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.
We have done piling under somewhat similar conditions. Piles having penetration more than 30'can be cast as augercast concrete piles. Even then as a safe practice we went for socketing for such piles in bedrock by 1Dia. of pile(i.e socket=1*Dia.of pile) in underlying bedrock. The point was that we wanted to increase load carrying capacity of pile for such short penetration.(I myself consider 30' penetration as short(10m)) There are various types of auger bits available for such type of piling and they can give you fairly acceptable base of pile for concreting. I would suggest to you that you conduct static load test of pile to make sure pile carries designed load. As far as piles with penetration less than 30'I can say that augercast piles are not suitable. I have done micropiling in such conditions. All the details including augercast piles which I have done are difficult to write however I would be happy to provide if you need them as reference in making decisions. My e-mail:email@example.com Thanks. Ashwin.