Auger Cast Piles
Auger Cast Piles
(OP)
A few general questions on designing foundations supported by augercast piles:
1) It appears augercast piles are usually specified with small diameters (10"-16" diameter range). Does a typical auger cast pile for gravity loading need to be reinforced, or is it assumed to derive all of it's structural strength from soil confinement and the compressive strength of the concrete? If reinforcing is required, would the geotech usually provide required depth of reinforcing so that the assumed axial capacity of the pile can be reached? What is tyically provided by the geotech when specifying auger cast piles should be used?
2) Are augercast piles acceptable in foundations supporting the lateral load system, If the foundation must rely on the augercast piles for shear and uplift resistance?
3) What is the standard protocol for designing foundations on auger cast piles supporting lateral load framing? Should the geotech provide a point of fixity below the top of pile to the structural engineer? Should the structural engineer then design the foundation as a pile cap on fixed based columns with a length equal to the point of fixity?
Thanks in advance for any insight!





RE: Auger Cast Piles
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RE: Auger Cast Piles
I have installed several thousand ACPs in the mid west (USA) and all are performing very well.
1. Pile sizes have been going up for the last several years. A common maximum size would be 18 or 20 inches, most large contractors should be able to install piles of that size. Last I heard some contractors can install piles up to 36 inches. As for reinforcing, I personally like to install a single bar to the bottom of the pile just to check the pile installation. However, for downward vertical support no reinforcing is required. If you have uplift, which the piles handle very well, then a single bar to the bottom of the pile is required. Also, if you have lateral loads then you will need a typical cage of reinforcing to handle the moment in the pile, more on this below.
2. ACP are installed and used every day to resist lateral loads.
3. The ACPs should be analyzed by the geotechnical engineer providing the design using a software program such as LPile. The group effects of the piles are very important as is the size and type of pile cap. This generally requires the geotech and the structural to work together as the process is iterative. In the end, the reinforcing cage should be installed somewhat deeper than the theoretical point where the moment and deflection in the pile goes to zero.
A few more points on ACPs. While I have designed, tested, and monitored the installation of literally thousands of auger cast piles, it is not the best foundation for all situations. Please review the soil and load conditions with the geotechnical engineer. You should also consider the part of the country where the work is located and the availability of contractors to do the work.
ACP installation and quality is VERY dependent on the contractor doing the work, they are not all equal. My preference is to require that a test pile be installed and tested to at least 200 percent of the design load. This not only validates the design but also proves that the contractor can install good piles under the conditions present at the site. If a test pile is installed, make sure that the contractor uses the same crew and equipment as will be used on the production piles.
Hope this helps.
RE: Auger Cast Piles
RE: Auger Cast Piles
cbosy, I don't like the low strain integrity testing of the piles. I have done a lot of it, but in my opinion if you do careful full time monitoring during construction, then the integrity testing is not necessary and can create more confusion and questions than it answers.
RE: Auger Cast Piles
RE: Auger Cast Piles
But perhaps I haven't seen the equipment that the contractors in your area are using - are they like a large HSA capable of inserting a full rebar cage through the concrete injection "pipe"? I've only seen rigs that can install a single (large) bar in the center of the AGP, or perhaps a very small rebar cage. In my view, a single central rebar - no matter how large - or small rebar cage is not sufficient for laterally loaded piers which develop a cracked section. You can't know where the damn thing is located within the pier cross-section. (A 60 foot long No 9 bar is really quite flexible.) It's too much 'by guess and by golly!' for me - I don't like to assume that much. I guess I've looked at too many failed structures to take that one on faith.
And please re-read my first post. In it, I said: I don't believe that auger-cast piles are appropriate where "significant" lateral forces are expected. Perhaps we need to discuss and agree on what is meant by "significant" lateral loads. In my view, significant lateral loads are those that would result in the development of a cracked pier section. In other words, the pier is designed to pick up the lateral loads by lateral soil resistance (a la p-y curves) and not by 'frame action', and a reinforcing cage is needed to handle the tensile forces within the pier.
I'm glad you use LPILE for your analyses, GeoPaveTraffic. In my view it's the only commercial program to use for performing lateral load analyses. Yet I am very familiar with the inherent risks with using p-y analyses; Lymon Reese was one of my professors and the second reader on my Master's thesis. (My topic was lateral load analyses of pile groups using p-y curves.) Based on your other posts on Eng-Tips, I'm convinced that you are using the program in an appropriate manner. Still, it is important to point out (to others) that the use of such a sophisticated tool does not obviate the need to use sound engineering judgment. The design parameters need to be chosen carefully, and the results carefully reviewed. To borrow from my computer science buddies, "garbage in - garbage out."
