Rock Mass Strength Research Articles

The Bologna Interpretation of Rock Bridges

One can only know where a rock bridge is once one measures it. In addition, to measure it, you need the rock mass to fail. This critical problem is ignored by many, and engineers continue to refer to rock bridges as geometrical distances between non-persistent fractures. This paper argues that this rather simplistic approach can lead to non-realistic failure mechanisms. We also raise the critical question of whether the inappropriate functioning of strength equations centred on the measurement of rock bridge percentages could result in misinterpreting the risk of failure. We propose a new interpterion, aptly called the Bologna Interpretation, as an analogy to the Copenhagen Interpretation of quantum mechanics, to highlight the indeterministic nature of rock bridges and to honour the oldest university in Europe (Bologna University). The Bologna Interpretation does not negate the existence of rock bridges. What rock bridges look like, how many there are, and where they are, we do not know; we can assume their existence and account for their contribution to rock mass strength using a potential analogue.

Laboratory-Scale Investigation on Shear Behavior of Non-Persistent Joints and Joint Infill Using Lattice-Spring-Based Synthetic Rock Mass Model

Discontinuities mainly control the mechanical behavior of rock mass and cause a significant reduction in the rock mass strength. Joint persistency and joint infill conditions are considered the most significant joint parameters that control the mechanical response of rock mass. In this study, numerical and statistical analyses were performed on pre-cracked specimens with two flaws to investigate the effect of joint persistence parameters on shear strength. In addition, an extensive study was conducted to explore the effect of infilled mineral strength, infill thickness, and infill wall roughness on shear strength. The Lattice-Spring-Based Synthetic Rock Mass (LS-SRM) approach was utilized to perform the numerical models. The results showed that the tensile crack propagation is limited at higher normal stresses as tensile damage is largely suppressed. The increases in rock bridge angle slightly increased the shear strength and caused a change in the failure mechanisms of the rock bridge from tensile to shearing. The results of the models with infilled minerals revealed that infilled minerals mainly controlled the shear strength of specimens when the infill thickness was 4.0 mm or greater. The infill wall roughness had no apparent effect on the shear strength. In contrast, it governed the failure mechanisms; cracks initiated at the asperity of the rough filling wall and propagated through the hosted rock mass.

Numerical Study on Deformation and Failure Characteristics of Rectangular Roadway

In the practical processes of coal mines and tunnel, rectangular roadways are widely used in the underground environment. However, the stress distribution law of the rock mass around the rectangular roadways is relatively complicated that is influenced by different parameters, such as the in situ stress, geological structure, and properties of the surrounding rock mass. It is extremely significant to conduct systematic and in-depth research regarding the stress distribution and deformation failure mechanism of the rock mass around the rectangular roadways. Based on theoretical analysis, the stress distribution law of the surrounding rock of rectangular roadway under different rock mass strength grades is studied directly through rectangular roadway in this paper. This further reveals the distribution law of secondary stress field after excavation of rectangular roadway. A coal mine intends to excavate a roadway in rock mass, and then, the rectangular roadway is simulated by FLAC3D software. Then, the strain distribution and failure types of roadway surrounding rock under different rock mass properties are obtained. The results show that the failure of rectangular roadway is mainly shear failure and tensile failure. The roof and floor of roadway are mainly tensile failure, and the side of roadway is mainly shear failure. Moreover, the stress shifting and distribution of the rock mass around rectangular roadways is a complicated dynamic variation process. The damage degree of the floor and roof of the rectangular roadway is greater than that of the two sides of the roadway. Its variation is mainly related to the strength of surrounding rock mass.

