Trenchless approach gives Blackburn flood relief

Installation of a new pipe, forming part of an integrated urban drainage scheme in Lancashire, required specialist auger boring. On behalf of Perforator, Ian Clarke takes up the story

As part of new flood alleviation scheme being undertaken by United Utilities in Blackburn, Lancashire, through its main contractor VJ Donegans Civil Engineering, a new 24m long drainage pipe needed to be installed beneath Gorse Street. The installation was to comprise a DN300 gravity sewer laid to
precise line and level parameters as set out by the design engineers at United Utilities.

Given the urban location and the need to ensure that the grade of the new sewer was maintained, it was decided to use a trenchless installation technique that could facilitate the line and level control necessary to successfully complete the installation. The contract for this part of the flood alleviation work was awarded to Trenchless Solutions of Wakefield, which chose to use a Perforator PBA85 Guided Auger Boring system.

This system was selected from those available on the market because it offered a relatively small operational footprint, which suited the urban location of the project, as well as having the capacity to handle the ground conditions expected to be encountered at the level of the pipe installation. The boring rig's line and level control system also meant that the critical line and level requirements of the bore could be accurately and precisely maintained throughout the installation.

Installation
Prior to the start of the installation works, cable percussive bore holes were used to undertake ground investigation survey using a Dando 2000 drilling rig. This investigation confirmed that the ground at the installation horizon comprised soft to stiff clay.

The Perforator PBA85 guided auger boring machine that was used in Blackburn was a complete system which included an HS56 power pack, pilot rods, as well as augers and casings designed to suit the parameters of the, in this case, 1m long Naylor-manufactured Denlok DN300 clayware pipe chosen for the installation. Directional control of the pilot bore was achieved using a monitor, an optical system with a charge-coupled device (CCD) camera and a diode target plate.

The PBA85 rig utilised a three pass system to complete the new pipe installation comprising an initial pilot bore which established the accurate line and level of the bore which would be followed by the subsequent installation phases. This pilot bore was then expanded using an auger chain with a cutting at the lead end.

The auger chain ran inside auger casings with excavated soil being removed from the bore to the start shaft via using the rotating augers. With the bore now at the required diameter the final placement by jacking of the Naylor Denlok DN300 pipe into the bore whilst displacing the auger chain at the reception shaft completed the installation.

The PBA85 guided auger boring rig offers some 850kN of forward thrust as well as a maximum 470kN pullback force and operate from a start shaft or pit of a minimum 2m diameter. The machine will install pipelines from DN150 to DN500. The unit can be extended by up to 5m where circumstances require and are permitted to handle the installation of longer pipe section in one operation.

Work started on the 24m-long sewer installation on 22 November 2010, with the installation being completed by 24 November, with no specific problems being encountered during any phase of the work. The use of the trenchless system for the installation also meant that despite the urban setting local residents and businesses were not inconvenienced in any way by the presence of the works.

Commenting on the installation, the contracts director for VJ Donegan said: "We have used Trenchless Solutions on several contracts and have been pleasantly surprised by their performance, attitude and flexibility.

"The company has in the past overcome unforeseen problems and successfully completed contracts with no additional costs. We would not hesitate to employ their services on future contracts."

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