Piles on the new 953m long A2/M2 Medway bridge were able to be shortened by up to 6m following preliminary pile tests, says WS Atkins chief engineer David Puller.
Two 1200mm diameter test piles were loaded up to 2500t, using four, 1200 mm diameter anchor piles for the reaction load. The test piles were each instrumented with 28 vibrating wire strain gauges fixed in seven levels to monitor load transfer along the pile shaft. WS Atkins used a 25% higher ultimate skin friction than suggested in Ciria Report 11, Foundations in chalk, as well as significantly reduced factors of safety, which allowed pile lengths to be reduced.
Piling was carried out by Mowlem Piling and Foundations.
Most of the bridge piers and the eastern abutment are supported on 1200mm diameter bored cast insitu piles sunk through fill, terrace gravel and socketed into the underlying chalk. Up to 3m of alluvial silt and clay was encountered in the river and here 2400mm diameter piles were installed.
The west abutment and pier 1 are supported by raft foundations on the underlying chalk. Mark Dawson explains that bearing pressures of 500kPa were required, which according to the Ciria guidance meant that Grade A/B chalk was needed. However, the chalk here is Grade C, so cement grout had to be injected to fill discontinuities in the chalk.
Geotechnical contractor Keller Ground Engineering injected grout in two stages: from ground level to 3m and between 3m and 6m. Because there was no overburden, a 150mm blinding layer was cast and relatively low grouting pressures were used to prevent blowing the chalk. The effectiveness of the grouting was confirmed by pre and post-grouting constant head permeability tests.
Foundations are now complete and the piers and abutments are under construction.