List of Abutment Design Standards: [Show]
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.
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:
Cast insitu porous no fines concrete
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.
Ideally the road alignment should have a continuous fall from one abutment to the other. Also decks should be made integral with the abutments to avoid any joints in the carriageway. If decks cannot be made integral then the fixed abutment should be positioned at the low end to minimise any surface water leakage through the deck joints.
The DMRB document CD 358 "Waterproofing and Surfacing Concrete Bridge Decks"says that the longitudinal gradient on the deck should be a minimum of 1 in 100.
Road gullies should be positioned at the rear of the abutment at the high end of the deck to intercept any water from flowing onto the deck. Gully spacing should be determined using DMRB document HA 102 although every effort should be made to avoid putting gullies on the deck. It is crucial to be involved in the early planning stages of the road alignment to advise on the benefits of providing adequate longitudinal fall to avoid providing gullies on the bridge.