Rock Engineering Applications Research Articles

New Method to Estimate Rock Mass Deformation Modulus Based on BQ System

The rock mass deformation modulus is one of the most important design parameters in a range of rock engineering applications. Its value is usually obtained directly through in situ testing or estimated indirectly on the basis of a rock mass quality classification system. Because in situ testing is generally costly, time-consuming, and presents operational difficulties, it cannot be carried out extensively, and many researchers have concentrated on developing indirect procedures to obtain information on the modulus of deformation, such as the RMR method, Q method, and GSI method. The purpose of this paper is to present a new system for estimating the rock mass deformation modulus called the BQ method, which is based on the BQ (basic quality) system. In this paper, the BQ system is first briefly reviewed, and then more than 60 in situ measurements from three large hydropower stations in China are used to develop a new relationship between BQ and the deformation modulus, based on a power function relationship. The paper also derives correlations based on the existing estimation formula and the relationship between BQ and other classification schemes, resulting in several recommended formulas for estimating the deformation modulus of a rock mass using the BQ method.

Mechanical testing and numerical simulation of intelligent terminal structure of rockbolt used as a deformable support and for safety monitoring in rock engineering

The nonlinear mechanical behavior perception and deformation control of engineering rock masses are crucial technical challenges in the fields of underground engineering support and safety monitoring. An intelligent terminal structure (ITS) of rockbolts was designed for deformable support systems and safety monitoring in rock engineering applications. It is a type of device installed at the end of a traditional bolt. The ITS comprises two key components: a memory tooth and a yielding ring. This study introduces the design principle of an ITS in deformable rockbolt support systems. It proposes design parameters for the ITS in relation to the deformations encountered by engineering rockbolts. Tests were conducted to measure the ultimate bearing capacity and deformation of the ITS under pull-out conditions. The results demonstrate that the ITS of rockbolt undergoes deformation in the form of elongation as opposed to conventional bolts. Furthermore, finite element numerical simulations were conducted to investigate the mechanism of interaction between the memory tooth and yielding ring of the ITS. The numerical simulation results are consistent with the experimental results. The load capacity of the yielding ring exceeded that of the memory tooth. The field test results indicate that the dilatation of the surrounding rock mass causes ITS deformation in the diversion tunnel. The axial stress load exerted on the rockbolt is transferred to the ITS, leading to memory tooth breakage and compression deformation of the yielding ring. This mechanism allows the load and deformation of the surrounding rock mass to be reflected. The ITS not only serves as a deformable support system but also plays a significant role in monitoring the stress and deformation of the rock mass.

Rock fissure pattern characterization by combining 1-D fractal dimension and statistical analysis

How to characterize rock fissures/fractures is significant for the measurement and analysis in a lot rock engineering applications. A new method for characterizing rock fissure patterns is studied in this paper. It is constructed by combining 1-D fractal dimension and statistical analysis for the whole rock surface in an image. In a binary fissure image, to characterize fissures accurately, all the possible fissures are skeletonized, and then the short lines or curves are removed and the gaps on the main fissure segments are linking up. After that, 1-D fractal dimension is applied for analyzing fissure patterns, in which, the fissure network complexity and the fissure quantity is characterized by statistical analysis. The testing fissure images were taken from a Swedish laboratory and a Chinese Mine, and the testing results prove that the fissure pattern characterization by the new method is promising.

Evaluating machine utilization times for roadheaders used in coal mines: Regression and artificial neural network analyses

Roadheaders are extensively utilized for tunnel heading rock engineering applications all over the world. To create a work plan and calculate costs, it is critical to forecast roadheader performance as precisely as possible. Machine utilization time (MUT) is required for the calculation of daily advance rate of roadheaders. This paper investigates the values of MUT for roadheaders used in underground coal mines. The performance measurements were conducted on fifty different locations for axial machines and thirty-nine different locations for transverse machines. MUT values vary from 15 % to 37.5 % with an average of 26.3 % for axial roadheaders, and vary from 6.9 % to 37.9 % with an average of 18.4 % for transvers roadheaders. The average MUT is 25.4% for all measurements. The percentage of average support time approximately equals to the average MUT. Multiple regression and artificial neural network models were also developed for estimating MUT. Concluding remark is that the determined MUT values and the derived estimation models for roadheaders will be very useful for coal miners.

