CA Milne and SA Solera, Hyder Consulting. This paper was first published in GE’s December 1999 edition.
Horsfall Tunnel near Todmorden in Yorkshire is a 250m long railway tunnel through a hillside on the Manchester to Normanton line. It is believed to have been constructed between 1837 and 1840, the western 40m by cut and cover methods and the remainder by boring into rock.
The cut and cover section has been subject to movements over at least the last 60 years. Initial feasibility studies evaluated remedial options on the generally held assumption that movements were related to the steep slope north of the tunnel. Options evaluated included refurbishment of the tunnel and methods of slope stabilisation to allow its removal.
To determine the form of the scheme, a comprehensive ground investigation and slope and tunnel inspection identified the probable cause of movement as the reactivation of a slip failure surface at the interface between coarse slope deposits and highly weathered rock, assisted by the periodic collection of surface water behind the tunnel. The investigations also confirmed the assumed cut and cover construction methods of the original tunnel. Based on the results of a trial piling exercise, which confirmed the feasibility of installing piles outside tunnel possessions (ie without closures), the construction of a reticulated bored pile wall immediately behind the tunnel lining prior to demolition was the preferred solution to stabilise the slope. This was later successfully adopted to allow tunnel reconstruction.
The successful outcome of the feasibility studies was greatly assisted by the high quality of the investigations carried out, which provided realistic design parameters for the subsequent remedial works. The scheme has since been awarded the 1999 Institution of Civil Engineers Yorkshire Association award for excellence in the concept, design and execution of civil engineering works.