Intact Rock Masses Research Articles

A new improved 3D Hoek-Brown criterion

The Hoek-Brown strength criterion has been widely used to estimate the strength of intact rocks and rock masses, and has been continuously developed. However, in the latest version of the standard, the criterion still ignores the influence of the intermediate principal stress. To study the different effects of intermediate principal stress on rock failure under triaxial or multiaxial stress states, this paper proposes a modified three-dimensional H-B strength criterion, which can be reduced to the H-B criterion, three-dimensional Priest criterion, Jiang’s (2012) criterion, and the Cai criterion. Multiaxial test data of six intact rocks were used for validation and applicability analysis. The results show that the proposed criterion can well describe the trend of multi-axial test data of six kinds of rocks, and the average mismatch of the six types of rocks is controlled within 10 MPa, which is much smaller than the fitting error of the original H-B criterion, Mogi criterion and Jiang’s criterion, indicating that the criterion has a good prediction effect on rock strength. The proposed criteria can provide a basic theory for the future construction of in-situ rock mass strength.

Failure mechanism of rock masses with complex geological conditions in a large underground cavern: A case study

Reasonable and accurate identification of the failure mechanism of rock masses is of great significance for achieving a comprehensive and thorough understanding of the fracture process in underground caverns under high geostress. Sophisticated microseismic (MS) monitoring was established in underground cavern groups to capture the excavation-induced microfracture signals of intact rock masses and rock masses with weak interlayer zones (WIZs). To clarify the mechanical mechanisms controlling the surrounding rock failure modes, the moment tensor inversion method is adopted to fully reveal the fracture types and focal mechanism solutions (strike, dip and rake) of the rock masses. The spatial distribution of the MS events shows that the excavation-induced microfractures of intact rock masses are concentrated in the working face of underground intersecting chambers. The fracture mechanisms of intact rock masses are dominated by tensile fracturing. Compared with those in intact rock masses, the microfractures induced by excavation in rock masses with WIZ form a strip-like cluster area along the WIZ, and the proportion of shear fractures is significantly higher. The moment tensor method quantitatively reveals the failure mechanism of excavation-induced microseismicity of rock masses with complex geological conditions.

Lithospheric strength and elastic properties for Mars from InSight geophysical data

Elastic moduli and unconfined compressive strength (UCS), material constants which help constrain the mechanical behaviors of rock materials, are useful for developing accurate models of geologic processes like fracturing and faulting. On Earth, UCS and elastic moduli for intact rock can be measured with uniaxial and triaxial compressive strength tests; however, when geological units are highly fractured, it is more appropriate to estimate these parameters for the rock mass, fractures included. At depth, these estimations can be obtained using seismic wave velocities where stronger more intact rock masses result in faster velocities. We use seismic wave velocities from the InSight Lander to derive the elastic moduli and UCS for the upper crust, lower crust, and mantle of Mars. These estimates could be used in developing models for lithospheric deformation on the red planet.

Grain-scale numerical simulation of crystalline rock fracturing using Soundless Cracking Demolition Agents for in-situ preconditioning

Soundless Cracking Demolition Agents (SCDAs) are becoming increasingly popular for near-surface underground rock fragmentation applications. Several studies have been carried out to accurately simulate the fracturing processes of SCDA under its volumetric expansion inside a borehole. These numerical simulations and experiments have been limited mainly to homogeneous and intact rock masses in most cases. In this paper, we present a numerical approach that assesses the influence of mineral heterogeneity and rock mass defects (pore structures) on the fracturing performance of SCDA at the grain size level. For the simulation, a numerical crystalline rock grain assembly was generated using NEPER (a polycrystal generation tool) to introduce spatial variability of grain size that closely mimicked the grain arrangement of granitic rock. The assembly was then imported to 3-Dimensional Distinct Element Code (3DEC) as a block-based model. Heterogeneity was introduced to the model in terms of both mineralogical spatial distribution and strength variation. Pore spaces were introduced to the model using random grain deletion in the rock model. Intergranular and transgranular fracturing of the assembly was also simulated by utilizing a dual-layer discretization technique in 3DEC. SCDA charged fracture simulation was carried out in the model using a single central injection well. The results suggest intergranular fracturing to be the dominant mode of fracturing with additional grain crushing (transgranular fracturing) in the vicinity of the injection well. Grain size and in-situ stress anisotropy largely affect the direction and geometry of radial fractures initiated around an injection well during SCDA charging. The stress concentrations introduced by the pore structures of the matrix were found to have a strong influence on crack deflection, additional micro-cracking in the matrix and final tortuosity of the fractures produced. The results presented in this paper suggest that the micro-mechanical heterogeneity of the rock mass significantly influences the final fracture pattern produced by SCDA charging and therefore should be given more attention during crystalline rock pre-conditioning applications.

