CONSTRUCTION OF pre-cast concrete foundations for pharmaceuticals organisation Pfizer's new headquarters in Reigate, Surrey presented a series ofchallenges to piling contractor Roger Bullivant, consultant Ove Arup and main contractor Laing.
Bullivant's Mike Tutt said it demonstrated how a combination of foundation engineering expertise, flexible installation techniques and a non-confrontational relationship could help deliver the project ahead ofprogramme despite significant difficulties.
Ground conditions consisted ofup to 1m of made ground, over up to 18m of firm to stiffclay with flints, all underlain by chalk.
Two main options were considered for the foundations: ground-bearing linked strip footings or precast driven concrete piles. Spread foundations and other piled solutions were considered inappropriate because ofthe combination ofsurface dissolution, surface weathering in the chalk and variable ground conditions.
The benefits ofboth methods were examined in detail and eventually driven precast piles were chosen. Essentially, the pile driving set provided the opportunity to test the ground at each location.
Detailed calculations resulted in specification of350mm square piles, typically 15m long (8m in clay with flint and 7m into the structured chalk). The piles had an allowable design capacity of650kN, associated with a factor ofsafety of2. 5 on ultimate resistance.
But as work got under way it became clear that there would be three main difficulties in meeting the original criteria.
First, demolition material in the piling platform made accurate positioning ofsome of the pre-cast piles difficult. Second, the depth to the clay/chalk interface was highly variable, resulting in a substantial variation in the length ofthe adjacent piles at the defined set. Finally, pile breakages occurred during installation.
To address the latter, Laing modified the pile installation process. This used the 'best-wayout'approach ofthe Observational Method as outlined by Peck (1969) and in CIRIA Report The Observational Method in ground engineering: principles and applications (1997).
'The approach is deemed to come into play once construction has started and a previously unforeseen problem looms or has occurred, ' said Tutt.
Bullivant proposed all pile locations should be prebored and the driving criteria relaxed to 50 blows per 250mm penetration (from the 95 blows originally specified by Arup - a reduction of about 47% of final driving energy).
In keeping with the 'best-way-out'approach - which allows changes to the installation and testing methods, provided they still meet the original design requirements - the following protocols were agreed:
All piles to be prebored to the surface of the chalk with a 300mm diameter auger, to reduce driving resistance in the clay with flint, reducing the initial blow count to provide a pile driven into sound chalk (thereby mobilising pile end resistance).
All piles to be driven to a set of50 blows for 250mm pile penetration at the end ofdriving and a selected number ofpiles were to demonstrate the driving resistance increased to 95 blows for 250mm on restriking the following day. The requirement for restrike test required set-up in the chalk to occur, providing a better resistance (in all cases the hammer was a 6t hammer falling 0. 55m).
1% of all piles to be subjected to a loading test to 1. 5 times the working load (all piles performed satisfactorily).
10% ofall piles to be CAPWAP dynamically tested to provide an estimate ofthe pile displacement at working load and the ultimate resistance ofthe pile (all piles performed satisfactorily).
Tutt said: 'These changes reduced pile breakage from 8% to 2% and prevented any uncertainty about the level of chalk.
'Such challenges demand close co-operation by all parties. The non-confrontational approach ensured that, although problems could not be prevented, they could be controlled and overcome, enabling the contract to proceed and in fact be completed ahead of schedule. '