Review on the Estimation of Static Deformability Modulus of Rocks and their adoptability in Different Rock Masses

The aim of this study is to review the different mechanisms employed in the estimation of static rock mass deformability modulus (𝐸𝑚) in rock engineering applications and to investigate the adoptability of the identified mechanisms in different rock masses. The paper discusses different evaluation criteria through experimental, empirical and other means, with their merits and demerits, including influential factors. It is known that deformability modulus of intact rock depends on the imposed stress, strain rate and the confining stress on the rock sample as well as the rock texture and structure. The results generated for 𝐸𝑚 by different in-situ tests are different and an appropriate in-situ test based on the rock mass conditions should be employed to obtain reasonable results. Empirical criteria are found to produce results of reasonable precision if appropriately adopted for specific rock mass conditions, while the back analysis method is widely adopted as an insitu estimation measure for the design of rock-sockets and tunnel support. It has also been reported that substantial reduction in 𝐸𝑚 occurs due to schistosity and larger test volumes, while it is sensitive to stress and discontinuity conditions. In this work, specific recommendations are made on the estimation of 𝐸𝑚 for different types of rock masses based on the findings and reviews reported in the literature.

Study of the Relationship between Mode I Fracture Toughness and Rock Brittleness Indices

Mode I fracture toughness (KIC) and rock brittleness are important properties that influence many rock engineering applications. Due to the difficulties in determining KIC experimentally, previous studies have investigated the relationship between KIC and rock brittleness indices. However, only rock brittleness indices (based on strength parameters) and KIC obtained from Chevron Bend and Short Rod test methods were considered. In this paper, regression analysis was carried out to investigate the relationship between KIC and rock brittleness using literature data collected from different rock types and core KIC test methods under level I and static test conditions. Rock brittleness was assessed using ten indices based on strength and pre-peak elastic parameters. The results showed that elastic-based indices were not good predictors of KIC, while strength-based indices correlated well with KIC. A comparison with previous studies revealed that the correlations between KIC and strength-based indices were significantly sensitive to the rock type, i.e., soft or hard, and the KIC test method. However, a brittleness index, based on both strength and pre-peak elastic parameters, was found to be the best index to predict KIC because of its lower sensitivity to the test method and rock type.

Slope-Scale Remote Mapping of Rock Mass Fracturing by Modeling Cooling Trends Derived from Infrared Thermography

The reliable in situ quantification of rock mass fracturing and engineering quality is critical for slope stability, surface mining and rock engineering applications, yet it remains difficult due to the heterogeneous nature of fracture networks. We propose a method to quantify and map the slope-scale geomechanical quality of fractured rock masses using infrared thermography (IRT). We use the Mt. Gorsa quarry (Trentino, Italy) as a field laboratory to upscale a physics-based approach, which was developed in the laboratory, to in situ conditions, including the effects of fracture heterogeneity, environmental conditions and IRT limitations. We reconstructed the slope in 3D using UAV photogrammetry, characterized the rock mass quality in the field at selected outcrops in terms of the Geological Strength Index (GSI) and measured their cooling behavior through 18h time-lapse IRT surveys. With ad hoc field experiments, we developed a novel procedure to correct IRT data in outdoor environments with complex topography. This allowed for a spatially distributed quantification of the rock mass surface cooling behavior in terms of a Curve Shape Parameter (CSP). Using non-linear regression, we established a quantitative CSP-GSI relationship, which allowed for the CSP to be translated into GSI maps. Our results demonstrate the possibility of applying infrared thermography to the slope-scale mapping of rock mass fracturing based on a physics-based experimental methodology.

