Unsupported Deep Excavation Next to a Brick Building
On a six-storey office project in Ulaanbaatar, a 6.0 m deep basement is being excavated 3.0 m from a four-storey brick building. The contractor proposes to slope the excavation at 1:0.5 with no shoring, based on 'good soil'. Dewatering will run continuously. Workers are entering the excavation to clean the footing trench while a crawler crane operates 4 m from the crest. Cracks have appeared on the neighbour's wall; the contractor says they are 'old cracks' and asks the engineer not to record them. A labourer was seen standing under a suspended concrete bucket. The excavation permit has not been issued. The engineer must decide whether work can continue and what actions are required.
1. Can excavation and crane work continue? Identify the immediate hazards and the engineer's duties.
Read the worked answer
Work should stop. The excavation is unsupported at 6 m depth, only 3 m from a brick building, and workers are entering it. The 1:0.5 slope is not a design, and 'good soil' is not evidence. The crane near the crest adds surcharge and vibration; a suspended bucket over a worker is an immediate danger. New cracks in the neighbour's wall may indicate movement and must not be dismissed as old. The engineer should issue a stop-work instruction for the excavation and crane operation, evacuate the trench, secure the area, notify the client and competent authority, and require an investigation before any resumption.
2. What support, monitoring and sequencing measures must be in place before work resumes?
Read the worked answer
Required measures: a shoring or support design by a geotechnical/structural engineer, such as sheet piles, soldier piles with anchors, or a properly designed slope with a verified setback; a dewatering and groundwater control plan; a crane exclusion zone and a lifted-load plan; and a monitoring program for the neighbour's cracks, settlement points and excavation walls. The sequence should install support before excavating below each stage. Work should resume only after the design, permit and monitoring are in place.
3. How should the engineer deal with the neighbour's cracks and the contractor's request not to record them?
Read the worked answer
Documentation and ethics: record the cracks with dated photos and a joint survey before work; report them to the neighbour and client; keep monitoring records; and do not alter or conceal evidence at the contractor's request. Prepare an incident and stop-work report, verify temporary support, and provide toolbox talks and supervision. Concealing the cracks would breach professional ethics and could delay the real remedy.