Trench Specification

When developers requisition mains from us they are encouraged to provide the trenchwork for any standard mainlaying within their site boundary; we will carry out mainlaying within the public highway or third party land.  Developers who choose to provide the onsite trenchwork should ensure that they adhere to the requirements listed below:

  • Developers should ensure that the trenches they provide allow for all mains to have 900mm of cover once the formation of the road/footpath/verge (under which the main is to be installed) is completed.  This is a minimum figure; slight increases in cover may be allowed but must be agreed in writing prior to the installation of the main.  Kerbing should be installed to give an indication of line and level for all installations.
  • The width of the required trench will primarily be dependent upon the diameter of the pipe to be installed.  Developers will be advised of the required width during a site meeting which will take place prior to the mains being installed, however the following is an indication of the minimum trench widths in which we will install mains:
DIAMETER OF MAIN (MM) MINIMUM TRENCH WIDTH (MM)
63 or 90 390
125 425
180 480
225 or 250 550
  • Mains are to be laid along the lines shown on the plans we have provided. Trenches should therefore be positioned to allow for this.  Any changes to the layout should be agreed with us prior to the proposed installation date, so that delays can be avoided. Our mainlaying crew will not start installing mains if the trenchwork provided does not correspond to the plans that they have been given.
  • Pipe installations should follow the National Joint Utilities Group’s (NJUG’s) Guidelines on the Positioning and Colour Coding of Underground Utilities’ Apparatus so that a minimum horizontal clearance of preferably 200 mm, or at the very least 150 mm, is provided between the wall of our pipeline and other utility apparatus, including chambers. Should this not be practical, an alternative arrangement must be agreed with us in writing prior to the start of the new mains installation.
  • All work should be carried out in accordance with HSE guidance HSG47 and HSG47.  Excavations should be supported, where required, and safe for the duration of our crew’s visit.  Any plant working on the site must maintain a safe distance from the trench.  Our mainlaying contractor may require you to sign a permit to work before they enter your excavation.
  • Sufficient trench is to be provided to enable our mainlaying crew to carry out uninterrupted mainlaying. We will charge for any waiting time.  Additional payments will also be due if we are unable to complete the installation and have to return to site to finish it.
  • We will install both a 100mm bed and a surround that extends to 150mm above the crown of the pipe. Developers should make sufficient stone dust available to our mainlaying crew for this installation.
  • After a new main is laid, a period of around two weeks is needed before the main can be made operational. This is to allow for the main to be chlorinated and for bacteriological and pressure tests to be carried out. During this period developers may completely backfill the trench, provided that they re-excavate around the connection point prior to our mainlaying crew’s return to site to carry out the connection. Developers should bear in mind that it may be necessary to re-excavate along the entire length of the main, if a problem with the main is detected during testing.
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