Top-to-Bottom Element Walls for Basement Construction

[crcivil]

In this method, intermittent/alternate permanent reinforced concrete panels are placed in a down-ward process as the excavation continues, and new elements of typycal size 3 m x 3 m are installed employing one prestressed temporary anchor per panel, and constructing a stripe foundation beneath the wall at final excavation level. This means we dig all the way down and start building back up the floor slabs at each level, at which point the anchors are not longer needed since the loads are transfered to the floor slabs/beams.

 

[crcivil]

As for the design of the fixed length of these temporary anchors, the load transfer from anchor tendon to the ground via grout the distribution of stress along the fixed anchor is non-uniform.

As Anthony D. Barley has stated; "In the vast majority of anchors, when applying the initial load, the bond stress is concentrated over the proximal length of the fixed anchor and at that time the distal component of the fixed anchor is
unstressed and redundant. As load is increased in the anchor the ultimate bond stress at either or both the
tendon to grout and grout to ground interface is exceeded and the residual bond stress at that location, after
interfacial movement, is generally of a lower order. When ultimate bond stress is achieved at one interface,
generally the grout to ground in soil anchors, then the bond stress at the other interface cannot increase
further. That unit length of anchor has reached capacity limit and subsequently the capacity will decrease."

"As load in the overall anchor is further increased the bond stress concentration zone progresses further
along the fixed anchor. Prior to failure of the anchor the load concentration zone approaches the distal end of
the anchor."

 

[hokie66]

Is there a question here?

 

[crcivil]

For Top-to-Bottom Basement Construction, we have succesfully completed a few projects so far installing prestressed ground anchors through these Element Walls without any free length, and then fully grouting the anchor which is prestressed to 30 - 50 t with no problem at all. We believe that by adopting this practice, we are avoiding over stressing the proximal lentgth of the theoretical bond zone of the anchor, achieving a much safer work by considering the shear strength that is also mobilized along the active wedge.

USGeotech, May 8th, 2013:

"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."

"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."

 

[crcivil]

Hold on hokie66, almost there. I Appreciate your patience.

 

[crcivil]

Harry Schnabel in Tiebcks in Foundation Engineering and Construction:

"We don't want any portion of our anchors to be made in the soil between the wall and the critical failure surfce. The physical significance of the unbonded length is to assure that no tendon load is trnsferred into this soil"

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

ANY THOUGHTS? Best Regards.

 

[concretemasonry]

Hokie66 -

I agree since it seems to be a theoretic discussion for major unique major installations and the possibility and problems for common problems/situations.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[PEinc]

The entire length of a tieback or tiedown anchor should be grouted. In the "old days," anchors were installed using two-stage grouting where the anchor was grouted, tested, and locked off after the stage 1 grouting was done only in the bond length. Then, the balance of the anchor was grouted in stage 2. The entire anchor length was either bare steel or coated steel with no greased or sheathed length (very little or no corrosion protection).

The later development of greased and sheathed unbonded lengths then allowed a single grouting of the entire anchor length This unbonded length is made long enough to assure that the bonded length is well beyond the theoretical failure plane and that no anchor capacity is being provided by the soil above the failure plane.

Both methods provide the required anchor capacity. However, two-stage grouting can be more expensive and time consuming than using a greased and sheathed unbonded length with a single stage of grouting. In addition, the greased and sheathed unbonded length, along with a properly designed and installed trumpet and anchor head cap and/or encasement, will provide the corrosion protection needed near the face of the wall for permanent anchors.

http://www.PeirceEngineering.com

 

[crcivil]

Thank you PEinc,

However, I'd like to hear your opinion on the practice some sub-contractors follow (including myself) of designing and testing ground anchors with no unbonded length, and being succesfull at it project after project.

As Harry Schnabel commented: "While we have not followed this design practice, I'm intrigued with the fact that it has always been succesful with others". I have to say I'm intrigued still, and I'm looking for a a better understanding of this design / construction approach, which has demonstrated to be correct in the field.

I feel very comfortable by following this practice because I see the positive results in my projects, and in the ones other subcontractors execute employing the same technique for so many years. I believe this is a safer approach than considering a free length and concentrating the all testing load within the first couple of meters of the bond length, specially in soils with medium to low shear strength.

Given that we know that these stresses' disribution in the anchor is not linear, I think we can avoid overstressing the tip of the bonded zone by taking advantage of the shear strength the active zone can provide.

