Groundwater ingress into excavations creates drowning hazards, destabilises trench walls, and can undermine adjacent structures. Dewatering systems remove water to maintain safe working conditions, but they must be designed and monitored to prevent ground settlement and environmental harm from discharged water. This talk covers the hazards of water in excavations and the controls for safe dewatering operations.
Key Hazards
Drowning in flooded excavations from rapid groundwater ingress or pump failure
Trench wall collapse caused by water softening soil and reducing stability
Ground settlement affecting adjacent buildings from excessive dewatering drawdown
Pollution from pumping contaminated groundwater into surface water drains
Control Measures
Design the dewatering system to match the ground permeability and expected water inflows.
Maintain pumps in working order with backup capacity available in case of primary pump failure.
Monitor water levels in the excavation and in adjacent ground using standpipe piezometers.
Ensure discharged water is treated or settled before entering watercourses or surface drains.
Obtain an environmental permit or exemption for groundwater discharge where required by the EA.
Inspect pump installations, hoses, and discharge routes before each shift and after heavy rainfall.
Do not allow workers to enter excavations with standing water until it has been pumped down to safe levels.
Monitor adjacent structures for signs of settlement caused by dewatering drawdown effects.
Brief the team on the dewatering system layout, pump locations, and the procedure if flooding occurs.
Emergency / Rescue
If pumps fail and an excavation floods rapidly, evacuate all workers immediately. Do not re-enter until water levels are reduced and the excavation support has been inspected by a competent person.
Remember
Maintain backup pump capacity in case the primary dewatering system fails during operations
Do not allow workers to enter excavations until standing water has been pumped to safe levels
Monitor adjacent structures for settlement signs caused by dewatering drawdown effects
Discharged water must be treated or settled before entering watercourses or surface drains
Inspect pumps, hoses, and discharge routes before each shift and after every heavy rainfall event
Applicable Legislation: CDM Regulations 2015 · Environmental Permitting Regulations 2016 · Water Resources Act 1991 · Health and Safety at Work etc. Act 1974