Design of Drainage Systems

The construction of new roads can cause impacts on the water resources of affected regions, causing irreversible effects in some cases.

Surface and subterranean water resources are finite and irreplaceable natural re­sources for survival, therefore their protection against abnormal flow and against pollution is of great importance, nowadays making their preservation an indispens­able part of a sustainable development policy. For this reason it is fundamental that a drainage system be developed that regulates the flow of effluents from the pavement platform, that controls the subterranean drainage and that minimises the hydrological impacts of the road on the environment.

An example of pavement sealing was seen in Slovenia for highways crossing very highly sensitive aquifers. There, the following requirements are used (Ajdic et al. 1999).

• In the asphalt layer the permeability can be controlled via the air void ratio (see Chapter 5). The wearing course of the asphalt layer should include not more than 5% air voids and the base course not more than 7% air voids.

• A stress absorption membrane should be constructed using a polymer modified bitumen in a layer of 1.5-2.0kg/m2 and appropriate fine aggregate. Junction sealing should be performed with bituminous tape.

An example of the use of a lining system beneath an embankment is Highway A-15 at Botlek in part of Europoort, Rotterdam (in the Netherlands). Approxi­mately 400 000 t of (municipal solid waste) incinerator bottom ash was used in an embankment for this major roadway construction. The ash was covered with a compacted sand-bentonite mixture with a minimum thickness of 20 cm to reduce water infiltration. The formation (founding level) of the embankment was shaped so as to bring any water seeping through the ash to a sampling point at which quan­tity and quality of seeping water could be monitored. The aim of the cover and lining systems was to prevent the contamination of underlying clean groundwater by infiltrating water that would have passed through the ash embankment material, potentially collecting undesirable contaminants on the way. In fact, due to the heavy industrial use of the land in Europoort over many years prior to the embankment’s construction, the natural groundwater is degraded at a regional scale, so use of the ash posed few risks of causing unacceptable contamination (Mank et al. 1992).

At another site in the Netherlands, a wind barrier was built at Caland (Stoelhorst, 1991). This project, built in 1985, used more than 650 0001 of bottom ash in an em­bankment 700 m long and 15 m high. The ashes were covered with a primary cover of 0.5 m of compacted clay with a sand drainage layer (0.5 m thick) and top soil (1m thick) overlaying the clay layer. The slope of the compacted ash was between 40% and 50%. As at the Botlek site, groundwater quality is monitored, in this case on both sides of the embankment.