Experimental study of the influence of wetting and drying cycles on the strength of intact rock samples from a red stratum in the Three Gorges Reservoir area

The influence of wetting and drying cycles (WDC) on the strength weakening of typical rocks in soft and hard interbedded rock masses is vital to comprehend the mechanism of numerous bank slope failures in soft and hard interbedded strata induced by water level fluctuations. How WDC affect the strength of intact rocks in red stratum of the Badong Formation, a typical sliding-prone interbedded stratum in the Three Gorges area, is a topic worthy of comprehensive investigation. In this study, the effects of WDC on the physical and strength parameters of silty mudstone and silty sandstone were examined by performing a series of experiments on a total of 116 rock samples. Models of strength weakening were developed for the two rock types to express the relations between the strength deterioration coefficient (ratio of the weakened and initial value of strength parameters) and the number of WDC. The results indicate that treatment with WDC causes increases in the water absorption rate, and decreases in longitudinal wave velocity, tensile strength, cohesion and internal friction angle for both silty mudstone and silty sandstone samples with the rate of change decreasing with an increasing number of WDC; the variations in physical and strength parameters are larger for silty mudstone compared with silty sandstone; the effects of WDC on tensile strength and cohesion are higher than on internal friction angle for the studied rock types; Intergranular cracks and intragranular cracks were observed to increase with the number of WDC, evolution mode of rock microstructures was proposed, and the main weakening mechanisms induced by WDC was summarized as microcracks growth by mineral expansion and contraction, which weaken the strength of the studied rocks with the impact depending mainly on the mineral contents and microstructures in rocks. Finally, the established strength weakening models were applied in the numerical model to investigate the influence of WDC on the strength of rock mass with silty mudstone and silty sandstone interbedded layers in the Badong Formation. The obtained results provide useful guidance to reasonably evaluate the long-term stability of soft and hard interbedded bank slopes in reservoir area.

Study on blast-induced ground vibration velocity limits for slope rock masses

Seismic waves induced by blasting can cause damage to slope rock masses and thus lower the stability of the slope. The vibration standards of many countries do not specify the allowable blasting vibration velocity for slope rock masses. “Blasting Safety Regulations” (GB 6722-2014) in China recommends the allowable blasting vibration velocity of slope rock masses. However, the strength of rock mass and geometric characteristics of slope are not considered in the criterion. Therefore, based on the wave equation of Rayleigh wave, the mathematical relationship between the additional dynamic stress of the rock mass near the slope surface and the peak particle velocity (PPV) on the ground is analyzed. Under the control conditions that no tensile and shear damage occur in the slope, a calculation method of allowable blasting vibration based on a simple slope model is proposed, in which the strength of rock mass and the angle of the slope are considered. The theoretical basis of the proposed method is verified by numerical experiments. Based on the field test results of the slope of Xiaowan Hydropower Station, the proposed method was verified further. The damage control target of the rock mass determined by the proposed method was in good agreement with the actual damage detection result.

Validation of a Continuum-based Weak Zone Joint Model for Simulating Anisotropic Rock Mass Strength

In recent decades, the approach to estimating rock mass strength and its anisotropic behaviour more accurately has been a growing topic of interest in the rock mechanics community. To evaluate jointed rock mass behaviour more accurately, sophisticated modelling techniques such as Synthetic Rock Mass (SRM) models are becoming increasingly popular. Still, these methods are computationally intensive and require detailed site characterization data that are often unavailable during the initial stages of project development and are not always the most practical option. This study developed an alternative continuum-based SRM-like model where thin contiguous regions of weak continuum material with equivalent joint properties are in place of explicit interface joint elements. The finite-difference program FLAC used these weak zone joints (WZJ) to generate a jointed rock mass continuum model (JRCM). The JRCM concept was applied to systematic sets of persistent joints, a single non-persistent joint, and two intersecting joints to validate the practicality of using WZJ. Results found that the concept produced reasonable anisotropic rock mass strength estimates for most cases, with some limitations. The model captured the anisotropic behaviour while reducing time requirements to develop and recalibrate a conventional SRM. The JRCM concept demonstrates a practical alternative to estimating anisotropic rock mass strength for specific projects where the quantity or quality of data is limited. Furthermore, the reduced run times could allow multiple iterations to perform a probabilistic assessment of the rock mass strength.

