Coping with the unexpected
Client: The consulting engineers
Unexpected ground conditions call for rapid problem solving.
Royal Close Worcester Park, Surrey
Purkelly faced just such a problem in a recently completed contract to deal with subsidence which had affected an attached garage at a property in Worcester Park, South London.
The consulting engineers, Angell Thompson, specified a contractor designed piled raft underpinning scheme. Whilst the engineers recommended a full site investigation, this was resisted on financial grounds as, generally firm to stiff clays were expected, requiring heave protection.
Purkelly designed an underpinning raft slab using auger bored piles sleeved to provide heave protection. This method would have been appropriate for open bored pile holes free of ground water.
However, the boring for the first pile revealed a different picture. Although clay was encountered, the clay cohesion readings were not as high as those assumed in the pile design. Ground water was also encountered in the bore and the lower part of the bore collapsed.
Purkelly proposed an alternative piling method using 150 mm diameter steel
cased driven piles installed within a 3.0 m pre-bored sleeved hole. This
was accepted by the engineer.
A total of 8 piles were installed to a depth of 9.0 m using a mini drop hammer
rig.
Installation of the RC raft involved breaking out the existing garage slab and constructing a 250 mm thick slab with a downstand detail to the external walls to ensure that the new slab would not be visible above external ground level.
The slab was constructed continuous below the external walls which were propped with temporary steel jacks placed on pre-cast concrete footing pads.
Polystyrene protection was placed around the jacks to form pockets in the slab to enable the jacks to be subsequently removed to maintain full heave protection provided by a 160 mm clayboard below the RC raft slab.
