CHESTER BRIDGE GAS MAIN
As a part of the National Grid’s long term investment programme of gas mains replacement to increase the capacity of the local gas supply network, 9000m of new medium pressure gas mains were being laid in Chester in the north west of England.
This included a 200m section of gas mains requiring replacement across the main span of the historic Victorian Grosvenor bridge over the River Dee, which forms a critical point on the highway network south of Chester. Tasked with undertaking the works, May Gurney engaged Radius Subterra to assist in maximising the gas flow capacity of the section whilst minimisin disruption to the road.
Opened by a 13 year old Princess Victoria in 1832, five year before she became Queen, the bridge is of historical and architectural importance to the city and thus any excavation that could damage the bridge had to be kept to a minimum.
The bridge also forms the main route in and out of the city centre and, positioned close to several key landmarks and venues, traffic culminates in the area, which makes long term works to the section infeasible due to the impact to surrounding travel networks. Furthermore, at 60 feet high, access to the site was restricted.
Replacement of the mains was rejected as it could potentially damage the bridge architecture and would cause unacceptable disruption to the road and surrounding commuters.
Whilst the use of direction directional drilling to cross the river adjacent to the bridge, was also discounted due to the high costs and elevations of thebridge and surrounding streets, which would have required an extended drilling length much greater than the existing bridge span to accommodate the angles needed.
As an existing gas main was buried within the shallow bridge deck, to avoid having to open cut the whole section across the bridge, May Gurney engaged Radius Subterra to implement their trenchless Subline DR close fit polyethylene lining system to minimise disruption and maximise the existing capacity of the old gas pipe.
Two pits were dug on either side of the bridge structure in preparation to access the existing main. A length of 200m polyethylene pipe was inserted into the Subline DR machine where rollers concentrically reduced the 12” diameter of the pipe by approximately 10%.
The pipe was then rapidly inserted into the old gas main bore in just one day, using traditional sliplining techniques, in one continuous pull across the central span before it was filled with cold water and pressurised to revert the pipe back to its diameter, forming a close fit against the host pipe for maximum capacity.
The total work on the bridge crossing took seven days to undertake from start to finish, including the excavation of the launch and reception, the welding of pipe, the reduction of the pipe diameter and the lining operation itself.
The trenchless solution caused the least possible disruption to the highway network and adjacent area, whilst ensuring the historic fabric of the bridge remained undisturbed.
These works were also completed faster than would have been possible with an open trench across the bridge deck and were significantly less expensive than the alternative of crossing the river by directional drilling.
Furthermore the capacity of the pipeline was maximised and National Grid was able to decommission several sections of adjacent metallic mains.Download: