Deep Excavation Adjacent to a Sensitive Masonry Building
A 12 m deep basement excavation in soft marine clay is supported by a 0.8 m thick diaphragm wall with three levels of steel struts. The adjacent six-storey masonry building is 8 m from the wall. Design ground movement trigger is 15 mm settlement and 25 mm wall deflection. During excavation of the last 3 m, survey shows the building has settled 22 mm, with 8 mm occurring in the last week. Inclinometer data show wall deflection of 38 mm at 10 m depth, and the maximum is moving downward. Strut loads are 20% below design, and piezometers show pore pressure 40 kPa above the pre-excavation baseline. A nearby water main is 12 m away. The contractor proposes to install ground anchors through the wall and continue excavation. The client asks whether to stop work, what mechanism is occurring, and what immediate measures are needed.
1. Compare the monitoring readings with the design triggers and identify the likely mechanism. Is the excavation safe to continue?
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Both triggers are exceeded: settlement is 22 mm against a 15 mm trigger, and wall deflection is 38 mm against 25 mm. The rate matters: 8 mm in one week is rapid. Deflection maximum moving downward with excavation indicates the wall is yielding below the current strut level, not simply elastic response. Strut loads 20% below design are not reassuring; they may mean the struts are not mobilizing load because the wall is moving away, connections are soft, or load is going into the soil. Pore pressure 40 kPa above baseline suggests excess pore pressure and consolidation of soft clay, which drives settlement. The combination points to basal heave/plastic yielding and consolidation, with possible water-main risk. Continuing excavation without stopping is not defensible: the trend is accelerating, triggers are breached, and the adjacent building is sensitive. Stop excavation, install additional support, and review before proceeding.
2. Evaluate the contractor’s proposal to install ground anchors through the wall and continue excavation. What immediate measures would you require?
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Anchors can add lateral support, but they are not an immediate fix: design, installation, testing, and lock-off take time, and anchors may not address excess pore pressure or basal heave. Continuing excavation while anchors are installed could increase movements. Immediate measures: stop excavation and unload or backfill the last 3 m if safe; install additional struts or preload existing struts to design load; check strut connections; control groundwater and verify piezometers; implement daily survey and inclinometer frequency; inspect the water main and notify the utility. Require a re-design by the temporary works engineer, including stability, anchor test loads, and a movement re-assessment. Set revised trigger and action levels and do not resume until movements stabilize and the building is instrumented. If the building is unsafe, evacuate or relieve it.
3. What changes to the instrumentation and monitoring plan would you make for the next stage?
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Add real-time or daily automatic monitoring of wall deflection and building settlement, with alarms at trigger and action levels. Increase inclinometer frequency to daily and add a second inclinometer closer to the maximum deflection. Install tilt or crack gauges on the masonry building and a settlement point on the water main. Monitor pore pressure at multiple depths in the clay, plus groundwater level. Measure strut loads continuously or daily and compare with design. Add strain gauges or load cells on new anchors. Use a linked dashboard with rate-of-movement alerts, not just absolute limits. Include a hold point: no excavation until two consecutive readings show stabilization. Review by an independent geotechnical engineer.