Why the debond length?

[youngstructural]

When specifying anchoring into rock, or similarly with soil nailing, you always see a purposefully debonded length... Other than causing a predictable location for yielding, what is the purpose of this debonded length?

I'm sure I've seen this discussed, or heard the reasoning before, but I just can't think of the reason...

Must be a Monday; But it's TUESDAY here!!!

Thanks in advance,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[cooperDBM]

Rock anchors are usually post-tensioned. The debonded length allows the strand to be tensioned over a sufficient length to give enough elongation to more than overcome anchorage set and other losses.

Monday's a holiday in Canada so Tuesday's Monday ... or something like that.

 

[hokie66]

YS,

Soil nails are not usually tensioned to my knowledge, so don't require a debonded length.

Ground anchors (rock anchors or soil anchors) are tensioned against a bonded length of anchor at the bottom. If you bonded the whole thing before stressing, the anchorage would be close to the surface, and that is not what you want.

 

[youngstructural]

Thanks for the advice; I'm not sure that I really need a debond length, as I am simply employing reinforcing bar grouted into bedrock as anchor bolts.

I can see the value in bonding deeper into the rock if my loads were higher, but I from the capacity charts I have for the product involved, I only have to grout 1.2m to get the strength required. The product literature recomends a minimum grouted length of 3m, so that is what I will specify, and although I'm certainly leaning towards providing a 600mm debonded length, I would rather eliminate if possible to reduce the total required drilling.

Would you think it necessary to debond a length for a case where we're just trying to get anchor bolts to develop in rock rather than concrete? Still not 100% clear I will require the debond length, but see the need/logic for many similar situations.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

There is no reason to debond if you are not tensioning the bars.

 

[apsix]

From 'Foundation Design and Construction' by M.J. Tomlinson;
"Progressive failure of the grout to rock bond due to stretch of the anchor is prevented by sheathing the upper part..."
I don't think stretch would be significant for a 1200 long anchor.

 

[hokie66]

Even if the grout bond did fail near the surface, I don't think that would differ much strengthwise from a debonded section, but corrosion protection could be a concern in some environments.

 

[youngstructural]

I'm using hot-dipped galvanized bars due to exposure, so I'm not too stressed by the idea of debonded grout permitting moisture access.

Thanks all, I'm satisfied I know where to go with this one.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[DRC1]

If you are using inclined anchors to resisist lateral load then the anchors need to be debonded over the zone through wich failure of the soil/ rock mass is anticipated. For vertical anchors it is still a good idea to have a debonded zone as the top of rock is somtimes not well defined, esp. in residual soils, or is often fractured or punky. The objective is to have 100% of your desired ultimate capacity developed in solid non yielding rock. This may not be true for a portion of the top of the rock.

 

[Robbiee]

On a recent project that we used rock anchors, the soil consultant suggested the following:
1- the capacity of the rock anchor is the least of a) the tensile capacity of the steel, b) the skin friction capacity between grout and rock, and c) the mass of a cone of rock that counteracts tension considering the group effects and the submerged unit weight of rock if UGW is higher than rock.
2- 10% of the anchors be tested for 130% of their design capacity.
The depth of the anchor was governed by the weight of the cone of rock.
As to bond or debond the upper part, it depends on the applicaiton. You definitly need to debond the anchor so that you can pretension it if you are anchoring the footing of a retaining wall for example.