Application of Specific Energy in Evaluation of Geological Conditions Ahead of Tunnel Face

In underground construction, especially tunnel building, a detailed knowledge of the geological conditions ahead of the tunnel face is essential for both safety and efficiency of work. Many tunnel collapses have been reported to occur because of a lack of accurate evaluation of the geological conditions. While conventional horizontal boring conducted from a tunnel face provides good accuracy, it is infrequently conducted due to its high cost and time-consuming nature. In addition, the tunnel seismic reflection method has limited practicality owing to the complexity of data processing. This paper presents a new approach based on the specific energy of the mechanical drilling rig to evaluate the geological conditions ahead of the tunnel face. In order to assess the geological conditions, rock mass quality index, buried depth of the tunnel and tunnel deformation investigation were undertaken, and the obtained data were compared to the specific energy. Results from the comparison reveal the evaluation criterion that if the distribution of specific energy in some areas deviates from the distribution of buried depth, it is considered that abnormal geological conditions exist in this area. This work can greatly contribute to the accurate and effective evaluation of the geological conditions ahead of the tunnel face

Experimental Study on Shear Behavior and Acoustic Emission Characteristics of Nonpersistent Joints

The shear behavior of rock discontinuities controls the stability of rock masses to a great extent. In this paper, laboratory shear tests were performed on rock-like materials with different cracks to study the effect of nonpersistent joints on the shear behavior of rock masses. The results show that the variation trends of the shear stress-displacement curves of specimens with different cracks are generally similar and have the same stage characteristics. When the crack length is relatively short, the elastic stage is prolonged, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength and the residual shear strength increases with the increase of the crack length. When the crack length is relatively long, the elastic stage is shortened, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength increases with the increase of the crack length. The peak shear stress gradually decreases with the increase of the crack length. The shear strength of the specimens with unilateral cracks is much higher than that of the specimens with bilateral cracks. The shear strength of the specimens is affected not only by the crack length but also by the crack distribution. The acoustic emission (AE) count peak occurs when the shear stress drops sharply and has an inverse "S"-type variation trend with the increase of the crack length. The inclination angle of the fracture decreases, the roughness of the fracture surface decreases, and the proportion of the wear area on the fracture surface increases gradually with the increase of the crack length. The AE source decreases with the increase of the crack length, and their locations are obviously asymmetric. This work can greatly contribute to the insight into the shear failure mechanism of rock discontinuities with nonpersistent joints

Discrete Element Simulation of the Macro-Meso Mechanical Behaviors of Gas-Hydrate-Bearing Sediments under Dynamic Loading

Under the action of dynamic loadings such as earthquakes and volcanic activities, the mechanical properties of gas-hydrate-bearing sediments will deteriorate, leading to a decrease in the stability of hydrate reservoirs and even inducing geological disasters such as submarine landslides. In order to study the effect of dynamic loading on the mechanical properties of hydrate sediments, triaxial compression tests of numerical specimens were carried out by using particle flow code (PFC2D), and the macro-meso mechanical behaviors of specimens were investigated. The results show that the loading frequency has a small effect on the stiffness of the hydrate sediment, while it has a large effect on the peak strength. The peak strength increases and then decreases with the increase in loading frequency. Under the same loading frequency, the peak strength of the hydrate sediment increases with the increase in loading amplitude, and the stiffness of the specimen decreases with the increase in loading amplitude. The maximum shear expansion of the specimen changes with the movement of the phase change point and the rearrangement of the particles. The maximum shear expansion of the specimen changes with the movement of the phase change point and the change of the bearing capacity of the particles after the rearrangement, and the more forward the phase change point is, the stronger the bearing capacity of the specimen in the plastic stage. The shear dilatancy angle and the shear dilatancy amount both increase linearly with the increase in loading amplitude. The influence of loading frequency and amplitude on the contact force chain, displacement, crack expansion, and the number of cementation damage inside the sediment is mainly related to the average axial stress to which the specimen is subjected, and the number of cracks and cementation damage of the sediment specimen increases with the increase in the average axial stress to which the sediment specimen is subjected. As the rate of cementation damage increases, the distribution of shear zones becomes more obvious

