Pipe jack option for difficult under-track crossing in West Sussex
Perco ESL recently undertook a difficult and substantial under-track crossing (UTX) at the Three Bridges railway station site near Crawley for client Network Rail. The existing railway corridor operation at Three Bridges runs north south and carries the Victoria to Brighton main lines which form part of the major commuter network serving the Capital from the Sussex area, so services could not be interrupted for the crossing installation works.
Having been approached to undertake the UTX works and having fully investigated both the requirements of the client and establishing the site and potential operational constraints of such an installation, Perco ESL offered two construction options and techniques to complete the installation.
The first option was that of Guided Auger Boring. However, after working with the client and taking into account the various risk factors associated with the site and the crossing route, this option was rejected. This was mainly due to historical evidence that previous Auger Boring attempts in similar circumstances had encountered unexpected buried obstructions that brought the advance to a stand-still.
The second option was the utilisation of a 1,200 mm diameter hand-excavated pipe jack installation. Close investigation of this option showed that the technique was more viable in that it had the greater capacity to cope with access to and removal of any unexpected obstructions. It also offered the potential to install extra ducts within the 1,200 mm diameter pipe at a later stage should they be required without the need for a new crossing installation.
The plan for the UTX was to install a 1,200 mm diameter pipe over a length of 55 m beneath the operating rail lines at the Three Bridges site. The operation would normally require a 3 m diameter reception shaft on the arrival side of the pipe jack run to recover the pipe jack shield used to excavate the ground ahead of the advancing pipe.
However, in this instance, due to site restrictions and the location of existing services and utilities in the ground Perco could not excavate this size of reception shaft for the recovery operation. The site would allow only a 1,800 mm diameter reception shaft to be accommodated. To overcome this problem Perco opted to make the steel pipe jacking shield sacrificial and not recover it at the end of the drive. To achieve this, the pipe jack was continued past the location of the initially proposed recovery shaft and the 1,200 mm diameter steel shield was simply buried in the adjacent ground and enclosed using a sealing grout.
Perco ESL was awarded the works in May 2014 with a start date of 16 June 2014. However, the discovery of contaminated ground on the proposed route forced an early delay whilst analysis of the contaminant was established and the correct personal protective equipment was procured. However, once fully underway Perco completed the tunnelled UTX in just 12 days.
The launch shaft for the pipe jacking operation, sited to the northeast of the rail tracks, was a 6.5 m deep, 6.5 m x 3.5 m rectangular, steel sheet piled and internally supported construction. However, even here the construction of the shaft was tight. There was limited space to excavate steel sheeted shaft, as the only working area that could be utilised was a small P-Way compound with tight constraints.
The route of the 1,200 mm diameter UTX ran under both Fast and Slow main rail lines at Three Bridges as well as two sidings lines. Monitoring of the tracks during the excavation was carried out by TPS, with automatic readings being taken every three hours on a system of 228 points that monitored any vertical, horizontal and twist movements on each track. Targeted warnings were set to advise the construction crew of any movements at various set parameters.
During the excavation process the advancing pipe jack shield was kept on alignment using laser guidance with the jacking shield and the jacking pipes being advanced using a jacking frame already owned by Perco.
The jacking pipes used for the pipeline installation were supplied by FP McCann. The pipes had a jacking force capacity of 144 kN/m. In the event, however, jacking forces on the jacking pipes did not exceed 60 kN/m at any point during the advance.
On completion of the main tunnelling work a central access chamber was installed to a depth of 4 m on to the 1,200 mm diameter pipe jacked tunnel. The 1,800 mm diameter shaft was constructed as an access shaft to aid installation of the 12 ducts that were ultimately to be installed inside the UTX pipeline. This shaft was completed as a vertical shaft of 1,800 mm diameter using bolted ring segments.
A second chamber on the Western side, in the area of the original reception shaft site, was designed as a timber frame construction due to limited access and existing utilities as discussed earlier. This was also to allow access for the 12 service ducts to be installed.
With tunnelling completed Perco then went on to install the 12 x 150 mm diameter ducts in the tunnelled pipeline. These ducts were then placed in holding frames which were bespoke manufactured by Perco ESL for this scheme. Once this installation was completed the remaining annulus was grouted to secure the ducts in place.
Ultimately the project was completed in the targeted period and handed over to Network Rail on 1 September 2014.
For Network Rail Graham Davenport, Engineering Manager said: “Throughout the operation the crew Perco proved to be experienced and capable of overcoming any problems with tenacity, and with the Client’s targeted date in mind when adapting the programme.”