earth engaging auger type anchor

[Mccoy]

Dear all,
long time no see, I've been not just busy, but overwhelmed by work lately since the rebuilding of the Italian city of L'Aquila after the April earthquake is starting. Among miriads of difficulties of course.

And I beg your pardon this time for posing a question related to soil mechanics rather than providing links to Jazz videoclips.

An engineer came out with this project of auger type anchors to be drilled into soft organic clays. That's to stabilize a wind-tower's foundation.

Now my doubt is how do I figure pull out resistance?

Auger will compress the clay on one side, will remold it on the other, so I envisage two opposing phenomena.

My gut feeling is to apply lateral friction (Su)to the cylinder materialized by the anchor's outer diameter. The soil cylinder should in fact fail as a whole and that's obvious.

But, which range of the in situ Su to apply, from residual-remolded to in site (Su 'undisturbed' is in the range of 50 kPa). IF the two opposing phenomena will balance out, nominal, in situ Su might just be the proper parameter to apply.

Any other aspects I'm not aware of?

This is still in the prelimiary stages, what's required is the minimum anchor's lenght, what I know now the design pull force ranges from 12 to 55 kN.

All other considerations related to the Eurocode provisions are to be developed after the proper method to calculate the anchor's resistance has been decided

 

[Ron]

In the absence of remolded strength data, I would discount the undisturbed Su value. How much? Not sure, but ratio of undisturbed to remolded might be as much as 2:1.

I agree with your cylinder approach. I would do a load test on a few anchors just to be sure.

 

[fattdad]

I'd load test each anchor to 125 percent of the design load.

Can't offer much other help.

f-d

¡papá gordo ain’t no madre flaca!

 

[jheidt2543]

Mccoy:

Take a look at this thread and the links to papers on helical anchors. I think you'll find what you are looking for.

thread256-259789AussieGeo (Geotechnical)
Displacement piles and effect on surrounding soil

 

[Mccoy]

Ah well, I see the bottom line here is stay more tuned into the eng-tips fora, it's a very recent and relevant discussion there.

I tried to print one of those articles and it's protected, my eyes are aching by overwork on the screen so I believe I'll resume the discussion later.
Sure some load tests are better scheduled, this appears to be one of those situations with high uncertainty.

 

[jheidt2543]

Mccoy:

I was able to get all three. For the first link, you do have to sign in, it's free and the download is free.

 

[jdonville]

Agree that you would want to look at helical anchor literature for the direction for analysis. The concept is that the helices (if discrete) screw into the ground and that they will follow the same path and minimize disturbance to the clay, such that the bulk of the soil will be relatively undisturbed. Apply normal bearing capacity as if each helix were a plate embedded in the soil.

Naturally, a continuous helix with many wraps would be a different story, and there are naturally checks that need to be performed for uplift resistance that do not apply to downward bearing applications.

Good luck.

J

 

[dgillette]

Hi McCoy. A couple of ideas (based on zero actual experience with anchors in clay until tomorrow*):

Can you get either vane shear test or cone penetrometer test done soon enough to check for sensitivity and remolded strength? Jdonville is, I'm pretty sure, correct about the disturbance being minor from the flights passing through, but if the clay is very sensitive, it might not be minor enough. My concern is this: if a large pullout force is applied, sensitive clay right at the auger flight could be strained past its peak strength (so its shear resistance is decreasing) before the rest of the failure surface is strained far enough for its peak strength to be mobilized. All depends on how sensitive the clay is, how much strain it takes to exceed peak strength, how much it is remolded by installation, etc. (Might make a good MS thesis for someone.) Using remolded strength everywhere with a high factor of safety is clearly quite conservative. Maybe you could design for FS>2.0 assuming peak strength everywhere AND FS>1.05 assuming remolded strength everywhere.

Clay immediately adjacent to the shaft of the auger would be pretty thoroughly remolded.

