In this paper, 114 uniaxial compressive strength tests were carried out on specimens of veined gneiss with diameters ranging from 14 mm to 100 mm. The main objective of the study was to investigate the influence of the specimen size on the strength behavior of this metamorphic, heterogeneous rock. This size effect results in an increase in strength from small 14-mm diameter specimens up to 50-mm diameter specimens. Thereafter, a mild decrease in results is observed from 50-mm to 100-mm diameter specimens, in line with traditionally considered size-effect approaches. The findings generally agree with previous studies on size effects in sedimentary and igneous rocks, suggesting that metamorphic rocks may also tend to follow the Unified Size-Effect Law (USEL), accounting for general and reverse size-effects in strength. Nevertheless, some variability observed in the results, mainly attributed to inherent heterogeneity of the studied rock could influence how clearly this ascending-descending trend is captured by the model.

As multifunctional structures, deep rock underground structures are built all over the world. Their lifespan is generally expected to exceed a hundred years. However, they are inexorable to rock creep which controls their lasting safety and stability. This article describes the strong factors affecting rock creep and the lasting stability of deep rock engineering, based on pertinent research works. It shows that temperature, water content, hydraulic pressure, stress level, and damage are major factors that significantly affect the creep behavior of host rocks of deep underground structures. Overall, such factors govern the creep life of rocks. Specifically, the increase in these factors causes an increase in the creep strain rate, a shortening of the steady creep phase, an acceleration of the onset of tertiary creep, and thus a shortening of the time-to-failure of rocks. This is due in particular to the fact that fluctuations in these factors disintegrate the internal structure of the rocks and make them increasingly weakened. As a result, the mechanical properties of the rocks are altered and the creep process proceeds at a higher rate than expected. Hence, the creep life of surrounding rocks is reduced, which significantly limits the prolonged stability of underground structures. Appropriate countermeasures, such as long-term monitoring, are strongly recommended.

This research aims to optimize ground support systems for underground tunnels in geologically challenging environments, specifically addressing the reduction of Fall of Ground (FOG) incidents in a gold mine in Mashava, Zimbabwe. The study integrates advanced detection and classification methodologies to enhance tunnel stability and safety. Tunnel Reflection Tomography (TRT) was employed to identify unfavorable geological structures ahead of excavation, while core logging at 20 locations on level 7 provided rock mass quality assessments using three classification systems: Bieniawski’s Rock Mass Rating (RMR), Laubscher’s Mining Rock Mass Rating (MRMR), and Barton’s Q-system. The results consistently indicated poor rock mass quality, informing the design and refinement of a robust ground support system. Fallout height data from past FOG incidents and probabilistic key block analysis using J-Block software further validated the support system's effectiveness. The findings significantly reduce collapse risks and downtime, enhancing operational safety and efficiency. This research contributes to developing practical strategies and tools for improving tunnel stability in complex geological settings, offering valuable insights for future advancements in mining support technologies. The study's necessity stems from the industry's growing demand for innovative solutions to enhance tunnel stability in adverse geological settings, particularly in regions with limited access to advanced technologies or methodologies.

This paper proposes a novel approach to rock mass characterization by adapting the coefficient of uniformity (Cu) and coefficient of curvature (Cc) from soil mechanics, redefining their mathematical formulations to align with RQD-based block size measurements. Specifically, Cu and Cc are calculated using D10, D30, and D60, derived from cumulative RQD data, while accounting for the increasing scale of block size distributions in rock masses. An illustrative case study demonstrates the application of this method, yielding insights into block size variability and heterogeneity in rock masses. The proposed coefficients enrich rock mass classification, offering enhanced quantitative tools for engineering and geological analyses.

Currently, the unified strength criterion (USC), the three-dimensional Hoek-Brown (3D H-B) criterion and the generalized unified strength theory (GUST) are the three types of typical rock strength criteria. In this paper, a comparative study of the three types of strength criteria is performed for rocks. Based on the nonlinear characteristics of rock strength on meridian and deviatoric planes, the USC can predict rock strength under triaxial stress state. The USC is composed of two failure functions on meridian and deviatoric planes. The failure surface of the USC in principal stress space satisfies smoothness and convexity. The predicted strength for five types of rock under the true triaxial tests were compared among the USC, the GUST, and the 3D H-B criterion. The results indicate that the USC can effectively reflect the influence of the intermediate principal stress on rock strength and accurately predict rock strength under both triaxial tension (σ₁=σ₂>σ₃) and triaxial compression (σ₁>σ₂=σ₃). Additionally, the conventional triaxial tests were conducted on other eight types of rock to measure the strength on meridian plane. The predicted strengths for the eight types of rock on meridian plane were compared between the USC and the original H-B, which suggests that the USC is suitable for various types of rock and provides higher accuracy.

