in use of the rock bolt an anchoring composition in a flexible container not shown is inserted into a borehole formed in a body of rock the leading end 16 of the rock bolt is then inserted into the borehole and the rock bolt is pushed fully into the hole the shear structure at the trailing end 14 is engaged with a device which rotates the shank as the shank is pushed home and rotated the blade 42 easily penetrates the flexible container due to its size and the sharp corners 50 the blade upon rotation of the rock bolt rapidly shreds the container irrespective of the material from which it is made the anchoring composition inside the container is released and is mixed by ongoing rotation of the blade the mixing members 44 which are simultaneously rotated help substantially in this regard the mixing members are relatively small compared to the blade and the likelihood that these members which has chamfered sides 54 will entrain parts of the flexible container is remote the blade on the other hand shreds the container and as noted continues with the mixing process the anchoring composition sets fairly rapidly and starts bonding to the shank and the leading end the rotational force required to rotate the shank increases and ultimately a point is reached at which the shear pin 22 shears it is then no longer possible to impart torque to the shank typically the nut 20 is automatically unscrewed from the shank alternatively the nut is manually released from the shank a load spreading washer if required can then be engaged with the shank whereupon the nut is re engaged with the shank the structure at the leading end 16 of the shank has been found in.