DUMMY SPAN SOLVES M55 HEADACHE

The author examines the problems which arose in the construction of the M55 motorway due to variable ground conditions between Blackpool and Preston. Layers of peat and running sand had to be shifted and replaced by rockfill up to 6M thick. At the Lancaster canal a sloping peat layer led to the use of a dummy span to prevent the canal from slipping which was inserted at a safe distance from the canal. The central pier, 56 steel-cased 450mm diameter piles were sunk 14-15M through the peat to bear on the stiff clay below. Each of the 22.7M spans consists of 20 precast prestressed box beams with reinforced concrete infill. Details are also given of the drainage layer, which consists of old stone sets and hardcore taken from a nearby wartime aerodrome. 6.5M further east, the M55 joins the northern end of the Preston bypass, which forms a spur for the M6 to the A6. The spur is being improved to take extra traffic, and a new fly-over carries the motorway over the A6 roundabout. Costs are quoted. /TRRL/