Rock Mass Research Articles

Methods of grout quality measurement in borehole exchangers for heat pumps and their rehabilitation

Methods and instrumentation for measuring grout quality in heat pump boreholes, including the measurement of groundwater flow through boreholes outside partly grouted borehole exchanger pipes, have been developed in the Czech Republic. A Semtex charge has also been developed to repair rock massifs, which reliably disconnects borehole exchanger pipes without severely harming the surrounding rock environment or buildings. The resulting hole can then be used for regrouting, thus preventing undesirable vertical water flow through the borehole.

A comparative study on machine learning approaches for rock mass classification using drilling data

Current rock engineering design in drill and blast tunnelling primarily relies on engineers' observational assessments. Measure While Drilling (MWD) data, a high-resolution sensor dataset collected during tunnel excavation, is underutilised, mainly serving for geological visualisation. This study aims to automate the translation of MWD data into actionable metrics for rock engineering. It seeks to link data to specific engineering actions, thus providing critical decision support for geological challenges ahead of the tunnel face. Leveraging a large and geologically diverse dataset of ∼500,000 drillholes from 15 tunnels, the research introduces models for accurate rock mass quality classification in a real-world tunnelling context. Both conventional machine learning and image-based deep learning are explored to classify MWD data into Q-classes and Q-values—examples of metrics describing the stability of the rock mass—using both tabular- and image data. The results indicate that the K-nearest neighbours algorithm in an ensemble with tree-based models using tabular data effectively classifies rock mass quality. It achieves a cross-validated balanced accuracy of 0.86 in classifying rock mass into the Q-classes A, B, C, D, E1, E2, and 0.95 for a binary classification with E versus the rest. Classification using a CNN with MWD-images for each blasting round resulted in a balanced accuracy of 0.82 for binary classification. Regressing the Q-value from tabular MWD-data achieved cross-validated R2 and MSE scores of 0.80 and 0.18 for a similar ensemble model as in classification. High performance in regression and classification boosts confidence in automated rock mass assessment. Applying advanced modelling on a unique dataset demonstrates MWD data's value in improving rock mass classification accuracy and advancing data-driven rock engineering design, reducing manual intervention.

System Reliability Analysis of Concrete Arch Dams Considering Foundation Rock Wedges Movement: A Discussion on the Limit Equilibrium Method

In this paper, a discussion on the applicability and limitations of the limit equilibrium method is presented. In this manner, the reliability of a concrete arch dam-foundation system under static loading is evaluated by considering a set of potentially moveable rock wedges in the foundation. The safety of the system is assessed utilizing a quasi-analytical method, which employs the limit equilibrium method and numerical analysis to calculate the sliding safety factors and the dam trust forces, respectively. The reliability is evaluated using the Latin Hypercube Sampling method. Random variables in the system are the friction angle, cohesion, and the Grout Curtain Efficiency Coefficient. In the end, the influence of two parametric variables of discontinuities, elastic slip and rock mass deformability modulus, on the rock wedges’ sliding safety factor is evaluated by comparing the results of the quasi-analytical method with the purely numerical method. The results show that in the case of complicated geotechnical conditions, the limit equilibrium method may not reflect real-world failure scenarios.

Trace elements in thermal waters of the northern Tien Shan: distribution and fate

The article presents new data on the abundance and ways of soluted trace elements (Si, Fe, F, Al, Sr, Br, B, Mn, Ba, Ti, Li, Rb, Mo, As, U, Th, W, Sc, Y, REE, Hf) in thermomineral, surface and groundwaters of the northern Tien-Shan (Issyk-Kul intermountain depression). It is established that trace element composition of thermomineral waters is able to be a marker of hydrogeological settings of water formation and flow: waters of sedimentary rocks of the intermountain artesian basin are enriched with Sr, Ba, Mn, B, Mo and U, while waters of rock massifs contain increased concentrations of F, Rb, W and Sc. Thermodynamic calculations performed for certain trace elements using Visual-MINTEQ 3.1 and GWB 14 programmes allowed us to identify the water migration patterns of the surveyed water points. Calculation of water migration coefficient showed the dependence of microcomponent accumulation rate on the type of water-bearing strata and hydrogeological conditions of water formation.

