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High stress in surrounding rock mass could cause serious stability problems such as larger squeezing deformation in soft rock and rock burst in hard rock. The support system applied in high in situ stress conditions should be able to carry high load and accommodate large deformation of rock mass. This paper presented a new yielding rock bolt, called tension and compression-coupled yielding rock bolt, which is promising to provide support for both squeezing and burst-prone rock mass encountered in mining or 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 segment of the rod has smooth surface. Two additional anchors were arranged on both ends of smooth segment. The bolt is fully encapsulated in a borehole with either cement or resin grout. The rough segment 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. First, direct quasi-static pull tests were performed to examine the load capacity of tension and compression-coupled anchor. The results showed that the coupling action of tension to the rough rod and compression on the inner additional anchor by grout in different positions can increase the ultimate bearing capacity of inner anchoring segment significantly. Second, the performance of the new bolt and the fully encapsulated rebar bolt was tested under fracture opening condition. Results showed that the load and strain concentration could result in premature failure of fully encapsulated rebar bolt. However, the smooth segment of TCC Yielding rock bolt can detach from the grout under pull loading and provide a larger deformation to accommodate rock dilations. Third, shear tests were performed to examine the deformation mechanism of the new rock bolt under fracture sliding condition. Results showed that the smooth section of the new bolt specimen can deform freely to accommodate the sliding of fracture. The maximum shear displacement of the new bolt specimen is much larger than the fully encapsulated rebar bolt specimen, which is promising a better ability to accommodate the large displacement sliding of fracture in engineering practice.

The shear performance of bolted rock joints under cyclic loading condition was studied through laboratory shear tests. The tests were performed on a cyclic shear testing apparatus. The fully encapsulated rock bolts were inserted in the rock joints. The upper block was fixed in the horizontal direction, and the cyclic shear load was applied laterally on the lower block. A total of 16 shear tests were conducted for two types of rough joints. The results showed that the failure mode of rock bolts in cyclic shear test was different with the case of direct shear test. When the cyclic displacement was small, the rock bolt played a very small role in the second-to-fifth cycles, the rock bolt remained unbroken during cyclic shearing. The shear resistance of the rock bolt would gradually recover after the shear displacement has exceeded the cyclic displacement in the termination cyclic after five cycles. When the cyclic displacement was large, the rock bolt broke during cyclic shearing, and the shear resistance could not be recovered in the termination cyclic. It is indicated that the rock bolt had completely lost its supporting role after cyclic shear loading. The shear strength reduction of bolted joints under the cyclic loading condition was much significant compared to the un-bolted joints. Therefore, the influence of cyclic loading on the shear resistance of the rock bolt was much larger than its counterpart of the rock joint itself. These results showed that the shear performance of a rock bolt inserted in a rock joint was strongly influenced by cyclic shear loading.

The actual convergence of an excavation located in fractured rock mass or the soft rock is largely different with the theoretical result in many cases. Experimental results showed that the influence of confining pressure on Young's modulus is very significant. This paper attempted to illustrate the influence of the confining pressure-dependent Young's modulus in the ground reaction analyses of mountain tunnel. Firstly, the relationship between Young's modulus and confining pressure was described as a non-linear function according to the test results. Based on the plane strain axial symmetry assumption and the incremental theory of plasticity, equilibrium equations and compatibility equations of rock mass around a circular tunnel were deduced theoretically. Based on fourth Runge-Kutta method, a semi-analytical solution was achieved. Considering the effect of confining pressure on Young's modulus, the stress and deformation of rock mass around tunnel was calculated by both analytical and numerical methods. The influence of confining pressure-dependent Young's modulus in surrounding rock was estimated quantitatively. Finally, Tawara saka Tunnel in Japan was taken as an example to explain the influence of confining pressure-dependent Young's modulus. The results showed that the error with respect to the monitoring data was largely reduced with the confining pressure-dependent Young's modulus model, which indicated the necessity of considering the non-uniform distribution of Young's modulus., Tunnelling and Underground Space Technology, 83, pp.135-144; 2019

