Mechanism of CTC-yield bolts and its experimental research

The high stresses acting on rock masses in deep underground can cause serious stability problems such as squeezing of soft rocks and rock burst in hard rocks. The support system applied under high in-situ stress condition should be able to bear high loads and accommodate large deformation of rock masses without experiencing serious damage. A specifically designed rockbolt, called combination of tension and compression type yield bolt (CTC-yield bolt), is proposed. The inner thread segment and anchor compose the inner anchorage section of the bolt that is firmly fixed at the bottom of the borehole, while the smooth section of the bolt will elongate in response to rock deformation when the load exceeds its capacity. Such special structure can help avoid premature bolt failure due to the stress concentrations caused by fracture/joint opening. The static pull tests show that the new bolt has larger ultimate bearing capacity and better deformation property than the conventional rock bolt. The stress distribution at bolt-grout-rock interface is effectively optimized by the combined state of tension and compression of the grout inside and outside the anchor, thereby improving the anchorage force significantly. The deformation capacity of this new bolt is closely related to the length of its smooth section between the two anchors. The new bolt is expected to provide support for both the squeezing and the rock burst encountered in tunneling and mining engineering.