A novel method for identifying rock interfaces and fragmentation zones based on drilling response characteristics

The rapid measurement and acquisition of the fracture characteristics and strength of the surrounding rock are crucial for evaluating the stability of underground engineering. In this study, a laboratory/coal mine universal drilling test platform was developed to enable real-time measurement of drilling parameters. The testing results demonstrate that under identical rotation speed conditions, both thrust force and torque increase exponentially with increasing drilling speed, while under identical drilling speed conditions, both thrust force and torque decrease linearly with increasing rotation speed. In intact rock layers, variations in torque, thrust force, and drilling speed with depth remain relatively stable, in rock layer interfaces and fragmentation zones, sudden changes occur within a drill displacement range of approximately 3–7 mm, which become more pronounced as strength differences between adjacent rock layers increase.Based on the cutting mechanics model of drill bit, the evaluation index of drilling energy consumption of unit rock mass (Eη) is derived. Subsequently, based on drilling speed, rotation speed, thrust force, torque and energy index, a machine learning model for predicting rock strength is established using the LS-SVM nonlinear regression approach. Field practice has shown that drilling platform can effectively identify both the rock interface and range of fragmentation areas. The experimental results are of great value to the selection of supporting types in the process of coal mine roadway excavation.

LightGBM integration with modified data balancing and whale optimization algorithm for rock mass classification

The accurate prediction of uneven rock mass classes is crucial for intelligent operation in tunnel-boring machine (TBM) tunneling. However, the classification of rock masses presents significant challenges due to the variability and complexity of geological conditions. To address these challenges, this study introduces an innovative predictive model combining the improved EWOA (IEWOA) and the light gradient boosting machine (LightGBM). The proposed IEWOA algorithm incorporates a novel parameter l for more effective position updates during the exploration stage and utilizes sine functions during the exploitation stage to optimize the search process. Additionally, the model integrates a minority class technique enhanced with a random walk strategy (MCT-RW) to extend the boundaries of minority classes, such as Classes II, IV, and V. This approach significantly improves the recall and F1-score for these rock mass classes. The proposed methodology was rigorously evaluated against other predictive algorithms, demonstrating superior performance with an accuracy of 94.74%. This innovative model not only enhances the accuracy of rock mass classification but also contributes significantly to the intelligent and efficient construction of TBM tunnels, providing a robust solution to one of the key challenges in underground engineering.

Investigating the impact of thermal treatment on fracture toughness and sub-critical crack growth parameters under mode II loading

Studying subcritical crack growth is crucial to investigating the long-term behavior of rocks under applied loads and evaluating the long-term stability of underground and surface structures in rock masses. While considerable research has been done to determine subcritical crack growth parameters in mode I, Studies on subcritical crack growth under mode II loading are limited despite its important applications in rock engineering problems. The purpose of the study is to understand the thermal effect on the fracture behavior of hornfels rock, which were heated at 25 °C (without thermal treatment), 250 °C, 500 °C, and 750 °C, respectively. The subcritical crack growth parameters were determined using the constant stress rate test and one of the available fracture mechanics tests for mode II loading, namely, the four-point bending test. Experiments were conducted at three fixed displacement rates of 0.06 mm/min, 0.6 mm/min, and 6 mm/min, and three experiments were performed in each case to ensure repeatability. The results showed that the fracture toughness of hornfels samples increased with increasing temperature up to 250 °C and then decreased with increasing heat treatment temperature. The fracture toughness decreased drastically due to the thermal breakdown of the quartz crystal structure and the creation of wider intergranular fractures. The study indicated that for the hornfels samples, the subcritical parameter A decreased and beyond this temperature, parameter A began to increase while parameter n remained relatively constant as the temperature rose to 750 °C. The subcritical crack growth rate was calculated using the subcritical crack growth parameters and the stress intensity factor under mode II loading. For a certain value of the stress intensity factor KII, the highest subcritical crack velocity occurred at the temperature of 750 °C (5.76×10−7–2.47×10−1 m/s), and the lowest velocity of the subcritical crack occurred at the temperature of 250 °C (3.30×10−11–6.84×10−5 m/s). The impact of inert strength, calculated at the highest loading rate, on subcritical parameters across various temperatures was examined. The findings indicate that the subcritical crack growth parameter A is reliant on inert strength.

