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The Design Manual for Roads and Bridges BD 30/87 requires surface water to be drained away from earth retaining structures or backfill. This will normally allow the retaining wall or abutment to be designed with zero ground water pressure on the back of the wall above the perforated drainage pipe level; this leads to a considerable cost saving. An instance where hydrostatic pressures will need to be considered is where there is a possibility of a burst water main in the vacinity of the wall.

Tower Bridge

Any water percolating through the fill is collected in a perforated drainpipe, not less than 150mm diameter, which is located at the rear of the vertical stem of the wall at the level of the top of the footing. Access to the pipe should be provided for rodding purposes from inspection manholes positioned at the foot of the wall. Weep holes are often provided as a safeguard in the event that the drainpipe is blocked; they also provide a visual check that the system is working.
Unless the backfill to the wall is highly permeable then a vertical drainage layer is provided at the rear of the wall and is connected with the perforated drainpipe.
The vertical permeable layer shown in the diagram above consists of hollow concrete blockwork, however it may also take the form of:

  1. Cast insitu porous no fines concrete
  2. or
  3. Granular drainage layer.

There are also proprietary systems on the market, such as Terram Geocomposite Drains, but they will need DfT approval to be used on a highway structure.
 

Special consideration to the drainage layer is required when the backfill contains materials susceptible to piping such as silt, chalk or PFA. Under these conditions then a granular drainage layer only is recommended; hollow blocks or no fines concrete are unsuitable.


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