June 27, 2007 -- Over the past few years, the sewer network beneath the city of Cardiff, the Capital City of Wales, has been undergoing some major improvement works. As part of this work, a major new concrete sewer pipeline has been laid near (parallel where possible) to the existing 1,676 mm x 1,118 mm brick egg-shaped sewer. Both new and old sewers will be used to increase sewer capacity during heavy storms.
One of the last sections of the 'sewer duplication' that required construction was in the Roath Park area of Cardiff.
Despite being only 250 m in length, along the route of the Roath Park section lay a series of major obstacles. These included a car park area serving council offices, the council office building itself, a railway embankment with an operating railway line, domestic garages and gardens, the residences connected with these gardens and garages (a row of terraced houses) and a roundabout serving a major junction of some five roads which lay on a major commuter access route to the city center. In the roads were numerous major services which in addition to the traffic problems would make any excavation very difficult.
In order to minimize any effect on any of these existing surface structures, services and traffic movement, AMEC Design and Project Services, the strategic partner of the Dwr Cymru Welsh Water Asset Management Alliance responsible for the project, decided, in conjunction with the project designers, Arup and DCWW Operations, that an inverted siphon sewer should be installed at depth. The inverted siphon sewer was chosen because it would provide the pipeline necessary, whilst ensuring that there would be minimum effect on the surrounding environment, population and traffic flows.
Hydraulic investigations showed that a 700 mm diameter pipeline would have all the necessary properties required for the inverted siphon, including flows that would ensure self-cleaning etc.
Planning
When planning the installation it was decided to utilize a pipejacking technique to install the pipeline, as excavation in the vicinity of any of all the surface structures was impossible. However, whilst it is not impossible to microtunnel a 700 mm diameter pipe over a single 250 m drive, this sort of distance is usually completed more successfully at larger diameters. The fact that the surface structures would make it impossible to sink a rescue/recovery shaft, should the microtunnelling shield become stuck, meant that AMEC opted to utilize a larger diameter (1,200 mm i.d.) for the installation.
Again, however, the hydraulic calculations showed that only a 700 m diameter final pipeline was needed for the inverted siphon to operate effectively. It was therefore decided to 'slipline' a 700 mm i.d. vitrified clayware pipe into the 1,200 mm diameter concrete 'casing' pipe that would be installed by pipejacking so as to ultimately maintain the required hydraulic integrity of the sewer.
Ground conditions
As the new pipeline was to be installed at depths of up to 14 m, the prevailing ground formation information was very significant. Initial investigations showed that beneath the general top soils lay what is commonly known as Mercia Mudstone. Normally this is a fairly weathered mudstone and when cut often 'clays' easily. It is usually relatively easy to excavate using a microtunnelling shield. However, during excavation of the shafts at either end of the sewer route, the mudstone in this particular area was found to be somewhat more substantial. Over the course of the works the unconfined compressive strength of the Mercia Mudstones proved to be as high as 70 MPa, relatively high for this geological formation.
AMEC DPS carried out all construction operations using directly employed labor and utilizing its specialist staff and operatives for the tunnelling and shaft sinking works.
The tunnelling work was to be completed using an Iseki-manufactured TCC 1200 Unclemole microtunnelling system, owned by AMEC DPS. The unit comprises a surface remote-controlled tunnelling shield, which utilizes slurry as the spoil removal and ground support system. Due to the apparent hardness of the Mercia Mudstones in the area AMEC engineers modified the standard cutting head of the microtunneller to include Tungsten Carbide tipped cutting teeth to handle the harder ground expected.
Whilst advance rates were not as high as originally expected, given the harder ground and careful driving by the machine operators, very acceptable advance rates of between 4 and 5 pipes per shift were achieved. The harder ground did make for a little nervousness in the operators cabin so to ensure that there would be minimum risk of the pipejack stalling in mid drive, two interjack stations were installed in the concrete pipe string. CV Buchan provided the steel banded joint, jacking pipes. In the event, neither of the interjack stations was required throughout the whole of the drive.
Ultimately the 250 m drive was completed in a total of 27 days, using a 12 hour shift system. Over the course of the drive jacking pressures did not exceed the machine capacity at any time, despite the nature of the ground.
Sliplining work
With the microtunnelling drive completed, all of the equipment within the drive was removed before commencing the installation of the 700 mm i.d. clayware pipe. The concrete pipe was then cleaned very thoroughly to minimize friction potential during the installation of the clayware pipe.
Naylor Drainage Ltd of Barnsley, Yorkshire, UK, provided the vitrified clay pipe. The pipe used was the Denlok jacking pipe system, which has an international reputation as a microtunnelling pipe in its own right. Given that the 'sliplining' pipe would need to be jacked into the concrete casing pipe over some 250 m, the properties of Denlok as a pipe specially designed for jacking applications made it an ideal choice. The Denlok pipes were supplied in 2 m lengths with a wall thickness of 75 mm, which gives the pipe a maximum jacking pressure capacity of 2,500 kN.
The Denlok pipes utilize 316 Ti stainless steel sleeves with EPDM seals to achieve a minimum 2 bar internal/external pressure capacity and come with a factory fitted particle board thrust ring to even out thrust pressures across the joint during the jacking process.
To ensure that the 700 m diameter pipe remained central within the concrete sleeve pipe project engineers designed a special skid mount. The skid mount comprised a steel ban with two small skid feet at the 4 o'clock and 8 o'clock positions around the circumference. The lead edge of each skid was up-turned to prevent the edges catching on anything in the concrete pipe. The first pipe in the string was mounted with two such skids attached around the pipe. Thereafter each pipe was fitted with one skid unit, the ends being supported at the pipe joints. As each skid unit entered the pipe lubrication grease was applied to smooth the jacking process.
The jacking frame from the Iseki microtunneller was adapted and utilized to provide the thrust to push the liner pipe into the casing pipe. The difference between the 1,200 mm jacking ring of the frame and the 700 mm diameter of the clay pipe was overcome by fashioning an adapter cone to sit between the two.
On completion of the sliplining operation with the clayware pipes, to prevent any untoward external pressures on the liner pipe during its operation, the annulus between the casing and liner pipes was ultimately grout filled to complete the installation. This operation required around 150 m3 of grout material.
Having been installed at depth, and to connect the new inverted siphon section to the existing pipe network, the inlet shaft end of the pipeline will be filled with concrete to form a large cone and direct the flows into the new 700 mm pipeline. At the downstream shaft, the 700 mm clay pipes will be connected to a stainless steel 'S' pipe which will transfer the flows back to the higher level.
The top end of the 'S' will be connected to the existing system by installing a new length of pipe, in open cut, to enter the adjacent manhole at a depth of around 4 m.
No movement of any structures were detected during construction. Network Rail was also happy with the outcome. Track levels were monitored continuously during the time the microtunnel drive passed beneath the railway embankment, without any sign of ground movement.
Arup commented: "We selected the Denlock pipe system as it was the only one that would give the required performance -- smooth bore with good resistance to scour from sewage debris at relatively high velocities and with a design life of more than a hundred years."
For Naylor Drainage Simon Marsh, Denlok product manager said: "This is the first occasion Naylors have partnered AMEC Design and Project Services in a trenchless project but with the experience of both companies a difficult engineering project was successfully completed."
Simon continued: "When I visited the site traffic on the busy roundabout was continuing to flow, trains were operating and the residents in the Victorian houses were not affected. This proves that trenchless technology is an extremely viable option for the installation of utility conduits."
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