When excavating below ground level on a project for earthworks, basement works or shaft/tunnel works, it is likely that you will encounter water.

When excavating below ground level on a project for earthworks, basement works or shaft/tunnel works, it is likely that you will encounter water. Most water encountered during excavation or standing water on a construction site is likely to be sediment/silt laden which means it is contaminated.

Causes of Excavation and Contaminated Water Risks

• Rainfall – Rainwater will result in the build-up of silt-laden puddles of water. This water can increase the risk of flooding and would also harm aquatic life if it entered a watercourse.
• High water table – The level of groundwater is close to or above the surface and therefore require special attention to ensure safe and efficient success in excavation projects.
• Projects Near Watercourses – Excavation near a river, lake or other body of water increases the risk of surface water run-off accidentally entering nearby watercourses.

Best Practice for Excavation

• Where possible, schedule excavation during dryer seasons. This can reduce the amount of water infiltration during digging, helping avoid delays and unexpected costs.
• A geotechnical engineer can conduct a detailed site survey to measure the exact depth of a water table at various points across your excavation site.
• Track weather patterns to help predict water levels and any knock-on-effect rainy seasons may have on water tables.
• Have a contingency plan in place to tackle emergency and unforeseen circumstances including sudden flooding, water surges and unexpected weather events.
• Use appropriate equipment such as dewatering pumps, hydraulic excavators, slurry walls and cofferdams to help handle waterlogged conditions.
• Implement proper drainage solutions such as French drains, sumps and pits and gravel backfill.
• Protect against soil erosion and instability by using geotextiles and geogrids, clay liners and shoring and bracing.

Don’t forget – a full water treatment solution may not be necessary, depending on the work being carried out.

If water is likely to be contaminated or you are discharging to a water course or sewage treatment, you probably need a water treatment solution. If you are pumping uncontaminated water run off to other areas of site, pumping and pipework solutions may be more suitable.

Speak with one of our water experts who can help advise what solution may work best for you >

A Working Example of Effective Dewatering During Excavation

A popular approach to excavation dewatering is to use a temporary water treatment system that filters out the sediment and treats the contaminated water, cleaning it to regulator-approved standards before it is discharged to surface water in-line with the obtained discharge permit.

RVT Group’s temporary water treatment systems are modular by design so parts of the system can be implemented when a complete solution is not required.

Step 1.

Contaminated water is pumped from the excavation site into a treatment tank.

Step 2.

Probes from the treatment tank informs the dosing unit of current measurements. The dosing unit automatically adds chemicals such as flocculant and coagulant to the treatment tank.

Step 3.

Agitators in the treatment tank mix the chemicals into the water. The chemicals attach themselves to the water molecules ready for silt separation and making the water pH neutral.

Step 4.

The pH neutral water moves from the treatment tank to the lamella settlement tank where solids and liquids are separated. He remaining clean water flows out of the tank.

Step 5.

Clean water is discharged in line with your environmental permit.

View Our Water Treatment Systems and Monitors >

Other Considerations

Excavation projects can have significant environmental implications if water is contaminated and left untreated. Disrupting the groundwater flow or introducing pollutants into a water table or local environment can affect surrounding ecosystems.

Because of this, you have legal requirements when discharging and disposing of contaminated water from a construction site. Before you start work, you will need a plan to identify how you will treat and dispose of water that enters any excavation. This includes obtaining the appropriate permit or license and, in all cases, you should contact your local environment agency for up to date and correct advice before excavation work begins.

Having the best dewatering solution in place for excavation works can be a minefield. From having the right permit through to having the most effective solution in place, there are a lot of moving parts. Reaching out to specialists and starting the planning process early is the best way to increase the chance of your excavation’s success.

RVT Group have a team of water-specific consultants to help you navigate through even the most complex dewatering projects. Speak to one of our specialists today >

Read our Best Practice Article for Compliant Eater Management on Site >

Read our Best Practice Article for Tips and Tricks to Help Manage Water on Site >

Read our Best Practice Article for Measuring Site Water Discharge >