THE DUBLIN PORT TUNNEL - WATER IN THE HOLE ?

The Dublin Port Tunnel on the Irish East Coast allows an opportunity to study the hydrological properties of the Dublin Limestone, classified as a Carboniferous (Chadian - Asbian) turbiditic limestone, as well as the overburden tills, sands, and gravels. Little has been published on the Dublin Limestone (Lucan Formation) also known as the Calp. It is interbedded with shale, clay rich, rare in outcrop and has structural complications. A 3D numerical groundwater flow model was constructed using site investigation data and MODFLOW 2000 with MODFLOW GUI interface. It is the first groundwater model in the Dublin Region. Limestone hydraulic conductivity (K) was the dominant variable within the model with assigned values from 3.6x10^-6 to 6.95x10^-7 ms^-1, which are within the range of observations. Glacial till units acted as confining or semi-confining beds creating nearly artesian heads within the underlying limestone. The uppermost glaciomarine/alluvial silts, sands, and gravels are restricted to the south of the model and act as a drain to the Irish Sea. A steady state solution was correlated to within 0.25 to 2.5m of observed head in 14 limestone test holes. A transient model was then constructed with a stress equivalent to the tunnel works applied. The 5 km, 11.7m diameter tunnel is to be a twin bored highway tunnel. The tunnel boring machine (TBM) will stop each night for 8 hours and will provide a drain up to 457m². The MODFLOW drain package was used, and a drain cell with a surface area of 457m² was allowed to accept water for an 8-hour period. Predicted flows ranged from 2.5 - 10 m³/hr. Drawdown in the uppermost, saturated drift was in the range of 6mm. The dominant controlling variable in water generation is the hydraulic conductivity of the Dublin Limestone with reduced flows in areas of lower K values, though connectivity to more transmissive units and head difference between the drain and limestone also played an important part. Local discontinuities and more importantly, their connectivity may generate more water than forecast. As the tunnel progresses the model can be validated and further refined to improve accuracy which will benefit the later return of the TBM and future tunneling and groundwater development projects within the Dublin Region.