Rock joints will undergo a sequence of cyclic shearing loadings during a seismic event. However, the effect of cyclic shear loading on the energy-absorbing rock bolts has never been studied before. Laboratory shear experiments were carried out to study the shear behaviour of rock joints reinforced by the energy-absorbing rock bolts under cyclic loading condition. The results illustrated that the support effect of the energy-absorbing rock bolts was very small after the first cycles in the cyclic shear experiments. In the case of small cyclic distances, the shear resistance of the energy-absorbing rock bolts will gradually recover after the shear displacement has exceeded the cyclic distance in the subsequent shear experiment after 5 cycles. In the case of large cyclic distances, no recovery of shear resistance was found in the subsequent shear experiment, indicating that the energy-absorbing rock bolts had completely lost its supporting role after cyclic shear loading. A new index of shear energy loss ratio (SELR) was proposed to evaluate the shear behaviour of energy-absorbing rock bolt and rock joint under cyclic shear loading condition. The results showed that the SELR of rock joints was commonly less than 20%. However, the SELR of rock bolts could reach nearly 100% when the cyclic distance was larger than 8 mm. When the cyclic distance was 4 mm or 6 mm, the SELR of the fully encapsulated rock bolts almost reached 100%. However, the SELR of the energy-absorbing rock bolts were located in the range of 50–80% for the same condition. The results indicated that the shear behaviour of a rock bolt inserted in a rock joint was strongly influenced by cyclic shear loading. The shear performance of the energy-absorbing rock bolts was better than the fully encapsulated rock bolts under cyclic shear loading conditions.

A large number of laboratory experiments have shown that the rocks in post-peak state present strain-softening behaviour under low confining pressure, and gradually evolved into elastic-perfectly plastic with the increasing of confining pressure. Neither the elastic-perfectly plastic model nor the strain-softening model can accurately describe the behaviour of rock mass surrounding the deep buried excavations. In this paper, a modified strain-softening model was proposed to describe the non-linear evolution of residual strength under the influence of confining pressure. The new model can realize the gradually transition from strain-softening behaviour to elastic-perfectly plastic behaviour with the increasing of confining pressure. The equivalent residual strength was defined to quantify the strength of rocks in post-peak stage. The parameters involved in the model were estimated via non-linear regression analysis upon a series of stress-strain curves. Based on the plane strain axial symmetry assumption and the incremental theory of plasticity, equilibrium equations and compatibility equations of rock mass around a circular tunnel were deduced. The equations were implemented in the Visual Basic development environment, and a semi-analytical solution was obtained. The influence of post-failure behaviour of rock mass was demonstrated through an illustrative case. The distributions of stress, displacement and transitional strength around the tunnel were presented. The differences between the elastic-perfectly plastic model, the strain-softening model and the new model were estimated quantitatively. In addition, the validity of semi-analytical solution was verified by numerical simulations. Parameters analysis showed that the tunnel convergence was influenced by the post-peak behaviour of rock mass dramatically., Engineering Geology, 240, pp.21-33; 2018

An interaction model is proposed to describe the interaction between the energy-absorbing rock bolt and the rock mass. Based on the plane-strain axial symmetry assumption and the incremental theory of plasticity, the equilibrium equations and compatibility equations of rock mass, as well as the response of the energy-absorbing rock bolt are deduced theoretically. The proposed method was programmed in a Visual Basic environment, and a semi-analytical solution for the coupling model was achieved. The reinforcement mechanism of the energy-absorbing rock bolt in conventional tunneling is clearly demonstrated through an illustrative case study. The reinforcement effect of the energy-absorbing rock bolt under different conditions was estimated quantitatively, and its mechanical work transfer ability is presented. In addition, the validity of the proposed method was verified through numerical simulations. Finally, a number of derivative cases were investigated to reveal the influence of the bolt and rock properties on the reinforcement effect and the bolt work transferred on the rock mass. In the case of higher in-situ stress or low-strength rock mass, the support effect of the energy-absorbing rock bolt is significantly improved, and the bolt absorbs more energy. Increasing the bolt installation density could always be helpful for the stabilization of the surrounding rock mass. However, additional rock-bolt length could hardly affect ground reinforcement because the bolt section embedded in the elastic region of the rock mass could barely help to constrain the elastic displacement release. The bolt should be installed no later than the stage of critical inner pressure, namely when the plastic region occurs., International Journal of Rock Mechanics and Mining Sciences, 103, pp.168-178; 2018