The grouting plugging mechanism of layered jointed rock mass considering the time-varying yield stress of grout

Slurry retention in fractures decreases after grouting is completed and the pressure supply is stopped, which affects the grouting sealing effect and prevents or restrains the occurrence of such adverse conditions. Based on the time-varying yield stress of grout, a theoretical analysis model of grouting diffusion decay is established, the decay height variation function and the minimum pressure stabilisation time calculation formula are derived, and the sealing mechanism of a jointed rock mass with multiple joints is studied. Moreover, a 3D visualisation laboratory test device for grouting diffusion decay of a jointed rock mass with layers was developed to analyse the diffusion and decay process of grout with different water-cement (W/C) ratios visually, and the correctness of the theoretical model was verified. The results show that: (1) the W/C ratio of grout should not be greater than 2, otherwise it is difficult for grout to remain in the cracks; (2) a large fissure opening easily spreads grout but is not conducive to the retention of grout; and (3) the time of stable pressure determines the retention rate of the slurry in the crack. When the horizontal crack opening width is 3 mm and the W/C ratio of the slurry is 1, the minimum stable pressure time to achieve a better sealing effect is 810 s. The research results can be used to reasonably plan the grouting time and improve the grouting efficiency to ensure the grouting effect, which is crucial for improving the theoretical system of grouting technology.

Experimental study on mechanical and acoustic emission characteristics of red sandstone containing combined flaws of hole-fissure under biaxial compression

To investigate the mechanical behaviour and failure mechanism of flawed rock masses under biaxial stress conditions, a series of biaxial compression experiments are conducted on red sandstone specimens containing combined flaws of the circular hole and perforated symmetric fissure combined with acoustic emission (AE) technology in this work. The results show a close association between the stress-strain curve morphology and confining stress, both the biaxial compression strength and elastic modulus exhibit an upward trend with increasing fissure angle and confining stress. The maximum AE energy and cumulative AE energy both increase with higher confining stress. The crack types in the specimens are identified by analyzing the AF/RA value distribution, revealing a gradual decrease in the percentage of tensile cracks with increasing fissure angle. An AE localization algorithm based on the least absolute value method is applied to pinpoint AE events during biaxial compression, and AE events distribution is analyzed through the Kernel density estimation (KDE). The crack extension behaviour is described based on the movement of the maximum kernel density points, which initially exhibits a progression from both ends of the specimen towards the central region and subsequently from the central fissure tip towards the two ends.

Experimental study on characteristics of gas seepage in broken coal and rock

AbstractThe characteristics of gas seepage in broken coal and rock composed of different particle sizes and grades were investigated in this study. On the basis of Darcy's law and non‐Darcy seepage theory, equations of gas permeability in the nonlinear seepage of broken coal and rock, as well as the porosity of broken coal and rock, under triaxial compression were determined. The stress loading path of gas seepage in broken coal and rock was developed. The characteristics of gas seepage in broken coal and rock composed of different particle sizes and grades were analyzed, and the results showed that the gas permeability after compression was proportional to the particle size of the broken coal and rock. Under triaxial compression, the gas permeability of the broken coal and rock composed of graded‐particle sizes was lower than that of the broken coal and rock composed of different single‐particle sizes. The gas permeability of the broken coal was lower than that of the broken rock mass, and the gas permeability and porosity of the broken coal and rock can be described by the exponential decay function. At a constant porosity, the gas permeability of the broken coal and rock was proportional to the size grading index under triaxial compression. The coefficient of viscosity and gravity of the flow are key factors influencing the flow permeability in broken coal and rock. This study provides a reference for on‐site practice such as the efficient extraction of gas in goafs.

