Due to the complex and poor rock conditions in Japan, the auxiliary methods are often used during the excavation by mountain tunneling method. Vertical pre-reinforcement bolt method, in which rebars are cast vertically to reinforce the ground above the tunnel before the excavation stage, is suitable for stabilizing the ground with small overburden, constraining subsidence of the ground surface and the behavior of unstable slope. However, a quantitative design method for this method has not yet been established due to the variety of design concepts such as its placing density and length, the variety of expected performance of the method itself, and the lack of clarity of highly effective rock conditions and so on. In this report, a model test and numerical analyses are carried out to examine the effect of the method, and a fundamental design approach was shown. In the model test, aluminum bar laminates were used as the ground material, and metal ball chains were set to simulate the vertical pre-reinforcement bolt. Internal displacement which simulated the excavation action was reproduced by pulling the PTFE sheets set around the tunnel model. Numerical analysis using the finite difference method was carried out for parametric study after the replication of the model test and to confirm the effect on the behavior of the ground and support structure of full-scale tunnel with vertical pre-reinforcement bolt around the tunnel. From the results of the model tests, the effect of integration of ground above tunnel in the reinforced area was confirmed, and the necessity of the reinforcement in the side area of tunnel was also shown if more stability of tunnel was demanded. From the result of numerical analysis, it indicated that its application might decrease the shear strain of ground and the change of apparent stiffness of ground might be brought by the integration of ground. Based on these results, an approach of a quantitative design was shown. It clarified that placing vertical pre-reinforcement affects the scale of support structure, and also a note to design the most suitable support structure and the relationship between the reinforcement conditions and the effect of this method were clarified.

In many occasions, especially when dealing with poor-quality rock masses, the importance of fast support placement is essential. If placement of the support is delayed, high convergences will develop due to the loss of confinement when creating the hole. As a result, the conditions of the geotechnical quality of the rock mass worsen and lead to excessively high pressures that require the rehabilitation of the support. The role of the support and reinforcement of underground roadways and infrastructures, is to control the convergences of the cavity and eliminate possible rockfalls. The support elements form a system composed of internal and external elements. The elements that work internally reinforcing the rock can be bolts and cables. The other part of the system is the external support that works on the exposed rock, as shotcrete, mesh or a combination of both. The aim of this work is to analyse the technical considerations to be taken into account to correctly design the support of roadways and infrastructures in underground mines and to propose a set of practical recommendations to enhance the support design. To explain the behavior of the different support elements, the role they play in each case is defined. The most common situations are considered according to the type of rock mass and stress conditions. When stability is conditioned by the structure of the rock mass, it is necessary to secure isolated blocks that are formed around the excavation. In this case, the bolts and cables must be able to anchor the blocks to the firm rock mass to transfer the load to the stable rock area, considering the calculations of the forces to withstand the static action of gravity in order to reach the safety factor prescribed in the ITC 04.6.05 currently in effect in Spain. The second role that the supporting elements can play is to reinforce the rock around the roadway or underground accesses so that an arch is formed and can withstand the tensile loads of the ground or to form a beam in the case of solid flat roofs on stratified rocks making them stable. The ultimate goal is to improve the safety of roadways by applying ground support that will maintain the excavations stable and reduce the need for future rehabilitation.

Ground support systems must be engineered to provide a solution for underground mines in static, quasi-static and dynamic geotechnical ground conditions. To prevent large deformations caused by rockbursts or squeezing ground conditions, rockbolts are widely used efficient counter measures. Self-drilling hollow rockbolts are gaining popularity as a means for ground support in rapid underground mine development. This paper describes a recent development in self-drilling rock bolt technology and introduces the Falcon Bolt. The Falcon bolt is a self-drilling R32 hollow bolt with a specially designed mechanical anchor that enables point anchoring and torque-tensioning prior to injection of a pumpable bonding agent. In rockburst-prone or squeezing ground conditions, a decoupled version of the bolt with a high elongation steel grade allows sufficient deformation to occur to dissipate kinetic energy from mobilised rock in a rockburst event or displace with squeezing ground movement. To determine the dynamic performance of the decoupled Falcon bolt, a campaign of mass impact tests were completed at the CANMET MMSL drop test facility in Canada. The results showed that the decoupled Falcon bolt is capable of withstanding 50kJ impacts, i.e. a 2.9 tonnes mass moving at a velocity of 5.9 m/s, without fracture. To further enrich our understanding, the authors performed a numerical study of the dynamic response of the Falcon bolt under dynamic loads using the finite element analysis (via ABAQUS). Replicating dynamic bolt response using numerical modelling techniques at the laboratory scale is a key stepping-stone towards more accurately simulating the complex bolt-rock interactions in a full-scale rockburst event.

