Rehabilitation of 140 metres of 20” cast iron water main
500 mm PE100 SDR17 pipe
The construction of the Crossrail link through London has required tunnelling under some of the capital’s most densely populated areas and prestigious commercial and historic districts. These works have required dewatering in locations where the tunnelling works were carried out below groundwater level.
This brought with it the risk of settlement of the overlying ground and structures, including buried utility pipelines located in the zone of influence. As a precaution, critical assets such as trunk water mains along the tunnelling route were surveyed both before and after the works to quantify settlement levels, and those exceeding defined thresholds of failure risk were earmarked for preventative maintenance works.
One such asset was a 20” cast iron main, part of Thames Water’s strategic network, located in Vallance Road, a heavily urbanised area in the Whitechapel district of East London. Due to the potential risk of failure on the affected length, Thames Water took the decision to replace the 140 metres of the pipeline.
The choice of renovation technique was strongly influenced by the need both to retain the maximum possible flow capacity of the pipeline, whilst at the same time keeping excavation to a minimum, so as to minimise the risk of damage to the high density of other buried utilities and associated plant in the vicinity. With the above considerations in mind Subterra’s Subline close-fit polyethylene (PE) lining technique was chosen as the preferred solution.
Rolldown® is a system for relining ageing and damaged pipelines with a structural PE pipe that is ‘deformed’ and inserted into the host, then reverted to its original shape.
Rolldown® is a system for relining ageing and damaged pipelines with a structural PE pipe that is deformed and inserted into the host, then reverted to its original shape to form a close-fit liner inside the main. One of the methods to deform the pipe is the concentric reduction of the diameter of the PE pipe. This entails pushing prepared strings of PE pipe through sets of specially-designed rollers. This induces a reduction in the diameter of the PE liner pipe of approximately 10%, which is subsequently retained without the application of any mechanical load.
The prepared host main can then be lined with the reduced-diameter pipe by conventional slip-lining techniques. Once inserted, the PE liner pipe is reverted using cold water under pressure to revert it back towards its original diameter, enabling a close fit with the host pipe to be achieved.
Due to site constraints, the affected main was split into two sections, with an insertion pit in the centre approximately 8 metres long and two smaller excavations at each end to allow winching operations and re-termination of the liner pipe to take place.
Once the 20” cast iron main had been decommissioned and cut at the centre pit, a CCTV survey was carried out to check the main for unforeseen bends, connections etc. On one of the sections an uncharted tee and a bend were identified. These, and a client requirement to fit a new tee branch on the section, ultimately led to an adjustment of the volume of lining works originally planned and the incorporation of two tee branches.
The DN500 SDR 17 PE100 liner pipe was supplied by Radius Systems in 12 metre sticks. An automatic PE butt fusion welding machine was used to fuse the PE pipe sticks into 2 strings, one of 72 metres length and the second of 48 metres length, to accommodate the two insertions.
The decommissioned 20” cast iron main was cleaned using the drag scrape and plunge technique. The bore of the prepared main was then surveyed using CCTV to confirm that it was in a suitable condition for lining.
After completion of the welding, the 2 strings were ‘rolled down’ and stored ready for insertion. Due to a relatively small site area, the liner processing operation was carried out in a single visit (1 day) to minimise site disruption.
The insertions were undertaken by drawing the processed PE liner pipes into the main from the central insertion pit to the reception pits at each extremity using a 10t winch.
Once the insertion was complete, the ends of the PE liner pipe were expanded and end terminations (standard 500 mm Aquagrips) fitted, along with flanges, to allow the liner to be pressurised hydraulically and reverted to a close-fit with the cast iron host pipe. The reversion process was also used as the final pressure test on the two sections.
The lining works, including an extra excavation required for the additional tees specified by the client during the course of the project, were completed over a three week period. Optimise’s Project Manager for these works, Brendan Melody, said “the Subterra team were able to use the operational flexibility of the Subline system to great advantage in the severely restricted site in Vallance Road. It has enabled us to ensure the security and capacity of our client’s asset has been maintained with much less disturbance to both the public and to other utilities’ buried plant than would have occurred with conventional open cut trenching or other sliplining solutions”.