Deep Drainage Excavation Beside a Boundary Wall
During construction of a retail park extension, a 28 m long, 0.9 m wide storm drainage trench is being excavated to 3.4 m beside a live car park. The adjacent 2.2 m high masonry boundary wall has an 8 mm wide diagonal crack. Utility plans show an 11 kV cable crossing at approximately 2.8 m depth and a 150 mm water main running parallel 1.5 m from the trench line. No cable locator or safe-digging permit has been used. Spoil is heaped 1.2 m from the trench edge to a height of 1.6 m. There is no edge protection; workers enter on a ladder tied to a scaffold tube. Groundwater is seeping from one face and the sides show signs of slumping. A gas monitor is on site but not in use. Two workers are in the trench and a third operates a 5 t excavator 4 m away.
1. Identify the principal hazards in and around this excavation and explain the control measures you would require before further entry. Prioritise the controls.
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The trench is the main hazard, but the wall and services make it a combined event. Priorities: (1) stop entry and establish an exclusion zone; (2) treat the 11 kV cable as live until located and proved isolated; (3) control the wall collapse risk; (4) support the trench sides; (5) prevent falls and plant strikes. Controls before re-entry: a safe-digging permit, cable locator and signal generator, hand-dig trial holes near the indicated crossing, and isolation by the utility owner. Do not rely on the drawing alone. The wall crack and the trench depth mean a temporary works designer must assess the wall foundation and specify support; a trench box or hydraulic shoring is likely, with the spoil set back and not surcharging the sides. Provide edge protection and a secured ladder, not a ladder tied to scaffold tube. Control groundwater and monitor the sides; a gas monitor should be used if a confined-space risk is identified. Keep the excavator and spoil away from the trench edge, and use a banksman. The order is deliberate: services and wall collapse could kill before the trench side fails, and entry must not restart until each is controlled.
2. Explain how you would decide the temporary support system for this trench, including the information you would provide to the temporary works designer.
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I would appoint a temporary works coordinator and provide the designer with: trench dimensions and depth (28 m by 0.9 m by 3.4 m); ground description and groundwater observations; the 8 mm crack, wall height and likely foundation depth; the 11 kV cable crossing at about 2.8 m and the 150 mm water main 1.5 m away; plant loading and spoil location; duration and access requirements. The designer should decide between a trench box, hydraulic shoring or a designed batter, and specify edge protection, monitoring points and permissible loading near the edge. Because the wall is a live car park boundary, the design should include movement monitoring and a trigger for stopping work. I would not let the site team choose a support method by habit. The designer's assumptions must be checked against actual ground conditions, and a handover should confirm installation before entry.
3. Describe the emergency arrangements you would put in place before allowing any person back into the trench, and explain why the adjacent wall changes your plan.
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Before re-entry: a written rescue plan, a means of raising the alarm, and a trained rescue team or emergency service arrangement. Because the trench is 3.4 m deep, retrieval of an injured worker is not a simple ladder lift; consider harness and rescue equipment only if trained, and prevent unprotected rescuers entering. Provide two protected ladders or a ramp, edge protection, and a gas monitor with pre-entry and continuous checks. Brief the retail park and control vehicle movement. The wall changes the plan: collapse could occur without warning and block the ladder, so set movement monitoring and a trigger for evacuation, and include the wall in the emergency scenario. Keep the exclusion zone in place until the temporary works designer and competent person confirm the trench and wall are stable. Emergency numbers and the location of the isolation point for the 11 kV cable should be displayed.