Foundation Piling have just completed their £600K piling and ground anchoring scheme working for John Sisk on client Ashfield Land’s ‘St Catherine’s Court’ project in central Bristol.

The whole site was steeply sloping and enclosed on all sides with the large Pro Cathedral building above, and Berkeley Place and the Triangle roads to the lower side. Some large and very old masonry retaining walls formed one edge to the site, and a steep footpath known as Pro-Cathedral Lane formed the other.

The logisitics of placing plant and materials onto the site were tricky from the start. The only access at the start of the piling works was between the old Pro-Cathedral and the existing Upper School Buildings, then by a smart left-turn into a temporary access formed through the old school building, and onto a narrow platform at the top of site.

In order to develop the site, Sisk’s plan involved working generally down the site from the high access mentioned above at an elevation of 60m, towards the lower boundary on Berkeley Place some 15metres below. This meant progressive piling and anchoring works from the top level to form a new platform at 51.5m, cantilever retaining walls or repair and support works to existing wall on the external boundary, then second-tier anchored pile retaining walls from this level down to the road level at 45m, where a second, easier access could be made directly onto Berkeley Place.

Ground conditions were well-known but expected to be variable, with the trend being 5-7m of made ground above Dolomitic Conglomerate, over the Upper Cromhall sandstone. For the initial retaining wall piles of 450mm diameter at 600mm centres, Foundation Piling mobilised two restricted access piling rigs to work together. A Klemm 708 with standard 450mm augers commenced by pre-augering the bores, then a Hutte 203 fitted with a 450mm down-the hole-hammer (DTHH) continued to full depth. This ensured full rock sockets could be formed as the expected rock profile became increasingly difficult to penetrate with depth. However, the going proved easier than expected, the DTHH only being used for final part of the bores.

Following the wall pile formation, the site was excavated to the successive ground anchor elevations, Foundation Piling using a Beretta rig to install either 12 or 9m long ground anchors, penetrating typically 3m into the Dolomitic Conglomerate or Cromhall Sandstone. Further down the site as more space was becoming available, Foundation Piling used the compact but powerful Bauer MBG12 rig to continue the piling, and finally in more space the larger Soilmec SR30 machine for the piles at the edge of site.

As the work progressed down the site, Foundation Piling moved onto the bearing piles to the bottom level elevation. In contrast to the higher elevations, harder rock was found than expected, which proved time consuming but not impossible to overcome.

Throughout the works and to complicate things further, the many centuries of previous history on the site provided some considerable challenges to all parties. More than 50 sets of human remains were found by Sisks, each having to be verified as ‘historic’ and carefully removed from site or left in situ if unaffected by the works. Across the site the existing structures were still in place to a variety of thickness and elevations- including vaulted arches and buttress walls that all had to be removed or incorporated into the works as it progressed.

Link to 'Ground Engineering' article October 2008.