THE MAGAZINE FOR THE DRAINAGE, WATER & WASTEWATER INDUSTRIES
CASE STUDIES
over 120min one continuous length. However, whilst on site the contractor was asked to undertake a further separate 12mlong bore. Initially designed as Timber Heading this option was soon abandoned by the main contractor due to the arduous ground conditions being encountered. This meant that an additional guided auger bore installation was to be undertaken by Allen Watson to complete this section. This in turn meant that the original Guided Auger Bore reception pit had to be converted to enable it to accept one of Allen Watson’s compact boring machines to complete this short run to tie the new replacement pipe back into the existing pipe network.
Prior to commencing completing the final design and commencing work two additional boreholes were sunk to reinforce the decision that the boring method selection was the correct one. These ground investigation boreholes were drilled at the proposed drive and reception shaft locations.
NEWPIPELINE
Once the bores were completed and the casing pipes were in place, the final product pipe was installed within the 610mmo.d. sacrificial steel sleeve. This was a 450mmi.d. SN8 Ridgisewer drain pipe.
This sewer pipe was held in position within the steel sleeve casing by specially designed and manufactured EPDMcentralising bushes that had been fabricated specifically to suit the steel sleeve internal diameter and the new sewer outside diameter. The special EPDMcentralising bushes that had been successfully used on previous Allen Watson projects were
FOLLOW US
manufactured and fitted to allow full annulus grout flow, giving a fully grouted support finish to the sewer pipe within the sacrificial steel sleeve.
Initial works on site for the main bore commenced in mid-February 2021 and works were completed in late March. The short second bore followed on once the main works were completed.
INSTALLTION CHALLENGES
The ground conditions at the site presented various challenges to the installation crew. During the pilot tube installation, injection of a polymer-based drilling fluid allowed the torque and thrust pressures on the boring machine to be maintained at suitable levels while overcoming the ground reactions from the highly swelling clays.
Equally, the conditions proved challenging during the auger boring process that followed. While these conditions were expected the experience, the correct choice of equipment and methodology used by Allen Watson allowed for these conditions to be successfully overcome.
One of the specific stipulations of the project was that the method should avoid all disruption to traffic using M27 motorway above whilst keeping below permitted heave and settlement predications at surface.
Stuart Baldwin Head of Planned Works for Cappagh Browne Utilities Limited stated: “The project to replace the damaged 600mm diameter sewer under the M27 was especially challenging. Excellent collaboration between Southern Water, CBUL, Cappagh Construction, Allen Watson Ltd and various stake holders was a
key to its success. Utilising the specialist techniques and skills available within our parent and sibling companies and the persistence from the Cappagh Construction team and the Allen Watson Ltd drilling team, enabled this extended drive to be completed without disruption to the busy M27 carriageway.”
Allen Watson’s Operations Manager was site- based throughout the project to provide support to the specialist auger team if needed. The operations manager stated that: “Based on the pressure readings gained from the pilot installation and those taken as the drive passed under the M27, the 120mlength crossing was initially planned utilising Allen Watson’s GAB155v machine. However, the high torque experienced during pilot bore was such that it made sense to switch to the contingency equipment after the pilot bore for the 610mmsleeve installation operation to avoid any downtime should pressures increase further.”
Commenting on the project for Allen Watson, Simon Marsh, Project Manager said: “There were some very tight alignment challenges on this project with the route of the new pipeline having to pass through a corridor that was just 1mwide to avoid existing piles, and this whilst maintaining the required gradient of 1 in 120 with an exacting tolerance over the 120m length. The ground conditions, consisting of swelling clays, were encountered during the pilot bore installation process. The challenges this presented were increased by the exceptionally long length of the crossing.”
Ultimately the project was a success across the board with the new sewer installed, no disruption to M27 traffic and a satisfied client.
July 2021 | 33
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