Predicting the Thickness of an Excavation Damaged Zone around the Roadway Using the DA-RF Hybrid Model

After the excavation of the roadway, the original stress balance is destroyed, resulting in the redistribution of stress and the formation of an excavation damaged zone (EDZ) around the roadway. The thickness of EDZ is the key basis for roadway stability discrimination and support structure design, and it is of great engineering significance to accurately predict the thickness of EDZ. Considering the advantages of machine learning (ML) in dealing with high-dimensional, nonlinear problems, a hybrid prediction model based on the random forest (RF) algorithm is developed in this paper. The model used the dragonfly algorithm (DA) to optimize two hyperparameters in RF, namely mtry and ntree, and used mean absolute error (MAE), root mean square error (RMSE), determination coefficient (R2), and variance accounted for (VAF) to evaluate model prediction performance. A database containing 217 sets of data was collected, with embedding depth (ED), drift span (DS), surrounding rock mass strength (RMS), joint index (JI) as input variables, and the excavation damaged zone thickness (EDZT) as output variable. In addition, four classic models, back propagation neural network (BPNN), extreme learning machine (ELM), radial basis function network (RBF), and RF were compared with the DA-RF model. The results showed that the DARF mold had the best prediction performance (training set: MAE = 0.1036, RMSE = 0.1514, R2 = 0.9577, VAF = 94.2645; test set: MAE = 0.1115, RMSE = 0.1417, R2 = 0.9423, VAF = 94.0836). The results of the sensitivity analysis showed that the relative importance of each input variable was DS, ED, RMS, and JI from low to high.

Experimental assessment of dynamic loading response of grouted non-persistent jointed rock

Grouting is a well-established engineering practice for stabilizing the jointed and/or fractured rock mass. This process may lead to the enhancement of the mechanical properties of fractured rocks. In the majority of the studies, the efficiency of grouting is determined under static loading conditions. Nonetheless, the grouted rock may be subjected to different dynamic loading from blasts or earthquakes. The present study explored the dynamic loading response, in terms of strength and fracture propagation, of a rock mass having a diagonal non-persistent joint (45° to the loading direction) subjected to impact loading to determine the efficiency of grout material. The focus of the study is to present a comparative assessment of different grout materials under dynamic loading. Split Hopkinson Pressure Bar (SHPB) was used to conduct the compressive impact tests on the synthetic rock mass with varying infill conditions (unfilled, cement-filled and epoxy-filled). The progressive fractures within the specimens were monitored by Photron fastcam analysis (PFA, a high-speed image analysis) and digital image correlation (DIC). The experiments highlighted that the strength of rock mass has an increasing trend with the dynamic strain rate. Epoxy resin provided a better strength enhancement than cement paste as the grout material. Due to the higher strength of epoxy resin than cement paste, the epoxy-grouted jointed rock demonstrates a similar response as the intact rock. In the case of both unfilled and cement-filled specimens, the nature of the primary crack was the coplanar shear crack. In contrast, with the injection of epoxy grout, the nature of the initial crack became the tensile or far-field tensile crack, which is often noticed in intact rocks.

Research on mechanical behavior of rock breaking of abnormal cutter based on PFC numerical simulation

Cutter wear is an important factor that restricts construction efficiency and cost in TBM tunneling, and abnormal cutter damage will even cause TBM jamming, seriously affecting the construction schedule. Therefore, using PFC particles flow numerical simulation method, two simulation schemes which will possibly cause cutter abnormal damage are set up, researching the mechanics behavior and efficiency of rock breaking. Otherwise, the influence of penetration, rock mass strength and parameters of the cutter itself on cutter force is also analyzed, and the corresponding engineering suggestions are provided in this article.