Analysis of Excavation Methods Against Deformation in Highway Tunnels

Cisumdawu tunnel is a highway tunnelling which composed of very weak rock mass conditions has an average compressive strength less than 1 MPa. It is located below groundwater level. The shape of tunnell is horseshoe, diameter of tunnel section 14,413 m and height 11,083 m. Cisumdawu tunnel is a shallow tunnel which has a maximum overburden of 52,8 m and minimum overburden approximately 14 m. In the case of shallow tunnel which has a large section could be lead an instability during tunnel construction. Tunnel deformation analysis is using an observation and numerical modelling approach with finite element method which is assisted by RS 2019 (Rocscience) software. Based on the research is obtained that excavation method greatly affects the level of deformation that occurs during tunnel construction. As tunnelling advanced, Vertical displacement in the longitudinal direction using three bench seven step excavation method is smaller than full face and bench excavtion method. The maximum displacement using three bench excavation method is -4.23 mm, the full face method is -5.51 mm and the bench method is -4.91 mm. The amount of displacement is affected by the length of excavation and stage in tunnelling. The rate of deformation is increasing as the length of excavation getting deeper, but this is applied in unsupported tunnel.

Failure characteristics and pressure relief effectiveness of non-persistent jointed rock mass with holes

In tunnel engineering, the rock mass contains a significant number of irregularly distributed joints, and typically exhibits high energy accumulation, thereby posing a risk of rockburst occurrence. Therefore, it is of paramount importance to investigate the fracture propagation behavior in jointed rock masses and assess the impact of borehole pressure relief on mitigating rockburst occurrences for effective prevention and control measures. This paper focuses on failure characteristics and pressure relief effectiveness of non-persistent jointed rock mass with holes through laboratory testing and numerical simulation. In laboratory experiments, rock samples are prepared to include a range of crack dip angles and circular holes. Then, the crack propagation law of crack inclination and circular hole is studied by AE and DIC technology. The experimental results show that with the increase of fracture dip angle, the peak strength and energy change of the sample decrease first and then increase. Due to the existence of holes, the crack propagation direction of the original crack is changed. After drilling, the strain energy of the sample is obviously reduced, which shows that the drilling pressure relief effect is obvious, which can effectively reduce the energy accumulated inside the rock mass and reduce the risk of rockburst. Finally, the PFC numerical simulation software is used to analyze the micro-failure process and energy change law of the sample from three aspects: the relative position of cracks and holes, the diameter of boreholes and the spacing of boreholes. Further understanding of the energy dissipation law and mechanical behavior characteristics of jointed rock mass provides a reference for exploring the pressure relief effect of rock mass and preventing rockburst.

The fracture propagation law of axially symmetric intersecting precut hydraulic fracturing in mine rock-breaking

As a mine rock-breaking technique, hydraulic fracturing technology can reduce the amount of explosives used, which enhance safety and reduce environmental pollution in mines. After precutting along the borehole axis, hydraulic fractures will expand along the precutting direction within a certain range and reduce initiation pressure. These hydraulic fractures cut through the rock mass, reducing its integrity and weakening its mechanical properties. Hydraulic fracturing with axially symmetric intersecting precut fractures not only controls the multi-directional propagation of fractures but also increases the fractures within rock mass. The lattice method simulated the hydraulic fracturing process, focusing on the parameters like angles between precut fractures and the minimum horizontal principal stress, the maximum horizontal principal stress, and angles between intersecting precut fractures. Results indicate that the hydraulic fractures propagate along intersecting precut fractures, forming main and interconnected secondary fractures. The directional cutting effect is influenced by the number of secondary fractures. With the increase in the angle between precut fractures and the minimum horizontal principal stress, the maximum horizontal principal stress, the angle between precut fractures, the area of secondary fractures decreased, and the expansion extent of main fractures along the precut fractures increased, indicating better directional effects. The study identifies relationships between initiation time, initiation pressure, and parameters. These findings provide valuable technical guidance for designing on-site construction plans for hydraulic fracturing projects involving intersecting precut fractures.

A prestressed anchor-grouting reinforcement method based on the novel HFQSME grouting material for deep high-stress fractured rock mass

