Avoiding disruption was a driving force when building a state of the art chemistry research lab in Oxford.
Alan Sparks reports.
Oxford University's status as having the largest chemistry department in Europe is set to be bolstered thanks to a brand new £60M research laboratory currently under construction.
World experts are already clamouring for the prime rooms - in the basement. 'Normally you get the top people wanting the highest office with the best view, ' explains consultant engineer Oscar Faber director, Peter Ayres. 'But here the powerful nuclear magnetic resonance machines need to be below ground, otherwise anyone who had a pacemaker on the street outside would keel over.'
Other cutting edge machinery to be used here will include magnetic lasers and mass spectrometers, with anti-vibration blocks in sump sections provided where necessary. Protection for neighbouring basements and future groundworkers will be provided by a steel shield within the basement wall.
Disturbance was a major factor in every decision made. For the basement's construction, a diaphragm alternative was chosen ahead of secant piles. 'Piling the whole 240m perimeter wall - which was proposed by some tender contractors - would have been too noisy with too much vibration for this job, ' says Ayres.
'People are generally only at university for a period of three to four years. We don't want to sour that experience through irresponsible engineering, ' he says.
Architect for the project is RMJM. Construction began in April of last year and is ahead of programme for its March 2003 finish. Future flexibility was also important, so modular construction was adopted - allowing easier rearrangement of the layout.
This permitted closely spaced columns, which in turn kept the floor zone down to just 325mm.
The above ground structure comprises two separate elements - the four storey concrete frame laboratories and a shorter three storey lightweight steel frame 'write-up' or office section so as to remain out of the college courtyard's sightline.
A 6.5m wide atrium links the buildings down to the lower ground floor level. The glazing facade is hung with cables from elliptical steel transoms, which has demanded a guaranteed level of tension throughout the construction process.
Fire safety demands in the basement caused the design team a real headache. 'Standard protection would have required extra vents the same size as the fume cupboards. This would have been physically unrealistic, ' adds Ayres. The issue was resolved by adding 'knock out' panels around the outer edge of the building.
Groundwork contractor, Bachy Solentanche continued the lateral thinking by using a polymer-based slurry instead of the usual bentonite temporary panel support medium. New to the UK, it is clean enough to be poured down the drain, so although initially more expensive, it brings a payback in disposal and reduced traffic.
With the 60m square, 15.5m deep diaphragm wall formed, a grid of 35m plunge columns were cast, and the ground slab was constructed to allow main contractor Laing to begin topdown construction.
'The slab worked better than any temporary support for the diaphragm wall and supports 650kN/m, ' says Ayres. 'This also reduced the maximum level of heave from the stiff clay.' Screw couplers were able to be cast into the wall, enabling a simple connection for these slabs.
The plunge piles are designed to take both tension and compression as they effectively restrain the maximum level of heave down to just 18mm.
Some disruption could not be avoided to Oxford's narrow historic streets in shifting the 36,000m 3of spoil from the 10m deep basement. Although lorry movements were kept to a minimum, there were still 800 a day.
The superstructure is going up at the same time as basement excavation, but far quicker than thought possible. A long lead-in time on design and planning enabled 'all problems to be ironed out' says Ayres. Contracts were then let out on an open book basis.
O'Rourke and Laing were appointed due to their level of understanding and the sheer quality of their proposals, according to Ayres. 'Other contractors offered cost savings on alternatives which were inappropriate. But O'Rourke and Laing really understood where we were coming from.' The proof of the pudding is that the concrete frame is seven weeks ahead of schedule.
A fully insitu frame would have taken too long, and a completely precast frame would generate a loss of continuity and robustness. O'Rourke proposed a partly precast, partly in-situ compromise which allowed moulds to be re-used and yet maintain the rigidity of a fully insitu construction.
'What set O'Rourke apart, was that they really love to build things, ' enthuses Ayres.
Monitoring model Any ground movement in constructing the diaphragm wall could have been disastrous for surrounding buildings, particularly the grade II listed Mansfield College which lies just 4.5m away.
In analysing potential movement of the very stiff clay which is topped with 3.5m of fill and deposits, a 3D model needed to be produced before a single spade was plunged into the old car park.
Linked to this was a system of sensitive inclinometers and gauges, to ensure constant monitoring.
This indicated the deformation of the wall, the Oxford Clay, the load applied to the supporting slabs and any movement of nearby buildings. 'When the computer detects a reading that oversteps the trigger levels, alarms are sounded and mobile phones ring via a modem link up no matter where you are in the world, ' explains Oscar Faber resident engineer, Keith Woods. One false alarm brought out Laing project engineer Keith Oram at 2am on Christmas Eve.
So precise are the monitors that temperature, humidity and rain effects can all be seen on screen.
Once the building is complete, the team hopes to publish a report into the accuracy of their model.
'Our analysis of the clay assumes much less ground movement as you retreat from the work face than the linear model would. At the moment we appear to be spot on, ' beams Ayres.
There are other benefits. As well as increasing safety, 'it allows the client and our neighbours to see that we are aware of the precise effects of our movements, ' adds Woods.