As I said, I just hope to grasp a better engineering understanding of how this practice can prove so succesful, although many practitioners do not recommend it, eventhough they do not provide a convencing reason for this, since the results speak by themselves.

Thank you for kindly sharing your knowledge.

 

[crcivil]

As for the two-stage method you've mentioned and agreed upon (I have employed it btw), at the end we don´t leave any free length either since the strands are bare-grouted, so part of the loads will eventually be transferred to the active wedge anyway, and in that way there is a close analogy between this and the method I'm talking about in this thread.

Regards

 

[crcivil]

From Anthony D. Barley of SBMA:

"It is widely acknowledged that, in the majority of circumstances where conventional anchors are used, debonding at the tendon/grout or the grout/ground interface must occur as anchor load increases and prior to any load being transferred to the distal end of the fixed length.

This phenomenon is commonly known as progressive debonding and is associated with grossly non-uniform distribution of bond stress along the fixed length at all stages of loading.

Information has been published by a multitude of researchers on this topic.

Progressive debonding generally results in a highly inefficient use of the existing ground strength. In circumstances where the ground strength far down the fixed length is being utilised, the ground strength above this has been exceeded and only some residual strength is available. This reduces the overall efficiency of the fixed length"

 

[PEinc]

crcivil,
Improperly installed tieback anchors may still perform satisfactorily because the design may be very conservative and/or the soil may be much stronger than assumed in the design. In other words, the contractor and/or wall designer may just be lucky. However, frequently being lucky is not an excuse for bad design or construction. I get asked to design lots of shaky contractor ideas that may have worked in the past and may even work again. However, I tell them that I don't design structures that MIGHT work. Earth retention is a risky enough business when done correctly. Doing something that might work is a recipe for disaster, eventually.

The difference between two stage grouting and single stage grouting of a bare tendon is that, with two stage grouting, the tieback load has already been tested and locked off in competent soil beyond the theoretical failure plane. The same goes for a single stage grouted anchor with a greased and sheathed unbonded length.

With respect to your last post concerning progressive debonding: Much has been written about uniform and non-uniform transfer of the anchor load along the bond length. However, if you properly test an anchor and monitor its creep as recommended by PTI (The Post-tensioning Institute), you can be assured that the anchor will perform as intended. In my opinion, there are three things that make an anchor a good anchor. They are 1) the anchor must hold the test load, 2) the anchor must have sufficient unbonded length, and 3) the anchor must not exceed the maximum allowed creep criteria. If all three of these conditions are met, who really cares about the actual load distribution along the bond length? It is very common that long tieback anchors that are bonded through variable soil layers will exhibit progressive debonding in the upper, weaker soil layers until the test load is finally held at a deeper, more competent soil layer. This happens all the time with long tiebacks for waterfront bulkheads where there may be significant layers of soft or loose soils in the "bond zone," beyond the assumed failure plane, but above a sufficiently competent layer. That is why PTI says that it is not necessarily a reason to reject a tieback anchor if its elastic elongation exceeds the unbonded length plus 50% of the bond length. After all, you may never know where the "real bond zone" is until the anchor is tested.

http://www.PeirceEngineering.com

 

[crcivil]

Thank you very much PEinc. Still trying to figure out statements like the following by Harry D. Schnabel:

"When tie-backs are used to stabilized a landslide, there may be no reason for an unbonded length. Since the failure surface is just that, it may even be desirably to bond the tie-backs on both sides of the slding surface. If this is done, the tendons and grout not only provide keys across the surface of sliding, but even a slight movement will mobilize the full tension on the tie-back"

Thanks again

 

[crcivil]

How about top-to-bottom basement walls construction? In this case, the total length of the anchors being installed at every new lift is literally within the passive wedge zone, so why should we bother in considering a free length then? When we prestress the anchor in this situation, what we are doing is testing the strength of the soil within the actual passive zone, all the way from the proximal through the distal ends of the anchor, so the provision of an unbonded length would be meaninless.

Regards.

 

[PEinc]

What Harry Schnabel was describing sounds more like a soil nail than a tieback anchor. I worked for Schnabel and we used tiebacks with unbounded lengths to stabilize landslides (at least on the projects with which I was involved).
With respect to your last post about the "passive wedge zone" behind a basement wall, I'm not sure what you are referring to. What passive zone is behind the wall? Tiebacks for a basement wall should still have unbounded lengths. The unbounded length should extend beyond the theoretical failure plane.

http://www.PeirceEngineering.com