In order to control the surrounding rock of deep high-stress roadways, a new prestressed anchor-grouting reinforcement method was proposed with an engineering background of the 1780 ventilation roadway of Shiyakou Coal Mine. A high-fluidity quick-setting micro-expansion (HFQSME) grouting material had been developed through superplasticizer, accelerator, and expansion agent. The results showed that the optimal ratio of HFQSME grouting material was superfine Portland cement of 95%, superplasticizer of 0.5%, accelerator agent of 0.5%, expansion agent of 5.0%, and water-binder ratio of 0.4. The fluidity, initial setting time and expansion rate of the grouting material were 47cm, 41minutes and 0.12%, respectively. The grouting reinforcement effect of superfine slurry and HFQSME grouting material on rock mass with different degrees of fragmentation was investigated. The peak strength of rock mass with two degrees of fragmentation reinforced by HFQSME grouting material was 166% and 233% higher than that of superfine slurry. The macro reinforcement effect of HFQSME grouting material on the surrounding rock of deep high-stress roadway was evaluated through industrial testing. The prestressed anchor grouting method was used to improve the original support design scheme, and data monitoring and analysis were conducted on site. Compared with the original supporting scheme, the deformation of the 1780 ventilation was significantly reduced. Therefore, HFQSME grouting material had good adaptability for the surrounding rock of deep high-stress roadways.

Contribution to the numerical modelling analysis of displacements caused by tunnel construction in discontinuous rock masses

This paper presents a comprehensive study of the stability conditions of a rock mass surrounding a tunnel, using numerical modelling analysis of displacements induced by the tunnel construction. The case study focuses on the Boukhadra iron ore mine in Algeria. This research employs empirical equations from literature based on geomechanical classifications to provide the most appropriate geotechnical input data for simulation. In addition to those equations, a novel correlation equation relating the rock mass ratio (RMR) and the rock quality index (Q) was developed. This equation gives a high regression coefficient, revealing a strong correlation (R2) of 0.878. It gives a better estimation of the rock mass characteristics: Young’s moduli (E), Poisson’s ratio (υ), cohesion (C), and friction angle (φ), compared to the existing literature-based correlation equations. The estimated parameters were used to pass from a discontinuous to a continuous equivalent medium, making the numerical modelling easier with the finite element method. Compared with intact rock and direct equations, the numerical simulation results based on the new equation fit well with the in-situ observations and provide a more reliable comprehension of rock mass behaviour. The new equation proposed in this paper provides a substantial evaluation of the rock mass parameters needed in the design process of underground structures once used with respect to their acceptable ranges.

The field-enriched finite element method with gravity effects for simulating the cracking behaviors of large-scale engineering rock masses

The field-enriched finite element method uses a scalar field defined as a field variable to describe cracks and characterize their impact on the displacement field and stress field of the solution model. It is capable of avoiding remeshing and employing level set functions to describe cracks when simulating the propagation of cracks. In this work, a field-enriched finite element model with gravity effects is proposed to simulate the large-scale failure process of engineering rock masses, and several numerical cases of geotechnical engineering are successfully analyzed. First, by introducing the unified tensile fracture criterion into the numerical model, the large-scale failure process of the intact slope is simulated. Second, the sliding process of rock slopes containing en echelon joints is numerically investigated. Third, the cracking process of the concrete dam is analyzed. Finally, the effects of joint and bedding plane inclination angles on the stability of tunnel chamber in transversely isotropic rock mass are studied. The numerical results indicate that the numerical method proposed in this work can accurately solve the large-scale failure process of rock masses.

Rock discontinuity extraction from 3D point clouds using pointwise clustering algorithm

Recognizing discontinuities within rock masses is a critical aspect of rock engineering. The development of remote sensing technologies has significantly enhanced the quality and quantity of the point clouds collected from rock outcrops. In response, we propose a workflow that balances accuracy and efficiency to extract discontinuities from massive point clouds. The proposed method employs voxel filtering to downsample point clouds, constructs a point cloud topology using K-d trees, utilizes principal component analysis to calculate the point cloud normals, and employs the pointwise clustering (PWC) algorithm to extract discontinuities from rock outcrop point clouds. This method provides information on the location and orientation (dip direction and dip angle) of the discontinuities, and the modified whale optimization algorithm (MWOA) is utilized to identify major discontinuity sets and their average orientations. Performance evaluations based on three real cases demonstrate that the proposed method significantly reduces computational time costs without sacrificing accuracy. In particular, the method yields more reasonable extraction results for discontinuities with certain undulations. The presented approach offers a novel tool for efficiently extracting discontinuities from large-scale point clouds.

