Fully Bonded Pre Stressed Ground Anchors
Fully Bonded Pre Stressed Ground Anchors
(OP)
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.
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.





RE: Fully Bonded Pre Stressed Ground Anchors
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.
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RE: Fully Bonded Pre Stressed Ground Anchors
www.PeirceEngineering.com
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
www.PeirceEngineering.com
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
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.
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
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RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
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.
RE: Fully Bonded Pre Stressed Ground Anchors
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.
RE: Fully Bonded Pre Stressed Ground Anchors
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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RE: Fully Bonded Pre Stressed Ground Anchors
The process is repeated for each subsequent stage of the excavation.
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
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RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
"In this case it is not only possible but advantageous to install the anchors without any free length"
Can you explain why would it be advantegeous? Regards.
RE: Fully Bonded Pre Stressed Ground Anchors
Tiebacks in Foundation Engineering and Construction by Harry Schabel Jr. McGraw-Hill
Can someone explain to me how a system of fully bonded anchors might be desirable or advantageous? Thank you
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
Still don't get it. What are the advantages? Why is it desirable? Why do some succesful companies follow this pratice? Regards.
RE: Fully Bonded Pre Stressed Ground Anchors
RE: Fully Bonded Pre Stressed Ground Anchors
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