A multi-scale roughness shear joint model using wear theory

Fracture is the weakest structure and dominates the shear strength of the natural rock mass. This structure potentially leads to many engineering hazards. Although various shear constitutive models have been developed previously their limitations have not been fully explained. Among them, there is an argument that the shear-off component added in the same manner with the basic sliding friction lacks physical meaning. Many models have been developed based on the lab-scale joint samples, thereby being less reliable when evaluating shear strength of field-scale joint. A new shear constitutive model incorporated with lab-scale has been developed in which the degradations of the irregularities are simulated based on the wear theory. Semi-empirical equations are proposed to estimate the wear coefficients to avoid the back-analyses of the experimental results in order to obtain input values. As different scaled roughness has different effects on both mechanical and hydraulic behaviours, multiscale roughness parameters are employed in the mechanical model. The proposed mechanical model potentially can be expanded to the development of the coupled hydromechanical model.

Explicit numerical models for the prediction of plastic and weakening rockmass behaviour around a circular tunnel in isotropic and anisotropic stress conditions

The Geological Strength Index, GSI, is an assessment system for rockmass structure conditions (blockiness and discontinuity condition) based on the visual examination of exposed rockmass surfaces. The Generalized Hoek-Brown failure criterion is widely used with GSI to estimate the strength and stiffness of jointed rockmasses in traditional continuum numerical modelling. Modern numerical analysis tools allow networks of discrete structure to be represented with assigned properties based on site investigation data. In this paper, models with defined explicit structure are shown to yield similar results to implicit GSI-based models and provide more detailed information on the spatial variability of tunnel response when reasonable rockmass input parameters are estimated and calibrated. Once plastic explicit models are calibrated to give similar response to plastic implicit GSI-based models, explicit models using weakening joint and intact rock elements are used to determine appropriate post-yield strength and dilation parameters for use in implicit GSI-based modelling.

Improving Rock Pillar Stability Guidelines Through Detailed Numerical Investigation

Pillar stability guidelines have been presented by numerous authors in the past, based on a combination of empirical observations (mostly visual) from mining applications and simple numerical (elastic or simple continuum plasticity) models. In many cases, the resulting design limits for pillar dimensioning are presented as universal for all rockmass conditions and stress regimes. There have been numerous developments in the past decades in mechanistic classification (brittle spalling, effective continuum rockmass, structural control) and mechanism-appropriate modelling and design metrics. These new tools and paradigms for rockmass behaviour allow for a modelling-based revision of the empirical guidelines from the past century that still form the basis of many performance critical designs. This paper includes the consideration of rockmass deformation and yield (rockmass strength approach), brittle strength, and damage behaviour where appropriate. This paper presents correlations for residual GSI and the effects of Hoek-Brown dilation on pillar behaviour. The investigation includes generating 3D FEM synthetic pillar models with equivalent elastic and plastic continuum.

Characterization of rock formation at Toshka fourth depression – Egypt

ABSTRACT Recently, the Egyptian south valley has been considered as one of the most promising developments for the mega projects such as power stations, roads and bridges. Fresh water channels were extended through to transfer water from Nasser Lake to the irrigated land in the western desert in order to save the groundwater storage for the later usage. Rock formations generally represent a great challenge in civil engineering due to their wide spatial variability, whether vertically or horizontally. This paper presents the results of a geotechnical study that was carried out across the Toshka fourth depression, Egypt. The study comprises a comprehensive investigation programme in the field and complemented with laboratory testing programme to evaluate the physical and mechanical rock properties. The geotechnical investigation included executing boreholes with a depth up to 100 m; core and bulk samples were extracted and recovered. Moreover, an extensive laboratory testing programme was conducted on the samples. Additionally, Rock Mass Rating System was used to identify the rock mass properties, considering the effect of discontinuities planes, layers joint roughness conditions, slaking rate and in addition to the effect of wetting and drying on Rock Mass strength parameters. The results indicated that the geological formation generally composed of limestone intercalated with layers of silty claystone or clayey siltstone. The results of this study cover, among others, rock mass strength properties that should be considered as a design parameters or regarded as guidelines for the design of any future development in the region.

