Mechanical Characteristics and Failure Modes of Low-Strength Rock Samples with Dissimilar Fissure Dip Angles

The mechanical characteristics and failure modes of low-strength rock sample with various fissure dip angles were investigated by conventional uniaxial compression test and three-dimensional (3D) crack reconstruction. The results indicated that compared with high-strength rock masses, cracks had different influences on the low-strength rock mass mechanical deformation features. Thereinto, the dip angle of fissures can cause post-peak failure stage of stress-strain curve change from swift decline to multi-step down, showing obvious ductility deformation and failure characteristics. Peak strength and elastic modulus owned an anti-S-shaped growth tendency with the growth of fissure dip angle, which was positively correlated and greatest subtle to the fissure dip angle α < 21° and α > 66.5°. The axial peak strain reduced first and enlarged rapidly with growing fissure dip angle, suggesting a V-shaped change trend. Increasing the fissure dip angle will change the sample failure mode, experienced complete tensile failure to tensile-shear composite failure, and ultimately to typical shear failure. Also, the crack start angle decreased with enlarging fissure dip angle, larger than that the high-strength rock mass fissure dip angle. The above research findings can complement and improve the study of fissured rock masses.

Mechanical Characteristics and Failure Modes of Low-Strength Rock Samples with Dissimilar Fissure Numbers

The mechanical characteristics and failure modes of low-strength rock sample with various crack dip angles and numbers were investigated by conventional uniaxial compression test and three-dimensional (3D) crack reconstruction. The results indicated that compared with high-strength rock masses, cracks had different influences on the low-strength rock mass mechanical deformation features. Thereinto, the number of fissures can cause post-peak failure stage of stress-strain curve change from swift decline to multi-step down and horizontally extended decline, respectively, showing obvious ductility and ductile-flow deformation and failure characteristics. Due to the structural effect, only under the condition of fissure α < 90°, the modulus of peak strength and elastic modulus lowered with the enhancement of fissure number and had a negative correlation. As the number of fissures increased, the axial peak strain increased first and then decreased, demonstrating a reversed V-shaped change trend. Fissure number can fully affect the crack propagation law only in the case of vertical fissures. The above research findings can complement and improve the study of fissured rock masses.

Prediction of Stope Stability Using Variable Weight and Unascertained Measurement Technique

A new model is established to analyze mining stope stability, using variable weight theory to calculate the index weight for each factor in different stopes and unascertained measure evaluation technique to predict the risk grade of stope stability. In this model, an evaluation index system by virtue of the 7 most important factors is established, including rock saturated uniaxial compressive strength, rock quality designation, rock joint and fissure, stope span, condition of pillar, groundwater seepage volume, and rate of supporting pit roof. And each index is divided into 5 grades by assignment value and the classification method of standardization. Accordingly, the analysis result is also classified into 5 risk grades. This model is used for the 6 main stopes from the -270 m section in Xin-Qiao Mine, China. The results, giving risk grade for each stope and guiding the use of corresponding measures, avoided the problem of state out of balance caused by conventional invariable weight theory models and have ensured no accident occurred in mining production in recent years. This model can be used in other mines widely, by assigning values for the 7 factors on basis of current in situ cases.

Of forming loads on the mounting of shaft shafts in the hierarchic block environment under the influence of modern geodynamic movements

The subject of the research is the lining of mine shafts and surrounding rock mass. The subject matter is the features of the stress-strain behavior of lining and adjacent rock mass in shafts and near-shaft underground openings in the Tenth Anniversary of Independence of Kazakhstan mine located in the tectonically high-stress and low-strength rock mass. The loading of the lining in the shafts and near-shaft openings in the Tenth Anniversary of Independence of Kazakhstan mine during construction is investigated, and the damages of the lining in the course of drivage in the nonuniform stress field are analyzed. The long-term periodic in-situ instrumental monitoring of stress variation in the lining of the mine shafts and near-shaft openings revealed the main influences on the process of load formation on the lining in the conditions of post-limiting deformation of surrounding rock mass. It is validated that the surrounding rock mass of the mine has a complex hierarchical block structure and is subjected to modern geodynamic movements. As the depth of mining is increased, surrounding rock mass of the mine shafts transfers to the condition of postlimiting stresses and strains. As a consequence, the lining of the shafts and near-shaft openings at different stages of construction experiences nonuniform concentrated loads, which violates integrity of the lining.

