Why the debond length?
Why the debond length?
(OP)
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
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...






RE: Why the debond length?
Monday's a holiday in Canada so Tuesday's Monday ... or something like that.
RE: Why the debond length?
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.
RE: Why the debond length?
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...
RE: Why the debond length?
RE: Why the debond length?
"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.
RE: Why the debond length?
RE: Why the debond length?
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...
RE: Why the debond length?
RE: Why the debond length?
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.