Drainage below ground


Excavations shall ensure that the invert levels and gradients required by the design are achieved. Issues to be taken into account include:

  1. setting out dimensions
  2. depth of trenches
  3. width of trenches.

Setting out dimensions

When setting out:

  • discrepancies in dimensions, and ground conditions which require design modification, should be reported to the designer
  • drain runs and depths should be set out from benchmarks previously checked and verified
  • resulting variations should be recorded and distributed to all concerned.

Depth of trenches

Excavate to the depths specified in the design.

Where any trench is excavated lower than the designed bottom level, it should be refilled to the designed level. Fill material should be:

  • granular material, or
  • concrete mix GEN1 or ST ½ (not for field drains).

Hard spots should be undercut and removed so that local stress points under pipes are avoided. Soft spots should be filled with suitable well-compacted material.

Width of trenches

Trenches should be as narrow as possible within working limits and allow a minimum 150mm working space on each side of the pipe.