Getting out of a hole
No matter the scale of operations, Wessex Water's preferred method for dealing with sewer problems is through trenchless technology where possible. But this process is not always straightforward
Over the past few years the name Wessex Water has become somewhat synonymous with the use of trenchless technology in the UK water industry, particularly in its dealings with the wastewater sector. Looking at some of the varying works that have been undertaken over recent months it is easy to see how this ethos of looking to use trenchless techniques has permeated the wastewater planning and construction hierarchy of the company.
It was not so many years ago that if a report of a blockage or sewer failure came into the office that the first question was ‘how big will the dig be?’. Now, at least within Wessex Water’s sewer teams, this approach has changed significantly with the first thought being ‘is there a suitable technical solution to this problem using trenchless technology?’. This thought process now runs through the company psyche, from larger scale operations to smaller diameter networks.
Redcross Lane, in central Bristol, is an old, narrow and partly cobbled alleyway lined by new-build terraced houses. When a sewer problem was reported to Wessex Water the response team investigating found that it was no ordinary blockage.
Having tried to jet the blockage clear with no result, investigation with CCTV found that the 300mm diameter vitrified clay sewer pipe, which was probably something in the region of 100-years old, was indeed blocked but not with the usual, easily jettable materials but with fill, stones and brickwork materials from a collapse at the crown of the pipe. This is where Stuart Soutar and his R&M team sprang into action.
Working with Onsite Central, Wessex Water’s framework contractor, a plan was developed to utilise an ultra-high-pressure (10,000psi) water jet cutter to remove the blockage material alongside traditional jetting equipment to remove the spoil created. This process, whilst removing the initial blockage, also enabled the CCTV operator to see the subsequent condition of the defective pipe.
Over the crown of the pipe there appeared to be a further collapse of material including some form of lintel or kerbstone across the pipe section that was preventing further collapse of material into the pipeline. This was somewhat to the advantage of the contractor as it enabled a full lining to be installed through the sewer subsequent to final removal of the original blockage. However, the lining itself was not an easy option.
The location and circum-stances of the old sewer were such that just before the damaged pipe section, the diameter of the pipe changed from 300mm to 225mm which meant that the pipe had to be accurately surveyed and the liner designed with a taper to fit exactly when it was installed. Furthermore, the liner had to be installed with access only from one end of the sewer as the exit point of the sewer was not accessible to the contractor.
Had this renovation option not been available the only other alternative was difficult with the excavation site being within the footprint of the two new-build houses at a depth of 1.8m. To complete an excavated repair this would have meant removing the stairway of one house, supporting the separating wall between them and excavating under this wall to the pipe depth within the confines of the house.
This would have been both a difficult and expensive option given that the residents would need to be re-housed for the duration of the works and extensive make-good works would need to be completed in the house after the pipe work was finished. The narrow alleyway and the location of the excavation also meant that access for excavation equipment would be virtually impossible.
Soutar says: “Given the alternative, we had nothing to lose in trying to use the trenchless option here. The costs involved with excavation option were prohibitive and the disruption that such works would have caused to the residents would have been immense. To some extent we had luck on our side as well, given the bridging of the defect by some of the materials above the pipe. Ultimately the trenchless option cost us less than £10,000 as compared to something in the region of £100,000 for the excavation option and all its additional customer care costs.”
Another recent problem solved using this mindset was the repair of a burst on an existing 100mm diameter cast iron rising main from Fisherman’s Walk Sewage Pumping Station (SPS) in Bournemouth earlier this year.
The 52m long rising main, laid in 1977, runs within a steep cliff face with the SPS located at the bottom between the cliff face and the promenade, adjacent to a toilet block and a number of beach huts. The 27m high cliff face is prone to movement and land slips, which is believed to have been the likely cause of the burst which lay some 6m into the cliff face from the wet well end of the rising main and at a depth of 4m.
Initial thoughts were that a new rising main could be installed using directional drilling but knowing that the steep cliff face was prone to movement could have made such an operation high risk and potentially dangerous as the drilling may have caused a significant land slip. Such a solution would also have involved significant temporary works, further adding to the costs.
The only other replacement option was to take the discharge to a different part of the sewerage network by installing a 950m long rising main within the Bournemouth promenade which would have caused widespread disruption and cost in excess of £950,000.
So it was decided that a rehabilitation solution for the existing rising main was preferable as it would not affect the surrounding soils thereby minimising potential for cliff instability.
Wessex Water approached Onsite with a view to establishing how this could be achieved. Working closely with the Wessex Water design team, Onsite was able to bring together a liner design, installed from the wet well at the base of the cliff, that would not only repair the burst in question but also provide a reinforcement of the rising main into the future.
The in-house design comprised a ‘dual skin’, steam cured, glass-fibre reinforced, 3.5mm thick liner, which was further reinforced by the introduction of a 2mm thick non-reinforced liner which allowed for easier installation and provide additional stiffness once cured.
This combination liner not only enhanced the overall stability of the liner but also provided high resilience to potential pressures fluctuations which might be experienced within a rising main of up to 3.5 bar.
Due to major corrosion of the rising main at the wet well and to enable the liner to be installed, both a 45o bend and a 17½o bend had to be removed from within the cliff. To enable this work a 1.2m square and 2m long timber heading was driven into the cliff face from the side of the brick wet well chamber. This also allowed an access platform to be erected in the bottom of the wet well.
Lining was achieved from within the wet well. The liner, being a composite twin-wall liner, was not inverted into the host pipe but winched through. It was then inflated and steam cured. On completion, the timber heading was backfilled with concrete and voids surrounding the timber heading grouted.
Following a routine CCTV survey, a 600mm diameter concrete surface water sewer pipe, running beneath a Victorian property in St Pauls Road, Salisbury and underneath a number of rear gardens locally, was identified as having collapsed over a length of some 60m.
During construction of the properties in the area, a local river was placed into a culvert over a length of some 93m and using a further three 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 2.4m 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 that would have included laying 110m of 600mm diameter surface water sewer using open-cut techniques at 2.5m deep in the adjacent highway, installing four new manholes, underpinning 8m of the property wall and utilising some 40m 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 2.4m beneath a property.
There was also an idea to slip line a smaller pipe through the 600mm diameter concrete pipe but due to the host pipe’s deteriorated condition and the hydraulic constraints of the surface water sewer this too was dismissed. 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 400mm wide and 1,000mm 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 600mm Ridgisewer pipe was installed in sections. This pipe was used because of its lightweight and easier access within the property’s limited working area. 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 scheme, contacted Onsite’s Richard Wortley to ensure that such a lining was possible and to discuss the potential lining options. Ultimately, a 60m length of 16mm 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.
Onsite says 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 have to be managed when the call comes in about what to the caller is a simple sewer problem.
This article first appeared in WET News.
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