Automatic interpretation of geometric information of discontinuities and its influence on the stability of highly-jointed rock slopes

The discrete fracture system of a rock mass plays a crucial role in controlling the stability of rock slopes. To fully account for the geometric shape and distribution characteristics of jointed rock masses, terrestrial laser scanning (TLS) was employed to acquire high-resolution point-cloud data, and a developed automatic discontinuity-identification technology was coupled to automatically interpret and characterize geometric information such as orientation, trace length, spacing, and set number of the discontinuities. The discrete element method (DEM) was applied to study the influence of the geometric morphology and distribution characteristics of discontinuities on slope stability by generating a discrete fracture network (DFN) with the same statistical characteristics as the actual discontinuities. Based on slope data from the Yebatan Hydropower Station, a simulation was conducted to verify the applicability of the automatic discontinuity identification technology and the discrete fracture network-discrete element method (DFN-DEM). Various geological parameters, including trace length, persistence, and density, were examined to investigate the morphological evolution and response characteristics of rock slope excavation under different joint combination conditions through simulation. The simulation results indicate that joint parameters affect slope stability, with density having the most significant impact. The impact of joint parameters on stability is relatively small within a reasonable range but becomes significant beyond a certain threshold, further validating that the accuracy of field geological surveys is critical for simulation. This study provides a scientific basis for the construction of complex rock slope models, engineering assessments, and disaster prevention and mitigation, which is of great value in both theory and engineering applications.

Study on influencing factors of controllable mechanical behavior of rock-like porous materials.

Due to the discrete and non-homogeneous of similar materials and the inability to realize large-size and original scale modeling, it is difficult to restore the structure and stress state of underground coal and rock mass in similar simulation tests. To solve this problem, a lightweight and suitable for large-scale modeling similar material, rock-like porous material has been developed. The quasi-static uniaxial compression experiment was carried out by using the large tonnage multi-module electronic control test system. And the influencing factors of controllable mechanical behavior of rock-like porous materials were studied. The results showed that, under uniaxial compression conditions, the material stress-strain curve exhibits three phases: elastic stage, failure stage, and platform stage. The uniaxial compressive strength, elasticity modulus, stress drop, and softening modulus of rock-like porous materials basically increase with the increase of density. The stress after peak strength changes from a slow decrease to a "stepped" or even "cliff like" downward trend. Polypropylene fibers have the effect of enhancing the uniaxial compressive strength, elasticity modulus, stress drop, softening modulus, shear deformation, and residual strength stability of rock-like porous materials. The rock-like porous material has a critical loading velocity, and it increases with density. At the critical loading velocity, the material shows obvious shear failure, and the shear inclination angle is the largest, and so is the uniaxial compressive strength. Through the experimental research, the influence laws of density, polypropylene fiber, and loading velocity on the failure mode, mechanical parameters, and mechanical behavior of the material are clarified, and the quantitative relationship between density and each mechanical parameter is obtained. The research is helpful to realize the accurate control of mechanical behavior of rock-like porous materials and further inverts the deformation and failure mechanism of underground coal and rock structures through indoor similar simulation tests.

Research on the evolution law and control technology of deviatoric stress in the surrounding rock of gob-side entry retaining

Gob-side entry retaining (GER) play a crucial role in enhancing coal recovery, improving roadway stress conditions, and ensuring safe and efficient mining operations. This study focuses on investigating the stability characteristics of surrounding rock using flexible formwork concrete (FFC) along the headgate of 3405 panel. According to the fracture pattern of roof, a mechanical model of GER was developed. This study derived expressions for the support resistance of the filling body and determined its optimal bearing capacity. A FLAC3D numerical model was employed to simulate the GER process, segmented into early, middle, and late stages. Simulation results demonstrate a progression in peak deviatoric stress and extent of plastic zones in the surrounding rock. This evolution includes an initial minor increase (early stage), followed by rapid escalation (middle stage), and eventual stabilization (late stage). The middle stage emerges as critical, characterized by pronounced mining-induced stress effects on the roof. During this phase, significant changes are observed in deviatoric stress curves, with plastic zones rapidly migrating deeper into the rock mass. Practical implementation of the findings revealed a maximum deformation of the filling body of only 198 mm, affirming the feasibility of employing FFC in GER. This research provides valuable insights into managing stability concerns in GER thereby enhancing safety and operational efficiency in coal mining under similar conditions.