Performance of Anchors inside the Failure Wedge 2

[Obregon]

Hello. As you might know, some companies grout between the tendon and the soil in the failure wedge, and some government agencies require such grouting, the unbonded lentgth of the tendon being zero. Then, when these anchors are tested, some of the measured anchor capacity is from the soil within the failure wedge.

Since the tiebacks in service at any time are near the interim bottom of excavation and since they are drilled at a downward angle, most of the length (if not the entire length) are outside the active wedge.
In this case it is not only possible but advantageous to install the anchors without any free length. At worst, the tiebacks near the interim bottom of excavation only need a very short unbonded length.

However, I'm still intrigued with design issues and the fact that this practice has been succesful with others all over the years and within our own practice, when we've decided to go this way.

Any experiences to kindly share?

 

[PEinc]

Get a copy of the Post-Tensioning Institute's Recommendatios for Prestressed Soil and Rock Anchors.
Without an unbounded or free length, you cannot guarantee that the anchor is developing its capacity beyond the theoretical failure plane, beyond the active soil wedge. Therefore, you won't know if you have a global stability problem. Check PTI's anchor testing acceptance criteria.

http://www.PeirceEngineering.com

 

[Obregon]

Good morning PEinc, thank you. In fact, I have a copy of the PTI and FHWA documents and I'm aware of their recommendations, rationale and generalized acceptance regarding this issue. Nevertheless, I don't know of any reported field tests talking about the inconvenience (with proof results) of installing and prestressing ground anchors without any or a very short free length, where a part of its capacity comes from the soil within the active (failure) zone.

Let me share with you that, as far as I know, this practice's been embraced succesfully by subcontractors within the USA, and, although I do not adopt it often, I have made use of it, succesfully, under certain circumstances.

As I understand, you used to work for Schnabel some years ago. Well, it's the same Harry Schnabel, in his book, who admits to be also intrigued and having knowledge of succesful subcontractors following this practice and government agencies requiering to follow it.

Is it possible that this could be a matter of theory and fashion vs field facts and experience? Thank you.

Carlos R. Obregón

http://www.sueloanclado.com

 

[PEinc]

You can't find any information because people do not consider the unbonded tieback anchor length as providing any anchor bond capacity throughout the active soil wedge. If it is not in PTI or FHWA or AASHTO, you probably should not be doing it. The soil that is trying to overturn a wall cannot also be supporting the same wall. If you want the entire anchor bonded, consider the anchor a soil nail.

http://www.PeirceEngineering.com

 

[Obregon]

If the tieback is fully grouted, then as you stress the strand, the first few feet will progressiveley break the strand to grout bond and/or the grout to soil bond, essentially forming an unbonded length.

What are your thoughts?

Now, this is what Mr. Schnabel says:

"For some tiebacks, it may be desirable to have a full-length anchor and no unbonded length. As one example, the reinforcing provided in a landslide area may be desirable for stabilizing the soil mass".
Tiebacks in Foundation Engineering and Construction by Harry Schabel Jr. McGraw-Hill

We´re talking about tiebacks, and this is the opinion of Harry Schnabel. I'm Still intrigued.

 

[Obregon]

Case History

A few years ago, we were engaged in a slope stability job requiring the installation of 500+ 60 ton-ground anchors in clayely soil (CL). For this project we were given the specs by an engineering firm working for the owner.

As I can recall, for the upper anchors the specifications called for a 10 m-free length and a 8 m-fixed length for a total length of 18 m; a 5"-bore hole diameter was employed. As we were progressing with the installation of the production anchors, we noticed that a few of them couldn't achieve the specified design load, this, apparently, because of an excesive creeping movement within the anchors and the need of a longer anchoring length.

I then suggested to shorten the free length dimensions instead of thinking of longer and more expensive anchors. Finally, it was decided to employ a 5 m-free length as opposed to the 10 m-free length that was called for in the engineering documents. The work progressed smoohtly and all the ground anchors were proof-tested successfully.

This is what my experience says, as well as the experience of others, in accordance with what some recognized people, like Harry Schnabel, have been able to observed as a succesfull practice within the USA.

H. Schnabel stated on this issue: "While we have not followed this design practice, I'm intrigued with the fact that it has always been succesful with others".

What do you think about this assertion made by such a respected man like Mr. Harry Schnabel?. Regards.

Carlos R. Obregón

http://www.sueloanclado.com

 

[PEinc]

Obregon, I was the Branch Manager for Schnabel Foundation Company's Philadelphia office during the 1980's until the end of 1991. While at Schnabel Foundation Company. I never counted on any anchor capacity along the unbonded length and within the soil's failure wedge. I am also unaware of other Schnabel engineers doing that. From which Harry Schnabel book(s) and page number(s) did you get the quotes?

