DRILLED MINI PILING

Drilled Mini Piling : Leighstone Court stabilisation

Drilled mini piling design : Branlows were  responsible for the detailed drilled mini piling design and installation of all drilled mini piling works which  comprised, 70 x 220mm diameter drilled minipiles socketed into sandstone at depths of up to 31 m.  The majority of the drilled mini piles were cored through the existing structural raft/floor slab.  Other drilled mini piles were supporting new reinforced concrete ground beams at critical corner location.  

Drilled mini piling : The existing 3 storey flats were constructed 30 years ago, built over vibro column foundations. Due to the depth and extreme looseness of the fill material, the vibro flotation failed causing subsidence to the flats.   In 1990, the insurance company responsible for the property brought in a specialist piling contractor to stabilise the building using a system of piling and overlying slabs.  Although the slab was installed adequately, unfortunately, the piles were not taken down to bedrock.  Where these pile depths have been determined through testing, they were found to be in the 6-1 Urn depth range.  With rock head varying on the site between 13-26 metres, the stabilisation works exacerbated the problem by increasing the dead load on the loose fill, triggering off further consolidation settlements.  

Exceptionally loose fill was encountered in the infilled quarry above rockhead, with N values normally below 5 and in some areas in the 1 -2 range.  Rockhead was found to directly underly the fill material in most areas although in some instances massive sandstone boulders were encountered just above rockhead.   The majority of the piles were installed within the existing building with a limited headroom of 2.5 metres.  The new works were undertaken within the four ground floor flats, foyer and stairwell areas.  The presence of numerous internal walls, door openings etc made access extremely difficult.

Drilled minipile 220mm diameter  : were used throughout and the mini piles were installed by employing an eccentric overburden drilling system to take the permanent 220mm diameter casing a distance of approximately 1 metre into solid bedrock.  A down-the-hole hammer of 170mm diameter was then used to form rock sockets of average depths of 4-5 metres.  Safe working loads of 320Kn were adopted and the permanent steel cased piles were infilled using tremmieing techniques utilising a cement/pfa C40 grout mix.  Reinforcement comprised a full length 25mm Dywidag bar with full strength couplers.  

Problem ground, very loose fill material, with N values commonly less than 5, were encountered in the upper 20 metres; in some areas the fill was present directly over the sandstone formation.  In other areas where the sandstone formation dipped to depths of 26 metres, large rock boulders were encountered in the base of the old quarry.  This caused  difficulties drilling for the piles as it appeared in numerous instances that natural bedrock had been encountered.  On commencement of the final rock socketing, we found the works for the piles went back into loose fill material below the sandstone boulders which necessitated reconnecting the overburden mini pile drilling system and taking the permanent steel casing deeper into the underlying competent bedrock.   

Twelve families had to be evacuated from the flats and were placed in alternative accommodation. These costs alone, together with the inconvenience to the residents, demanded the shortest possible time scale for the works on site.  Due to the restricted access and limited headroom on the site, static load pile testing was not undertaken but the mini piles had two dynamic load tests carried out using Capwap analysis.  Settlements of 3.0mm and 3.07mm were recorded at working load and settlements of 4.51mm and 4.28mm were recorded at one and a half times working load on the two test piles.  On removal of the load, a 95% recovery in the pile deflection was observed, indicating that the drilled pile settlement was of a purely elastic nature.