Chicago has some weird City codes - like the restriction that only bearing or skin friction - but not both - can be used for pier design. Unless to field verify, of course. An arcane requirement in my book. And a waste of money. Glad that's not on my 'beat.'
Don't feel too bad about disagreeing with me, CBosy - my 22 year old does all the time. I don't take it personally - and I've got a pretty tough hide!
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Auger Cast Piles
RE: Auger Cast Piles
RE: Auger Cast Piles
I agree that the center bar does not add to the ACP for lateral load resistance. However, a typical reinforcing cage can be installed in an ACP. It is normally installed after the pile is pumped and the surface spoil has been removed. It is important that these tasks be completed as quickly as possible and the cage installed. The longer the time between the grout being placed and the cage being installed, the harder it is to install the cage.
You made reference to full cages. I don't know if you mean full length cages or cages that are say 6-inches less in diameter than the pile. From the projects that I have worked on, the average required length of cage was on the order of 20 feet. Some were longer some shorter. The cages were typically installed using a "cherry picker" or similar small crane and inserted into the pile within 15 minutes or so of the grout being pumped. The cages are centered in the pile using rollers that are attached to the cage. The rollers are really just plastic wheels that provide the 3 inches of grout cover between the steel and the soil. They are not perfect, but all in all work very well.
When I write the specs, if the cages cannot be installed, the pile must be redrilled and the process started over again. This happens maybe 1 percent of the time if the contractor is working well and not having problems. If there are a lot of equipment or crew problems, it can happen a lot. Given the price of the grout that is typically installed, the contractor tends to correct those problems quickly.
Using these methods, I have designed, installed, and tested ACPs on many sites. Measured lateral loads at 0.5 inch or less of deflection have been up to 30 tons on a single pile. I don't recall off hand what we gave the structural's for allowable capacities or what the cage geometries were. But to me 5 or 10 tons of allowable lateral load per pile is a significant lateral load.
Also, just for general information. When these piles were tested for vertical load capacity, reaction piles were used in uplift. These reaction piles were reinforced with a single bar, usually a 1-inch Dywidag.
If you have any other questions about how I have seen thing done feel free to ask.
RE: Auger Cast Piles
What diameter AGPs are you installing? What's the L/d for the 'reinforced' section? How strong are the near-surface soils? Magnitude of lateral loads? Pier performance criteria?
We don't see AGPs in central Texas - soils are hard and quite expansive. (San Antonio was singled out in Terzaghi & Peck in their discussion of expansive soils.) "Minimally" reinforced piers (< 1% steel) have been yanked in half in less than a year - we even require the contractor stagger the rebar splices. Our active zone is 15 to 20 feet deep; I've found ½-inch infilled shrinkage cracks at depths of 23 feet. The method of construction that you describe won't work here. The cages will have to penetrate at least 35 or 40 feet.
And AGPs aren't making much penetration in Houston, either. I haven't seen them any closer to my home than Alabama or Mississippi, so far - although they may be in use in southern Louisiana.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Auger Cast Piles
I have read your responses with great interest, not having much experience myself with Auger Cast Piles. We are currently looking at a site in PA, USA where ACP's seem to be the preferred alternative. As I have gone through the literature on compression and tension design in cohesionless soils, I note that the skin friction factors appear to be much higher than if you were to use Ko tan(fe). I am referring specifically to the O'Neill and Reese FHWA manual on Drilled Shafts (1999) where they say that the beta can range from 0.25 to 1.2. Could you please give me some discussion on the physical reason why skin friction factors can be so high? Is it that the surface is so "rough" that the effective pile diameter is much larger and that the failure occurs in the sand/gravel away from the pile? Just a guess.
Any thoughts would be appreciated.
Thanks,
Glen
RE: Auger Cast Piles
Another factor that can greatly effect the capacity of the piles is the installation technique. That is the reason that I always recommended a pile load test.
For design the method you quote is often used, as are driven pile formulas. Another option is to talk to an ACP contractor who has installed piles in the area and see what capacities they got. They may also have some load test results that they are willing to share.
Hope this helps.