The presence of rock mass fractures has always been a subject of study for the prevention and control of related natural disasters. To understand the effects of different dip angles and horizontal distances on crack development, numerical simulation experiments on Brazilian disks under uniaxial compression were con-ducted using the PFC2D particle flow program. A function module was utilized to monitor the expansion and quantity of cracks. The numerical simulation results under 0° conditions were in good agreement with the experimental results, validating the rationality of the numerical simulation. The simulation results indicate that: under single fracture conditions with different dip angles, samples with angles between 30° and 60° produced typical wing-shaped cracks. At 0°, cracks propagat-ed through the center of the fracture, while at 90°, cracks initiated from the tip of the fracture and propagated through the sample. The peak stress and the number of cracks in the samples first decreased and then increased with the increase of the dip angle, reaching a maximum at 90°. For samples with double fractures and varying horizontal distances, all produced wing-shaped cracks. Their peak stress and the number of cracks increased monotonically with the increase in distance, reaching a maximum at a distance of 30mm. The experimental results confirmed that the PFC2D program can effectively simulate the process of crack initiation and development, and the research findings provide a reference for correctly understanding the fracture mechanics of fractured rock masses.

The determination of reserved deformation is typically based on referring to relevant codes or the engineering analogy method, lacking a certain theoretical approach. The purpose of this study is to provide a theoretical basis for determining reserved deformation and to analyze the variation law of the surrounding rock affected by reserved deformation. Considering the reserved deformation under the condition of asymmetric load, the expression of optimal reserved deformation, the expression of support resistance reflecting the strength of surrounding rock, the strength of support material and the magnitude of in situ stress, the displacement expression of surrounding rock are derived by approximate solution based on the classical elastic–plastic theory. Numerical simulation software is used to simulate the displacement expression of the surrounding rock considering the reserved deformation and the expression of the optimum reserved deformation under the condition of asymmetric load. The results of the numerical simulation were compared with those of the analytical solutions, and the analytical results show that the errors between the two are within 12% and that the consistency is good.

The block caving is one of the most commonly used methods in underground mining of massive, low/high grades, and deep deposits. As a result of undercutting, caving occurs and propagates toward the upper part of the ore block. It is essential to predict the height of caved ore and its volume based on shape of the caved zone to make an economic decision. This paper describes the mechanism of height of the caving zone development. Consequently, four new mathematical models (prismatic, parabolic, cubic, and hemispheric) are proposed to estimate the height of this caving zone. The results are compared with the existing approaches. The proposed methods have proven that the height of caving zone has a linear relationship with the uniaxial tensile and compressive strengths and the height of undercut as well. Also, the height of the caving zone has a negative exponential relationship with unit weight of undercut roof materials and its expansion factor. The Height of the caving zone of the prismatic model is 1.5 times higher than the parabolic model and 1.3 times higher than the hemispheric model. The cubic model estimates the lowest height of the caving zone (0.3 times the height estimated by the prismatic model). The average height of the caving zone, taking the average unit weight of 27 KN/m3 into account, is about 69.4 times the tensile strength, 5.8 times the uniaxial compressive strength of the rock mass, and 18.8 times the height of the undercut.

Dimension stone and terrazzo are quarried rocks following specific dimensions, generally huge in tonnage than ore minerals. Dimension stone and Terrazo resource development opportunities in the Tigray Region, Ethiopia, are studied regarding their potential to make dimension and terrazzo, geology and deposit nature, physio-mechanical properties, rock petrography, and opportunity for exploitation. The granite and marble are quarried as dimension stones and they meet the quality parameters of the rocks, which include, being durable, easy to quarry, work, cut, and polish. It was conducted through the integration of new geological descriptions, interpretation of thin sections, and physico-mechanical tests. Both surface and quarry rock samples were used for the study. 15 from granite and 15 from marble deposits in total 30 samples were used for petrographic analysis and physico-mechanical tests. Based on petrographic and physicomechanical results, granite is harder than marble this may be due to silicates being less dissolved than carbonates. The accessory minerals in both granite and marble deposits limit the quality of the deposits as being impurities. The integrated results indicate that the marble and granite deposits in the area are good in quality and quantity. In addition, they are geologically widespread, technologically do not need sophisticated technology and economically they do not need huge investment and have less effect upon the environment. Diamond wire saws and diamond belt saws separated the dimension stone, which are oblate to curved in shape with 1m width. In the area, investment has been at a relatively low stage more investment will significantly increase production of the dimension stone for local usage, export, and economic growth of the region.

A rock mass in-situ rotary cutting test system (RMIRCS) was developed to estimate the mechanical properties (i.e. compressive strength and elastic modulus) of rock masses. The pressure-on-bit and torque-on-bit of the RMIRCS were investigated in detail over a wide range of revolution speed and penetration rate. A simple experimental model is proposed to describe the qualitative relationships between rotary cutting test data and mechanical parameters. The existence of a linear constraint between pressure-on-bit and torque-on-bit and the depth of rotary cutting exhibits good agreement with the proposed model. The mechanical properties obtained from the quasi-static compression tests were in agreement with those of the RMIRCS at lower revolutions rate of 300 rpm and 350 rpm, which suggests that the developed RMIRCS is valid. Furthermore, the experimental test results on rock mass demonstrated that the mechanical properties of rock masses are revolutions speed-dependent and drilling rate-dependent.