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.

 

[hokie66]

The economics can vary. Top-down construction has a specific definition as USGeotech described it, and can indeed be economical in many instances. But one reason it is economical is that it generally does not require any tiebacks...the floors brace the retention walls as the construction and excavation proceeds down. This normally involves diaphragm walls or secant pile walls, with only one level exposed at a time.

 

[crcivil]

What I have seen in top-down construction method, and the way it is done down here consists in 1)digging to the bottom of the first floor, 2)drilling and installing fully bonded temporary ground anchors (usually 2 or 3 0.6" strands per anchor), 3)place rebar and shotcreting a permanent basement wall, 20-30 cm. thick (having previously left a pvc sleeve in each anchor position and the anchor plate against the back rebar layer of the wall), 4)post-tensioning the anchors(usually 30-50 tons), 5)dig to the next floor and so on repeating the operation in each level. At the end you go from down up building your floors at each level against the walls as permanent supports.

 

[hokie66]

That is not top-down construction. Suggest you google for descriptions of that method. What you are describing is anchored shotcrete excavation support.

 

[crcivil]

USGeotech,


"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.

 

[crcivil]

"For some tiebacks, it may be desirable to have a full-lenght 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

Can someone explain to me how a system of fully bonded anchors might be desirable or advantageous? Thank you

 

[FixedEarth]

It depends on how you want the wall design to function. If you want the wall system to engage once a movement occurs (passive system), then we go for soil nail walls which have full length grout and no stressing length. However, if you want to lock in a 90% to 110% load of anticipated force, we than need a grouted tieback with threaded bars or strands (active system.) For the latter wall type, we need a stressing length to account for the elongation that will occur once the anchor is stressed.

 

[crcivil]

We are talking about ground anchors, as opossed to soil nails, anchors that are actually pre-stressed without having an unbonded length and are fully grouted.

 

[hokie66]

When you grout an anchor fully and then stress it, it will create its own unbonded length. For a temporary anchor, that might not be a bad thing, but for a permanent anchor, corrosion problems would be expected.

 

[Mikerk]

Note that tensioning a strand anchor with no free-length can present some real problems with properly locking off the wedges at the desired force. Normal wedge seating ussually results in a minimum loss of steel elongation of 0.5 inches. If you do not have a minimum (real)unbonded length of 15 feet or more, chances are pretty good that all the force in your tieback is lost upon the wedge seating process. If a fully bonded anchor is desired, I would suggest grouting the borehole in two stages. First stage (bottom of anchor with minimum length necessary to develop the maximum test load. Top of first stage grout should be a minimum 15 feet below the top of the borehole (and below the theorhetical failure plane). Allow first stage grout to set and reach adequate strength, then test load and lock-off the anchor as prescribed by the Post-Tensioning Instititue. After final lock-off the remaining portion of the borehole can be grouted up, providing a true fully bonded and properly locked of tieback anchor.

 

[crcivil]

Harry Schnabel Jr. in "Tiebacks in Foundation Engineering and Construction" (McGraw-Hill) states that for some tiebacks it would be desirable to be fully grouted and with no unbonded length, as in the case of landslide stabilization. He also mentions that some companies within the USA follow this pratice succesfully.

Still don't get it. What are the advantages? Why is it desirable? Why do some succesful companies follow this pratice? Regards.

 

[hokie66]

Looks like your answer is not forthcoming from this panel. I believe Mr Schnabel is no longer with us, but the company which bears his name is still a going concern. Perhaps an enquiry there?

 

[PEinc]

Mikerk makes some good points. Also, I worked for Schnabel Foundation Compny for 11 years. We did not install tieback anchors without unbonded lengths (except for short toe ties which are more like rock bolts).

http://www.PeirceEngineering.com