The McAllister Group’s UK Rehabilitation team was on site in leafy Hertfordshire on the 20th June tasked with relining two sewers which lie above the Hadley Wood Tunnel. The tunnel lies on the route from King’s Cross to Shaftholme Junction where trains can reach speeds up to 115m per hour at this point. The two sewers lie just above the railway tunnels – one was a 225mm clay Surface Water sewer which crosses over the Great Northern Railway (East Coast Mail Line) Hadley Wood tunnel which was built in 1850. In 1959 the railway was widened from two to four tracks with the addition of a second tunnel. There is a parallel 225mm clay foul sewer which also crosses the track. Both sewers are 5m in depth but do not pass under the railway. Their proximity to the tunnels are perhaps the cause of the number of breaks, fractures and deformations in the sewers which have occurred from ground movement. The sewers were probably constructed after the original 1850 tunnel, perhaps from 1885 when the area development began but certainly before the commencement of the 1958 second tunnel construction works.

CCTV surveying of the sewers highlighted heavy build-up of calcium scale from the joints. If untreated this calcite build-up can cause multiple problems including reducing the diameter of the sewer by as much as 90% with the formation of both stalactites and stalagmites. In both sewers, the calcite build-up had hardened sufficiently for our team to deploy the robotic cutter. Once removed our team was able to swiftly install 70 m and 90m of 225mm CIPP liner to rehabilitate both sewers. In just over a decade the McAllister Group UK operation has firmly cemented its reputation as an Industry specialist in the rehabilitation of sewer networks for major utility and transport clients.