Special Works
The defective sewer beneath St Paul’s Road, Salisbury.
BELOW: The massive underpinning works are completed in the St Paul’s Road property to allow installation of the new lightweight pipe.
Installing the St Paul’s Road
liner which added structural strength to the new pipeline.
concrete pipe but due to the host pipe’s deteriorated condition and the hydraulic constraints of the surface water sewer this too was dismissed.
property in St Pauls Road, Salisbury and underneath a number of rear gardens locally, was identified as having collapsed over a length of some 60 m.
During construction of the properties in the area, a local river was placed into a culvert over a length of some 93 m and using a further 3 brick culverts the river was transferred underneath the local railway line before entering a nearby river. The ground over the culvert was made up to a depth of 2.4 m over the existing river bed.
The CCTV survey however revealed that the concrete pipe covering part of the diverted river course had fractured in a classic ‘4-point’ collapse and the invert of the pipe had risen to the centreline. Whilst flow was still occurring through the pipe it was obvious that structure was close to full collapse and was in need of urgent renovation, with a trenchless solution being preferable due to the location adjacent to the old Victorian properties.
To utilise more traditional techniques would have required a diversion route which would have included laying 110 m of 600 diameter surface water sewer using open cut techniques at 2.5 m deep in the adjacent highway, installing four new manholes, underpinning 8 m of the property wall and utilising some 40 m of sheet pile retaining wall. The construction cost to undertake this diversion would be approximately £450,000.
Various trenchless options were also investigated including Pipe Bursting but this was ruled out due to the forces required and the potential for foundation movement beneath the buildings. The project was further complicated by the fact that the damaged pipe was just 2.4 m beneath a property. There was also an idea to slip line a smaller pipe through the 600 mm diameter
Ultimately, having completed a ground investigation over the site, which showed the ground comprised made ground over water bearing gravels over chalk, it was decided that even the use of a timber heading to install the new pipe would not have been safe as the temporary tunnel would have been in the water bearing gravels and therefore a high risk strategy.
It was therefore decided that there was no other option than to remove the tenant from the property to allow for the pipe replacement from within the property. To allow for the pipe to be replaced significant underpinning works needed to be undertaken as digging straight down onto the pipe was also not a safe option and it would also ensure the structural integrity of the terraced property into the future.
Wessex Water’s in house structural design team completed an underpinning design. The design consisted of 37 concrete filled steel piles tied into a number of cast in situ C35 concrete beams each 400 mm wide and 1,000 mm deep. The underpinning included 30m3 of concrete. On completion of the underpinning the ground floor of the property was completely renewed to meet current Building Regulations.
Once the structural underpinning was completed, the defective concrete pipe was broken out and a 600 mm Ridgisewer pipe was installed in sections. This pipe was used due its lightweight and easier access in the limited working area within the property. However, using the Ridgisewer pipe was only possible provided that, subsequent to installation, a structural liner was also installed over the pipe length.
Ashley Williamson, the Wessex Project Engineer in charge of the project and who received the coveted UKSTT Young Engineer Award for his work on this project, contacted Richard Wortley of Onsite to ensure that such a lining was possible and to discuss the potential lining options. Ultimately a 60 m length of 16 mm wall thickness Premier Pipe CIPP structural liner was installed from an existing manhole at the front of the property using hot water cure techniques. The lining operation was completed in just four days.
Overall the cost comparison between the diversion option and the renovation option, even with the huge amount of property underpinning required, showed a saving of some 40% (£181,000).
These are just three of the challenging projects that have been undertaken by the teams at Wessex Water over recent times. They show the diversity of expertise as well as the open-mindedness of the teams in finding the creative solutions needed by the range of defects and circumstances that the teams have to manage when the call comes in about what to the caller is a simple sewer problem.
24 drain TRADER | September 2015 |
www.draintraderltd.com
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