I will soon be involved with the foundation design of a 5 story concrete parking structure within 200' of a hospital building complex. The ground conditions, which have not been explored yet, are expected to consist of alluvial clays over weathered sedimentary rock lying at depths of 15' to 35' below the ground surface. The structural engineer and I are leaning towards supporting the structure on end-bearing augercast piles bearing on the rock. Two foundation contractors have told us they can install high capacity end-bearing augercast piles.
We are concerned about getting a clean surface for the end-bearing piles to rest on. I have seen some augercast piles installed in Oregon and all of the auger bits were deep "V" shaped with the concrete discharge point at least 1' above the bit tip (with these piles end-bearing was not important). These holes couldn't have been very clean.
Are there other auger bits being used in the industry that are very good for obtaining a clean hole and reliable? Are there bits being used which allow concrete to be discharged at the very bottom of the hole?
I would appreciate it if someone would share their personal thoughts and experiences with this or point me to some publications that deal with this subject. Thanks.
Kirk has covered most of the important points. However, you should make sure that the piling contractor has experience in installing augercast piles into stiff bottom layers.
If the torque is sufficiently high to penetrate deep enough into the bottom layer, there should not be a problem even if there is some spoil at the bottom as the pile will carry most of the load along the shaft, rather than at the bottom.
You may want to contact DFI headquarters and inquire some relevant publications which address the practical as well as design aspects.
You will find a link to DFI in the Market Guide section of Geoforum.com.