Joint analysis of rock mass stress state and elasticity modulus during shaft sinking

The article presents an innovative procedure for the joint stress-strain and elasticity modulus analysis in high-strength rock masses with spacing from a few to tens meters. The procedure includes measurement of elastic convergence of rock walls due to deeper penetration of the foot of a vertical shaft and the analysis of measured displacements of check points along the shaft cross-section perimeter with subsequent two-stage solution of an inverse geomechanical problem. In the first stage, in the lobed diagram of measured displacements of check points, the azimuths of axes of the principal horizontal stresses in surrounding rock mass are determined. In the second stage, the process of deeper penetration of the shaft foot is modeled with different scenarios of the rock mass stress-strain behavior set as varied principal horizontal stresses at the known azimuths of their main axes. Then, the model and in-situ measurement results are compared using the analysis of variance ANOVA. The wanted variant of the stress-strain behavior and the associated modulus of elasticity, such that deviation of the actually measured displacements of check points from the model values is minimal, is identified by the extremum analysis of the experimental diagrams. The procedure was successfully tested in Vspomogatelny and Skipovoi vertical shafts of the Tenth Anniversary of Independence of Kazakhstan mine within Donskoy Mining and Processing Plant, in qualitatively different geological conditions: high-strength rock mass areas categorized as unstable and stable. In unstable rocks, the measured elasticity modulus Е = 3,5 ± 0,7 GPa made 6 %-16 % of the elasticity modulus in samples. In the stable rock mass, the measured modulus Е = 36,6 ± 7,7 GPa almost coincided with the elastic modulus of samples.

Influence of Rock Strength on the Propagation of Slotted Cartridge Blasting-Induced Directional Cracks

Based on theories of explosive mechanics and rock fracture mechanics, the influence mechanism of rock strength on the propagation length of the primary crack in the directional fracture blasting with slotted cartridge has been investigated deeply to propose the relation equation between the rock strength and the propagation length of the primary crack. Theoretically, the maximum length am of the primary crack increases with the enhancing rock strength parameters. The explicit dynamic analysis software LS-DYNA has been used to simulate the slotted cartridge blasting in the mudstone, sandstone, and granite with different strengths in order to reveal the effect of rock strength on the propagation length and velocity of the primary crack and the stress distribution characteristics in rock. The numerical results show the primary crack easily bifurcates and attain a much shorter propagation length in the mudstone with the minimum strength, and there are radial cracks appearing in the nonslotted direction. When rock strength rises, the propagation length, velocity, and duration of the primary crack and the concentration degree of effective stress in the slotted direction will all increase in the sandstone and granite, but there is an opposite influence trend of rock strength in the stage of the initial guide crack’s formation. The cracking velocity has an overall oscillation downtrend whose swing amplitude enhances clearly with the increasing rock strength, signifying the more unsteady propagation of the primary crack in the higher strength rock.

The solution of geomechanical tasks in stage technology disclosure of the workings of the great migration in low-strength rock

The materials presented in the article will be used in the dissertation for the postgraduate of technical sciences. The paper introduces the construction situation of urban subways in China, and points out that the construction of the station complex adopts closed approach on the newly built subway lines, usually on rocky soils with low-strength. This paper provides research data on the support of the large-span station roadway and the stress deformation of the surrounding soil layer. These data are obtained by numerical analysis based on the finite element method. The main attention is paid to the degree of influence of the sequence of execution of tunneling operations on the sustainability of production when developing the soil enclosed between the internal diaphragms. It is noted the feasibility of these works with the simultaneous destruction of the temporary lining of the side of the pilot tunnels. The numerical simulation results is evaluated according to the surface settlement value, the nature of the soil stress-deformation state, the temporary support of the contour (primary lining) and the vertical and horizontal displacement values of the feature points on the inner wall, and the anchoring force values. The results obtained are presented in tabular and graphical form and analyzed. Established numerical simulation methods are used to comprehensively assess the stability of large-span tunneling, which will help to make reasonable engineering decisions in the construction of a single station on a subway line.