CASE STUDIES
The River Parrett Twin CIPP Pressure Lining Renovation
Francieli Thums, BEng (Hons) MEng Senior Engineer, Sewer Rehabilitation Team, ESD Olly Elworthy, Senior Engineer, Networks Team, ESD Dr Ian Bateman, BEng (Chem), PhD (Chem Eng), Owner, Bateman Consultants Pty Ltd
Introduction
Large diameter rising main deterioration has become a big capital expenditure because of the risk of major infrastructure failure. The costs to repairing these assets in the event of failure are likely to be very high and characterise a large problem for water and sewerage companies (W&SCs) in the UK. Wessex Water’s critical crossing review had identified the River Parrett twin rising mains in Bridgwater as medium to high risk of bursting with a high consequence, resulting in severe environmental pollution events and large operational disruption. The crossings were originally built in 1976 and were replaced in 1981 with steel pipe construction.
The River Parrett passes through the town of Bridgwater in Somerset and discharges into the Severn Estuary, which is subject to a 13-15m high tidal range, one of the highest in the world. The twin mains are of 450mm steel inverted syphon rising mains with a number of 22.5 degrees bends. They convey flow from Bristol Road Sewage Pumping Station (SPS) to the Chilton Trinity Water Recycling Centre (WRC) crossing under the River Parrett, with a shallow depth below bed level. Figure 1 shows the long section.
An initial option study was undertaken and concluded that wholesale replacement of the rising mains would be extremely challenging both technically and financially, due to the bed and embankments geology. The existence of a deep artesian bed of gravels within the proposed catenary of drive was further complicated by a tortuous potential drill path under Larssen sheet piled walls supporting both levee embankments. The initial concept of directional drilling two new replacement mains raised concerns over the possibility of ‘frac out’ in the gravels and loss of drilling fluids and steerage. In addition, the
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Figure 1. Twin rising mains long section crossing River Parrett, Bridgwater
launch sites for the drilling were very confined, and to locate the drilling rigs, a private house would require purchase and demolition. This also complicated the positioning of any HDPE lines for insertion, due to the A38 (Bristol Road) passing the site.
Alternative options were therefore considered including Cured In Place Pipe (CIPP) lining. It is known that there have been relatively few CIPP pressure liners installed by the UK water industry since their development in the Far East in the 1980s, and at the present there is no recognised international design standard for pressure liners.
Wessex Water’s Sewer Rehabilitation Team and their framework contractor Onsite Ltd explored the alternative concept of a CIPP pressure lining and highlighted the need to line the mains whilst they were fully surcharged, to avoid catastrophic floatation of the steel mains at low tide. This would require the CIPP to be delivered with submarine considerations, displacing the water surcharge as the liner went, believed to be a first in the UK.
| July 2021 |
www.draintraderltd.com
The River Parrett twin rising mains assessment and enabling works
The internal condition of the twin mains was unknown and surveys were required to assess the lining feasibility, to confirm the internal diameter, any deposits within the pipe and to retrieve samples of pipe for testing. Flows from the sewerage pumping station (SPS) to the water recycling centre (WRC) had to be maintained at all times, and the potential of redundancy was available to us if a fully functioning crossover between the two mains was constructed allowing diversion of flows. This required some enabling civils whilst recognising concerns that the mains were expected to be corroded and potentially fragile.
Two new shafts were constructed on either side of the riverbank with 1.5m long rollout sections installed in the pipelines to enable reciprocal flows for diversion dependent upon the main to be lined. The sections removed from the mains were then tested at an independent laboratory to determine the level of corrosion and the remaining wall thickness and thus a prediction of
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