Auger Cast Piles 2

[shepherd]

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!

 

[Focht3]

Here's my 2 cents' worth -[ol][li]I would specify reinforcement. But not all do - and not all auger-cast rigs can install it. In fact, most can't.[/li][li]I don't believe that auger-cast piles are appropriate where "significant" lateral forces are expected. The "jury is still out" on the question of using this foundation type to resist uplift.[/li][li]I wouldn't provide a point of fixity - lateral loading isn't appropriate for this foundation type.[/li][/ol]



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[GeoPaveTraffic]

I have to disagree with Fotch3 on each point.

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.

 

[cbosy]

I couldn't agree more with GeoPaveTraffic (I can't believe I am disagreeing with Fotch 3). The only thing I would add is to do some low strain integrety testing on maybe 10% of the piles and specify full time monitoring.

 

[GeoPaveTraffic]

Thanks cbosy, I can't believe that I didn't comment on the need for full time monitoring during installation of the piles. I would add that the monitoring needs to be conducted by an inspector experienced in pile installation or if this is the first ACP project for the geotechnical firm, then the inspection should be by an engineer who fully understands the installation methods and risks AND experienced inspector to watch the general construction activities.

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.

 

[cbosy]

Low strain testing is good for piles with an l/d ratio less than 30 so caution should be exercized when recommending it. Can you believe that the City of Chicago requires testing of every auger cast pile? That is why low strain testing is used when the auger cast piles are short and usually gives pretty good results.

 

[Focht3]

My biggest issue with ACP is the installation of the reinforcing cage. In my view, a proper cage is needed in order to really pick up lateral loads. And I've seen lots of attempts to install them "after the fact"; most were unsuccessful, with the exception of pretty short piers and large vibratory hammers. And these made me nervous - no way to know the amount of concrete cover...

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 N[sup]o[/sup] 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, [blue]GeoPaveTraffic[/blue]. 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 [green]Eng-Tips[/green], 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, [blue]CBosy[/blue] - 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?"

 

[shepherd]

Thanks to all for the input. Is the low strain integrity test tied to an ASTM desginated test by chance?

 

[dirtsqueezer]

Just a small comment if you do decide on reinforcement. You may also want to include a center bar in addition to the cage. I have been on several projects that called one out, and one that did not. I honestly don't know its specific function (possibly to minimize shrinkage cracking?), but it turned out to be required by the code, and got pretty nervous when it was never put in. If made to choose, I usually sway toward more steel rather than less. Good luck!

 

[GeoPaveTraffic]

Focht3,

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.

 

[Focht3]

You refer to 'grout' - what material properties are you using for the concrete? (I assume the maximum aggregate size is [¼]-inch.) Are you using a flow box, or still using slump cones? Retarders, water reducers and/or superplasticizers?

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 [&frac12;]-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?"

 

[gandersen]

GeoPaveTraffic:

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

 

[GeoPaveTraffic]

Glen, the design of ACPs still has a lot of art and not as much science as we all would like. However, you are correct in stating that part of the reason that the friction factors are so high is the roughness of the surface. Also, depending on the soil type, the diameter of the pile can be several inches larger than the diameter of the auger due to compression of the material by the grout.

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.

 

[GeoPaveTraffic]

Fotch3,

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.

 

[Russellccooper]

Typically a Auger cast pile will contain rebar dependant on the load type typically you should at least expect to have (4) #6 with #3 Rings @12" o/c and this is defiatly the case with a lateral condition With some 18" Piles I have work with lateral loads as high as 50Kips in the lateral and typically i would expect to see at least 2'-0" projection into any cap bearing a lateral load. Please bear in mind that cage design should taken very seriously in lateral cases and you should employ a structural engineer to confirm and discuss any ideas you may be thinking of going with. I can be contacted @ rcooper@berkelapg.com if you have any work that you would like me to take a look at!

 

[Focht3]

Hmmm,

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.

[blue]GeoPaveTraffic[/blue]: 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 -



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[Hookem]

Shepard:

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.

 

[Focht3]

The geotech's role does not include rebar. The geotech provides end bearing and skid friction values.

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...



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[gandersen]

Focht3:

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

 

[Focht3]

Hmmm,

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?"