More is different: On the emergence of collective phenomena in fractured rocks

Fractures widely exist in crustal rocks and form complex networks dominating the bulk behaviour of geological media. Thus, understanding how fracture networks affect subsurface processes/phenomena is highly relevant to many rock engineering applications. However, the large-scale behaviour of a fractured rock mass consisting of numerous fractures and rocks cannot be predicted by simple applications of the knowledge of individual fractures and/or rocks, due to upscaling complexities involving the hierarchy of scales, heterogeneities, and physical mechanisms as well as the possible emergence of qualitatively different macroscopic properties. In other words, macroscopic phenomena in fractured rocks arise from the many-body effects (i.e. collective behaviour) of numerous interacting fractures and rocks, such that the emergent properties at the fracture system scale are much richer than those of individual components. Hence, more is different! This paper gives a discussion on the mechanism of emergence in fractured media from a combined statistical physics and rock mechanics perspective, and further presents a multiscale conceptual framework to link microscopic responses of single fractures/rocks to macroscopic behaviour of rock masses consisting of many fractures and rocks. This framework can serve as a useful tool to bridge experimentally-established constitutive relationships of fracture/rock samples at the laboratory scale to phenomenologically-observed macroscopic properties of fractured rock masses at the site scale.

Study on the Damage Mechanism of Sandstone under Different Water Content States

Understanding the failure mechanisms of rocks that are exposed to different water contents is important for rock stability in rock engineering applications, and the quantitative analysis of rock behavior is necessary for predicting and preventing the occurrence of rock failure due to water effects. Mechanical tests using real-time acoustic emission (AE) technology were carried out to reveal the damage evolution in sandstone rocks in a dried state, natural state, and saturated state, which includes a quantitative analysis of AE characteristics and cracking properties. The testing results indicate that with the growth of water content, sandstone rocks show a decreasing trend in strength and tend to experience gentle damage with relatively fewer fractures. The crack morphology of the main fracture surfaces is quantitatively described, including a fractal dimension calculation and cracking length measurements. As the water content rises, when rock failure occurs, a higher AE b-value can be obtained, revealing an increasing proportion of large-scale cracks. The fractal dimension of the acoustic emission hit rate shows that the evolution of rock damage and deformation has self-similarity, that is, the transformation from order to disorder to order, and it is affected by different water contents. The AE waveforms of the sandstone have two dominant frequency bands (0~75 kHz and 75~150 kHz) no matter which water-bearing state they are in. The increase in rock water content has resulted in the decline of AE waveforms located in the range of 200–300 kHz, whereas the rise of AE waveforms is located in the range of 0–50 kHz. The findings of this study deepen our understanding of the mechanism behind rock failure and provide a meaningful reference for disaster assessment and control.

Experimental study on the role of clay mineral and water saturation in ultrasonic P-wave behaviours across individual filled rock joints

Understanding wave propagation across rock joints is of great importance in rock engineering, petroleum engineering, and other geophysical applications. Previous studies have shown that wave behaviours across filled rock joints are highly affected by the properties of filling materials. However, how clay minerals and water saturation of the infilling affect wave attributes across filled rock joints is still poorly understood. In this study, ultrasonic tests were conducted on kaolinite- and bentonite-filled rock joints to investigate the roles of clay minerals and the water saturation of infillings on elastic P-wave behaviours across individual filled rock joints. The test results show that the infilling's water saturation and mineralogical compositions have combined effects on P-wave signatures of single rock joints filled with clayey soils. Particularly, for the kaolinite-filled rock joint, P-wave velocity and attenuation first increase and then slightly decline with the increasing water saturation. By contrast, for the bentonite-filled rock joint, an increase in water saturation causes gradual decreases in velocity and attenuation. The integrated interpretation of the experimental results indicates that the difference in heterogeneity saturation due to clay hydration is the first-order reason for the discrepancies in saturation-dependent P-wave attributes between kaolinite- and bentonite-filled rock joints. The findings of this study can help further understand wave propagation and attenuation across rock joints filled with fluid-saturated clayey soils.

Micro-macro damage, deterioration and cracking of heterogeneous composite rock masses with non-penetrating cracks under uniaxial compression