RE: Auger Cast Piles
I don’t have access to the specific mix design; however, a ¼-inch max aggregate is about right. The mix had a 28 day design strength of 5,000 psi, 15 to 30 percent fly ash, and a job added plastizier. Most breaks were 5,000 psi in 5 days with about 6,000 psi in 28 days. The grout is tested with a modified (1/2 inch opening) flow cone and cubes.
The pile diameters were 16 to 20 inches; don’t remember the length of the cages. Near surface soils were generally medium to medium stiff lean clays with sand.
RE: Auger Cast Piles
RE: Auger Cast Piles
A 16 inch diameter pier with a rebar cage that's 20 feet long has a L/d ratio of 15; a diameter of 20 inches yields a L/d of 12. These fit in the category of 'rigid' to 'transitional' piers; they certainly aren't 'flexible' - unless the loads are fairly light i.e. the shafts are very unlikely to crack under load. The portion of the pier below the rebar cage may or may not be 'in play' from a lateral capacity aspect; it really depends on the type and combinations soil properties, pier properties, loads and the frequency of occurrence of each range of loads. The statistical aspect of pier design is in its infancy; and LPILE - or any other lateral load analysis tool - can't make this assessment. That requires the engineer's judgment.
GeoPaveTraffic: can you post a 'typical' case in this thread for discussion? Please include the relevant soil and pier properties as well as the design loads. Your results would also be of interest -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Auger Cast Piles
Natural Gas Compressor Stations (our work) typically use augercast piles of 24" diameter, as required by soil conditions. We usually provide for upper rebar cage for lateral loads and for uplift with either freezing soils or with swelling clays. If vertical load only, no rebar is theoretically required; however, this is seldom the case. The geotech's role does not include rebar. The geotech provides end bearing and skid friction values.
RE: Auger Cast Piles
This is typically, but not always, true. For instance, I frequently include a requirement that the rebar splices be staggered when dealing with deep, expansive soils. And I've been known to design - and seal - the pier foundation drawings for some kinds of projects. Then again, my undergraduate studies included a lot of structural engineering courses.
Don't make the all too common mistake and believe that the geotechnical engineer knows "nothing" about designing pier reinforcement. We have lots of opportunities to see what does - and doesn't - work from both a design and construction standpoint...
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Auger Cast Piles
Interesting perspective. I too agree that some geotechnical firms/engineers will get involved in foundation design drawings. I have always thought that this was more a business/marketing decision rather than a technical capability decision. In other words, if the company's business model is to perform some design, then the geotech's will seal design drawings. One potential downside of this is the negative marketing that can happen if some of the firm's clients (structural design firms) perceive that the geotech is taking business away from them.
I have seen this happen.
Glen
RE: Auger Cast Piles
When challenged on that point, I usually provide a rather long laundry list of areas that structural engineers 'tread on my turf.' Not in an angry tone, just as a point of departure for the discussion that follows.
Most readily get the point: our subdisciplines have overlap. In many cases it doesn't make any sense to involve a structural engineer in what is primarily a geotechnical engineering design. After all, they make geotechnical engineering decisions all the time...without the involvement of a geotechnical engineer.
And for what it's worth, we are usually working for the owner, not the structural engineer. We frequently get referrals, of course - but most structural engineers are really interested in being involved with a quality project team, not in the 'loss' of a few hundred dollars' worth of work. In my experience, the complainers aren't necessarily the most desirable clients or team participants, anyway.
Yes, I too have heard the complaints. But very infrequently.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Auger Cast Piles
RE: Auger Cast Piles
For static loads, I have installed battered piles on a hand full of jobs.
RE: Auger Cast Piles
I have logged many 16"-24"-dia piles 60' - 90' deep, with full-length reinforcing, with no serious difficulties. There is only one condition that affects reinforcing placement: if permeable material is encountered below about 30', and the cage is not inserted quickly after the auger is withdrawn, it will be a problem. The grout will bleed out into the sand or gravel very quickly, and the cage gets more resistance with depth as it is inserted. When it reaches the stiffened grout, it drags to much to advance further. 5 minutes delay can be a problem, by 10 minutes, plan to reinsert the auger. I have heard of people misinterpreting the blockage at a sand layer as a mixing of the sand and the grout at that point, and therefore a degradation of the the integrity of the pile. That is NOT the case. Think it through. There is always too high a head of grout pressure to allow that to occur.
I'm a great fan of auger-cast piles. I'd enjoy hearing from others about projects where they proved advantageous over CIDH, driven, etc. I have also had great experiences with geo-piers.