Study on disc cutter chipping of TBM based on field data and particle flow code simulation

Using tunnel boring machines to excavate high-strength intact rock masses is becoming more common. Due to the interactions between disc cutters and rocks, abnormal wear of disc cutters, especially cutter chipping, has become a common phenomenon. Existing research has mainly focused on normal wear of disc cutters without addressing abnormal wear cases. This study used the disc cutter consumption data of a tunnel project in China to investigate the abovementioned problem based on field research. According to the failure patterns and fracture surface characteristics, the cutter chipping patterns were mainly categorized into four types: granule chipping, patch chipping, primary collapse, and secondary collapse. To further simulate the evolution of disc cutter chipping, based on the linear plastic bond model, a new contact model called the modified plastic bond (MPB) model was proposed to solve the metal simulation problem in Particle Flow Code software. To this end, a set of uniaxial tensile and compressive tests were initially conducted to verify the applicability of the MPB model. Then, a series of three-dimensional rock-cutting simulation tests were conducted to reflect the evolutionary processes involved in each type of cutter chipping. The cutter chipping mechanism and morphological characteristics were classified and summarized in detail. The results revealed that the cutting speed and penetration growth led to a rising trend in the probability and intensity of the cutter chipping. The presence of initial defects also induced an adverse effect on the service life of the cutter. The results indicated suitable working conditions for the cutter and suggested ways to control tunneling parameters and avoid frequent cutter chipping cases.

Linear thermal expansion behavior of compacted bentonite buffer materials

In a geological repository system, buffer is indispensable to ensure the safe disposal of high-level radioactive waste (HLW). Because heat generated from spent nuclear fuel in a canister is released to the surrounding buffers, thermal properties of such materials are fundamental in determining the overall disposal safety. Specifically, given that thermal expansion causes thermal stress to canisters and intact rock masses in the near-field location, it is imperative to evaluate the thermal expansion characteristics of the buffer, particularly when bentonite is used. This study investigates the linear thermal expansion properties of Kyeongju bentonite buffer, a type of Ca-bentonite produced in South Korea. The linear thermal expansion coefficient of dried bentonite was measured considering the heating rate, dry density, and temperature variation using dilatometer equipment. The linear thermal expansion coefficient values of the KJ bentonite buffers were found to be 4.0–6.2 × 10⁻⁶/°C. Based on test results, a numerical analysis was conducted, and the thermal strain values were similar between the test and numerical analysis. The overall linear thermal expansion coefficient of the KJ bentonite, considering radially confined or unconfined conditions and dried or saturated states, was predicted to be between 3.2 × 10⁻6/°C and 1.0 × 10⁻5/°C.

Back-analysis of Donghekou landslide using improved DDA considering joint roughness degradation

Currently, the dynamic behavior of high-speed long-runout landslides triggered by earthquakes remains unclear due to the complexity of such phenomena. Degradation in the shear strength of rock masses is considered a primary factor contributing to the high velocities and long runout distances of these landslides. In this study, the strength degradation induced by changes in rock joint morphology is analyzed considering a numerical model of the Donghekou landslide, by using discontinuous deformation analysis (DDA). First, an exponential joint roughness degradation function (i.e., JRCm) that depends on displacement is incorporated with the nonlinear Barton–Bandis (B–B) shear strength criterion, and this modified B–B criterion is combined with the DDA. Moreover, a virtual joint is introduced in the DDA to model the propagation of cracks in intact rock masses, in order to simulate the dynamic phenomenon of earthquake-induced landslides. Subsequently, this improved DDA (IDDA) is validated via simulations of a shear test and a sliding block over an inclined plane. Finally, the IDDA is employed to simulate the Donghekou landslide, which was induced by the 2008 Wenchuan earthquake in China. Furthermore, the entire dynamic process of the landslide, including crack propagation, instability, initiation, and motion, is depicted. The simulated deposit patterns obtained via the IDDA are in good agreement with the results of field investigations. The simulation results also prove that the IDDA can reflect key movement behaviors of Donghekou landslide and that the degradation of rock joints significantly influences the long runout distance and high velocity of sliding mass.

