Structural and geothermal piles, plus chalk with an identity crisis in Cambridge. By Kevin Walsh
Main contract Value: £159m (BAM)
Piling subcontract value: In excess of £1m (May Gurney)
Client:Medical Research Council (MRC)
Main Contractor:BAM Construction
Piling Contractor:May Gurney
Start Date: March 2009 (BAM on-site) 20th April 2009 (May Gurney)
Completion Date: January 2012
Staff at the existing lab will surely mourn the loss of the facility which has produced no less than 13 Nobel Science Prize winners in the last five decades. However the new state-of-the-art laboratory will be a welcome addition to the hospital, boasting enough lab space to accommodate 400 scientists, in addition to 200 support staff.
A new concrete frame structure covering 106,608 sq m has been designed with social interaction at the forefront of design, with several meeting areas providing a friendly working atmosphere. A steel frame roof structure containing a restaurant with panoramic views will also sit on top of the concrete frame.
To ease the level of disruption to research during building maintenance, each floor of the three storey building will be built to 6 m in height, providing room for 2.75 m tall interstitial steel floors in between each storey that will house all M&E services.
BAM Construction project manager Shaun Harris says: “Each floor of lab space has its own space above it, and all the M&E pipes and ventilation equipment for the lab is installed there, above it.
“It means that if in the future they want to refurbish their lab space, they can access the services while the lab stays live. They can refurbish all of the plant that’s on the floor above it easily, because it’s a walk on floor. It’s got all the ducting, the cable trays, and all the pipework above the actual lab areas. It works out far better than having it above a false ceiling.”
This layout also conveys benefits during the construction period, as BAM Construction will be able to work on building the laboratories while simultaneously installing the necessary M&E distribution services.
But by far the most ambitious aspect of this project is that when completed, the heating and cooling systems in the building will draw on the largest and most extensive use of geothermal boreholes in the country.
The closed loop system will use roughly 250, 150mm diameter boreholes, up to 100m deep. The closed loop pipes in the geothermal boreholes will use a thermal medium, such as a Glycol mix, which is pumped through them to absorb and transmit the thermal energy from the ground to the buildings heating and cooling systems.
This will provide 1.4 MW of cooling and just over 1.5 MW of heating, which BAM intends will constitute 10 per cent of the laboratory’s annual power consumption. This is in addition to power from the energy centre being constructed adjacent to the main building.
BAM is currently deciding on bids from the tender, valued in excess of £2.5M, with a decision imminent.
Initially, the firm intended to include a network of energy piles in piling contractor May Gurney’s subcontract. But when considering of the levels of energy generation required, the impracticality of using geothermal piling among the foundations became clear.
BAM’s construction manager on site, Jim Power explains: “If you imagine boreholes and piles in terms of linear metres, you have around 250 boreholes and these go up to 100m depth. The 850 or so building piles are all somewhere between 17m and 27m depth. At 13m or so below ground level you derive a constant temperature of approximately 12.5ºC.
“This means that boreholes deliver a more feasible solution as the thermal transfer fluid is exposed to a greater depth and is present in the constant temperature over a greater length than it would be with energy piles. The more linear metres the more energy you can recover from the ground. If you wanted to use energy piles you would have to make massive piles or increase the number of piles way beyond what is required for the structural solution. It would be uneconomical.”
As a result, the decision has been taken to move the borehole scheme to a location off the critical path, beneath a planned car park to the south of the building within the site boundary.
Even though the geothermal borehole system being removed from their scope of works May Gurney has a lot to be getting on with. Despite the Medical Research Council initially pushing back the start date, May Gurney is confident of making good headway on the project with piling work just beginning this month.
“At the moment everything is going extremely well; it’s exactly how we’ve foreseen,” says May Gurney operations manager Steve Bursnell. “Our programme of works was to get in and do the preliminary test piles, get them tested the second week and do some probing. Then when we got the test pile results within the parameters of the design, which they are, and all the pile designs had been approved, we planned to start the main works today. From here it should take 12 weeks to complete.”
The availability of only one access and egress point on the site, which leads onto a private road could have easily have put paid to May Gurney’s 12 week project timeline, especially with heavy goods traffic limited to between the hours of 10 am and 4 pm. So in anticipation of the large concrete pours needed throughout the project, a Hanson on-site batching plant was hired to minimise traffic to site.
Chalked up to experience
Having worked on the Addenbrookes campus several times before, Mr Bursnell is all too familiar with the troublesome ground conditions across the site. With the main contractor’s sister company BAM Nuttall simultaneously working at the back of the site, on Cambridgeshire County Council’s 43 km long £86M guided bus route, there’s no shortage of information regarding the local geology available to the team.
As a result of careless disposal of excess construction material by previous contractors in years long past, the client insisted on pre-boring checks for any obstructions across all of the pile positions. This was to ensure any obstructions that had been backfilled over by local farmers would be uncovered prior to the commencement of piling.
However the biggest difficulties on-site are not caused by obstructions in the first few metres of the ground but by the roughly 10 m deep layer of chalk marl, overlying gault clay, which exhibits some unusual properties.
May Gurney estimating and design manager John Earp is no stranger to the awkward strata on this site. He explains: “We have had this problem before, we describe it as a chalk with an identity crisis, because the chalk behaves more like a cohesive material. It’s better [to drill] than you’d expect a normal clay to be, but not as good as what it would be if it was chalk, so we came up with a mid-way design based on that.”
“It took a lot of time and trouble to actually identify how this site behaves, and how the piles performed, because when we originally designed for chalk, we got far better results during design than what we actually did in the ground. Through a lot of R&D and test piles on the site we managed to come up with our own design parameters for this strata, so it was quite a complex thing to come up with our final design parameters for this.”
On this site May Gurney conducted four preliminary test piles, two at 1200 kN, and two at 2000 kN. With these tested to two and a half times their working loads, there were also five working pile tests.
The end result was three different pile design cases for the 859, 600 mm diameter CFA piles going in the ground, with the longest pile being used on site reaching 26.7 m deep. All piles on site are being cut down to between 600 mm and 5 m in the ground.
Of these, 831 are load bearing piles, while the remaining 28 are for the contiguous piled basement wall, going down up to 15 m for a retained height of 4.5m.
With detailed knowledge of the site’s strata, and detailed reliable design borne out of experience achieved, Mr Bursnell expects progress on site to continue without a hitch.
“The two drilling rigs we’ve got on here are the Llamada P150 and the Soilmec CM50 and the drilling is superb really, we don’t foresee any problems now. Hopefully we’ll be out within our programme period and everyone will be happy.”