By S Drew, TRL
This paper was first published in GE’s January 1978 issue.
In pull-out tests conducted during the tunnelling trials in chalk at Chinnor, Oxfordshire, 31 mechanically anchored bolts 1 or 2m long failed at between 9 and 62kN with an average load of 28kN; 19 resin bonded rock bolts again 1 or 2m long were also tested and failed at loads between 19 and 52kN with an average load of 32kN. These loads lie at or below the bottom of the range of pull-out loads quoted by Franklin and Woodfield.
Failure occurred at the anchor/rock interface in all cases. Loads were never developed that were great enough either to snap the bolt or fail the rock en masse, which implies that there are gains to be made by ‘improvement of the method of anchoring.
In this article the author describes 40mm diameter mechanical anchors which have been designed specifically to function in the weaker rocks similar to the chalk encountered at Chinnor.