Experimental Study of a New Energy-Absorbing Rock Bolt Under Static Loading Condition

High stress in surrounding rock mass can cause serious stability problems such as squeezing in soft rock and rock burst in hard rock. The support system applied in high in-situ stress condition should be able to carry high load and accommodate large deformation of rockmass. This paper presents a specifically designed rock bolt, called Tension and Compression Coupled Yielding bolt, which can provide support for both squeezing and burst-prone rockmass encountered in mining and tunneling at depth. The new bolt mainly consists of a steel rod and two additional anchors. The steel rod is a round shape bar with varying surface conditions. The inner segment is processed into rough surface, while the middle of the rod has smooth surface. Two additional anchors were welded on both ends of smooth segment. The bolt is fully encapsulated with either cement or resin grout in a borehole. The rough rod and the inner anchor are firmly fixed in the bottom of the borehole, while the smooth segment has no or very weak bonding to the grout, which can stretch to accommodate rock dilatation. Static pull tests show that the load and strain elevations could result in premature failure of conventional rock bolt, as it is strongly bonded to the grout. However, the smooth segment of TCC Yielding bolt can easily detach from the grout under pull loading and provide large deformation to accommodate rock dilations. The coupling action of tension and compression of grout in different position can increase the ultimate bearing capacity of inner anchoring segment greatly. The stress dispersion structure also makes the load of rough rod lower than the smooth rod, preventing the premature failure of steel rod at inner anchoring segment. Finally, a simple method was developed to predict the deformation ability of the new bolt. The bolt elongation will be 386~754mm for 2500~5000mm long bolt at high load level equal to the strength of the material, thereby absorbing a large amount of energy to maintain the stability of surrounding rockmass.