Fully Bonded Pre Stressed Ground Anchors 1

[crcivil]

A local and well known contractor avoids having a free length when installing ground anchors during top down construction activities. In this manner, the tendon is fully grouted and no sheath breaker is used to have an unbonded section. With this procedure, a few inches of free strands are left between de back of the concrete wall and the face of the supporting plate, which are used to fully pre-stress the anchor.

I have read in the literature of some contractors within the US following the same practice succesfully, so I wonder if leaving a free length behind the critical surface is just a matter of choice. Hope you guys can help me with this issue.

 

[PEinc]

You really haven't given enough information for me to know if these are grouted tieback anchors or just strands to a deadman. Can you post a sketch?

Tieback anchors should have an unbonded or free length when they are stressed and locked off. This can be done with an bond breaker in the unbonced or free length or can be done (although less frequently these days) with two stage grouting. The second stage grouting would be done after the first stage grout has cured and the tendon has been tested and locked off.

If you stress a fully grout bonded tendon, the anchor capacity may not be developed in the stable soil beyond the theoretical failure plane. Therefore, the anchor may not perform as needed. Also, grouting a bare tendon up to the back of a wall may cause just a short length of tendon to be tested just against the back of the wall. This would not show that the tendon is satisfactorily bonded into soil behind the failure plane.

http://www.PeirceEngineering.com

 

[PEinc]

If the tendons are actually soil nails instead of tieback anchors, then there would be no unbonded or free lenght required. Also, soil nails are not normally prestressed. However, sometimes (but not often) soil nails are stressed a small amount to "tighten up" the wall facing.

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[crcivil]

Thank you PEinc. These are actually fully grouted tieback anchors, and fully pre-stressed as oposse to soil nails. I just keep on wondering what the rationale is on doing the job this way and being succesful at it, eventhough the need for a free unbonded length has been, generally speaking, recognized by the industry. Regards.

 

[PEinc]

I am unaware of any tieback references, recommendations, or specifications that advocate or allow fully grouted and bonded anchors.

http://www.PeirceEngineering.com

 

[crcivil]

Thank you very much PEinc.

 

[Mikerk]

The Post-Tensioning Institute's (PTI)Reccomemmdations for Prestressed Rock and Soil Anchors are the most commonly accepted standards for anchors in the United States. These recommendations call for a minimum unbonded length of 15' (for strand anchors), and 10' (for Threadbar anchors). The unbonded length should extend beyond the theoretical failure plane. The PTI Recommendations explain in detail why shorter unbonded lengths are not a good idea. It's hard to say what the Contractor you are referring to is doing, but I can tell you they are not installing what one would consider a typical anchor.

 

[crcivil]

Thanks Mikerk for your helpful comments.

 

[InDepth]

Well, let's go back to some fundamental behavior of strands....if it 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. Combine this with contraction of the strands and voila.

Note that the strand would probably need to be sized to accomodate high strain over a short bonded length (temporary) in order to avoid failure of the strand and ensure failure of the grout to soil bond as the unbonded length is formed.

 

[crcivil]

Thanks InDepth. So do you think it is ok to leave the anchor fully bonded for practical purposes? Ultimately, as I understand, an unbonded length will be formed anyway. What do you mean when you say we need to ensure failure of the grout to soil bond as the unbonded length is formed? Regards.

 

[hokie66]

I gave an answer in the post-tensioned forum, but another issue is whether the anchors are temporary (for ground support until the structure is complete) or permanent. If permanent, I would be very concerned about corrosion protection where the grout cracks.

 

[crcivil]

The anchors are temporary, employed in top-down basement construction hokie66. Thanks.

 

[PEinc]

Without an unbonded length, there is no way to assure that the anchor is obtaining its bonded capacity beyond the theoretical failure plane. Therefore, you don't know if the stable soil behind the failure plane is holding the anchor and the wall or if the anchor is firmly bonded to the same soil that is trying to push over the wall. The unbonded length assures that you are testing bond capacity and creep in the stable soil and not in the active soil wedge.

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[crcivil]

I can see what you mean PEinc, thanks a lot for sharing your thoughts.

 

[USGeotech]

One point that is missing from the discussion is the fact that the anchors are being used in top-down construction. I'm assuming that once the permanent supports (eithier the full slab or just the permanent building beams) are in place, any given level of tiebacks is redundant and could (but probably won't) be removed or detensioned.

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.

An analogous situation commonly occurs when an excavation is made through mixed face conditions (soil overlying bedrock). We often use soldier piles driven to the top of bedrock. Assuming the bedrock is hard and massive, the active soil "wedge" starts at the top of the bedrock. The lateral load at the toe of the soldier pile is resisted by installing a toe bolt into the rock prior to excavating the rock below. The toe bolts are never installated with a free length in the rock.

I do agree that I have never seen any publication or can provide any justification for anchoring a post-tensioned tieback in the "active" soil wedge directly behind an earth support wall.

 

[dcarr82775]

USGeotech,

I do not disagree with your reasoning for the toe anchor condition. But for the upper and intermediate level anchors the excavation will proceed downward and the active wedge changes so that a significant portion of the anchor is then inside the active wedge.

PEinc's point about verifying your resistance is behind the active wedge after the hole is dug is to me the biggest reason to use a free length.

 

[PEinc]

In response to the comments from USGeotech, I respectfully submit the following comments:
1. The first paragraph is irrelevant to the original question.
2. I disagree with the second paragraph. While it is true that, when an anchor is being installed from a temporary bench elevation, it is basically installed outside the active wedge, the anchors still need the unbonded length for the final, full height condition. Therefore, the anchors need their full unbonded length. Toe ties, however, are a different situation. See next comment.
3. While it is true that toe ties in rock are often installed without an unbonded length, the rest of the paragraph is also irrelevnt to the original question.

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[USGeotech]

In top-down construction, each floor level is cast on-grade then the soil below is mined out to get down to the next lower level. I am assuming the temporary anchors provide stability of the wall at that next lower level until the slab and/or floor beams are constructed at that level. Once the load is transfered to the slab/floor beams, the anchors at that level are no longer needed and could be removed/detensioned before excavating deeper. As the excavation becomes deeper, the extent of the active wedge increases so that the tieback level above is within the expanded active wedge. But by then the tiebacks no longer provide any lateral load resistance.

The process is repeated for each subsequent stage of the excavation.

 

[dcarr82775]

For that type of construction I guess I agree with you, but I have done a fair number of multi-level tieback jobs and never once has that method been used or even discussed. In my experience they dig all the way down, and then start building back up. The costs are astronomical to build a floor then dig out below it for another level, and repeat.

 

[PEinc]

crcivil did not define "top down" construction. It can have one of two meanings - (as USGeotech has assumed) building a permenent structure from the original grade down as the excavation progresses or (as dcarr82775 has described) building temporary sheeting from the original grade down and then building the permanent structure from the bottom up. This is by far the more common and economical method of construction.

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