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Construction Technology - SUBSOIL DRAINAGE


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BUILDING REGULATION C2 Resistance to moisture

Subsoil drainage shall be provided if it is needed to avoid:

  • the passage of ground moisture to the interior of the building;
  • damage to the fabric of the building.

The ideal site (see Fig. 1.3.1) will not require any treatment, but sites with a high water table will require some form of subsoil drainage. The water table is the level at which water occurs naturally below the ground, and this level will vary with the seasonal changes.

The object of subsoil drainage is to lower the water table to a level such that it will comply with the above Building Regulation, i.e. not rise to within 0.25 m of the lowest floor of a building. It also has the advantage of improving the stability of the ground, lowering the humidity of the site, and improving its horticultural properties.

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MATERIALS

The pipes used in subsoil drainage are usually dry jointed and are either porous or perforated pipes. The porous pipes absorb the water through their walls and thus keep out the fine particles of soil or silt, whereas perforated pipes, which are laid with the perforations at the base, allow the water to rise into the pipe, leaving any silt behind.

Suitable pipes

  • Perforated clayware: BS EN 295-5.
  • Porous concrete: BS 5911-114.
  • Clayware field pipes: BS 1196.
  • Profiled and slotted polypropylene or uPVC: BS 4962
  • Perforated uPVC: BS 4660.

Note: Porous concrete is rarely manufactured now, therefore the BS has been withdrawn.

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DRAINAGE LAYOUTS

The pipes are arranged in a pattern to cover as much of the site as is necessary. Typical arrangements are shown on the plans in Fig. 1.3.2. Water will naturally flow towards the easy passage provided by the drainage runs. The system is terminated at a suitable outfall such as a river, stream or surface water sewer. In all cases permission must be obtained before discharging a subsoil system.

The banks of streams and rivers will need protection against the turbulence set up by the discharge, and if the stream is narrow the opposite bank may also need protection (see Fig. 1.3.3). If discharge is into a tidal river or stream, precautions should be taken to ensure that the system will not work in reverse by providing an outlet for the rising tide. On large schemes sediment chambers or catch pits are sometimes included to trap some of the silt which is the chief cause of blockages in subsoil drainage work. The construction of a catch pit is similar to the manhole shown in Fig. 1.3.4 except that in a catch pit the inlet and outlet are at a high level; this interrupts the flow of subsoil water in the drains and enables some of the silt to settle on the base of the catch pit. The collected silt in the catch pit must be removed at regular intervals.

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Figure 1.3.4 Outfall to surface water sewer or drain.

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