Passive groundwater control system for a motorway

Considerable public discussion was generated during the planning stages of a rural motorway in Ireland regarding the possible effects of the road drainage system on an ecologically important fen, which was about 5 km distant. To overcome this potential problem, a passive groundwater level control system was devised to limit the drawdown by maintaining the water levels 1·75 m above the road surface. This groundwater level corresponded approximately to the seasonal low water levels recorded in the aquifer prior to the construction of the motorway. This is achieved by constructing the motorway within a sealed cross-section with a permeable layer beneath this section, called the underdrainage system, which is connected to drains that outflow at the higher level. This underdrainage system controls the water pressures and flows beneath the sealed section. This paper presents the concept of the passive groundwater control system in a non-homogeneous glacial aquifer, and analyses the effects of this control system on the groundwater around the motorway. The selection of the design criteria and site control methods for the material within the sealed section is also discussed. The results of site monitoring that show the successful performance of the system in operation are also given.