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.
Rick: The Deep Foundations Institute (DFI) puts on an Augercast Pile Specialty Seminar about once per year. Tracy Brettmann of Berkel does a talk that deals with the installation of augercast piles and equipment. Based on information provided by Tracy, there are several different types of bits available to augercast piling contractors, including: standard Pengo bit, basic rock bit, knock-out bit, and clay flight bit. You may want to talk to Tracy directly (281-344-1090). We install piles into weak limestone here in north Florida, and usually it is the torque of the drilling equipment rather than the bit that governs available penetration. Our limestone is layered, with weak and hard seams. A torque of 35,000 ft-lbs or more has allowed for some penetration into the rock.
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.
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.
Seasons greetings from Australia. I came across your problem whilst installing geoforum on a new laptop.
We have recently completed a large project in Melbourne which involved the use of 800 mm diameter end bearing auger cast or a we call them CFA (continuous flight auger) piles to end bearing on a hard basalt flow at around 28 m through soft clays.
End bearing was vital and to achieve it we modified the tip of our auger and replaced the traditional V bit centraliser with a side discharge plug. We also modified the end of the auger to be flat and fitted it with Tungsten carbide bullet type teeth.
The project went very well with the first 3 piles all dynamically tested to in excess of 7000 kN, confirming good end bearing was achieved. We used a Pile Dynamics PIR-A monitoring system, and kept drilling until we achieved a high torque and low penetration rate on the basalt, indicating we had good contact on the basalt. We then used the PIR - A readings as a control for the remainder of the pile installation, which was carried out after the first 3 piles were test loaded.
We also often install CFA piles onto/into sandstone rock at shallower depths in our Sydney office. To ensure good base contact we start pumping grout and withdraw about 500 mm, then stop pumping and go back down thus mixing the grout at the bottom, and achieving good base contact before recommencing pumping and extraction. This method works well.
Your comment is justified, as I have had plenty of experience in what is called grout injected piles. The only way to be sure that a full bearing is achieved at the toe of grout,or concrete injected piles, is to, Inject a small quantity of grout or concrete,as the auger starts to lift. I have used one linial meter as a guide. Then stop injecting, and redrill into the injected material, insuring that the auger reached the original depth. The start injecting again, and complete the pour. This method insures that any loose material that is left as the auger is lifted first time, is mixed into the grout or concrete. We have extracted test piles, to inspect the shape at the toe, and the piles where redrilling was performed, had full flat bottoms. The piles that were poured in one lift, all were miss shapen, in some way, for the most part, they had only about half of the disturbed area concrete filled. Regards, Bill McGarry
Gary Chapman mentioned a PIR-A for monitoring augercast piles. To provide a little more information, the PIR-A records drilling time as a function of depth, and then monitors the grout volume pumped as a function of depth during the grouting phase. It does this with a depth sensor attached to the auger (keeping track of position or depth), and a magnetic flow meter to monitor grout volume. Further information is available on request, or simple look on our website www.pile.com for the PILE INSTALLATION RECORDER FOT AUGERCAST PILES (PIR-A) in the PRODUCTS area.