Taking a partnering approach on a stormwater storage construction scheme has helped deliver £1M of savings for the client and created a smooth-running project
Fleetwood’s new stormwater storage construction team may describe what they are doing as just putting concrete into the ground, but it is the preparation that has made the work straightforward. The work currently being delivered on site by main contractor KMI – a joint venture of Kier, Murphy and Interserve – and Bachy Soletanche for United Utilities (UU) is the result of a year of partnering and planning that has reduced costs and minimised risks.
The scheme is needed to reduce the number of unsatisfactory intermittent discharges to the River Wyre in Lancashire as part of the Fylde Coast Strategy to comply with the Bathing Water Directive and the Shellfish Waters Directive. UU asked KMI to design and build a facility to store 15,000m3 of water at its existing waste water treatment works site in Fleetwood. How the solution was delivered was up to KMI.
The solution calls for two shafts – one for the stormwater storage and a smaller shaft to house pumping equipment – and a tunnel connecting the shafts to the other storage tanks at the site. The shafts – named tank 3 and tank 4 to tie in with existing infrastructure – will extend to 31.4m and 28m respectively, but tank 3 has a diameter of 41.75m and tank 4 is smaller at 9.75m.
“We started to work on the design with KMI a year ago and what is being delivered on site is a collaboration of our skills and we have evolved to become a team,” says Bachy Soletanche general manager (north) Richard Piggin.
“The main challenge at Fleetwood has been the design and the implications of constructing new storage tanks close to existing ones. It wasn’t just a case of copying the design of the existing tanks either – they are 1,000m thick walled shafts with domed slab bases that could not be safely delivered under Eurocode 7 and other industry changes.
“The closest tank is 1m away from the new tank, so Donaldsons undertook lots of modelling using Plaxis to look at how the construction work would affect the existing tanks. The checking did not influence the design but has resulted in the use of instrumentation and monitoring during the diaphragm walling and excavation work.”
The overall project is valued at £20M with Bachy’s element of the scheme coming in at £3.1M, but it could have easily been much higher without the design improvements delivered by the partnering approach.
“We started to work on the design with KMI a year ago and what is being delivered on site is a collaboration of our skills and we have evolved to become a team”
Richard Piggin, general manager (north), Bachy Soletanche
“The original concept was to use 50m long, large diameter tension piles to overcome the uplift pressures on the tank, but these would have cost around £1M extra on top of the diaphragm walling costs,” explains Piggin. Instead, Bachy worked with KMI to use the base slab and top slab as dead weights for the tanks.
“The first counterweight designs involved using 10,000m3of concrete to form the slab in the larger shaft but we have been able to bring this down to 4.75m thickness using 6000m3 of concrete by adding a 4m top slab to the design,” says KMI senior site manager Mick Boyle.
The change in approach to managing the uplift pressures led to the main shaft being widened to 41.75m from 40m to maintain capacity without having to go deeper when the roof slab was added to the design.
“We have still got an issue with heave at the formation level but analysis has shown that it won’t be a problem as long as the shaft is excavated and slabs formed within a six week period,” says Boyle.
But before work could progress to excavation, Bachy needed to create the shafts. Diaphragm walling work started on site in September and Bachy Soletanche contracts manager Barrie Arkwright expected the final panel to be completed as this issue of GE went to press.
Other than some fine-tuning of the concrete mix design, everything on the scheme has gone well. “Hope Construction Materials is the only supplier in the area and we needed 40 to 45m3of 50N DC4 concrete an hour to meet the programme here. This demand created a few issues with strength and flow retention initially,” explains Arkwright.
Ground conditions also presented some challenges with the site being located on an old landfill, so the diaphragm walling had to pass through 4m to 5m of fill material before reaching natural glacial clay which varied from a silty sand to a sandy clay to a gravelly sand over boulder clay at 24m below ground level. Groundwater was located at 2.5m below ground level.
“The shafts have been formed using 1,200mm thick panels excavated by KS hydraulic grabs mounted on Liebherr 855 base units,” explains Arkwright. “Tank 3 has 20 panels, while tank 4 has five but both shafts were formed with starter, intermediate and closer panels.”
Soft eyes have been cast into the panels to enable the tunnelled connection between tanks 1 and 3 at 23m below ground level.
“The ground conditions meant that we had a few problems with overbreak in the fill material that resulted in the loss of bentonite and clogging of the desanding unit but this was not unexpected,” says Piggin.
“There was also some concern that the glacial clay may have been gravelly at the toe of the diaphragm wall so there was a provision to extend the panels if this material was encountered at the base. In the end only one panel needed to be extended by 1.5m to overcome the issue.”
The design called for verticality of the diaphragm wall panels to be within 1:200 but, according to Bachy Soletanche site engineer Ruth Webster, the site team have bettered this on all of the panels. “The grabs are fully instrumented and there is an on screen display in the cabs of the rigs so we knew we were within the tolerances during the construction phase,” she says.
With the final panels now constructed, KMI is just about to start work on the excavation phase which will be undertaken by long reach excavators. Once excavation reached 4m, KMI will construct a corbel to carry the precast concrete beamed roof slab which will eventually be grassed over.
Tunnelling work to create the 37m long, 1.8m diameter shaft connection is expected to start in May next year but it will be early 2015 before the whole scheme is completed. However, the expertise gained on the partnering approach at Fleetwood may yet be put to further good use as Boyle believes that UU is considering adding a further storage facility at the site