Foundation geology of the River Dee estuary cable-stayed bridge, Flintshire, North Wales

A new highway crossing of the River Dee incorporates a major cable-stayed bridge with a single A-frame tower and a main span of 200 m. Support for the bridge is provided by barrettes that pass through 30 m of superficial Quaternary formations into bedrock strata of the Carboniferous Bettisfield Formation. Lithology of the bedrock stratigraphic sequence varies greatly over short distances and required a flexible response for foundation design. Below the north tower leg, barrettes were socketed some 7 m into massive sandstone that provided relatively good foundation bearing conditions. However, only 50 m away, below the south tower leg, each barrette required individual design to accommodate much weaker and diverse lithologies including siltstone, mudstone, coal, seatearth and disturbed beds. Here, certain barrettes extend as much as 30 m into the mixed coal measures strata. This case history illustrates the need to appreciate the localised nature of the ground conditions when carrying out a site investigation and designing foundations in areas of complex sedimentary geology.