THE PROBLEM

The Deeside Road Link/Rail Crossing is Phase 1 of the proposed A548 Shotton - Connah’s Quay Bypass in North Wales. The subgrade consists generally of alluvial deposits of silt and sand of the Dee Estuary. However, the alignment of the Link road meant a section of the works passed the site of the former Shotton steelworks.

One area of the steelworks site had been used for the disposal of rubble and masonry which had arisen from the demolition works. This area presented an obstacle to the construction of the proposed scheme. The demolition had left areas of reinforced concrete foundations and basements infilled randomly and uncompacted which meant that there was a high probability of voids being present in the materials.

THE SOLUTION

The alignment of the carriageway could not be altered to avoid this area therefore the Engineer was faced with the option of either removal of the problem material or the formation of a raft to carry the construction. The option to remove and dispose of this material was found to be economically unattractive and complicated. The decision was therefore made to form a reinforced granular raft over lose, irregular materials.

PROJECT DESCRIPTION

Tensar SS grids have been developed for use over weak subgrades where it is possible to reinforce unbound granular materials and reduce layer thickness. However, in this case the geogrid reinforcement was employed in a multi-layer construction to create a flexurally stiff raft over a length of variable sub-grades and to cater for local loss of support by the raft’s ability to span voids.