A2/A282 DARTFORD IMPROVEMENT - RETAINING WALL

The Costain Group was awarded £138 million by the UK’s Highways Agency for the widening works to add extra lanes to the M25 and A2 near Dartford, Kent.

The new lane development saw improvements made to the M25 between Junctions 1b and 3 near Dartford, and the A2 between Bean in Kent and the M25 (M25/A282 junction 2).
The £138 million contract included the widening of the M25 between junctions 1B and 3, south of the Dartford Crossing, from three to four lanes, a total distance of four miles. The £66 million project was the first from a number of proposed schemes to widen the remaining three lane sections of the M25.

The A2/A282 Dartford Improvement saw the A2 widened from three to four lanes between Bean and its junction with the M25/A282, a distance of 1.5 miles. The £72 million scheme included a link road from the M25/A282 in the north to the A2 in the east.

FACTS

Amount of material: 2050m3

Interesting Fact: This project required mass full to a rear bridge abutment and approach embankment over sub soils.

Delivery Method: Walking Floor

Main Contractor: Costain Group

Overview of Project

The specification of LECA LWA was required as a backfill to the east abutment of Structure S01 in order to reduce loads imposed on the bridge abutment. The use of LECA LWA reduced loads acting on the pile cap and differential settlements between the bridge and the backfill to the abutment. The bridge abutment is to be founded on Upper Chalk. The groundwater at the location of the abutment is 10 m AOD, or about 14 m below the base of the abutment.

The use of LECA LWA provided a minimising differential settlement between piled bridge abutments and the backfill to the abutment. The reduced earth pressures also allowed for substantial savings in the design for the retaining walls and bridge abutments.

LECA LWA offers a number of advantages over natural fill materials as a structural backfill in bridge abutments and retaining walls. It is inert, durable and easy to place and compact. The very low density of LECA LWA compared to natural fill materials also led to greatly reduced earth pressures and settlements, both in the short and long term.