mm diameter contiguous bored piles at 1.2 m spacing, propped by a single level of steel struts at 1.5 m below ground. A 30-year-old four-storey reinforced concrete building sits 3 m from the excavation boundary. The building is on shallow footings 1.5 m below ground. Monitoring after excavation to 6 m shows 8 mm of settlement at the building corner, 12 mm of pile head deflection, and a new 0.3 mm wide diagonal crack in a ground-floor beam. The contractor wants to complete excavation to 9 m before installing the second strut, arguing that the measured movements are within the trigger level of 15 mm. The design predicted 10 mm maximum settlement. A nearby water main is 20 years old and 1.5 m below ground.
1. Evaluate the contractor's proposal to excavate to formation before installing the second strut.
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No. The contractor's argument misuses the trigger level. A 15 mm trigger is an action level, not a permission to continue. At 6 m depth the building has already settled 8 mm, the pile head has deflected 12 mm, and a new 0.3 mm crack has appeared. The design predicted 10 mm maximum settlement, and the excavation is only two-thirds complete. A single strut at 1.5 m leaves a 7.5 m cantilever below it, which will deflect disproportionately as the final 3 m is removed. The building is only 3 m away on shallow footings, and the water main is old. Excavating to 9 m before the second strut would likely increase settlement and could cause structural damage. The second strut should be installed before further excavation.
2. What immediate actions and investigations would you require?
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Immediate actions: (1) stop excavation and install the second strut at the design level; (2) carry out a detailed crack survey and photographic record of the building, and install tell-tales or crack gauges; (3) check the water main for leakage and notify the utility; (4) review all monitoring data and compare with predictions; (5) inspect the contiguous piles for gaps or loss of ground; (6) inform the building owner and the authority. If movements continue after the strut is installed, carry out temporary propping or ground improvement behind the piles. Do not allow the crack to be plastered over before it is recorded.
3. How would you revise the observational method for the remainder of the works?
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Revise the observational method: (1) define green, amber and red limits for settlement, horizontal deflection, strut load and crack width; (2) specify the action for each limit, with named responsibilities and response times; (3) require the second strut before excavation below 6 m; (4) increase monitoring frequency to daily during excavation and twice weekly afterwards; (5) add inclinometers in the soil behind the piles and strain gauges on the struts; (6) carry out a review by the designer at each stage. The original 15 mm trigger is too loose for a building 3 m away; a 10 mm amber and 15 mm red would be more appropriate, with a mandatory stop at amber.