AUGER BORED MINI PILING

Auger Bored Piling : JOB TITLE : Clifton Ford Hotel extension, Westminster, London

160 x 300mm diameter auger bored piles x 23m maximum length, installed at 450mm centres to form perimeter contiguous piled wall.   100 x 450mm diameter x 24m plus length, installed at 600mm centres to form perimeter contiguous piled wall plus internal bearing piles.   12 x 900mm diameter x 22m plus length, featuring plunge column reinforcement sections.  

The existing basement was founded directly onto London Clay that improved with depth from firm to stiff.   Groundwater was absent other than minor seepages.  During the works, heavy rain caused difficulties with water collecting locally within the site.  Temporary casing was used as required to overcome areas of fill, softened clay and surface water.  The site had  a highly restricted access.  The small site area was bounded on three sides by busy, narrow streets.  The  ramp into the existing basement level commenced directly from the narrow site entrance, complicating traffic management problems.  Deliveries, particularly of ready mix concrete required careful co-ordination and timing. The work areas were limited as two rigs plus a large diameter rig were operating.  Access was further complicated by the staged release of work areas and the temporary works propping arrangements.  The ramp took up a large area of the site and the position changed as the works progressed to allow access while maintaining entry for deliveries and muck away.

Contract gives Branlows limited headroom

In most areas the existing basement wall extended beneath the boundary roads and the associated services within the pavement.  Through constant liaison with the Engineer, a final approach involved a temporary works cantilevering arrangement to support live services under which, at the lowest point, the  mini piles were installed the contiguous wall in 2.5m of headroom.  Most of the remaining perimeter wall piles were installed in a headroom of 3.5m or less.  Reinforcement cages were required in short sections to suit headroom limits and were lapped during installation.  The temporary propping arrangement to the existing perimeter wall was spaced to allow our mini piling rigs to install a section of the contiguous wall.  Two pile rigs worked in an overlapping sequence of bays allowing the prop positions to be moved sequentially, whilst at all times, providing the required level of support to the surrounding structures.  In order to achieve the required four week programme, careful planning on a daily basis was vital to ensure site congestion did not hamper efficiency.  Plunge columns : The 900mm diameter internal piles incorporated plunge steel section reinforcement, with stringent constructional tolerances, to form the column stanchions for the next level of the basement.  A conventional cage was also required to reinforce the piles below the low level concrete cut off.  The stanchion sections and cage were prefabricated off site into a single unit and delivered to site at intervals as required.