Research on Drilling Test of Grouting Rock Mass Based on Cutting Strength

Grouting is an effective way to control the fractured surrounding rock in underground engineering. Quantitatively obtaining the strength parameters of grouting rock mass is very important for the grouting parameters rational design. Drilling tests of grouting rock masses with different water-cement ratios and particle sizes are carried out using the rock digital drilling test system. Based on the rock cutting mechanical model, the cutting strength of grouting rock is obtained. The response laws of different water-cement ratios and rock particle sizes on drilling parameters, cutting strength, and strength parameters are analyzed. On this basis, combined with the analysis of the relationship between grouting sandstone specimens’ strength and drilling parameters with different water cement ratios, a while-drilling testing model of grouting rock mass strength parameters based on cutting strength is established. It is a theoretical basis for quantitatively evaluating the strength of grouting surrounding rock while drilling in underground engineering.

Influence analysis on the shear behaviour and failure mode of grout-filled jointed rock using 3D DEM coupled with the cohesive zone model

Grouting is one of the primary methods used to reinforce the strength of fractured rock masses. In this study, a three-dimensional discrete element method (3D Co-DEM) grouted joint shear model, which consists of upper and lower intact rock blocks, 3D real joint surfaces and a grout-filled layer, was developed with the cohesive zone model. The 3D real joint surface was imported into the model using 3D scanning point cloud data, and the grout-filled layer was described by a cohesive zone model. Then, a numerical simulation algorithm was developed to implement the numerical direct shear test and grout-filled layer cracking simulation. Also, the parameters of the proposed method were calibrated, and the effectiveness of the cohesive zone model and direct shear numerical test were verified by comparison with analytical and experimental results. Finally, grouted jointed specimens with regular sawtooth were used to analyse the influencing factors of shear characteristics. Numerical tests showed that shear peak strength and residual strength decrease with increasing grout water-cement ratio but increase with increasing JRC. Specifically, with increasing sawtooth inclination angle, the shear peak strength first decreases and then increases, while the peak shear strength increases as the sawtooth height increases.

Block Caving Mining Method: Transformation and Its Potency in Indonesia

The block caving mining method has become increasingly popular in the last two decades. Meanwhile, Indonesia has several potential ore bodies which have not yet determined suitable mining methods. The references to block caving mining projects worldwide and the potency of metal deposits in Indonesia were reviewed to determine the requirements of ore bodies suitable for mining using the transformed block caving method. This method can be applied on a blocky ore body with a thickness of 200–800 m, various rock mass strengths until 300 MPa, from low to high (from 0.3% Cu until more than 1.0% Cu), but of uniform grade and at a depth from 500 to 2200 m. The technical specifications for running block caving mines have been synthesized, including preparation methods, undercutting strategy, mine design, mining equipment and monitoring. Considering the requirements and the successful practice of the block caving project in the Grasberg Caving Complex as a role model, the Indonesian government should concentrate on the detailed exploration of porphyry deposits and feasibility studies on applying the method to the prospective ore bodies, i.e., Onto, Tambulilato, Tumpangpitu and Randu Kuning. In addition, the exploration method, cost, operation, environment, mining policy and social geology are important aspects worth noting.

Regional seismic landslide susceptibility assessment considering the rock mass strength heterogeneity

Rock mass strength parameters are critical when applying the permanent-displacement analysis to landslide susceptibility assessment. Previous studies use limited measured data to assign a constant value for each lithology. Since the rock mass strength is spatially heterogeneous, assigning a constant value to each lithology cannot effectively reflect the spatial heterogeneity of rock mass strength, reducing the reliability of seismic landslide susceptibility assessment. Here we present an analysis of a well-documented seismic landslide inventory and the lithological environment, and develop an empirical model to quantify the rock mass strength heterogeneity. The rock mass strength heterogeneity was subsequently taken as an input of the permanent-displacement analysis to assessing regional seismic landslide susceptibility. Results on two case studies show that considering the rock mass strength heterogeneity would significantly improve seismic landslide susceptibility assessment. The findings of this study are valuable for earthquake emergency rescue and post-earthquake land plans.

