Permeation grouting work has paved the way for structural stabilisation to start on a Lancashire rail tunnel which should see full rail services restored this spring
Long-term ground movements had led to distortion of a Victorian-era rail tunnel in Lancashire to the extent that Network Rail had been forced to cut line speeds from 64kph to 32kph through the 250m long tunnel. However, over the last few months grouting and anchoring work has been carried out to halt the ground movements that have affected the Holme Tunnel and stabilise it enough for structural support to be installed.
For the past three months, contractor Amco Rail has been injecting permeation grouting and installing ground anchors to stabilise a mix of mudstones and siltstones behind the Victorian tunnel’s masonry lining during nightly track possessions. But from mid-November, 16km of track between Burnley Manchester Road and Todmorden has been closed to allow the contractor and engineers from Network Rail’s LNW (London North West) North region to begin constructing a system of steel rib and sprayed concrete reinforcement.
This is the latest in a series of strengthening projects within Holme Tunnel, which passes through debris from a series of historic and prehistoric rotational landslips beneath the Thieveley Scout escarpment south of Burnley.
From the mid-1980s to the early 1990s, distortion of the tunnel led British Rail to install a reinforcement system of 152 steel ribs. Since then, a regime of movement monitoring has been keeping an eye on the structure’s integrity. The southern half of the tunnel remains unstable so Network Rail is investing £17M in what is expected to be a permanent reinforcement solution.
As soon as Amco Rail had full possession of the line, it began to construct the new strengthening system from a point 150m from the tunnel’s southern portal.
A 6m long transition section was the first to be installed, comprising an array of radial cross-stitching of the existing masonry lining with 600mm long 12mm steel bars, followed by a sprayed concrete over-lining.
The contractor will then build the main reinforcement solution working sequentially 12m at a time towards the southern portal. The track bed and existing tunnel invert structure will be broken out and temporary propping erected to allow the old steel ribs to be replaced by heavier 305 x 305 x 198 UC steel ribs.
Each rib will consist of five UC sections pre-bent to the curvature of the masonry lining and connected insitu with hinged brackets to form a steel frame approximately 9m in width and 10m in height.
“The steel rib spacing has been designed at 1m centres, but because of the vulnerability of the ground strata overlying the tunnel, we have included contingency options for decreasing the spacing if necessary. A bespoke movement monitoring system will be used to keep tabs on how the overall system is performing as it’s installed,” says Network Rail senior project engineer Andy Dunn.
The monitoring has been designed by Datum Monitoring to a red, amber, green traffic light system whereby green levels of movement represent the all clear and red recordings will signal evacuation.
A new reinforced fibre sprayed concrete lining will be applied between and covering the steel ribs and the tunnel invert reconstructed from 500mm thick 4m x 1m precast concrete slabs, which will also form a new drainage channel beneath the track bed.
“The sequence will then be repeated nine or 10 times until we reach the southern portal,” says Network Rail LNW North infrastructure projects manager Stephen Townley. “We are also looking at installing a new fibre optic movement measurement system for future monitoring of the tunnel.”
Network Rail is planning to have the Holme Tunnel project completed in time for rail services to resume on 24 March 2014. When Amco Rail’s work is completed the 32kph speed restriction through Holme Tunnel will be lifted to match the 64kph limit applied to the rest of the line