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Briefings

Member Briefing: De-watering of excavations

Date: 06/12/16

De-watering of excavations

Introduction 

The dewatering of excavations on construction sites is a relatively straightforward and at times, a quite common occurrence, especially those sites affected by high groundwater levels. Granular soils and alluvium can be particularly problematic, similarly perched groundwater in granular soils when interbedded with clay soils. Excavation induced groundwater flow, which can also be associated with construction on steeply sloping sites, especially when sands and gravels are present, together with basement construction, are construction settings that may encounter high levels of groundwater infiltration. In the latter case, this could include groundwater under artesian pressure.

Sewerage infrastructure excavations are perhaps the more frequently encountered instances when some degree of groundwater control may be necessary. This may take the form of low quantity ‘pumping out’ from discrete sumps or in more extreme/difficult cases, the installation of a positive well-pointing system. Certain types of contaminated site may also require ‘pump and treat’ methods for groundwater remediation but in many respects such processes are covered by a more appropriate Regulatory Regime, typically, a Mobile Treatment Licence. This information note merely covers the relatively simple/routine instances when some form of dewatering may be required.

Regulatory Position

Importantly, the Environment Agency (EA) have a say in how we approach the de-watering of excavations both from a quantitative and qualitative perspective. That said, the EA have long accepted that dealing with water entering excavations is a common occurrence and as such, they have never sought to over-regulate what is in effect a relatively routine part of construction. Notwithstanding, the EA considers that there is still a need for a degree of control.

To assist the industry and to avoid excessive requests for ‘permits’ under the Environmental Permitting Regime, the EA produced what they call a Regulatory Position Statement (RPS). This has been in existence for a number of years but in January 2016 it was updated; for example a maximum dewatering discharge ‘quantity’ is no longer stated  – see EA web link below:

https://www.gov.uk/guidance/dewatering-building-sites-and-other-excavations-environmental-permits

Regulatory Position Statements remain something of a grey area but in many instances and providing the correct procedures are followed, in particular by confirming/recording the actual date when de-watering is to commence, (and likely finish) then the principles of the RPS should hold good. However, it should be noted that the duration of any discharge to a receiving water body other than a public sewer, (continuous and/or intermittent discharges specific to works being constructed) must not exceed three months. If this period is likely to be exceeded, then an Environmental Permit will be required. As can be seen from the guidance referred to above, non-compliance with this basic timescale requirement can lead to a possible prosecution by the EA.

The way forward

The key to successful compliance, whilst also avoiding the need for an Environmental Permit, starts at the land acquisition, technical due diligence stage, in particular the desk study and ground investigation part of the process. The importance of robust, qualitative and quantitative geotechnical data that also includes details of the prevailing groundwater conditions needs no further elaboration. Coupled with the topographical characteristics of the site and knowing the likely depth and extent of any subsequent excavations, this should enable easy decisions to be made on whether dewatering will be necessary, similarly, the approach to be adopted. One of the key considerations is whether the limited timescale will meet the requirements of the RPS. If the basic requirements of the RPS cannot be met then a formal Environmental Permit from the EA will be required. If in any doubt, then it would be prudent to default to a ‘permit’ – it is relatively inexpensive but these permits can take a couple of months or so to obtain. Hence, early application for a permit is prudent.

Moreover, when seeking tenders for the construction of roads and sewers there may be considerable merit in making it a condition of contract for the Contractor to apply for a ‘permit’ if dewatering over an extended period is likely and/or close to the three month time limit.

Other considerations

The EA’s RPS requirements do not state a maximum rate of dewatering discharge but this is not something that should be ignored. High volume, long duration discharges are a concern for the EA and if this is a likely outcome then discussion with the EA is recommended. In many respects, it is prudent to include in the brief to the geotechnical consultant that they investigate and report on groundwater issues, whilst also being called upon to provide recommendations for dealing with infiltration into any excavations where this is likely to be anything other than localised/discrete. The plant used for both pumped discharges and well-pointing systems can often be accompanied by approximate discharge rates.

Water quality also remains a concern for the EA. The passage of silt and/or suspended solids into a watercourse/water body is not permitted. To do so may again leave Developers and their Contractors open to prosecution by the EA. As part of the discharge arrangements it may be necessary to also introduce temporary settling tanks prior to discharge. Further guidance should be available from the EA.

This short information note should help inform members but it is no substitute for undertaking an informed and comprehensive approach to dealing with groundwater entering excavations. Notwithstanding HBF would welcome feedback on its content, similarly, feedback on any related issues raised by the EA. 

HBF London

30th November 2016