Stability Control and Quick Retaining Technology of Gob-Side Entry: A Case Study

In Chinese coal mines, gob-side entry retaining, an efficient technique for coal mining, has been widely used. However, severe roadway deformation and slow retaining speed have gravely restricted the popularization and its application. Hence, in order to solve the existing problems, the deformation mechanism of gob-side entry was studied. Then, a new approach for gob-side entry retaining technique (GERT) was proposed to increase the speed of gob-side entry retaining. Finally, the application effect of the new GERT method was tested and analysed. The results show that the rotation and subsidence of roof key block B lead to severe deformation of roadway. And the proposed gob-side entry stability control technology can affectively resist the severe roadway deformation. Compared with the conventional support method for gob-side entry retaining, GERT has completed the complex wall construction work in advance, thereby simplifying the process of gob-side entry retaining and increasing the speed of gob-side entry retaining. When retaining the entry in panel 183U04 with the new support method, the entry retaining speed significantly improved from 0.25 m/h to 1.0 m/h compared with the traditional method, and then the problem of gob-side entry severe deformation was solved

Experimental Study on the Shear-Flow Coupled Behavior of Tension Fractures Under Constant Normal Stiffness Boundary Conditions

This study experimentally investigated the effects of fracture surface roughness, normal stiffness, and initial normal stress on the shear-flow behavior of rough-walled rock fractures.A series of shear-flow tests were performed on two rough fractures, under various constant normal stiffness (CNS) boundary conditions. The results showed that the CNS boundary conditions have a significant influence on the mechanical and hydraulic behaviors of fractures, during shearing. The peak shear stress shows an increasing trend with the increases in the initial normal stress and fracture roughness. The residual shear stress increases with increasing the surface roughness,normal stiffness, and initial normal stress. The dilation of fracture is restrained more significantly under high normal stiffness and initial normal stress conditions. The hydraulic tests show that the evolutions of transmissivity and hydraulic aperture exhibit a three-stage behavior, during the shear process-a slight decrease stage due to the shear contraction, a fast growth stage due to shear dilation, and a slow growth stage due to the reduction rate of the mechanical aperture increment. The transmissivity and hydraulic aperture decreased, gradually,as the normal stiffness and initial normal stress increase

Study on a High-Efficiency Mining Technology System for Gas Outburst in Coal Seams—Example of an H Coal Mine

Coal will continue to play an important role in China’s economic development and social development in the coming decades. However, due to the complex distribution conditions of coal resources, the mining of coal resources is subject to various restrictions. Coal and gas outburst is an important issue in coal mining, and the threat to the mining of coal resources caused by high gas outburst activity has been receiving more attention. In order to solve the problems related to safe and efficient mining under coal seams with gas outburst, such as mining difficulties, large amounts of work, resource waste, no guaranteed gas treatment time, and low economic efficiency, it is necessary to innovate mining technology and methods for managing gas outburst in coal seams to improve the efficiency of coal mines and to solve the above problems. This study proposes a green mining technical method system known as the “L-H method”, which is applicable to the safe and efficient mining of coal seams with gas outburst based on combined theoretical analysis and numerical simulation. The following research results are achieved: (1) The “L-H method” is proposed, and a mining area model of coal seams with gas outburst is established. The specific details of the method and the implementation process are introduced. (2) Examples of H coal mine applications are presented, and the effects of the implementation of the “L-H method” are analyzed and summarized through mine pressure observations from roadways, and it is concluded that the implementation of top-cutting and pressure-relief technology has a good control effect on the roadway-surrounding rock and that gas extraction reaches the national standard of less than 8 m3/t for protrusion prevention; this ensures safety and also achieves efficient mining. This study will provide a good reference for the implementation of green mining methods to similar coal and gas outburst mines

Rheological Behavior of Rockmass Supported with Rockbolts Based on Viscoelastic Analysis Method

In this paper, we present the viscoelastic solutions for rockmass supported with discretely mechanically or frictionally coupled (DMFC) rockbolts to reveal the coupling rheological mechanisms. The analytical solutions are first acquired by applying the Laplace inverse transforms. The effect of different viscosity coefficients and supporting parameters on the coupling model rheological behavior are then investigated. It is concluded that the variation of the rockbolt axial force or rock mass stress and displacement have a close relationship with rheological parameters and support parameters. In addition, the variations of mechanical states of rockbolts and rock mass are closely related to the rheological model