Best Practice Guide to Dewatering Excavations

For construction projects, housebuilders & roadside utility maintenance 

Dewatering an excavation

Whether you manage a construction site or a roadside maintenance team, at some point you’ll need to empty an excavation that has filled up with rainwater or ground water. Particularly in city centres where deep pilings and basements mean the water has nowhere else to go.

Long gone are the days when you could simply just pump the water out and discharge it into the environment. Legislation ensures that operators not abiding by pollution prevention guidance are at risk of prosecution and hefty fines.

However, the expense and disruption associated with some dewatering remedies can lead to malpractice and contamination of surface water.

Fortunately, if you are well-prepared you can dewater most small to medium-sized excavations quickly, cost-effectively and compliant with environmental regulations.

This Best Practice Guide to Dewatering Excavations tells you how. We’ve broken it down into 3 steps:

Step 1: Do You Need a Discharge Permit?

There are two main issues here: how clean is the water and where are you going to discharge it?

Silt left on road & pavementIf the water is sediment-laden, you can only discharge a minimal quantity to land without a permit. Make sure that dirty water doesn’t overflow onto pavements and roads as this is a pedestrian and traffic hazard not to mention unsightly for residents. This is a common and illegal practice of roadside contractors.

If you filter out the sediment first, to regulator-approved standards, you will be able to discharge to surface water. You might still need a permit. If your dewatering activities are going to continue for more than three consecutive months, you must apply for a permit.

If the water is contaminated with hydrocarbons, you should try to discharge it to the foul sewer. You will need a permit from the public sewerage undertaker which will incur a fee and there will be a process of approval. However, if you are performing emergency works, approval will usually be granted quickly.

If this is not an option, you will need to remove the waste water using vacuum tankers or clean it yourself before dewatering. Vacuum tankers can be prohibitively expensive – in the region of £1,000 per load – and may cause disruption to your project. If the level of contamination is high, you may have no choice.

If the water is clean, you could discharge to land or surface water. You can only discharge clean water to a watercourse for a set time period under EA exemption. Beyond that you will most likely need a permit (EA or local council). You can discharge to land without an environmental permit. If you cause pollution to the land, your dispute will be with the landowner.

In all cases, contact your local environment agency for advice. It is against the law to be operating without a permit when you need one. Permits can take three or four months to obtain so plan well in advance. Advice can also vary depending on who you talk to so don’t assume the advice you received in one area of the country will be the same in another area. (How to apply for a discharge licence in England, Wales, Scotland & Northern Ireland)

Make yourself familiar with the Temporary Dewatering from Excavations to Surface Water regulatory positioning statement (Updated 30 April 2020) or the equivalent in your jurisdiction. There are other factors, including your proximity to protected areas, which will determine whether or not you need a permit.

Step 2: How to Test for Hydrocarbon Contamination

Oil sheen on waterWater in excavations can easily become polluted with oil from machinery or run-off from refuelling areas. Or you may have been working on oil-filled cables inside the excavation. A rainbow sheen or slick will indicate that the water is polluted.

However, to be safe you should test the water with an Oil Detection Strip. These indicate the presence of hydrocarbons (1ppm) by changing colour from light blue to dark blue. Simply wave them around in the water for a few seconds. Depending on the concentration of contaminants, the colour change may range from light speckling to instant transformation – so check carefully.

Oil detection stripsThese strips are a low-cost and effective method of detecting contamination and far more convenient than sending samples away for third-party testing.

Note that there is a type of harmless bacteria that can form a multi-coloured sheen on the surface of standing water, similar to that caused by oil. The easiest way to check is to drag a stick through the material. Oil will usually remain as a constant layer on the surface of the water. If the sheen is caused by bacterial growth, it will break up into smaller pieces.

If you have detected hydrocarbons, you’ll need to clean the water or get a third party waste contractor to remove it. If you decide you can clean the water, do this before dewatering.

Absorbent pillows are a fast and inexpensive way to soak up oil and fuel. Check out Green Rhino’s smart polymer pillows – these are more effective than traditional absorbent pillows and generate less waste. The Green Rhino Quick Response Dewatering Pack includes an Oil & Sediment Filter, Oil Retention Pillows and a pack of 100 Oil Detection Strips.

If you suspect that the water in your excavation may be contaminated with anything other than hydrocarbons or sediment you should send samples for testing.

Step 3: Dewatering the Excavation

Oil & Sediment Dewatering Filter

The size of your excavation is going to determine how you dewater it. Very large excavations with a high volume of sediment may require one or more settlement tanks like Siltbusters. Dewatering filters (also known as sediment filters or silt bags) are the best solution for small and medium-sized excavations but can also be an effective solution for reasonably large excavations. They are low-cost and easy-to-implement.

The type of filter you need will depend on the type of contamination and your location. If you’ve confirmed that no hydrocarbons are present, a Sediment-only filter will be fine. Filters generally remove sediment particles down to 150 microns which is within regulatory guidelines.

If you’ve had to clean hydrocarbons out of the water beforehand, or if you just want to protect against accidental spill, you should use an Oil & Sediment Filter. These will filter out low levels of hydrocarbon contamination. 

In a sensitive area, such as an SSSI, you may be required to remove smaller particles such as silt and clay in which case you’ll need an Ultra-fine Sediment Filter

When pumping, be sure not to disturb the bottom of the excavation. This will suck thicker mud and any settled hydrocarbons into your filter. If hydrocarbon pollution is severe – with thick polluted mud at the bottom of the excavation – you may still require a vacuum tanker but a dewatering filter will enable you to process the majority of the water before the tanker is needed.

Place filter close to discharge channel

Don’t operate machinery in the excavation before dewatering is complete. You risk leaking oil or washing other pollutants off the equipment and into the water.

Make sure too that your filter is placed well away from the excavation so that discharged water cannot run back in. If discharging to surface water, place the filter close to a surface drain or natural gully. Allowing the discharge to run over grass first can remove some of the smaller sediment particles.

An important condition of discharging into the water system is that the banks and bed of receiving channels should not be damaged. We’ve seen the East West Rail Alliance place their dewatering filters on a makeshift bed of gravel to slow the discharge down and reduce scour in the channel. 

Gravel Trap

Good dewatering filters are reusable; you can empty them out when full of sediment then carry on pumping. The emptied spoil can be reused on site.

For lengthy dewatering jobs, filters will eventually clog so make sure you have spares on site. Check the specification of your filters for how much sediment and oil they will retain, and how often they can be reused, to gauge how many you might need. Green Rhino ManifoldThe type of geology will also affect the life of the filter; very fine sediments like clay will clog filters more quickly.

If you want to increase the speed of your dewatering operation, consider using a Green Rhino Manifold System to harness four, six or eight dewatering filters.

How to Encourage Compliant Behaviour

Most responsible constructors and contractors understand their environmental obligations but it can be a challenge getting buy-in at ground level. Faced with the prospect of project disruption or unbudgeted costs, operatives might be tempted to take shortcuts. You can encourage compliant behaviour by providing easy-to-implement solutions like Dewatering Filters, Oil Retention Pillows and Oil Detection Strips.

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