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As best I can tell this effect occurs with or without the edge effect as John has noted in his initial post.  The An/Ano ratio decreases, approaching unity (1) for large embedment depths.  At large embedment depths the projected failure surface for a single bolt is nearly identical to the projected failure surface of a closely spaced bolt group.  At shallow embedments the the failure surface of a 4-bolt pattern is almost 4 times the failure surface of a single bolt.  Hence the unexpected effect.  Equation (D-5) should still behave properly however since Nb increases so dramatically with embedment depth and overides the reduction in the An/Ano ratio.  The capacity of the bolt group increases with embedment depth when the edge distances are held constant.   
Christopher A. Banbury, PE
Vice President
Nicholson Engineering Associates, Inc.
PO Box 12230, Brooksville, FL 34603
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