With respect to your job where the unbonded length was decreased, essentially, you increased the bonded length which allowed the anchors to pass their test criteria. However, what was the basis for specifying the original 10 meter unbonded length? Perhaps 10 meters was the correct length - maybe not. PTI calls for 15' (4.6m) minimum unbonded length for permanent anchors. If there was a sound engineering reason for 10 meter unbonded length, then 10 meters should have been installed and longer anchors should have been installed with 10 meter unbonded length and bond lengths longer than 8 meters. I hope that the unbonded lengths were not reduced just so the contractor didn't have to buy new anchors with longer bond lengths or have to add coupled bond length to the encapsulated anchors that must have been already on site. I do not think that the job you just described is an example of allowing bond within the failure wedge of soil.

http://www.PeirceEngineering.com

 

[Obregon]

PEinc

The 10 m-free length specified for that project was, of course, a result of an anchor design elaborated by a well-known geotech firm, and I believe the decision we made to shorten such length was, based on the results, the right one, being these kind of assertions encouraging:

"While we have not followed this design practice, I'm intrigued with the fact that it has always been succesful with others".

Harry Schnabel Jr., Tiebacks in Foundation Engineering and Construction. Mc Graw Hill 1982, page 145.

 

[PEinc]

Another of Harry Schnabel's quotes in his section on Design of Individual Tiebacks (in both editions of his book): "We do not want any portion of our anchors to be made in the soil between the wall and the critical failure surface."

http://www.PeirceEngineering.com

 

[Obregon]

That´s right. This is why he tells us his company don´t follow this design practice. Nevertheless, he actually acknowledges, as a fact, that the method has always been succesful with others who follow this approach, meaning that he knows about it and admits it.

Interesting topic to take into consideration, exploring and recognizing its practical applications and advantages at least under certain circumstances, don't you think so PEinc?

 

[r13]

The soil that is trying to overturn a wall cannot also be supporting the same wall.

I think this statement tells all. You'll have bond strength in the active zone during stressing, because the soil simply isn't in the active state. But once the active state is reactivated, the bond is largely lost, at least, it is unquantifiable.

 

[r13]

Ha ha, that could be a good example. Image the soil in the active zone as fluid (liquor), does it hold anything?

 

[jimstructures]

Obregon,

Insulting people that are trying to answer questions you have raised is NOT a way to generate helpful responses.

Jim

 

[PEinc]

Obregon, a tieback anchor without an unbonded length is just a long soil nail. I don't see the advantage of installing a lot of long, low capacity, soil nails versus installing much fewer, long, higher capacity, tieback anchors with unbonded lengths (unless there is great cost to install soldier beams or SSP). Fewer tieback anchors are easier to attach to a soldier beam or SSP wall that many soil nails on a soldier beam or SSP wall. If you install soil nails, maybe you can also eliminate the soldier beams or SSDP by using shotcrete. The question is: Which wall is easier and more economical to build - a tiedback soldier beam or SSP wall or a soil nail shotcrete wall?

http://www.PeirceEngineering.com

 

[Obregon]

I see your point PEinc, but remember that niether Mr. Schnabel nor myself are talking about long, low capacity soil nails, which btw we do employ, but tiebacks, i.e., prestressed full-length ground anchors with no unbonded length. I hope you can consider the fact that, as Mr. Schnabel has asserted, it is possible to do things in a different way and still being right.

 

[Obregon]

Good morning r13

jimstructures is right, please accept my apologies.

Carlos R. Obregón

http://www.sueloanclado.com

 

[PEinc]

Obregon, if you test a fully bonded "tieback" anchor, the only way that you can tell if the anchor is getting its capacity beyond the failure plane is by checking the amount of apparent elastic elongation of the tendon during a load test. If the amount of elastic elongation does not extend approximately to or beyond the failure plane, your anchor may not be long enough to perform as needed. For example, if you have a 20 feet distance along the fully bonded tieback to the failure plane, and the anchor tests with little to no apparent elastic stretch, you won't know the capacity of the anchor once the soil becomes active. Essentially you are relying on "sky hooks" to retain the active soil wedge. I know what is written in the 1st and 2nd edition Schnabel books. As far as I know, Schnabel has not and does not use fully bonded tieback anchors. Give Hank Schnabel (Harry's son) a call. He is the co-author with Harry for the 2nd edition. Maybe he can give you some guidance or opinion. FYI, I don't think I ever saw or designed a tiedback wall that needed a 10 meter unbonded length. I can't remember ever needing more than a 6 meter (20') unbonded length. Your wall must be very high (or you have very conservatively written anchor specifications).

http://www.PeirceEngineering.com

 

[MTNClimber]

Obregon mentioned that the tiebacks were for slope stabilization. I'm guessing the 10m spec could of been where the theoretical critical slip surface was located.

 

[Obregon]

Thank you again for your kind attention PEinc, I sincerely appreciate it. I understand that, by following this approach, we wouldn't be able to get a minimum apparent free length value, i.e., at least 80 % of the design free length, based on elongation measurements during proof/performance tests.

How can we then deal, design phylosophies aside, with the FACT that this "wierd" method of doing things is regarded by Harry Schanabel as SUCCESFULL?

 

[Obregon]

You are correct MTNClimber. In fact, considering this free length, the potential slide surface must had been located at around 8.50 m from the slope's surface.