7.5 Earthworks drainage systems

7.5.1 Pre-earthworks drainage

The earthworks designer should assess the requirement for pre-earthworks drainage.

In cuttings there is likely to be a requirement for drainage in the form of a v-ditch (or where insufficient space is available, a filter drain) along the crest of the cutting to intercept surface water liable to flow towards the cutting. For the crest drainage to be effective the system needs to be able to discharge to a suitable outfall in order to prevent water overtopping the drain and subsequently damaging the cutting slopes. The pre-earthworks drainage design should include consideration of:

  • location and form of crest drains;
  • requirements for lining of drains to prevent infiltration from the base where this might lead to slope instability or groundwater pollution (the effectiveness of lining systems is often limited and sediment within v-ditches will form a natural seal so in many soil types establishment of vegetation may be the most appropriate form of seal);
  • requirements for scour protection on steeply inclined drains; and
  • details for the interception of field drains that are present within the footprint of the earthworks.

Before an embankment can be constructed, existing watercourses, ditches, subsoil agricultural drainage, springs, ponds, etc., should be dealt with so that the earthworks can be carried out without detriment to the existing ground water regime. Existing field drains should be intercepted by collector drains in the form of open-jointed or perforated pipes laid in a gravel-filled trench.

In the case where a new culvert is provided, its size, gradient and invert levels should be agreed with the appropriate water authority or agency to ensure that possible run-off from future areas of development can be accommodated and any future re-grading of the watercourse can be carried out both upstream and downstream of the embankment crossing.

NOTE 1 Where it is necessary to provide a pipe under the embankment, it is always prudent to provide one of sufficient size to permit blockages to be cleared by working from the ends of the pipe.

NOTE 2 To avoid damage by earthworks construction plant to pipes laid at existing ground level or at shallow depths, it might be necessary to protect the pipes by means of a concrete surround or by other methods.

7.5.2 Drainage during construction

Adequate temporary drainage should be provided during the construction of earthworks. The assessment of the drainage measures required is generally the responsibility of the contractor undertaking the earthworks who should use their skill and experience to ensure the temporary drainage provided is adequate to ensure the success of the earthworks by maximizing the suitability of excavated material and minimizing the potential for deterioration of materials or instability of the works. When temporary drainage issues of particular note for the scheme are identified during the design of the permanent earthworks and drainage these should be recorded so that the earthworks contractor can make adequate provision.

COMMENTARY ON 7.5.2

Issues of note that can be addressed by temporary drainage during construction include:

  • provision of adequate permanent or temporary approved outfalls during the works;
  • use of measures to reduce flow rates and remove silt from earthworks run-off drainage;
  • an earthworks methodology that allows for temporary fill surfaces to be sealed and shaped to shed water and operational restrictions during periods of rainfall;
  • installation of temporary v-ditches in cuttings as the works are progressed improves the stability of side slopes and working surfaces in silts and sands below the groundwater table;
  • planning the works so that cuttings in permeable soils and high groundwater table are excavated so as to gradually lower the groundwater table and maximize suitability of excavated earthworks materials;
  • there can be significant benefits of advance earthworks activities in some ground conditions, such as installing filter drains in advance of trimming to formation to draw down the groundwater table to avoid softening of the formation, or installation of well points in inter-layered silts/fine sands and clays); and
  • during earthworks construction, care has to be taken to avoid blocking permanent filter drainage with silt from surface water run-off.

7.5.3 Embankment under-drainage

In certain situations the designer should ensure that lower levels of the embankment are relatively highly permeable, for instance an infrastructure carrying embankment in a flood plain. A common form of this may be the inclusion of a granular starter layer (see Table 8) to aid construction and accelerate consolidation. In other situations, e.g. flood defence, the inclusion of such an arrangement would be inappropriate. Under-drainage may also be required where band drains are being used to accelerate consolidation or to mitigate uplift.

7.5.4 Earthworks drainage requirement

The earthworks designer should liaise with the drainage design team to establish suitable site constraints and practical limitations, that is:

  • define the objective of the drainage system;
  • assess the catchments (surface water and groundwater);
  • determine the outfalls (site and environmental constraints – EA agreements);
  • observe/conceptualize the existing flow paths;
  • conceptualize future post earthworks flow paths;
  • design to capture significant flows (see Figure 8);
  • consider the likely construction and maintenance regime;
  • review the consequences of system failure (may justify overdesign to avoid unacceptable flood risk);
  • review potential knock on effect on adjacent users; and
  • size the ditches and pipes (use simple systems wherever possible), see 7.5.5.

Where drainage is proposed as a remedial technique for the stabilization of unstable slopes the earthworks designer should give particular consideration to the most appropriate forms of drainage to suit the site conditions. The use of flexible and open drainage systems can often prove advantageous. Particular attention should be given to the consequences of system failure, in particular whether on sections of slope liable to significant movement the risk of drainage failure might exceed the potential benefit. Reference should be made to publications such as CIRIA C591 [29] for general cases, and Bromhead [22], and published papers for advanced techniques in differing ground conditions.

Figure 8 Design of earthworks drainage to capture significant flows
Design of earthworks drainage to capture significant flows

NOTE There are many potential sources of water. Drainage requirements commonly include:

  • crest drain – surface water (land drains);
  • toe drain – surface water/groundwater (installation disturbance);
  • pavement drain;
  • slope face drains – seepage points/groundwater profile.

COMMENTARY ON 7.5.4

Guidance in relation to the provision of earthworks drainage is contained in Carder et al [30].

Guidance on the design of soakaways is provided in BRE Digest 365 [31].

7.5.5 Hydraulic design

NOTE Guidance on the design of pipe and open channel sizes is widely available in civil engineering texts (e.g. BS EN 752-4) and does not need to be repeated here.

The drainage designer should make a realistic and adequate assessment of:

  • design return period;
  • catchment size and type;
  • run-off rate/time of entry (use of the Flood Estimation Handbook [32] for assessment);
  • maintenance issues (e.g. scour erosion of open channels); and
  • climate change.

7.5.6 Sustainable drainage (SUDS)

COMMENTARY ON 7.5.6

Surface water drainage systems developed in line with the ideals of sustainable development are collectively referred to as sustainable drainage systems (SUDS). At a particular site, these systems are designed both to manage the environmental risks resulting from urban runoff and to contribute wherever possible to environmental enhancement.

SUDS objectives are, therefore, to minimize the impacts from the development on the quantity and quality of the run-off, and maximize amenity and biodiversity opportunities. The philosophy of SUDS is to replicate, as closely as possible, the natural drainage from a site before development. There are various documents on the subject of sustainable drainage; a good starting point is CIRIA C697 [33], which provides best practice guidance on the planning, design, construction, operation and maintenance of SUDS to facilitate their effective implementation within developments.

From an earthwork perspective the most notable aspect of SUDS is to provide permeable surfaces that encourage surface water infiltration into the ground (rather than runoff from impermeable surfacing such as tarmac).

SUDS should be considered in all planning applications.

Whilst SUDS design is outside the scope of this standard, the attention of the earthworks designer should be drawn to the potentially deleterious effects that poorly planned SUDS can have on either pre-existing or newly designed earthworks, and, as far as local developments are concerned, SUDS are being included as the main form of drainage in the majority of projects being designed these days.

The earthworks designer should encourage use of SUDS to reduce the volume of surface water runoff, but consider the potential impact of SUDs on earthworks stability, i.e. aim for infiltration areas away from at risk areas to gain the benefit of SUDS and avoid additional risk.

BS 6031:2009 Code of practice for earthworks