Classical rock support methods for underground engineering emphasize the importance of investigating and controlling the unloading deformation of surrounding rock, such as in the New Austrian Tunnelling Method (NATM) and Norwegian Method of Tunnelling (NMT). Current engineering practice in underground tunnels and caverns under high geostress conditions indicates that stress-induced collapse and rock bursts are the major engineering hazards. Prior to this kind of failure, crack initiation and propagation occurs. Thus, a new support concept, termed the crack-restraining method, has been developed based on the NATM and NMT for underground hard-rock engineering under high geostress conditions. This method includes three key technical points:
The length of the rockbolts is slightly greater than the maximum depth of cracking;
the optimal supporting time of the rockbolts is decided based on the crack evolution and deformation tendency;
shotcrete with high tensile strength is used to ameliorate the stress state of the rock mass' surface.
The aim of these supporting efforts is to restrain the subsequent cracking and enhance the stress-bearing capability of the rock mass. Application of this approach to the Baihetan underground cavern (the largest hydraulic cavern in China) and other deep underground tunnels indicated that this crack-restraining support method is appropriate for determining the rockbolt length and supporting time.
Increasing attention is being given to deep-level excavation and mining due to the gradual exhaustion of shallow mineral resources and recent discoveries of deep mineral resources. Most South Africa gold mines have reached depths of more than 2000m, and the mining depth at Mponeng almost 4350m (Kwiatek, Plenkers, and Dresen, 2011); the Creighton mine in Canada has reached 2400m in depth (Snelling, Godin, and McKinnon, 2013); the Kolar gold mine has progressed to 2400m (Mishra and Panigrahi, 1999). Greater depths are also being attained in civil engineering projects–some traffic tunnels and hydraulic tunnels have been excavated at depths greater than 1000m (Zangerl et al., 2008, 2010, 2014). All these engineering projects have shown that the unloading behaviour of hard rock under high geostress is very complex. The support scheme for hard rock under the conditions encountered in these projects obviously presents a great technical challenge.