Strength test of 3D printed artificial rock mass with pre-existing fracture

Discontinuities or structural planes are widely distributed in natural rock masses and significantly influence their geo-mechanical and geometric properties. Herein, a batch of rock samples, each with a single structural plane, is created using a 3D printer equipped with two robotic manipulators. One of the manipulators is connected via a nozzle to a concrete pumping truck, which can extrude brittle rock-like material to form layered intact rock masses; the rock-like material is mainly composed of cement, silica fume, sand and water. The other manipulator features a knife, which can carve discontinuities onto each layer of the printed model. By means of this system, rock masses with discontinuous joints are formed, and the failure strengths of rock masses with different joints are demonstrated via uniaxial compression tests and direct shear tests. The results thereof obtained via digital image correlation technology show that discontinuities lower the strength of the rock mass models significantly. With the increase of the angle between the fracture and horizontal plane, the uniaxial compressive strength first decreases, and then increases. During shear testing, the shear strength of the rock mass models increases with the surface roughness of the preset joint. These test results indicate that the influence of artificial joints on the mechanical properties of the models is consistent with that of natural rock mass joints. Using digital modeling and 3D printing technology, cracks hidden in a rock mass can be reproduced.

Revisiting statistical correlation between Mohr-Coulomb shear strength parameters of Hoek-Brown rock masses

Mohr-Coulomb shear strength parameters (i.e. cohesion c and friction angle ϕ) have been widely applied to analyses and designs of rock engineering (such as tunnels, underground caverns, and rock slopes), and they are statistically correlated. Such a correlation plays a vital role in the reliability and risk assessment of rock engineering, and is often quantified by their correlation coefficient ρc,ϕ. Observations on ρc,ϕ for intact rocks and rock masses are, however, conflicted, indicating that the ρc,ϕ is negative for intact rocks while positive for rock masses. Reasons for such a confliction are unrevealed, posing difficulties in determining a proper ρc,ϕ value for reliability and risk assessment of rock engineering. This study revisits the statistical correlation between c and ϕ of rock masses based on Hoek-Brown (H-B) failure criterion and triaxial test data of intact rocks. The rock masses, whose c and ϕ are derived from H-B failure criterion using triaxial test data of intact rocks, are referred to as “H-B rock masses”. Various uncertainties affecting ρc,ϕ of H-B rock masses are first identified, and their effects on ρc,ϕ are systematically explored. It is shown that the positive correlation between c and ϕ of H-B rock masses is attributed to the simultaneous decrease in c and ϕ with the decrease of Geological Strength Index of rock masses and the increase in lumped uncertainty in triaxial test data of intact rocks. Nevertheless, the negative correlation between c and ϕ of intact rocks is due to the decomposition of a certain shear strength of intact rocks into cohesion and friction parts. The increase in one part leads to the decrease in the other part so as to obtain the same shear strength of a given type of intact rock. The generalized H-B failure criterion allows exploring the correlation between c and ϕ of intact rocks and rock masses under a unified framework.

Propagation Characteristics of Blasting Stress Waves in Layered and Jointed Rock Caverns

The existence of joint fissures makes explosive actions between rock masses more complex. Therefore, it is of great significance to carry out experiments studying blasting stress waves propagating in jointed rock masses. Based on the Froude Similarity principle, the geological mechanical models of intact rock masses and jointed rocks have been proposed. A blasting vibration experiment was carried out and demonstrated that the propagation of the blasting stress waves and changing structures have an important relationship. A numerical simulation of the blasting stress wave propagation law in a layered jointed rock mass was carried out. This study found that with an increase in the joint angle, the peak velocity of blasting stress wave, transmission coefficient and reflection coefficient all gradually increased, while the attenuation coefficient gradually decreased. With an increase in joint spacing, the attenuation rate of the blasting stress waves increased.

