Harbury Tunnel Portals

Harbury tunnel portals in Leamington Spa
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In the spring of 2015, the UK suffered some of the highest levels of rainfall seen on these shores in a number of years. Whilst unpleasant for the average person, levels of rainfall such as this have a far more detrimental effect on the railway network. The railway network experienced flooding and instability of railway cuttings and embankments in various regions as a result of the extreme weather.

One such case of instability arose at the Harbury tunnel portals in Leamington Spa; where, in May 2015, more than 350,000 tonnes of earth slid from the 150m stretch of cutting. This resulted in the line being closed between Leamington Spa and Banbury for a period of six weeks, causing disruption for thousands of passengers making the journey between Oxford and Birmingham.

Network Rail emergency earthworks contractor, J Murphy and Sons were appointed as the principle contractor to firstly rectify the problem and subsequently oversee a more permanent solution. JMS oversaw the stabilisation of the embankment, the installation of an improved drainage system and various other civil engineering solutions to restore the track to an operational state.

Murphy Group appointed Force Contracting Services Ltd. as a specialist Geotechnical and Rope Access contractor to aid in the stabilisation of the cutting.

SOIL STABILISATION

Under the guidance of JMS, and FCS project manager Jim Broe, Force Contracting installed 300 R32 soil nails above the tunnel entrance and around the portals. Due to the challenging environment, access if extremely difficult. Rope access techniques were utilised reach the work face, position the drilling equipment and complete the task of installing R32 hollow bar soil nails into the cutting.

FCS mobilised its specialist team of roped access personnel to complete the project. Rope access operatives abseiled down the slope, winched the drilling equipment into place and began to drill100mm diameter cores, to depths of approximately 6m, into the unstable cutting.

32mm diameter hollow reinforcement bars were then inserted into the freshly cored bore holes. The anchors are then secured in place by pumping thixotropic grout, fast setting grout into the voids before plating the ends. The installation of the bars helps to provide an anchoring system to ensure the ground remains stable even in harsh weather conditions.

The installation is then completed by the application of a geotextile wire meshing system to the entire surface area, which is fitted to the anchors. This acts as a safety net to trap any falling debris in the event that the anchors should fail.

In order to help prevent any re-occurrence of the land slip, FCS installed a new ACO drainage channel around the top of the tunnel portals to compliment the larger system installed by JMS, and to collect any surface water running down the embankment.

To complement this work, Force has constructed a new drainage channel around the top of the tunnel portals to capture water flowing off the embankments. FCS also installed a set of Kwik-Step access steps to enable future access to the top of the tunnel portal from the top of the embankment. This was completed with Kee-Klamp handrail, which has been installed around the profile of the portal to ensure safe access for future inspections and remedial works.

TUNNEL PORTAL REPAIRS

The large amount of earth caused significant damage to the tunnel portals and wingwalls themselves during the land slip, damage which FCS helped to rectify.

Shotcreting techniques were used to treat the affected areas and to restore stability. Finally the damaged and weathered brickwork around the entrance to the tunnel was replaced to restore the tunnel to full working order.

Force Contracting Services continue to work very closely with J Murphy & Son on several key projects to help deliver safety and efficiency on UK railways.

PROJECT DETAILS

Client: Network Rail
Location: Leamington Spa
Year Completed: May 2015

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