National Grid continues to build the gas pipe to improve services throughout the UK. The new pipeline has been completed linking Asselby to Aberford in South Yorkshire using open cut pipelaying and trenchless methods. All were completed with minimal disruption to everyday life and more importantly, the environment in this rural setting.

The main contractor Laing O’Rourke brought in a specialist subcontractor, F & B Ltd of Doncaster, to provide rail, canal and road crossings in the Selby area. A total of 500 metres of the 1,500 mm diameter pipe crossings were completed between May and September 2008.

Bore holes along the intended route of crossings revealed soil conditions consisting of very stiff clay, sand and a water table within two metres of the street level. This left no option but to use microtunnelling as the method to install the carrier pipes under the rail and road network that crisscross the area. In mid-January 2008, F & B contracted Wirth to provide a new RVM1500 machine for the project. F & B took delivery of the new cutting head during early May 2008.

The crossings were under rail tracks and busy roads around the A63 Selby on Leeds trunk road. Therefore a very efficient traffic management scheme was introduced at each crossing to minimise disruption to local traffic, whilst heavy equipment moved supplies around the different sites. Due to the rural situation and close vicinity of the Selby A63 bypass, traffic management was crucial to allow everyday life to carry on.

Overcoming obstacles underground

In May 2008, Wirth delivered a new concept microtunnelling system to F & B. Working together onsite, the project was delivered on time and within target. To ensure a timely completion, staff manned each crossing for 24 hours per day. Production peaked at 20 metres per twelve hour shift which, considering the soil condition was exceptional, proving the efficiency of the new slurry handling system designed by Wirth.

To deal with the heavy clay and consequently very dense slurry, F & B constructed two large lagoons: one lagoon for settling out the fines decanted from the slurry tank and another to store water taken from the dewatering system around the shaft area. Water supply was never going to be a problem as the saturated soils provided a free and plentiful supply. However, halfway through the project, nature and civil engineering clashed in the form of a swallow that built a nest in a storage container on the job site! Adhering to strict rules about the environment, the container was locked up with a warning that the nesting birds must not be disturbed.

The shaft for the main rail crossing had to be at a much deeper level than the expected six metres. This was due to the discovery of an area of disturbed soil caused by excavation works some years earlier. The shaft team excavated to 13 metres before stable soil was found in an area of very stiff clay. The disturbed area also meant that the distance would be 92 metres instead of 60 metres, as F & B had to keep clear of the problem soil area.

The carrier or casing pipes ordered for the microtunnelling crossings were from F P McCann of Alnwick, Northumberland. They were standard DN 1,500 jacking pipes with an outer diameter of 1,780 mm. Every sixth pipe installed was a bentonite pipe with three ports to lubricate the line as it advanced.

For the trenchless sections of the job, special fabrications of pipe sections into elbows, allowed the 48 inch product pipe for the gas line interface between the shallow open cut sections and the deeper microtunnelling shafts.

The rail track is very sensitive to any possible subsidence that may occur during operations under the main line between Leeds and Selby, a main route for coal and steel freight trains and the main passenger line linking two major towns. Special monitoring systems were installed on the tracks to indicate any movement on the rails caused by movement or subsidence from tunnelling operations. As a precautionary measure, Network Rail introduced an adjustment to the speed of passing trains. The train speed was reduced over the zone of interference until the microtunnelling machine had clearly passed under the active rail track. In all cases no major disturbances were recorded.

Wirth and the microtunnelling market

Wirth said that currently new machines are under construction for old and new markets. The modular design approach adopted by Wirth has resulted in very fast response times to both enquiries and deliveries. Using common engineering across the range of machines means long lead items can be ordered in advance, saving time and costs. Standard installed power is 200–250 kilowatts making this type of machine the most powerful available in this range.

A state-of-the-art laser guidance system provided by Centreline of Germany ensured that the tunnel was driven within the high accuracy laid down in the specification. The flexibility offered allows the contractor to tailor-make the system to suit different project requirements, long distance pipes and curved pipe jacking.

The slurry system was highly effective in the heavy clay soil conditions. This was due to the unique distribution pipe work installed in the head which kept the crushing chamber free from the build up of clay normally experienced in these soils. As a testament to how well the system kept the cutter head clear, when the machine emerged from the 92 metre crossing, the original paint was still on the cutting tools.