Stitch one mill one

It's been a busy summer for BAM Nuttall. Alongside its Southern Civils Framework colleagues from Network Rail, the company has been involved in high-profile remedial works in two tunnels on Kent's south coast.

Only four miles apart, the structures at Abbotscliffe and Dover Harbour both required maintenance to strengthen and repair sections of their lining. Working closely with the Southern Civils team and designers Donaldson Associates were BAM Ritchies, provider of specialist geotechnical and sprayed concrete skills.

Abbotscliffe milling
Trials were carried out in Abbotscliffe Tunnel in January this year, with the purpose of determining the milling capabilities of the equipment intended for use. Further development work then took place at BAM Ritchies' depot. The main works were successfully undertaken during a five-week blockade in July and August, involving around-the-clock operations and a workforce of more than 70 people.

Bored between 1840 and 1844 as part of Sir William Cubitt's main line from Redhill to Dover, the tunnel was driven through chalk immediately to the east of Folkestone Warren. The lining is six bricks thick throughout. Abbotscliffe is a substantial structure - 1 mile 182 yards long, 24 feet wide and 19 feet 6 inches high. Its alignment, close to both the cliff face and high tide level, allowed construction to take place both from conventional vertical shafts and 13 horizontal galleries cut into the cliff from a road which was dug along its base. These were used for spoil dispersal into the sea. A significant spring, Lyddon Spout, burst through at one point and had to be incorporated into the drainage system. When access, services and ventilation had been established, work started with the removal of 153m' of brickwork from the tunnel walls. This was done by both a bespoke milling tool - fabricated from a reduced boom - and a Webster wheel (road header) mounted on a road-rail vehicle and hydraulic breakers carried by Case 998 RRVs. Up to 5-metre wide panels were removed using a variable technique which allowed the spray concrete to gain sufficient strength as well as linking into existing sprayed concrete in the crown. These works used a site- batched dry mix which generated more working time and increased flexibility. The mix contained locally-sourced damp sand with high-tensile steel fibres. Water was added at the nozzle. In total, 177mm3' of spray material was used. This figure including some overspray works required in the crown.

Staff involved in BAM Nuttall's ongoing tunnel maintenance contract made best use of the blockade to complete a significant amount of additional repair work that would have not been possible within normal midweek night possessions.

Stitching in the Harbour
The 687-yard Dover Harbour Tunnel was constructed in 1863 to connect the stations at Dover Priory and Dover Marine. It was driven through chalk and lined with four rings of local yellow stock bricks. Annual examinations had identified deterioration in the condition of the lining; subsequent intrusive investigations found voiding behind the brickwork.

Donaldson Associates was appointed to develop remedial options and carry out the civil and structural engineering design elements of the project. It proposed the use of radial stitching, stitching and grouting, and the installation of a 150mm thick fibre- reinforced sprayed concrete lining using Bekaert Dramix Steel Fibres. The approach was deemed to improve the overall integrity and strength of the arch.

The works involved the installation of 720 stitching bars - each 0.7m long - in the crown and haunches. Holes for the bars were drilled using a Hilti drill and filled with Hilti HY-70 resin. Also fixed were 60 dowel bars, 350 L- bars and 80 Fibregrip rockbolts from Weldgrip Geotechnical - these were fitted using BAM Ritchies' purpose-built rig mounted on a rail trailer and anchored the concrete lining into the chalk rock mass behind the brickwork, preventing localised overstressing. Test Rockbolts were installed within the tunnel during an earlier phase so bond strengths could be determined. The analysis and design was carried out using a linear elastic finite element model in combination with in-house software developed by Donaldson Associates.

A subsequent phase of work involved the spraying of 35m3 of concrete in a layer 150mm deep, measuring 6.7m x 34m.The concrete mix was produced by a local BAM Ritchies batching plant near Dover and delivered by lorry in one-tonne bags. An RRV and concrete pump completed the concrete's journey to the works.

Andrew O'Donovan, BAM Ritchies' Project Manager, described this as "a key project, clearly demonstrating the benefits of teamwork with all BAM divisions working closely together to ensure that the full scope of the multi-discipline works were completed successfully within the limited time available.'


Article courtesy of The Rail Engineer - November 2009