Drainage below ground

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Drainage systems shall be adequately durable and protected against damage. Issues to be taken into account include:

  1. loads from foundations
  2. bedding of pipes
  3. loads from overlying fill and traffic
  4. drainage under buildings
  5. chemicals in ground and ground water.

Loads from foundations

Drains should be located so that foundation loads are not transmitted to pipes. Where drainage trenches are near foundations:

  • foundation bottoms should be lower than adjacent trenches, or
  • the drain should be re-routed to increase separation.

Where the bottom of a drainage trench is below foundation level, the trench should be filled with concrete to a suitable level.

Bedding of pipes

Bedding should be in accordance with Clause 5.3.15.

Loads from overlying fill and traffic

Special protection may be required where pipes are near the ground surface or where they could be damaged by the weight of backfill or traffic load from above.

For flexible pipes, and where greater safety is needed, the bedding class and grading of backfill should comply with BS EN 13242, BS EN 1610 and BS EN 752.

When using proprietary systems assessed in accordance with Technical Requirement R3, pipes should be supported accordingly.

Drainage under buildings

Pipework support should take account of the ground conditions and ensure that the drainage is not adversely affected by ground movement.

Pipework under suspended floors should not be supported on ground or fill that is susceptible to movement without adequate provision being made to:

  • maintain minimum design gradients
  • protect against backfall
  • protect against leakage.

Where drains are located beneath raft foundations or where ground movement is likely, the design of the pipework and support system should be carried out by a suitably qualified engineer in accordance with Technical Requirement R5.

See Clause 5.3.14 for ‘Pipework passing through substructure walls’.

Chemicals in ground and ground water

Where the ground or ground water contains sulfates, concrete and masonry work may require special precautions.