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Rock Anchors

Rock Anchors

Rock Anchors

Need some help with grouting of rock anchors. We are using 1-3/4" diameter 150 ksi Dywidag bars in a permanent application to resist uplift in a pile cap.

The commentary to section of the 5th Edition PTI manual states that "For normal applications the bond betwen the tendon and anchor grout is not critical." But it doesn't define a "normal" application.

ACI 318-99 section 12.2.2 gives absurdly long development length for fy = 150 ksi and fc = 4000 psi (because it has no provisions for fse). Section 12.9 states that it is only appropriate for 3 and 7 wire strand (but it gives lengths for the bar that would seem reasonable considering PTI's statement concerning normal applications).

So, can anyone offer assistance to determine the embedment required into the grout (for a given grout strength)? Is ACI  section 12.9 also appropriate for high-strength bar tendons?

RE: Rock Anchors


Recommendations for Prestressed Rock and Soil Anchors,
PTI, Phoenix 1986,
p. 29

has a formula for the Load per unit length, depending of quality of the anchor and nature of the soil.

For sand/gravel, 10-20 kip/ft. For rock and the best standard of construction, then, likely 20 kip/ft of load.

The table lists this as Ultimate Load, and then should be compared with factored loads. It is said before "the following may be used for calculationg the working load...". I preferably assume oneself is to devise a suitable safety factor, for ultimate means to me true limit.

Respect what safety factor, in bond is usual the embedments have between 2 and 3 safety factor. Other thing is that you choose to use just 1.6 or so but I would think it would be against the preferred practice.

RE: Rock Anchors

hi dougantholz,

Regarding ground anchors, IMHO, forget the 5th edition of the PTI Manual - it is more than 10 years old and needs updating urgently. In 1996 the PTI introduced the "Recommendations for Prestressed Rock and Soil Anchors" (see www.post-tensioning.org) and it revises, considerably, ground anchor design and construction. Unfortunately, my copy is in transit from our other office, but i do have my British Standard, which i do consider to be superior than PTI in this area.

Littlejohn and Bruce have done considerable work in ground anchors and their work is extensivley quoted in the British Standard BS 8081: Ground Anchorages. For the grout/tendon interface their research is repeated in the BS appendix and states the working and ultimate bond values for varying tendons configurations (strand, bar, wire etc) based upon actual test results from around the world. For ultimate bond values to the BS(assumed to be uniform over the tendon bond length - although in practice it is NOT) are as follows:

   1)  plain wire/bar:                  1.0 N/mm2
   2)  clean crimped wire:              1.5 N/mm2
   3)  clean strand or deformed bar:    2.0 N/mm2
   4)  locally noded strand:            3.0 N/mm2

Based upon 30 MPa (4400 psi) grout strength.

For your case, an ultimate value of 2.0 N/mm2 (300 psi) is applicable.
Actual bond behaviour (resistance verses slip) is dependent on adhesion, friction and shear (mech interlock). The grout strength and the nature of the tendon surface will determine the bond characteristics.

Appendix G of the BS covers this area well - i can fax it to you if you wish.

In my experience, the ground/grout interface has not governed the design BUT you do indeed have to check it.

Also, since these are permanent anchors you are deemed to do performance and proof testing - which will be testing this interface - but it may also be worthwhile to consider some test anchors prior to production, or at least some bond tests of 1 3/4" bar in some sample holes with the specified grout.

Re ACI 318 12.2.2 - i have never used this clause for this type of application and i do not think it is relevant - i seem to recall that these equations were limited to max bar strengths based on fy = 75 ksi or less (but i may be wrong on this).

I would also refer you to the PTI's new publicaiton on grouting - authored in 2001 I believe. It is worth getting a copy of this document and see what parts of it will be relevant for your anchor project.


RE: Rock Anchors

You might want to check out manufacturer's of specialty grouts for rock anchors such as Fosroc - they should have bond information for their product readily available.


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