Rock masses with non-penetrating cracks are widely found in nature. Their mechanical behavior plays a key role in rock engineering applications. However, previous studies have concentrated on the single lithologic layer, and few studies have reported the crack coalescence mechanism in heterogeneous composite rock mass with non-penetrating cracks (HCRC). In this study, uniaxial compression tests were performed on HCRC by discrete element numerical simulation experiments. Through the uniaxial compression simulation tests, the stress–strain variation and acoustic emission (AE) characteristics of HCRC are revealed. The instability and failure characteristics, energy evolution law and crack coalescence process of HCRC are also studied. And the influence of the crack numbers and crack positions (in weak body or strong body) on the mechanical behavior and failure processes in heterogeneous composite rock mass is investigated. The results show that the mechanical properties of HCRC are not all weakened with the increase of the crack numbers. As the number of cracks increases, the failure modes of the HCRC changes from brittle failure to ductile failure. The number of cracks will regulate the relative relationship between the strength of the two bodies of heterogeneous composite rock masses, so the crack coalescence forms, instability forms and failure characteristics of the HCRC will also change accordingly. If the number of cracks in the weak body is large, the damage of the HCRC mainly occurs in the weak body. The strong body produces a rebound energy release to the weak body, accelerating the destruction of the weak body. At this time, the overall strength of the HCRC is low. Conversely, increasing the number of cracks in a strong body will weaken the strength of the strong body. At this time, the strength difference between the strong body and the weak body decreases. The HCRC shows that the overall bearing capacity is not lower but becomes stronger. The research results provide a theoretical reference for the stability and disaster prevention of jointed rock mass engineering.

AN EFFICIENT CALIBRATION PROCESS FOR THE PREDICTION OF ROCK STRENGTH THROUGH MACHINE LEARNING ALGORITHMS

Numerical models based on the discrete element method (DEM) have been widely used to predict the mechanical behaviors of rocks in rock engineering applications. Nevertheless, calibration of the model parameters is done by running numerous simulations and this time-consuming simulation process precludes the numerical platforms to be used as a practical tool in such applications. This study aims to accelerate the calibration process of the micro-parameters of three-dimensional (3D) numerical models built based on DEM and facilitate the generation of an efficient database by using machine learning algorithms in the prediction of rock strength. Namely, these algorithms are linear regression (LR), decision tree (DT) regression, and random forest (RF) regression. The appropriate methodology for predicting the uniaxial compressive strengths (UCS) of certain rock types was investigated using a dataset consisting of micro-parameters of 87 DEM-based rock models, generated through an open-source code, Yade. The performance of such methods was evaluated by using metrics including R-squared score (R2), mean squared error (MSE), root mean squared error (RMSE), and mean absolute error (MAE), and then their statistical discrepancies were analyzed. The most accurate prediction of UCS was obtained in the LR method and the lowest percentage of performance was derived from the RF algorithms. LR method provides the results efficiently during calibration of the micro-parameters of a DEM-based rock model.

Experimental investigation on rock microfabrical effects on dynamic hardness behaviour

Rocks' physicomechanical properties are influenced by their textural qualities. Rock mass stability is largely governed by these variables. Many geoengineering facilities depend on the capability to assess both short and long-term rock behaviors based on the interaction between different textural characteristics and physicomechanical properties. Rock hardness is frequently used in excavation engineering and is measured by several methods with different mechanisms. The Leeb dynamic hardness method has been recently applied in rock engineering applications as a new, inexpensive, non-destructive, and portable technique. This paper aims to show the interaction between the quantitative microfabrical features and the Leeb hardness of the rocks. For this purpose, 33 various rock types were chosen by considering their physicomechanical properties and microfabrical characteristics. Thin sections were prepared and their microphotographs were analyzed by ImageJ software. Four basic characteristics, including grain perimeter and area and length of minor and major axes, were measured. Then, seven textural quantities, including equivalent diameter (Dequi), shape factor (SF), aspect ratio (AR), grain compactness (C), texture coefficient (TC), interlocking index (g), and grain size homogeneity (t), were calculated based on those basic parameters and statistically analyzed. The findings indicate that the Leeb hardness increases with increasing the grain area and perimeter, equivalent diameter, grain compactness, homogeneity of grain size, grain interlocking, and TC. In contrast, Leeb's hardness decreases with increasing the AR and SF. The texture coefficient reveals the most meaningful relationship (R2 > 0.8) with the Leeb dynamic hardness test among the studied microfabrical characteristics in sedimentary and igneous rock samples.

