Excavation Dewatering — Safe Work Method Statement
Safe Work Method Statement for dewatering excavations and trenches using sump pumps, spear point (wellpoint) systems and bores, including pump set-up, discharge control and monitoring for ground instability.
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Dewatering keeps an excavation workable, but taking water out of the ground changes how the ground behaves. Water flowing into a trench can wash fines from behind shoring, cause the base to boil or heave, and undermine adjacent footings or pavements. Switching pumps off, or a pump failing overnight, can flood an excavation and saturate the walls. Dewatering is High Risk Construction Work when it is done in or near a trench or shaft deeper than 1.5 metres, where there is a risk of drowning, or where powered mobile plant such as an excavator or jetting rig is working. A SWMS is required.
The task-specific hazards are electrical (submersible pumps and leads in water), carbon monoxide from petrol or diesel pumps placed in or near the excavation, drowning in a flooded excavation, collapse caused by seepage or loss of pore pressure, settlement damage to neighbouring structures, and the environmental and legal consequences of discharging sediment-laden or contaminated water. In coastal areas, lowering the water table can expose acid sulfate soils. Groundwater on former industrial land may be contaminated.
Discharge usually needs an approval. Depending on the site, this may come from the local council, the water authority (trade waste) or the state environment regulator. Check before you pump. Electrical work on pumps and control panels must be done by a licensed electrician. Install the system to the design from the geotechnical engineer where one is required, and monitor groundwater levels and neighbouring structures where the design calls for it.
The SWMS should cover how water is removed and treated, what happens if a pump fails, who checks the system and how often, and how the crew gets out if the excavation starts to flood.
Customise this template with the excavation depth and dimensions, the dewatering method and pump types, the geotechnical advice, the discharge approval and point of discharge, the sediment treatment method, the monitoring frequency, and the after-hours alarm and call-out arrangements.
Personal Protective Equipment
High Risk Construction Work Types
- •Work in or near a shaft or trench deeper than 1.5 metres, or a tunnel
- •Work in an area with movement of powered mobile plant
- •Work in or near water or other liquid involving a risk of drowning
Risk Assessment
1.Planning, geotechnical advice and discharge approval
| Hazard / Risk | Initial Risk | Control Measures | Residual Risk | Responsibility |
|---|---|---|---|---|
| Dewatering causes settlement of neighbouring buildings or services | High | Obtain geotechnical advice on dewatering method and drawdown limits for excavations near structures. Carry out dilapidation surveys of adjacent structures before starting. Install settlement and groundwater monitoring points where the design requires. | Moderate | Project Manager / Geotechnical Engineer |
| Unapproved discharge of sediment or contaminated water | High | Confirm discharge approval and conditions with council, water authority or environment regulator before pumping. Test groundwater where contamination or acid sulfate soils are suspected. Treat through sediment tank or filter as required by the approval. | Low | Supervisor / Environmental Officer |
2.Installing spear points, bores or sumps
| Hazard / Risk | Initial Risk | Control Measures | Residual Risk | Responsibility |
|---|---|---|---|---|
| Struck by jetting rig or excavator installing spears | High | Exclusion zone around plant. Spotter for reversing and slewing. Workers enter the zone only with operator's acknowledgement. | Low | Plant Operator / Spotter |
| High-pressure jetting water injury | Moderate | Inspect jetting hoses and couplings before use; whip checks fitted. Depressurise before disconnecting. Keep hands clear of the spear tip while jetting. | Low | Jetting Operator |
| Manual handling of spears, header pipes and pumps | Moderate | Use plant to lift pumps and header pipe bundles. Two-person lifts for spears and headers. Keep loads close and avoid twisting on soft ground. | Low | Crew |
3.Pump installation and electrical supply
| Hazard / Risk | Initial Risk | Control Measures | Residual Risk | Responsibility |
|---|---|---|---|---|
| Electric shock from submersible pumps and leads in water | Catastrophic | Supply from a circuit with RCD protection; push-button test RCD daily. Leads inspected and tested and tagged; no joins in water. Leads elevated off the ground and protected from plant. Licensed electrician for any work on panels, float switches or hard wiring. Isolate before handling pumps. | Low | Supervisor / Licensed Electrician |
| Carbon monoxide from engine-driven pumps | Catastrophic | Never place petrol or diesel pumps in a trench, pit or enclosed area. Position at ground level downwind with exhaust directed away from the excavation. Use electric submersible pumps inside the excavation. | Low | Supervisor |
| Fuel spills during refuelling | Moderate | Refuel with engine off and cool, using a funnel and drip tray. Bunded fuel storage. Spill kit on site. | Low | Crew |
4.Operation and monitoring
| Hazard / Risk | Initial Risk | Control Measures | Residual Risk | Responsibility |
|---|---|---|---|---|
| Excavation wall collapse from seepage, piping or base heave | Catastrophic | Competent person inspects excavation walls, shoring and base at the start of each shift and after rain. Watch for fines in discharge water, boils in the base and cracking at the surface. Stop work and get everyone out if any are seen; get geotechnical advice before re-entry. | Moderate | Competent Person / Supervisor |
| Pump failure floods the excavation | High | Standby pump on site. High-level alarm or regular checks at intervals set in the plan, including after hours where required. No entry into an excavation that has flooded until it is pumped out and inspected. | Moderate | Supervisor |
| Discharge water exceeds approval limits | Moderate | Visually check and test discharge at the frequency required by the approval. Keep records. Stop discharge if limits are exceeded. | Low | Supervisor / Environmental Officer |
5.Working in a wet excavation
| Hazard / Risk | Initial Risk | Control Measures | Residual Risk | Responsibility |
|---|---|---|---|---|
| Drowning or being swept by inflow | Catastrophic | Ladder access within 9 m of any worker in the excavation. Nobody works alone in a wet excavation. Stop work if water rises; workers exit immediately. Rescue plan in place and briefed. | Low | Supervisor / All Workers |
| Slips and trips on wet ground and hoses | Moderate | Route hoses and headers along defined paths, ramp over where crossings are needed. Keep access ways clear and drained. | Low | Crew |
6.Decommissioning
| Hazard / Risk | Initial Risk | Control Measures | Residual Risk | Responsibility |
|---|---|---|---|---|
| Excavation fills too quickly when pumps are switched off | High | Only turn off dewatering after backfill or structure is to the level set by the engineer. Remove spears and seal bores as designed. | Low | Supervisor |
| Contaminated sludge from settlement tanks | Moderate | Dispose of sludge at a licensed facility. Gloves and eye protection when cleaning tanks. | Low | Crew |
Relevant Codes of Practice
Worker Acknowledgement
By signing below, I confirm that I have read, understood and agree to comply with this Safe Work Method Statement.