Strengthening secures tunnels’ future

Ore Tunnel remedial works
BAM Ritchies working with BAM Nuttall carried out remedial stabilisation in Ore Tunnel, East Sussex to address lining deformation and water ingress problems. Where bulging of the side wall had occurred, affecting the train kinematic clearance envelope, the solution consisted of 6.0m long glass-fibre anchors. Drilling was completed by a road-rail excavator- mounted drill mast. Grout-socks were used to limit grout loss into the voids behind the tunnel and open rock joints.

The ground conditions surrounding the tunnel consist of weakly cemented sandstone with bands of silt, as well as mudstone with persistent, strong ironstone bands. Owing to access restrictions, the ground investigation information was limited so the root cause of the tunnel's problems was difficult to diagnose. BAM Ritchies worked collaboratively with BAM Nuttall, White Young Green and Network Rail to develop the design as information became available through the project duration.

To deal with water ingress at various locations, holes were drilled to 5m depth and 112mm perforated pipes installed with geotextile socks to conduct water to pipework leading to an existing drain. Sizeable voiding between the tunnel lining and the surrounding ground - which occurred at various locations - posed a risk to the lining as sandstone blocks could fall and cause damage. The solution was to drill holes and inject a polymer void-filling grout which also sealed the tunnel, reducing water ingress.

Folkestone tunnels
A series of stabilisation measures were carried out by BAM Ritchies at Abbotscliffe and Shakespeare tunnels, forming part of major ongoing maintenance on the 160 year-old Folkestone tunnels. Abbotscliffe is a 1,800 metre long twin-track tunnel. With increasing frequency, the locally famous Lyddon Spout erupts, suddenly releasing a large build-up of water from within the surrounding chalk. An adit - used to drain the groundwater that enters the tunnel, transferring it out to the sea - was showing signs of distress as a result of rock fall failures. Stabilisation was required to ensure the safety of the maintenance crew working on the drainage channel so 108 resin-fixed glass- fibre rock bolts were installed and tested. Rock fall mesh was placed along the bolted section and a tell-tale monitoring to record future movements was set up.

Shakespeare is a twin-bore tunnel, approximately 1,200 metres long, which exhibit a Gothic influence as both bores consist of high arches. During previous works, when tie bars were installed through the pier between the two tunnels, a series of voids were found. Network Rail required these voids to be filled to ensure the tie-bar scheme would perform as designed.

100 holes were drilled from one bore towards the other and perforated pipes installed. Neat cement grout was then injected through these to fill the voids.

Rodney Street tunnel strengthening
In Edinburgh, BAM Ritchies Concrete Techniques section worked on strengthening an old railway tunnel as part of the Sustrans National Cycle Network.

Sustrans is constantly reviewing the status of the National Cycle Network and is working with local authorities and other partners to bring all sections of the network up to standard. The Rodney Street project at Canonmills was part of this programme.

The tunnel was originally built to carry trains to the Granton Ferry before the Forth Rail Bridge was constructed. The rail line was closed in 1968 and the area beneath the tunnel was partially infilled. As part of the development of the National Cycle Network, Sustrans strengthened and reopened the tunnel to make it safe for use by cyclists.

The future aspiration, in partnership with the council, is to use the tunnel as part of Route 75, enabling cyclists and pedestrians to avoid the busy road junction that sits above it.

Prior to any works being undertaken, traffic management reduced live traffic above the section of tunnel to be lined to a single carriageway as cover to the tunnel crown was only 300mm. It was necessary to survey the road levels to monitor any potential road settlement.

The strengthening works were undertaken in two phases to coincide with the traffic management regime. Each phase consisted of daily monitoring of road levels, excavation of backfill, application of initial steel fibre reinforced sprayed concrete, fixing of two layers of steel reinforcement and application of 400mm sprayed concrete. A final work layer of polymer fibre-reinforced sprayed concrete was applied to the whole area to provide fire resistance.

Elsenham footbridge
In 2005, two children were killed crossing the railway at a level crossing at Elsenham Station. As a result of this tragedy, a footbridge was designed to take the public safely over the line.

BAM Ritchies installed 34 mini piles - each 7m deep and with a nominal diameter of 350mm - using auger drilling and casings to support the new bridge. The piles were designed to take the weight of the constructed footbridge and also withstand a collision load if a derailed train hit the structure.

Piles were reinforced with a full- length reinforcement cage in three short coupled sections owing to adjacent overhead power cables and then grouted with a neat cement grout. Although the piles were close to the track, work was completed without disruption to traffic or inconvenience to the public.


Article courtesy of The Rail Engineer - May 2009