Carriageway Drainage

In order to intercept rain water away from the final road surface, a system of pipes and chambers is installed beneath the carriageway margins. The pipes usually range from 150mm plastic poly pipes to 900mm pre-cast concrete pipes. Carriageway drainage is an important feature of a road because of the need to channel water away from the traffic flow as quickly and efficiently as possible.

Before any drainage works takes place data sheets are prepared by the site engineers and located at the position of each chamber providing the information required to implement the design.

Excavation of the main drainage trench is generally carried out by a 20 tonne, 360º tracked excavator with either a ½ or 1 tonne bucket attachment. As each 3 metre section of trench is dug, a support box is lowered into the sides of the trench. This is particularly important in wet ground conditions and provides a stable working area for the site team. Once the box is in place, the excavation of the trench can be completed inside the support until the required depth is reached. The bottom of the trench is lined with an average of 110mm layer of single size gravel which forms the bedding for the pipes.

The next stage involves lowering the pipe sections into place and joining them by pushing together the ‘male’ and ‘female’ end joints. A gasket around the pipe end seals the joint. At intervals of between 75 to 100 meters the trench is widened to accommodate concrete manhole chambers that will regulate drainage flow and provide important access for future maintenance. The pipes are joined to the sump by cutting a hole through the rings and sealing it with grout to ensure the joint is watertight and airtight.

Steps are fixed inside the manhole chamber and are orientated so that an operative emerging from the manhole will be facing oncoming traffic and will be able to see and be seen. The trench can now be backfilled around the chambers and above the main pipes and the area compacted. A final test is made to the drainage system by filling the pipes with air and measuring any drop in pressure over a five minute period, which should amount to no more than 5%loss of pressure.

The drainage installation is now complete and the final carriageway earthworks and pavement construction can progress.