One of the biggest challenges facing Stephenson, apart from unruly landowners and farmers opposed to the railway construction, was the infamous Chat Moss, described by the writer Samuel Smiles as “an immense bog of about twelve square miles, a mass of spongy vegetable pulp”. Stephenson armed his work force with planks strapped to their feet to stop them sinking into the Moss – the closest thing to PPE for the time.
Over its five mile stretch, the peat can reach up to six metres deep and was a formidable obstacle for Stephenson. After trying to backfill the area for several months to provide a stable base, preference was turned to ‘floating’ the railway across the area using heather bundles, brushwood mattresses and timber hurdles, each about 2.5 metres long and 1.2 metres wide, placed in layers to form a raft. This proved to be successful, and victory went to the famous engineer.
Stephenson’s technique, however, posed a significant challenge to today’s engineers as the timber layer installed by Stephenson was not to be damaged during construction. Careful survey work was carried out to position foundations away from the areas where the raft existed. This, however, resulted in foundations being placed anything up to eight metres from the running edge.
The Chat Moss stretch of the line required two different solutions for the electrification structure foundations: deep tubular steel piles of up to 14 metres in depth, and a mini-pile arrangement for areas where the presence of underlying rock made it impossible to drive the steel piles. There are approximately 120 mini-pile arrangements and 200 deep steel piles throughout the five mile stretch of railway across the moss, each with its own unique topographical and sub-strata issues, making this one of the railway’s most challenging geotechnical hurdles.
Deep steel piles, made from two halves connected together by high strength bolts, were first vibrated then hammered into position using rail-mounted plant. This was a very quick and economical solution and a number of piles could be installed during a single night-time possession.
The mini-pile solution was used when rock was identified from ground investigation boreholes or when the distance from running edge was beyond five metres (this being the maximum physical distance a deep steel pile could be installed using the on-track plant employed on this project). The mini-pile system consists of four or six concrete piles topped off with a concrete slab to which the steel mast is connected. The piles are generally anchored into the rock and provide a push/pull support arrangement for stability.