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Kobus pulls the cables for Yorkshire Water WTW upgrade

Upsizing a PE gas main to a 90mm PE supply for Yorkshire Water's Sandall Sewage Treatment Works upgrade should have been an easy task - if it had not have been for the dual-carriageway that was in the way

The final short excavated sectionThe final short excavated section

U Mole recently provided two products from its hire range to help on a challenging dual-carriageway for client Manterfield Drilling that required the replacement of a 25mm diameter PE gas main with a new 90mm diameter PE supply. The scheme is part of upgrading works at Yorkshire Water’s Sandall SewageTreatment Works.

The two systems were a Kobus Cable Puller and a PB30 cable-based pipe-bursting system.
The existing gas main ran beneath a section of the A630 Wheatley Hall Road dual-carriageway near Doncaster. As well as upsizing the existing 25mm diameter PE supply to 90mm diameter PE, the new supply main had to be tee’d off a nearby existing 16” diameter gas main. The dual carriageway crossing length was some 18m.

Concerns within the local authority and highways for the maintenance of the existing road condition meant neither HDD techniques nor the use of a 110mm diameter impact moling technique could be considered for the direct installation of the new gas supply.

Manterfield did attempt to use a small diameter service mole to create a bore for the PB30 cable to be installed into for subsequent upsizing to 110mm diameter rather than utilise the Kobus pipe pulling system. However, during the course of these works the moled crossing had to be aborted due to extremely compact ground conditions beneath the roadway.

Obstacles
The fact that the existing service ran beneath a dual carriageway was challenging enough in itself, however it was found that other obstacles increased the site challenges even further.

The 25mm diameter service was found to change level substantially part of the way through its route. Not only this the pipe changed from being initially PEX material at one side to being PEA at the other. Also, the old single carriageway road surface that used to run across the site still existed under the new dual carriageway surface.

There were also access restrictions imposed on works at the site.

As a result Manterfield looked at utilising the Kobus system to initially pull out the old service and at the same time pull in the 19mm diameter compacted wire rope that was used with a HammerHead PB30 cable-based pipe bursting system. Once this was installed the plan was to use the PB 30 to upsize the bore from 25mm diameter to 110mm diameter to enable the new 90mm diameter PE service to be installed along the same route as the old supply.

To extract the existing gas service pipe a Kobus consumable kit was utilised with 20m of pulling cable and a burst head. The cable was inserted into the old pipe with Kobite being injected into the annular space to lock the cable in place. The Kobus puller was then utilised to pull out the PE service and install the compact PB30 cable.

After just 3m of the pull-back operation the process stalled. Tension was maintained for ten minutes with only very slow movement. The PE pipe at the Kobus puller end of the arrangement yielded (at a joint it was later discovered) but the wire held and it was decided to continue with the pull.

Over an hour the cable was retrieved until around 13m into the pull when the unit stalled again. It was decided to slowly increase the tonnage on the Kobus system in a final effort to get rope extraction completed and the PB30 rope in place. Another 2 to 2.5m of pull was achieved but then the rope failed. At this point the PB30 cable was some three-quarters of the way through the route.

The failure was down to exceptionally compact ground and the old carriageway cover. Under these circumstances the Kobus cable was simply not substantial enough to take the high tonnage required during the pull.

To recover the project, it was decided to dig down on the point of the break to attempt to get to the PB30 cable so that three-quarters of the job could be completed. Within the time restrictions for open-cut access to the carriageway, the end of the cable was located and the excavation backfilled to allow traffic flow.

Backfilled
The next day the first lane of the carriageway was closed off and re-excavated back to the PB30 cable. A 6” diameter PVC duct was installed and the PB30 cable pulled through to the grass verge beyond the kerb. The lane was then backfilled and reinstated to ensure it could be reopened within the access window allowed by the local council.

Once the PVC duct was in place the PB30 was positioned into its pull-in / receive pit and the upsize operation commenced. With the new PE pipe connected to the cable behind the expansion head the upsize commenced. Progress was slow and steady with the PB30 running close to its maximum pullback tonnage for most of the upsize run. This was a further indication of just how compacted the surrounding ground was.

By early afternoon the work was completed and the new 90mm diameter service pipe was installed.

The carriageway was fully reopened within the council’s stipulated time and the client ecstatic to have installed the new pipe along the line of the old 25mm diameter PE pipe within the client’s tight deadline.

Topic: Trenchless
Tags: maintenance , Yorkshire Water , water treatment , trenchless technology , pipelines

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