A cost-effective alternative for piled foundations is being used at the home of F1, Silverstone.
Silverstone Estates is building a technology park as part of a programme of development at the home of Formula One racing in Britain.
South Northamptonshire Council recently granted planning permission for the park, which will house firms associated with motor sport.
One early project is for a building on a greenfield site close to a balancing pond. Originally a piled/suspended slab was proposed to address potential shrinkage and swelling of the subsoil desiccated clays which could be further affected by nearby trees.
Although the ground was capable of taking the bearing pressures required at depths of about 2m, the piles would have extended well beyond this to avoid the potential effect of heave forces caused by the expansive properties of the clay.
It would have also needed compressible layers around the piles and under the ring beams together with a suspended floor slab to absorb any potential movement. This approach was rejected on cost grounds. Main contractor Mainland Construction appointed Con-Form to help develop an alternative.
Working with designer Complete Design Partnership, Con-Form proposed an £80,000 E-Found foundation solution which involved reengineering the ground through the addition of lime and rolling dynamic compaction across the 1,800m 2 building footprint. The contract included cutting and filling to level and subbase replacement.
Lime modifies the structure of the clay, enabling the desiccated material to be compacted at its optimum moisture content, and rolling dynamic compaction consolidates it at depth uniformly across the site.
Con-Form managing director Andrew Armstrong says: 'It's a hybrid solution that integrates soil stabilisation and rolling dynamic compaction to multiply the benefits of both.
'We use the lime to dry the soil out so we can increase the energy put into the ground and the lime increases the soil [strength] characteristics anyway.' Testing ensured sulphates, which might cause heave during the stabilisation process, were not present.
A maximum 25mm settlement criterion was established at the frame positions with no more than 15mm differential settlement between adjacent frame legs.
The E-Found system required the treatment of the building footprint plus 3m around the perimeter to a minimum 2m depth. The material removed was stockpiled and plate bearing tests of the competent strata base layer gave readings in excess of 200kN/m 2, well above the speci'ed 125kN/m 2.Checks ensured there were no roots or desiccation below the 2m treatment level, but to be certain of its competence, Con-Form treated this base layer with lime in the same way as the subsequent layers above.
This saw site workers adding water to achieve a moisture content that was wet of optimum moisture content. The moist clays were then brought back to optimum through the addition of the lime with the soil rotavated to a 300mm depth.
Calcium in the lime reacts with the clay's silica and alumina causing a slow, cement-like, pozzolanic hardening reaction. This changes the clay's structure, limiting the potential for further shrinkage or expansion, and increases the unconfined compressive strength of the modified soil. Testing verified the target CBR of 7% to 10% by three days for the base level.
Once the insitu level had been treated, the excavated soil was backfilled in layers of 300mm and treated with lime and water as required.
Con-Form achieved 95% compaction using a traditional roller with nuclear density meter tests verifying the results.
Rolling dynamic compaction was then used across the building footprint to minimise the potential for further long term and/or differential settlement taking place.
The true value of the rolling dynamic compaction was shown when an additional settlement of 75mm was induced in the material already compacted to 95% of its dry density. Further plate tests confirmed insitu minimum bearing pressures of 125kN/m 2.After the testing, a 250mm capping layer was laid out, treated with lime then lightly compacted and allowed to mellow for 24 hours. The layer was then remixed and water added as necessary to achieve the optimum moisture content.
One further round of rolling dynamic compaction was used with plate testing verifying the results.
The surface layer was then trimmed to form a capping layer and testing gave results of about 20% CBR.
Rolling dynamic compaction was also used in the car park area to induce settlement and to highlight potential soft-spots that would not have been identified through traditional rolling.
The area was finally cut to the required finished level and treated insitu using lime and cement to form a 250mm 30% CBR layer. The whole site was then overlaid with 75mm of imported granular material and trimmed to final level.
Con-Form's E-Found foundation works at Silverstone, which took about two weeks, were completed last month.
Armstrong says: 'There's a lot of manual labour with piling and this is less labour intensive. With steel prices going up the savings are greater as well, so we have a lot of jobs in the pipeline including a 170-home mixed housing development in Norfolk.'