THE MAGAZINE FOR THE DRAINAGE, WATER & WASTEWATER INDUSTRIES
CASE STUDIES
thickness’ versus diameter and length of eversion and the possibility of a ‘liner on liner’ composite, to aggregate the design thickness required.
Taking into consideration the tests results, CCTV and sonar surveys and using the equations to confirm thickness design, the liner used was ‘NordipipeWPP’ by Norditube Ltd, as an ISO 11295 Class A ‘ Independent’, fully structural liner. The liner was made of 4 layers of ECR glass epoxy liner, equating to a wall thickness of9mm after cure.
Figure 2. Project plan.
remaining life. The chambers were designed to have suitable dewatering sumps to allow for removal of residual foul flows and purging water (Figure 2).
Sonar survey was carried out from the new chambers to determine the internal diameter of the main and the type of bend installed. It highlighted the need to inspect and line the rising mains whilst they were fully surcharged, to prevent floatation of the steel mains due to low cover and fluvial tides. This was considered the most challenging aspect of the project. To establish the amount of deposits, the main was purged with clean water to allow a visual CCTV survey, with a low turbidity, while keeping the pipe full. Wessex Water have their own CCTV survey department and a suitable camera was to be deployed with resistance to over a bar of water pressure, suitable for their otherwise hostile environments.
Prior to CCTV surveys, hydraulic analysis had predicted sedimentation due to the low velocities in the Braidney main and these calculations were confirmed as the surveys identified one of the risers contained gravel deposits up to 30% of the diameter. These were predominantly cemented and varied in size from an estimated 3-50mm. The second riser, known as the Bristol Road Main, had little sedimentation. The sonar survey established the configuration of bend construction, perceived to be welded ‘Lobster Back’ at the time of planning, and both mains were protected by a thin wall of bitumen, and it was concluded the mains were deteriorated but could be cleaned and lined.
Design and delivery of pressure liners
When we consider the design tools available to us as a W&SC, we have enjoyed the benefits of ASTMF1216, the WRc SRMandDWA143 for
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gravity CIPP solutions, enhanced by the ISO 11295 and ISO 11296, but the design and procurement of our first CIPP pressure liner at Christchurch WRC in October 2017 highlighted a number of submissions from contractors with differing mechanical material properties which we had to align.
We now have ISO 11297, but in the absence of a worldwide recognised design method for the design of CIPP rising main liners, we commissioned an innovative design methodology from Dr Ian Bateman of Bateman Consultants Pty Ltd in Australia, which allowed us to dictate the mechanical properties for clarity and comparison. The design of thickness provided by the supplier was assessed and compared against Short Term Pressure Rating, Long Term Pressure Rating, Surge Pressure (Occasional), Surge Pressure (Cyclic), Surge Pressure (Vacuum) and External Loads. The designs explored and encompassed concepts such as overcoming eversion frustration of liner
The designs allowed for a maximum operational pressure (MOP) of 8 bar down to an absolute vacuum of -1 bar. The MOP in the existing rising mains, including surge, was well below this value. Should the host pipe eventually corrode and subject the liner to a hydrostatic high tidal load, and for any reason the syphon CIPP lining was emptied, the short term and long-term flexural conditions were also computed. In addition, fatigue due to cyclic loading was also a consideration.
Another challenging element in this project was the number of 22.5 degree bends and the concern of removing compacted heavy mineral detritus, whilst not using excessive jetting pressures that could damage the host pipe. A large capacity water tank was brought to site and was constantly being filled with clean water to gently displace and purge the main. To achieve the required cleanness, OnSite Ltd used an extraction method integrating balanced air scour amongst other techniques, to clean the riser suitably for the pressure liner.
The total head required to evert the liners was 13m above rising main invert. This was achieved by installing a scaffold tower of 10m high, as shown in Figure 3. The liners were slowly everted through the main over a distance of 100m from the SPS side, constantly displacing the clean purging water surcharge as the liner went into the receiving shaft, which was pumped away. The curing time was
Figure 3. Eversion Tower with levee in the foreground
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