Field Observations of Rock Bolts in High Stress Rock Masses

The loading conditions and the failure modes of rock bolts are presented through field observations conducted in cut-and-fill mines. With the cut-and-fill mining method, slopes are mined in 3-5 m high slices along the strike of the ore body by breasting using drill jumbos. After a slice has been mined out it is backfilled with waste rock and tailing. The observations revealed that in situ rock bolts are most loaded in the portion close to the collar of the borehole. In a dilating rock mass the maximum bolt load usually occurs at a distance of about 0.5 m from the face plate. Rock bolts are subject to both pull and shear loads in situ. The shear strength of a rock bolt is equally important as its tensile strength. The major shortcoming of the conventional fully bonded rebar bolt is its small deformation capacity. In high stress rock conditions, rock bolts should not only have a high load-bearing capacity, but also should be able to accommodate large rock dilations.