- Roof bolts / studs are mechanically point anchored within the rock using 3 leaf bail-type expansion shells attached to the inserted end of the bolt. Expansion shells are supplied screwed onto the end of the rock tendon and are inserted into the hole. When tension is applied to the tendon the tapered plug within the shell forces the leaves to move outwards, thus wedging them against the walls of the borehole. This means that the mechanical bolt is functional immediately on installation. In many applications the installation is post-grouted for additional stability. The complete rock bolt assembly includes the threaded bar, bearing plate (washer), nut, seat and expansion shell.