Fully Bonded Free Stressing Length in Ground Anchors

[Obregon]

Hello everyone. I wonder if you can comment on the applications, advantages, disadvantages, of two-stage ground anchor grouting, leaving a fully bonded free length for corrosion and performance issues of retaining structures, besides mass structures, i.e., dams, etc., Your insights are well appreciated.

 

[Obregon]

G'Day. Here, I share with you a figure of a partialy bonded free anchor length. If this is advantageous in the way that it provides redundancy of structure-anchor load transfer, would that be a good idea to consider this method routinely?

 

[Obregon]

http://www.sueloanclado.com

 

[PEinc]

Two-stage anchor grouting in soil usually requires that the temporary casing pipe remain in place while the grout cures and until the anchor is tested. If not done carefully (too much grout), you may mistakenly grout inside the casing pipe and possibly lose one or more sections of casing. If you have a greased and sheathed unbonded length, then you can grout the entire anchor length and extract the casing before the grout sets. Whether or not you do two-stage grouting, the entire tendon will be protected by grout. There is nothing wrong with doing two-stage grouting but you want to keep the drill installing other anchors. You don't want to have to return the drill several days later to extract casing (or try to extract grouted casing).

http://www.PeirceEngineering.com

 

[Obregon]

Thank you much PEinc. If you agree, I would also like to know your thoughts on leaving a fully or partially bonded free length, i.e. redundancy of interaction between the structure and the anchors.

 

[PEinc]

I do not think that I fully understand your question. However, it is my opinion that drilled holes should not be left open unless in very hard, competent bedrock. If you drill a hole for an anchor, put the tendon in and grout the hole ASAP. Do not risk collapse or deterioration of an uncased drill hole.

http://www.PeirceEngineering.com

 

[r13]

IMO, other than the length required to develop the strength, the grout is there to protect the anchor only, thus strength-wise it is not required. Partially/unbonded anchor can be easily restressed later, if necessary.

 

[Obregon]

Thank you again John, and I will try to clarify my point. From PTI DC35.1-14 Recommendations for Prestressed Rock and Soil Anchors Paragraph C6.4.2 Fully bonded, partially bonded or unbonded anchors.

Fully bonded free stressing lengths force the anchor to strain with the structure. Partially bonded free stressing lengths provide redundant load transfer at the anchorage, while at the same time leaving a certain amount of unbonded free stressing length (figure attached). This requires that grouting be accomplished in two stages - the first to grout the bond length and the second to grout the free stressing length after the anchor has been stressed.

Do you think that, if partially bonded free length is advantageous in the way that it provides redundancy of structure-anchor load transfer, would that be a good idea to consider this method routinely?

Best regards.

Carlos R. Obregón

http://www.sueloanclado.com

 

[PEinc]

r13, but what does your experience say?

http://www.PeirceEngineering.com

 

[Obregon]

Hello r13. Aside the obvious requirenment of a minimum compressive strength value for these grouts, so that the necessary shear strength can be developed trough the interface between the grout and soil or rock, the W/CM ratio has a dual role to play in grout durability. A lower W/CM ratio increases the strength of grout and hence, improves its resistance to cracking from internal stresses that may be generated by adverse reactions.

In summary, the compressive strength of cement-based grouts in ground anchoring is indeed a very important parameter to take into account during the design phase of these retention systems.

Regards

 

[Obregon]

By the way, once you have decided to install partially unbonded-free length anchors (same thing as saying "partially bonded-free length"), there is no way you can properly re-stress these elements.

 

[r13]

I don't understand how the redundancy is achieved once the tendon is fully stressed, unless it is stressed in two stages. No comment.

 

[Obregon]

On the other hand, the permeability of cement grout plays an important role in durability because it controls the entry rate of moisture that may contain aggressive chemicals. Permeability, in turn, is strongly dependent on the W/CM ratio, i.e. compressive strength.

What are your thoughts?

 

[r13]

There is a lot of things to worry about wet application. Minor moisture entry near the ground surface can be prevent through better sealing, or other measures.

 

[Obregon]

What do you mean by "wet application"? What other measures?

 

[Obregon]

Hi PEinc "Two-stage anchor grouting in soil usually requires that the temporary casing pipe remain in place while the grout cures and until the anchor is tested. If not done carefully (too much grout).." Do you really mean it John?

No need to leave the casing (if needed at all) until the anchor is tested. That would become a very unpractical, uneconomical and burdensome operation.

Good night

 

[r13]

Wet - under water or seepage before grout is set.

 

[Obregon]

r13 "Wet - under water or seepage before grout is set."

Sorry, but you shouldn't dare to install a ground anchor under water or seepage conditions within the borehole.

 

[r13]

Only to remind you, the application has been done, but lots of things to watch out for.

 

[Obregon]

Thank you r13. Could you kindly share your experiences?