Civil Engineering Reference
In-Depth Information
7.5 Cast-in-place screw piles
These piles share some of the potential problems that can occur with CFA pile in as
much that the extraction of the auger is required at a controlled rate in order that
concrete fills the void created by the screw auger completely and that the screw-form
is created correctly. Unless the screw-form is properly formed it will not contribute to
the pile capacity. It is possible for the threads to be badly or partially formed and these
will shear off from the pile shaft at low stresses. This is illustrated in Figure 7.24 which
shows severe pile necking likely to have been the result of poor control of extraction
rate of the auger. It is interesting to note that ultra-sonic integrity testing did not reveal
this fault.
Rig instrumentation is therefore essential to control the auger extraction and con-
creting process. In those screw piles in which there is no attempt to form a thread by
back-screwing in the original downward path cut by the auger, the soil is remoulded
during extraction and concreting and whilst the dimension of the pile is greater by
the outer screw diameter, cohesive soils are at a remoulded strength. Pile capacities
Figure 7.24
Necking of a screw pile shaft likely to have been the result of poor control of extraction
rate of the auger.
