Driven piling beats a bad ground problem
Client: Main Contractor via Structural Engineer
Purkelly used 12m steel cased piles driven by vibratory hammer when soft silty gravelly clay was encountered to a considerable depth.
Brook Drive Ruislip, Middlesex
The structural engineers determined that a piled raft solution was the correct approach to dealing with subsidence which had badly affected this bungalow property.
Initial shallow depth site investigations with a hand auger indicated that the bungalow wall foundations were founded on soft silty clay with gravel. The maximum depth of the boreholes was 2.0 m where increased gravel content prevented the boreholes being progressed any deeper.
The external wall foundations were found to be spread brick footings on concrete to a depth of 590 mm
In addition a conservatory at the rear was to be taken down and rebuilt on a piled slab. The piled rafts were to be to the specialist contractor’s design.
Purkelly was sub-contracted to the main contractor to design and construct the piled raft schemes. We opted to use a 150-mm diameter steel cased piling system with piles being driven using a vibratory hammer.
For the bungalow and garage, the piles were used with a 250 mm thick RC raft. The rafts were needled into the external walls on a hit and miss basis. The raft ran continuously through the internal walls with sacrificial jacks being used as temporary propping to the internal walls.
The conservatory foundation comprised a 250-mm thick RC flat slab on 150-mm diameter piles. The maximum pile safe working load on the whole scheme was calculated at 120 kn.
Before commencing on site, Purkelly was instructed to carry out a test bore to a depth of 8.0 m to confirm the design and depth of the piles. This was achieved using a tracked mounted mini piling rig with a 200-mm diameter hole being auger bored to a depth of 10 m.
Generally, the ground was found to comprise soft silty gravelly clay to 9.0 m over what appeared to be a chalk strata. Ground water was encountered in the bore at about 2.0-m after which the returns were minimal and the bore collapsed when the augers were extracted.
Piling commenced following a general strip out and preparation by the main contractor. The tender estimated depth was 9.0 m with piles being driven to a set of 25 mm in 1 minute. However, the initial pile was driven to 12.0 m without reaching the designed set. It was decided to terminate the driving at this point so as not to exceed the recommended pile slenderness ratio of approximately 75 x diameter.
At this point we felt that it was necessary to verify the pile load which would be achieved on the 12.0 m piles and we recommended to the consulting engineer that two test piles be installed and load tested by a dynamic pile test system, this work being carried out by Test Consult Ltd.
For speed and to maintain continuity the pile testing was carried out on the installed casings prior to concreting. The pile test indicated that the piles would carry well in excess of a working load of 180 kn. (1.50 x SWL).
The pile installation proceeded on the basis of installing all piles to the 12.0-m depth regardless of their required safe working load in the piling scheme and to the original design layout.
The piled raft to the main bungalow structure was separate to the garage and conservatory raft slabs and this was constructed in two sections to minimise the extent of the bungalow internal walls being supported on jacks.
Following the concreting of the main bungalow raft slab, the brickwork above the slab was made good and the walls dry packed off the slab.
A total of 37 No. piles were installed at the site, plus two test piles. The whole of the works were completed in a 10-week period at the site.
