Anchor depth in Rock 1

[Adeel7]

I want to calculate the depth of anchor in rock foundation.

I have found one relation in EM 1110-1-2908 (Page 9-3), which is:

D = ((FS)x(F))/(unit weight x L x s)

where
FS is Factor of safety
F is required anchir force
L is anchor row spacing
s is center to center spacing of anchor


My question is why from above equation if we increase anchor spacing depth decreases and if we decrease spacing anchor depth increases. Is this is due to Group action or for some other reasons? What can be the maximum and minimum spacing criteria for anchor?

Any other literature regarding design of anchor in rock will be helpful.

Thank you

 

[BigH]

There are several modes of anchor pull-out failure in rock. The first is bar to grout; the second is grout to rock. There is also, of course, the structural strength of the bar in tension. The one you are probably concerned with is the "cone pullout". Take the bottom of the anchor and, if I remember correctly, take a cone 30deg from the vertical. This gives you the weight of a cone to offset the pull-out. You will base your depth of anchor on the weight of cone required to resist the pullout - normally, I take it up to about 2 m from the surface of the rock and then straight up - like fractured surface. For groups, the "cones" will interact. This is shown in Tomlinson's book Foundation Design and Construction. It is also shown in Bell's Ground Engineer's Reference Book, Chapter 48 although they have a bit of a different take on the location of the apex of the cone. I've seen this also in NAVFAC 7.1 or 7.2 as well. Sorry, but I don't have Tomlinson directly in front of me at the moment.
See:

http://www.williamsform.com/Ground_Anchors/Designing_Ground_Anchors/proper_anchor_length.html

 

[PEinc]

I start my cone at the tip of the grouted anchor. The file that BigH attached starts the cone at the top of the grouted bond length - too conservative unless you are selling tendons.

http://www.PeirceEngineering.com

 

[BigH]

I follow PEinc's method as well - but I was just pointing out that others have a different take - both Bell's reference written by two Swiss Engineers and the Williams reference "conservatively" take the "top" approach.