The term "wind tower" suggests to me that there will be many cycles of loading on any windy day, and many windy days over the design life. If the cyclic loading from the wind gets anywhere close to the monotonic peak strength, there might be permanent strain in the clay, even if the peak is not exceeded.

I don't know much about organic clays. Truth is, I don't have very many answers for clays at all, but I sure as hell do know a lot of questions!

Bon chance!
DRG

* Tomorrow they will be installing the helical anchors for my house repair and improvements. If I go home and watch, my experience will change from 'zero' to 'very little,' which is a huge increase on a percentage basis.

 

[Mccoy]

Great food for thought, folks!

And I really, really forgot about the field VST, so little used over here but so much useful in this particular instance.

By the way, Ron's concept of 'discounting' the Su_peak by a certain value is also what I have been doing in a few instances to get Su_r to be used in foundation sliding analyses. Of course a basic knowledge of the involved clay helps. If sensitive, things might get tricky.
Also, there is the plot on Kulhawy and Mayne 1990, Su_r related to LI, which unfortunately appears to be pretty much on the conservative side, very much conservative for high LI values.
I'm going to attach it ASAP...

Dave, hope you yard's clays are not too much of the sensitive type!!

 

[dgillette]

No, no sensitive clay problems. My house is high on a hillside in a dry climate. The clay is unsaturated. Its big problem is swelling and shrinking of montmorillonite with changes in the weather.

Went home to watch them work for a while this morning. To everyone's surprise, the helical piers went much deeper into the weathered claystone before reaching refusal (defined by 50 mm square solid rods twisting). They were expecting refusal at a total depth of about 5 m (2 m into "rock"), but they went as deep as 9.5 m total. Live and learn.

First placement of concrete occurred today (or was planned to occur) with second tomorrow. I'm having a lot of fun watching all this. My wife is not an engineer, and to say the least, she does not see it as fun. Some people just don't understand.

 

[Mccoy]

My wife is not an engineer, and to say the least, she does not see it as fun. Some people just don't understand.

Just wait until she can translate the concrete jobs into wider home room, more furniture, better floor tiles & finishings.

Even if it's all unrelated to that, she may find a way.

Even if there isn't a way, she may be inspired and suggest some serious job entailing wider home rooms >>>> more furniture >>>> better floor tiles >>>>>> and more >>> and more

 

[jdonville]

dgillette:

The shafts are not supposed to twist. They should have a torque indicator on the drive head and stop before twisting occurs. It appears that you have what I refer to as some "hillbilly engineering" going on there.

We are very careful to size the shafts for the expected installation torque (capacity is roughly proportional to the installation torque) and have enough extra shaft length on hand to provide the design capacity should the soils not provide the torque resistance - in this case, the embedment depth alone (gamma' x D from the bearing capacity relation) gives you the capacity.

J

 

[dgillette]

Thanks, Jeff. It wasn't the full length of the shaft that yielded, just a little separate piece at the top. Maybe that was a "fuse" to protect the real shaft? Looked like about the same diameter (~50mm square), but may have been smaller, or softer steel.

They did have some sort of torque indicator inside the cab, which I didn't look at too closely. It may have just been a pressure gauge on the hydraulic pump. It jumped up pretty suddenly as the refusal depth was reached. Not too surprising, I suppose, since the upper part of the weathered bedrock had unadjusted SPT N not much over 20.

Regardless, the bearing requirement is real small for the two on the front of the house, since two of them support half of one room and a small piece of the roof (one end of a few roof trusses that are supported at midspan as well as the ends). On the back of the house, they need pullout capacity only, to hold down counterfort walls to restrain the basement wall against further movement caused by swelling clay and lousy construction. There are going to be void forms on the underside of the counterforts (and the walls of the front room) so swelling clay won't apply any upward force on them. (My son looked out the front windows and saw the void forms, and asked whether the contractor was trying to rip us off by sneaking cardboard boxes into the bottom of the formwork to save concrete.)

DRG