The influence of shear rate on the mechanical behavior of rough rock joints prepared using a molding method was examined by testing jointed samples of four rock joint groups, which were obtained from the underground caverns of the Huangdao State Oil Reserves in China. The tests were performed using a JAW-600 coupled shear-flow machine under constant normal load (CNL) conditions. Shear rate and joint roughness coefficient (JRC) were the two primary considerations in this study. The results indicate that the peak shear strength is controlled by shear rate and joint roughness. Shear rate has a nonlinear relationship with the peak shear strength, whereas joint roughness exhibits good linearity with a high correlation coefficient (>0.97, except for results under a shear rate of 24.0 mm/min). Furthermore, shear rate affects the damage incurred by the rock joints, as illustrated by the acoustic emission of rock joints monitored by a PAC acoustic emission system. The joint surface breaks earlier with increased shear rate. Furthermore, because of the more rapid generation of macrocracks under higher shear rate, the corresponding cumulative acoustic emission (AE) count and AE energy are inversely proportional to the shear rate. A new shear strength criterion that considers both shear rate and joint roughness was developed based on the experimental results and Barton’s shear strength criterion. Then, the validity of this proposed criterion was verified with an average estimated error of 10.539% using the shear strength values of rock joints proposed by Barton, indicating that the new criterion is capable of providing an accurate estimate of shear strength for rough rock joints under different shear rates.

長崎大学学位論文 学位記番号:博(工)甲第44号 学位授与年月日:平成29年9月20日

In this paper, the time evolutions of the mechanical characteristics of a discretely mechanically or frictionally coupled (DMFC) rockbolt and surrounding rockmass are studied. A conceptual model is developed to analyze the interaction between the DMFC rockbolt and rockmass, and analytical solutions are presented for a tunnel supported with DMFC rockbolts. The formulation is based on the following assumptions: (1) a deep tunnel with a circular cross-section; (2) an axisymmetric problem, i.e., having a lateral pressure coefficient K a = 1; (3) homogeneous and isotropic ground; (4) small deformations; (5) rockbolts simulated as two cylindrical surfaces and oppositely distributed forces, i.e., one at the head and the other at the bolt bond; and (6) viscoelastic material assumptions, i.e., a one-dimensional viscoelastic model for the rockbolt and a three-dimensional viscoelastic model for the rockmass. The analytical solutions of the rheological model are obtained by assuming the constitutive models of the rockmass and rockbolt in terms of a Maxwell model and Maxwell model (M–M), Kelvin model and Kelvin model (K–K), and Kelvin model and Maxwell model (K–M). The analytical solutions of the M–M model are compared with the results obtained from numerical simulations with FLAC3D. The analytical solutions are in good agreement with the numerical results in the case of small rheological rock deformation and close bolt arrangement. Furthermore, the variation of the axial force along the rockbolt and the evolution of stress and displacement in the rockmass are closely related to the rheological parameters and support parameters.

High stress in the surrounding rock mass can cause serious stability problems. The applied support system used under high in situ stress condition should be able to carry high loads and also to accommodate large deformation without experiencing serious damage. This paper presents a specifically designed rock bolt for energy-absorbing applications, which can provide support for squeezing and burst-prone rocks often encountered during underground excavation in the tunneling and mining community. The bolt mainly consists of a smooth steel bar with an anchor near the bottom end of its body. The anchor is firmly fixed within a borehole using either cement grout or resin, while the smooth section of the bolt inserted in the anchor can slide in response to rock deformation once the load exceeds the pre-set capability. Static pull tests on the new bolt show that it can elongate to any expected length at a high load level, thereby absorbing a large amount of energy to maintain the stability of surrounding rock.