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!

 

[haynewp]

Why can't you batter the auger cast piles for lateral resistance?

 

[GeoPaveTraffic]

ACP's can be battered to a point, however, most of the lateral load that I have encountered with these piles has been earthquake induced. Most these structurals don't like battered piles for earthquake loads since the piles tend to punch through the caps.

For static loads, I have installed battered piles on a hand full of jobs.

 

[lallatin]

Just a couple of points about procedures during installation: I have found the utilization of a Pile Installation Recorder (PIR), manufactured by PDA, to be indispensable for the accurate logging of drilling and grouting procedures. I found the PIR to be absolutely accurate. There is no other basis for determining grout volume with confidence.

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if permeable material is encountered below about 30', and the cage is not inserted quickly after the auger is withdrawn, it will be a problem.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.

 

[eric1037]

Glen:

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

 

[GeoPaveTraffic]

lallatin;

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.

 

[lallatin]

In being brief as I rushed out to a soil-nailing project, I didn't expand on my comments, and expected even more comments than I got. I am familiar with the standard of practice of "calibrating" a concrete pump by filling a known volume and counting strokes, and have listened to a contractor's braying about "I'm wasting grout-where-is-it-going-I-counted-400 strokes on that pile!!" on many days when I knew from the PIR, which can "calibrate" the pump on every pile by counting strokes as the PIR records the stem fill volume. Uniformity of pump volume under a wide variety of conditions is a fantasy, and it can undermine confidence in the procedure and the integrity of piles when no problem exists, especially if the displacement of weak soils could account for excessive grout take. (My usual response to the contractor was "I hadn't noticed that that was a 14-yard truck.") As far as a pile "pinching", it would take a total lack of attention by the operator and the inspector for a withdrawl of a near-empty stem to occur, and withdrawl of a full stem, pump interrupted, of even a couple of feet, won't allow a pinch to develop. There are a number of people involved who are being paid to prevent that. Again, a PIR should be SPECIFIED, or somebody is going to be doing guessing about pile installations, and they won't be in agreement with the soils investigation data.

 

[pmkPE]

The following link to Foundation Technologies out of Atlanta has some innovative cage guides for installing in cast-in-place piles:

http://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).

 

[lallatin]

re: centralizers. They were used on about 1400 piles of various lengths and diameters that I observed, and they worked very well; even staying intact when cages had to be pulled because of excessive drag during insertion [as described earlier.] If the soil profile includes zones of soft material, make sure the centralizers will end up in zones of stiff/dense material. If you want more information, let me know.

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]

 

[Inzynier]

All:
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?

 

[GeoPaveTraffic]

In my opinion 6#6 bars will be difficult to install in a 65 ft. pile in most soil conditions. If the soils are stiff clays for the full depth, then it would not likely be too difficult, otherwise, it will likely be a bear.

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.

 

[lallatin]

I think you will find that any contractor will be comfortable with placing whatever cage the design calls for. The heavier the cage, the easier it is to place to full depth. It's a light cage that's hard to insert. Talk to experienced contractors. They have nothing to gain by making false assurances. Berkel is probably the most experienced in the US. You can use my name re their Moss Landing, CA project in 2000-2001.

 

[Inzynier]

I have to disagree with lllatin's comment that "any contractor will be comfortable with whatever cage the designs calls for". That may be the case in those regions where ACIP piles are prevalent type of deep foundations....

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.

 

[Focht3]

This, indeed, is an interesting thread!

ACIP piles are not preferred choice for applications where resistance to significant lateral loads is anticipated.

I concur completely!

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.

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?"

 

[lallatin]

Somehow I wasn't clear on my main point re auger cast piles: any cage can be installed if appropriate procedures are followed. The biggest question will be the proper basis of comparisons between auger cast and driven piles, or CIDH, or geo-piers, or.... Capacity won't be the final determinant in many cases.

 

[GeoPaveTraffic]

I have to disagree with Inzynier and Focht3 with respect to ACIP not being a good foundation when significant lateral loads are expected. As indicated previously, these piles can take substantial lateral loads.

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?

 

[pmkPE]

What comes to mind regarding driven piles versus acip is quality control and prestressing.

 

[Focht3]

Hmmm,

I'm still waiting for problem specifics, [blue]GeoPaveTraffic[/blue]. (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?"