When deep tunnels are excavated in poor ground, squeezing conditions occur and the design of supports must follow the yielding principle. To this aim, special elastic-plastic elements embedded in the preliminary support can be employed. The presence of the elastic-plastic elements radically modifies the ground-lining interaction mechanisms making necessary the use of numerical analyses. Particularly relevant is the case of the anisotropic geostatic state of stress. The paper reports and discusses some results obtained by 2D numerical ground-lining interaction analyses of yielding preliminary support with initial non-isotropic stress field. Results of classic rigid support and isotropic state of stress are also reported and compared. Specific attention will be given to the effect that the stress anisotropy has on the lining bending moment.

Skin support provides resistance to the shearing and spalling of exposed rock layers/blocks, as well as protects the surface from open atmospheric contact. Polymeric liners are one such skin support that can be applied to the roof of excavation to enhance load-deformability behaviour. Understanding the reinforcement mechanism of the polymeric liner to the rock in laboratory experiments and numerical models is key to measuring its performance for field applications. The failure behaviour of hydrostone plaster beams when coated with a thin layer of polymeric liner was studied experimentally and numerically. The flexural bending experimental results for load-deformation behaviour of polymeric liner-coated samples showed remarkable difference from the specimens without polymeric liners. The lined beams exhibited Strain hardening behaviour, whereas unlined beams showed tensile-brittle failure. The average load-carrying capability increased nearly 2.5 times for un-notched samples. The substrate material and the interface between the polymer and the substrate was modelled numerically using a cohesive-zone based interface model, to understand the damage behaviour of the samples with deformation as observed during experiments. The results showed that once plaster had yielded, cracks started to generate in the beam. However, the polymeric liner restricted the growth of micro-cracks and their propagation. From the plastic shear strain development it was also evident that plaster started to fail in shear from supporting rollers. As a result, a distinct diagonal shear crack started to form between the loading and the support roller points, causing the ultimate failure. Also, at the interface of liner and plaster, cracks propagated laterally from the roller support points as observed during experiments.

A unified quantitative index and criterion for the stability evaluation and control of tunnel surrounding rock support system composite structures based on plastic complementary energy and over force have been established. It will provide scientific basis and guidance for the selection and quantitative regulation of dynamic support schemes under different conditions. When the rock mass is good or the ground stress is low, the damage evolution is less than the self equilibrium evolution, and the plastic complementary energy eventually tends to stabilize. Support is not required or simple support can be provided as needed. When the rock mass is medium or poor, or the ground stress is medium or high, the damage evolution is greater than the self equilibrium evolution, and there is a minimum value of plastic complementary energy. It is necessary to provide appropriate or strengthened system support in a timely manner. When the rock mass is extremely poor or the ground stress is extremely high, the damage evolution is far greater than the self equilibrium evolution, and the plastic complementary energy increases sharply. It is necessary to immediately strengthen the system support or even advanced support is required. A method has been established to determine the optimal support timing and reinforcement force. The timing of support has a significant impact on the effectiveness of support. The earlier the support time, the better the surrounding rock support effect, but the greater the stress on the support structure, which may damage the support structure. The later the support time, the worse the support effect of the surrounding rock. Even the support cannot suppress the evolution of damage to the surrounding rock, ultimately leading to instability and failure. The optimal support timing is when the plastic complementary energy reaches its minimum value, and the required reinforcement force is optimal, ensuring the support effect without damaging the support structure.