Case Study: Temporary Sheet Pile Pipe Trench, Harrowside, Blackpool

Temporary Sheet Pile Pipe Trench, Harrowside, Blackpool

Sheet Piling (UK) Ltd. were employed by Land & Marine for the design, installation and extraction of a temporary sheet pile Pipe Trench on the beach at Harrowside West, Blackpool. The sheet piled trench was constructed between high and low water mean springs and was 400m in length. A further cofferdam was formed above mean high water springs adjacent to the existing promenade and formed a Safe Haven on which equipment could be stored.

The purpose of the temporary sheet pile trench was to facilitate the installation of a new 2.1m diameter Outfall Pipe from the Lennex Gate and Harrowside Pumping Stations. The project is part of United Utilities £125 million investment to improve the sewer network along the Fylde Coast over the next 5 years.

Working closely with Land & Marine from an early stage to develop an optimum sheet pile design and sheet piling installation solution, Sheet Piling (UK) Ltd proposed to preauger the pile line prior to installation and install the piles using powerful leader mounted vibratory hammers on tracked piling rigs.

The length of steel sheet piles varied throughout the project from consisting of Arcelor Mittal PU23 sheet piles and Sheet Piling (UK) Ltd used second hand sections (Nippon Steel NSP.IIIW and NSP IVW). The piles for the main pipe pull trench were generally 12.0m long and designed in cantilever to accommodate the floating into position of the new Outfall Pipe. The Safe Haven piles were generally 11.0m long and were tied by a waling beam and tie bar arrangement. All piles for the Safe Haven were supplied from Sheet Piling (UK) Ltd.’s extensive stock of used and second hand sheet pile sections which are available for external sale or hire.

Due to the works being in the tidal zone, co-ordination to maximise the workable hours at each section had to be undertaken to achieve the programme. In order to do this two telescopic leader rigs and two 30 tonne 360° excavators complete with Movax attachments were used throughout the project. This involved working split shifts and also night-time tidal work during a 7no day working week.

During the works, Sheet Piling (UK) Ltd also provided a service gang to transport piles from the promenade to the works location.

Once the sheet pile trench had been installed and excavated, the 2.1m diameter pipe, which has been preassembled in Holyhead with concrete anti-floatation concrete kennels attached, was floated into position on a falling tide.

The pipe was then flooded and further concrete saddles were attached along the trench to prevent floatation.

Once completed, the trench was backfilled, pipe connected into the overflow chamber and the sheet piles were extracted.

The installation works were completed over an 8no week period between May & June 2015 and the extraction over a 6no week period between September and October 2015.