Evaluation of input geological parameters and tunnel strain for strain-softening rock mass based on GSI

The regression analysis method is being widely adopted to analyse the tunnel strain, most of which ignore the strain-softening effect of the rock mass and fail to consider the influence of support pressure, initial stress state, and rock mass strength classification in one fitting equation. This study aims to overcome these deficiencies with a regression model used to estimate the tunnel strain. A group of geological strength indexes (GSI) are configured to quantify the input strength parameters and deformation moduli for the rock mass with a quality ranging from poor to excellent. A specific semi-analytical procedure is developed to calculate the tunnel strain around a circular opening, which is validated by comparison with those using existing methods. A nonlinear regression model is then established to analyse the obtained tunnel strain, combining twelve fitting equations to relate the tunnel strain and the factors including the support pressure, GSI, initial stress state, and critical softening parameter. Particularly, three equations are for the estimation of the critical tunnel strain, critical support pressure, and tunnel strain under elastic behaviour, respectively; and the other nine equations are for the tunnel strain with different strain-softening behaviours. The relative significance between the GSI, the initial stress and the support pressure on the tunnel strain is assessed.

Effect of nano-Al2O3/epoxy resin composite on the shear strength recovery of fractured rock masses with various crack widths and SCA interfacial treatments

The mechanical properties of grout and the effect of the bonding between grout and the repaired material are the two main factors affecting the shear strength of the material. To study the effect of repairs carried out with nano-Al2O3/epoxy resin composite (NAEC) on the shear strength of rock masses, cracks with different widths and treatments by silane coupling agent (SCA) solutions with various surface treatment concentrations (kSCA) were considered. The mass percentage ratio of nano-Al2O3 in the composite (kAl) was optimized based on the obtained uniaxial compressive strength (UCS). The selected NAEC was used to fill cracks of different widths, and the bonding effect was analyzed using a bilateral shear strength test that effectively avoids the bending effect. Solutions with diverse kSCA values were used to treat the rock surfaces with the 3-mm-wide cracks. The results showed that the UCS of the NAEC tended to increase with increasing kAl when the mass ratio (kCE) of the curing agent to epoxy resin was 0.25. When kAl= 0.01, the UCS of the composite peaked at 97.82 MPa. With increasing crack width, the shear strength first increased and then decreased. When the crack width was 3 mm, the shear strength reached a maximum of 29.02 MPa. However, without nano-Al2O3, the shear strength was 23.66 MPa, and the shear strength of ultrafine Portland cement (UPC) material was only 8.37 MPa. With increasing kSCA, the shear strength tended to first increase and then decrease. For a kSCA of 2%, the shear strength reached a maximum of 39.40 MPa, corresponding to an increase of 35.8%. Nevertheless, the composite shear strength as degraded at 5% kSCA. In summary, the NAEC exhibited a sound bonding effect, SCA surface treatments effectively improved the shear strength, and the bonding shear strength was related to the crack width.

Influence of the Preexisting Fracture Network on the Mechanical Properties of Rock Mass and the Secondary Crack Propagation

Preexisting fracture network (PFN) is ubiquitous in natural rock mass and is one of the key factors affecting the mechanical properties of rock mass. This research employed a combined approach of theoretical analysis and finite element numerical simulation to investigate the mechanical behavior of PFN-containing rock mass. Based on the sliding crack model and fracture mechanics theory, the initiation mechanism of closed and open cracks driven by compressive and shear stress was theoretically analyzed. Rock mass models with different inclination angles (β) of PFN were built in Fast Lagrangian Analysis of Continua in 3 Dimensions. The complete stress–strain curves of rock mass and the secondary crack propagation were obtained. Results indicated that the increase of β reduced the strength and elastic modulus of rock mass. When β is less than 45°, this influence is more prominent. Based on the load-bearing capacity of PFN-containing rock mass, the stress–strain curves can be classified into two types: type I curve has multipeak and is strain-softening in the postpeak stage when β is 15°, 30°, 45°, 60°, and 90°; type II curve is unimodal and with brittle failure when β is 60° and 75°. Secondary crack propagation can be classified into four main types: type I is the coplanar crack, type II is the wing crack around PFN, type III is the near-field wing crack, and type IV is the macroscopic shear crack.