Compendious Index for Drillability: a rapid tool to assess drill penetration rate and bit life for rock engineering applications

Compendious Index for Drillability: a rapid tool to assess drill penetration rate and bit life for rock engineering applications

Analysis of Anisotropic Strength and Wellbore Unstable Zone in Shale Formation Relating to Water Content

During oil and gas exploitation, wellbore stability has always been the focus of many scholars. Instability of wellbore will lead to washouts, breakout, stuck pipe, mud loss, and so on, even the abandon of the well. Shale rock is widely distributed in nature and must be taken into account in some rock engineering applications. In order to evaluate the strength anisotropy and the influence of moisture content of the wellbore integrity, the LMX outcrops are divided into three groups to carry out triaxial strength experiment. Based on the fitting method of collaborative search, the strength parameters of different moisture contents are determined; then, the effect of strength anisotropy and the moisture content of the LMX reservoir on wellbore collapse pressure and wellbore integrity are investigated. The results show that shale strength, especially cohesion, is more sensitive to the influence of water. With the increase of water content, the inclination angle of the minimum value obtained by shale strength is deviated to the left, and the collapse pressure and instability area increase significantly. At the same time, it is necessary to adjust the direction of borehole drilling to avoid the occurrence of the zone of bedding slip instability in the direction of the minimum in situ stress, and the inversion of in situ stress should be corrected by considering the effect of bedding on the location of the maximum in situ stress. The study on the effects of bedding and water content on the instability zone can provide suggestions and guidance for the optimization of borehole trajectory, the control of bottom hole safety pressure, the inversion of in situ stress, and the drilling and completion of wells.

A prediction method for abrasion loss rate of some Egyptian carbonate rocks due to cyclic salt crystallization weathering using physico-mechanical deterioration: insights from laboratory investigations

Salt crystallization is the most important weathering processes that causes problems for rocks used as building stones. Therefore, assessment of the physico-mechanical properties of stones against cyclic salt crystallization is a critical issue for rock engineering applications in salty environmental conditions especially prone to abrasion. This research aims at investigating the relationship between the degradation of various physico-mechanical properties including apparent porosity, point load index, unconfined compression strength and abrasion loss rate for rocks during salt weathering process. For showing rock weathering in the salt crystallization process, five kinds of carbonate rocks were sampled from different areas of Egypt and subject to cyclic salt crystallization, up to twenty cycles. The variation rates of their physico-mechanical characteristics and abrasion loss values were calculated after each five cycles of salt weathering. The results showed that the studied rocks are suitable for use as building and decorative stones in areas prone to crystallization of salts, but attention must be paid to rocks with high porosity and low strength characteristics such as El-Minia limestone specimens. It was also found that there are strong linear correlations between the physico-mechanical degradation parameters and the abrasion loss rate of the studied rock samples at the end of the twentieth cycle of salt weathering. These relationships may be used to estimate the abrasion loss rate of the studied rock types against the cyclic salt weathering conditions and hence to make a rapid evaluation of the stone durability.

Mechanical Properties and Failure Behavior of Dry and Water-Saturated Foliated Phyllite under Uniaxial Compression.

Phyllite is widely distributed in nature, and it deserves to be studied considering rock engineering applications. In this study, uniaxial compression tests were conducted on foliated phyllite with different foliation angles under dry and water-saturated conditions. The impacts of water content and foliation angle on the stress-strain curves and basic mechanical properties of the Phyllite were analyzed. The experimental results indicate that the peak stress and peak strain decrease first and then increase with increasing foliation angle as a U-shape or V-shape, and the phyllite specimens are weakened significantly by the presence of water. Moreover, an approach with acoustic emission, digital image correlation, and scanning electron microscopic is employed to observe and analyze the macroscopic and mesoscopic failure process. The results show that tensile microcracks dominate during the progressive failure of phyllite, and their initiation, propagation, and coalescence are the main reasons for the failure of the phyllite specimens. The water acts on biotite and clay minerals that are main components of phyllite, and it contributes to the initiation, propagation, and coalescence of numerous microcracks. Finally, four failure modes are classified as followed: (a) for the specimens with small foliation angles α = 0° or 30° (Saturated), both shear sliding and tensile-split across the foliation planes; (b) for the specimens with low to medium foliation angles α = 30° (Dry) or 45°(Saturated), shear sliding dominates the foliation planes; (c) for the specimens with medium to high foliation angles α = 45° (Dry) or 60°, shear sliding dominates the foliation planes; (d) for the specimens with high foliation angles α = 90°, tensile-split dominates the foliation planes.

