Bachy Soletanche phase two of Barrow contract

Bachy Soletanche returned to the seaport town of Barrow-in-Furness to complete phase two of its groundwork contract which is part of United Utilities sewer flood alleviation scheme.

Originally, the leading geotechnical firm constructed a circular shaft for a storm water tank in the heart of the town using the sophisticated diaphragm wall technique.

Twelve months on and with the shaft excavated and cast, Bachy Soletanche is back to install tension piles at the shaft's base to resist uplift load or flotation of the tank.

When complete, it will be used as an underground storage tank, capable of holding 2.3million gallons of water - enough to fill nearly five Olympic-sized swimming pools.

The project is part of a GBP14million major improvement being undertaken by main contractor KMI Plus on behalf of the client, United Utilities for sewer flood alleviation in the town.

The area is particularly prone to flooding and with precipitation levels rising more so each year, flood alleviation schemes such as this will be valuable to Barrow's residents and businesses.

At the same time, it will allow Untied Utilities to bring the sewer network into the 21st Century.

Work for first stage of the project commenced in April 2007 when Bachy Soletanche constructed the circular concrete shaft for KMI Plus.

The 23.6m internal diameter shaft, which forms the outside of the water tank, was installed using the diaphragm wall technique rather than the more commonly used secant piled wall shaft.

Bachy Soletanche's Barrie Arkwright was the Contracts Manager for this phase of the project.

He explained: "We favoured diaphragm walling due to the 30m depth required for the shaft".

"By utilising a crane-mounted grab, which we use for this particular technique, we have the ability to build a deeper shaft than is possible with a traditional piling rig while maintaining verticality and using fewer joints".

The slurry trench method was utilised for this project where a narrow trench is excavated and then kept full of bentonite fluid.

The fluid exerts hydraulic pressure against the trench walls and acts as a shoring to prevent collapse.

The trench is excavated in panels with 12 used in all for the shaft at Barrow.

Each panel was 1m thick and approximately 6.4m in length and constructed using the rope operated grab.

Once the excavation was complete, the bentonite digging support fluid was cleaned in the panel to reduce its density ready for concreting.

However before the concreting takes place, three reinforcement cages are tandem lifted by two cranes and placed in each panel.

These cages were 30m in length and therefore reached the full depth of the shaft.

Once they were in place, the concrete was poured into the panel through tremie pipes which gradually get removed as the concrete rises.

Furthermore, the concrete displaces the bentonite slurry mix which is then pumped from the top of the panel, de-sanded and reused for subsequent panels.

Each panel took approximately 200m3 of concrete and six hours to cast.

This stage of the project completed in August 2007, allowing 12 months for the excavation of the shaft and associated tunnelling works to be completed.

At that point, Bachy Soletanche was back on site for the final four week contract to install test piles and 30no.

tension piles using the anchor drilling technique.

Speaking of this phase of the project, Contract Manager, Daniel Barnard explained: "The tension piles will provide resistance to the long term flotation forces resulting from groundwater pressures whilst the tank is empty".

The tension piles were drilled into the base of the storage water tank through pre-cast holes for easier drilling access.

To do this, the rig was craned into the 30m deep shaft, which then drilled each bore hole to the 140mm rock socket.

A pre-stressed bar, with a 47mm diameter, was then placed into the bore and grouted into place.

Finally, a head plate was installed to fully transfer the load from the concrete slab to the tension pile.

The double corrosion protected anchor heads will then be encased in the final pour of the finished base slab.

Now that Bachy Soletanche has completed both project elements, there are still several stages to complete for the Spring 2009 deadline.

Now that the new tunnel has been constructed from approximately 12m down the shaft, KMI Plus will be putting the finishing touches to the scheme with the final sewer connections.

The shaft will then be covered, restored and landscaped as part of the overall project specification.

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