Inky black darkness swallows up the headlights of Paul David's car. The May Gurney Rail construction manager is driving along a disused railway tunnel, which cuts through the Pennines at Standege in Yorkshire.
The 5km long passage has been out of action for over 50 years but has been pressed back into service by Network Rail and contractor May Gurney in order to gain access to the parallel, live Standege railway tunnel. This twin track tunnel carries trains between Leeds and Manchester Airport, as well as services to villages on the Lancashire/Yorkshire border. It was recently discovered to be distorting at two points and a rescue plan is now underway.
The ability to get plant, materials and manpower to the rail tunnel without creating a major disruption to train movements has proved invaluable.
'It is a bit creepy down here, ' comments David, stopping his car at a cross passage. The still, cold air is suddenly whipped to a gale by a passing passenger train.
'The tunnel movement has been off and on for a few years, ' explains Network Rail project manager Bob Lowery. 'There are two geological faults running through the rock, and the strata are moving differently in a couple of locations. We installed monitoring devices in 1998 and since then we have seen the horizontal tunnel clearance reduce by 15mm, ' he says.
In brief, the tunnel is undergoing a slow progressive collapse. Its horseshoe shaped walls are bulging inwards, lifting the crown and pinching the invert inwards at two points. These mobile sections are each about 30m long and lie 400m apart. If the deformation is not stopped it is feared the gauge could ultimately alter so drastically that trains would be unable to use the tunnel.
To arrest movement, Network Rail decided on a three phase recovery plan which began in 2001. A new 30m long piled foundation beam, was installed on either side of the tunnel to act as a restraint at the toe of the wall. The C23/40 beam was supported by a row of 50, 219mm diameter steel cased piles, placed at 600mm centres.
Had this prevented further movement the work would have been complete. 'Unfortunately, monitoring showed that the ground beams alone were not adequate to stabilise the tunnel completely, ' says David.
So in late 2004 Network Rail framework contractor May Gurney began work on phase two. 'This involves placing reinforced concrete slabs to act as a prop between the piles, ' says May Gurney project manger Andries Liebenberg (see diagram).
Due to the limited working time during a possession, May Gurney has opted to minimise the use of insitu concrete, instead using 2.8m wide, 2.1m long prefabricated reinforced C32/40 concrete slabs, transported into the tunnel by rail-mounted carriers. Separate slabs are placed under each set of rails and connected to a trough running on the tunnel's centre line.
'Initially a precast trough was installed in the centre of the tunnel between the two tracks, ' says David. 'The concrete slabs were then installed one line at a time, ' he says.
A major challenge for the team has been excavating ground below the tracks the 1.5m necessary to place the slabs.
Such major intervention initially looked as if it might cause the tunnel to collapse.
'While there is no structural invert to the tunnel, it is possible that the passive pressure provided by the earth between the tunnel walls is contributing to its stability, ' David explains.
'Consequently, a temporary propping system was designed to maintain the integrity of the tunnel during installation.' 'The excavation and propping proceeded in front of the construction of the new invert, ' says Liebenberg. So at no time would the open section be unsupported.
The only piece of insitu concrete construction is the connecting reinforced concrete stitch between the sub-track slabs and the centre trough. Lenton rebar couplers were chosen to minimise the complexity of the joint detail and to speed up installation, notes David.
'Once the insitu joint is poured and the initial set has taken place, we lay steel plates over it to decrease the waiting time required to replace the track and reopen the line, ' says Liebenberg.
So far only one of the two unstable tunnel sections has undergone this propping operation and the other section is still being monitored. 'The section at tab 262 [3km in] was moving more than the other, ' says Lowery. But it looks likely that the same operation will be carried out at the second location next year.
'We have it pencilled in to the 06/07 programme, ' he says.
The most expensive and time consuming operation has been saved as a last resort. If the tunnels are still moving after the invert propping is completed, Network Rail will install an entire new tunnel lining, consisting of a steel lattice arch, which will be fully connected to the new ground beams and invert slab. 'We are hopeful that it wont come to that, ' says Lowery.