RE: Auger Cast Piles
Another reason that the skin friction values are higher is that in ACPs, the grout is injected under pressure and, therefore, induces a passive state where the grout moves toward the soil, rather than an active state such as in drilled piers where the soil moves toward the concrete.
Eric
RE: Auger Cast Piles
I generally agree with most of your comments. However, I do not agree that the ONLY way to get an accurate grout volume is by using a PIR. A competent inspector with proper training and observation CAN accurately determine the amount of grout pumped in a pile and the distribution of that grout in the pile.
I also disagree with your statement "There is always too high a head of grout pressure to allow that to occur." If the auger is pulled too quickly or the grout pump misfires or a hose breaks or the pump runs out of grout, etc. then there can be a place in the pile where the pile has caved in. I have seen this only a few times in many thousand piles, but it does happen.
Your comment about a sandy layer "sucking" the water from the grout is VERY true and is one of the biggest problems during many ACP jobs.
RE: Auger Cast Piles
RE: Auger Cast Piles
www.foundationtechnologies.com
Look under "products" at Spacers and Centralizers.
If any one has used them before I would be interested in your opinions. They appear to address some of the issues brought up in earlier posts to this thread regarding clearance/cover of the steel. (A salesman brought a spacer into my office probably more than 10 years ago, but I have not worked with auger cast piles since then; they appeared practical and the concept was well thought out).
RE: Auger Cast Piles
re: lateral resistance. A thick lean concrete "rat slab" of 1'+, below a structural mat was used on the project mentioned above, simplifying the cage design, which is more an issue of handling/hanging/placing, then protecting during ongoing installations. The work sequence was: pile intallation; excavation to rat-slab+4" subgrade; pour rat-slab; complete structural mat. [cages are grade-60]
RE: Auger Cast Piles
Great discussion.
Here are my questions for experienced participants of this forum:
1)Are there any practical limitations for the length/diameter of reinforcing cage? DFI spec Sec. 1.3.indicates that piles with slenderness over 30 require experienced design professionals and pile contractors.
I have recently reviewed a project where design engineer specified installation of full length reinforcement
6#6 w ties w #4 ties @ 12") in 65 ft long augered cast-in-place piles 18 in. and 24 in. diameter. Any comments?
2) What is % cost increase for the above referenced full length reinforcing installation vs. single bar reinforcement?
3) DFI specs section 2.1.2 indicate that "steel pipe has been successfully utilized to reinforce ACIP piles". Is this a reference to steel casing installed after auger removal?
RE: Auger Cast Piles
I don't have any idea how much the contractor would want for installing that type of cage.
Have never seen pipe used, however, I assume the pipe would be at least 6 inches smaller in diameter than the pile and would be installed after the auger is removed.
RE: Auger Cast Piles
RE: Auger Cast Piles
The real issue is - why design engineer would select a full length reinforcing cage for ACIP piles? It appears that full depth reinforcing cages are specified as a result of very conservative approach to ACIP pile design.
ACIP piles are not preferred choice for applications where resistance to significant lateral loads is anticipated.
It appears, that if ACIP piles are intended to resist flexural loads, reinforcing cage does not need to be any longer than total distance from surface to theoretical point of fixity + reinforcement development length - in most cases 20 ft+/- should be adequate.
RE: Auger Cast Piles
I concur completely!This gives me lots of indigestion. Why, in this modern age and with all of our accumulated knowledge about soil-structure interaction, are we still using point of fixity in design?
Perhaps that question should be posted to a new thread. Any thoughts?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Auger Cast Piles
RE: Auger Cast Piles
I would also ask; do you believe that driven piles should be used in situations when significant lateral loads are expected? If so, what is the difference?
RE: Auger Cast Piles
RE: Auger Cast Piles
I'm still waiting for problem specifics, GeoPaveTraffic. (See my April 15, 2004 post.) Without them, I remain unconvinced.
For those of us that have been involved with offshore and heavy civil/marine facilities, 22+ ft tall cantilever drilled shaft walls in clays, and electric transmission line monopole structures supported on drilled piers in soils, "significant lateral loads" has a pretty specific meaning. In my book, a 1,000 kip lateral force acting on a protection dolphin at El. +8 ft with a mud line at El. -45 ft in the Port of Houston - now, that's a significant lateral load!
I can't judge the merits of your claims without specifics...
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"