A465 Widening: Nant y Bwch Underpass


Carillion Civil Engineering





Artesian conditions coupled with flooded mine workings and complex geology had created a flooding risk during construction of a new underpass on the A465 Heads of the Valleys road near Tredegar. A permanent passive dewatering scheme was designed by Arup comprising 48 water wells in a highly fractured quartzitic sandstone aquifer.

Our solution involved:

  • Drilling: 48 boreholes to 20m depth comprising 273mm diameter casing and 200mm open hole in heavily fracture highly abrasive quartzitic sandstone.
  • Well Installations: 168mm diameter drinking water quality well screen with 5mm washed sand filter pack.
  • Grouting: Cement grout sealing upper levels to prevent water seepage beneath the carriageway.