Adoption of ASTM A956-06 Leeb hardness testing standard to rock engineering applications

The Leeb dynamic hardness test was originally developed for metallic materials and is now widely used in rock engineering and engineering geology. This study aims to fundamentally investigate the application conditions of the Leeb hardness test in rock engineering because it is a high-precision, fast, nondestructive, and portable method. Therefore, four main limitations of the Leeb method mentioned in ASTM A956-06 have been further analyzed. The important challenges, including scale effect, temperature effect, surface roughness effect, and the effect of physical properties on the Leeb method, have been studied. For this purpose, 33 rock samples with a wide range of hardness and different geology origins were used. Based on the results, the minimum thickness of the block samples of 5 cm (or volume of 500 cm3) and the length-to-diameter ratio of 1.6 on core samples were suggested. Moreover, block samples were exposed to various temperatures, including −30, 0, 20, 50, 100, and 150 °C. The results indicated that with increasing the temperature, the Leeb hardness decreased, but the trend of the Leeb hardness variations is unnoticeable. The roughness analyses using a laser profilometer indicate that the Leeb hardness of rough samples can easily be determined with high reliability using the Leeb hardness of polished samples. Also, the Leeb hardness test for rough surfaces with JRC <4 (classes 1 and 2) can be used properly and performed locally in classes 3 to 10. Finally, the statistical analyses showed that the density and porosity have reasonable effects on Leeb's hardness.

Coupled Hydro-Mechanical Behaviour of Rock Joints During Normal and Shear Loading

The hydraulic conductivity of rock joints is an important parameter controlling fluid flow in various rock engineering applications. The shearing and normal loading have significant effects on hydraulic conductivity of rock joints, the property of which is mainly controlled by hydraulic aperture. Despite the importance of hydro-mechanical behaviour of rock joints, the fundamental micro-scale processes leading to macro-scale observations remain unexplored partly due to difficulties with in situ measurement of hydraulic aperture and its complex relation to roughness and contact area. Therefore, in this study, a series of experiments coupling fluid flow with normal deformability and direct shear are performed on joints with varying controlled roughness at different normal stresses. Along with measuring stress and flow rate, the time-lapse X-ray micro-computed tomography is carried out to explore the evolution of joint aperture and contact area during the experiments. The results of the normal deformability experiments show that the joint conductivity is well correlated to the mean hydraulic aperture of joint profiles. Such correlation, however, is not apparent for the shearing experiment where under high normal stresses, the flow rate decreases continually indicating that damaged asperities hinder the fluid flow. Despite the trend in the average mechanical aperture not following the flow rate in some cases, the trend in the contact area follows the flow rate very closely throughout the shearing process. In addition, the results reveal that despite an increase in contact area with increase in normal stress, it is not physically possible to reach full contact even for the artificially well-mated samples at a high normal stress of 10 MPa. Finally, a new correlation is proposed to relate the hydraulic aperture to joint average mechanical aperture, contact area and roughness. The correlation estimates the experimental flow rates at both normal and shear loading conditions with good accuracy.

Automatic arrival-time picking of P- and S-waves of microseismic events based on object detection and CNN

Automatic, quick and accurate picking of the arrival-times of both the P- and S-waves of microseismic (MS) events is essential to real-time data processing for MS monitoring. This study proposes a two-stage arrival-time picking method that involves extracting the region of interest of the waveform and P- and S-wave arrival-time prediction. The dataset collects 6107 MS signals with low and high signal-to-noise ratios to train and test the proposed method. The YOLO network is employed to extract the MS signal region to narrow the range of arrival-time picking and significantly improve the problem of serious interference caused by excessive negative samples in the MS signal. Then, the convolutional neural network is utilised further to precisely pick the arrival-times of both the P- and S-waves. The results showed that the proposed two-stage method is more accurate than the one-stage method in picking the arrival-time. The effects of different convolutional kernel sizes and fully connected layers on the two-stage model are also discussed. Furthermore, a new multi-index comprehensive evaluation method that considers both the accuracy and the size of the model is proposed to further evaluate the proposed model's practicality. This study presents a new method suitable for both P- and S-wave arrival-time picking to enable MS monitoring, which is valuable for various rock engineering applications.