Fractal characteristics and acoustic emission of anisotropic shale in Brazilian tests

The strata of shale contain structural weak planes such as laminations and joints, which do not have the same mechanical properties as intact rock masses. Tensile strength is a critical parameter that determines the capacity of a rock and its resistance to deformation and failure. Focusing on tensile strength and acoustic emission (AE), the characteristics of shale were investigated at various orientations of the laminations with respect to the loading direction. By coupling Brazilian test and the AE technique, the mechanical properties and damage patterns of shale can be explored. The shale exhibits clear laminations and contains a high proportion of brittle minerals by XRD and SEM analysis. The stress–time curve of the Brazilian test can be divided into three stages with distinct brittleness characteristics, and the tensile strength exhibited undulatory trend as bedding angles increases. At low bedding angles, the compaction of fissures and pores within the shale is not significant, and the cumulative AE count–time curves exhibited a flat-to-sharply rising trend. By contrast, the curves showed a gradually increasing “stepped” tendency at high bedding angles. Analysis of the AE time sequence based on fractal theory reveals that fractal dimension values fluctuate with increase of the stress, signifying the initiation of complex microcracks within the shale. The fractal dimension values sharply dropped when approaching the limit of tensile strength, signifying the occurrence of major cracks. The sudden drop of AE time sequence correlation dimension values can serve asan early warning for the coming failures. The research findings could be instrumental in the monitoring of rock mass instability, microcrack mechanisms, and earthquake sequences.

Analysis of support requirements for underground water-sealed oil storage cavern in China

The aim of this study is to provide a sound support design for a large-scale underground water-sealed oil storage facility in China. The lithology of this study area consists of reddish-gray, medium-coarse grained granites interpenetrated by diabase, amphibole dioritic porphyrite and aplite. In order to obtain the geotechnical properties of intact rocks and rock masses, detailed field and laboratory studies were carried out. Rock masses at three sites were characterized in terms of Rock Mass Rating (RMR), Rock Mass Quality (Q-system) and Geological Strength Index (GSI), and then rock mass properties of underground caverns were estimated, accordingly. The support systems obtained from empirical methods were analyzed using FLAC3D commercial software considering the effect of the water curtain system. The maximum thickness of plastic zones and the maximum total displacement occurred around underground caverns after installing the support systems suggested by the empirical methods were compared to the unsupported case. The results show that more reliable support design could be obtained by using both empirical rock mass classifications and numerical analysis method.

Analytical Models for Estimation of Slope Stability in Homogeneous Intact and Jointed Rock Masses with a Single Joint

Analytical Models for Estimation of Slope Stability in Homogeneous Intact and Jointed Rock Masses with a Single Joint

Role of Brittle Behaviour of Soft Calcarenites Under Low Confinement: Laboratory Observations and Numerical Investigation

The strength decay that occurs in the post-peak stage, under low confinement stress, represents a key factor of the stress–strain behaviour of rocks. However, for soft rocks this issue is generally underestimated or even neglected in the solution of boundary value problems, as for example those concerning the stability of underground cavities or rocky cliffs. In these cases, the constitutive models frequently used in limit equilibrium analyses or more sophisticated numerical calculations are, respectively, rigid-plastic or elastic–perfectly plastic. In particular, most of commercial continuum-based numerical codes propose a variety of constitutive models, including elasticity, elasto-plasticity, strain-softening and elasto-viscoplasticity, which are not exhaustive in simulating the progressive failure mechanisms affecting brittle rock materials, these being characterized by material detachment and crack opening and propagation. As a consequence, a numerical coupling with mechanical joint propagation is needed to cope with fracture mechanics. Therefore, continuum-based applications that treat the simulation of the failure processes of intact rock masses at low stress levels may need the adoption of numerical techniques capable of implementing fracture mechanics and rock brittleness concepts, as it is shown in this paper. This work is aimed at highlighting, for some applications of rock mechanics, the essential role of post-peak brittleness of soft rocks by means of the application of a hybrid finite–discrete element method. This method allows for a proper simulation of the brittle rock behaviour and the related mechanism of fracture propagation. In particular, the paper presents two ideal problems, represented by a shallow underground cave and a vertical cliff, for which the evolution of the stability conditions is investigated by comparing the solutions obtained implementing different brittle material responses with those resulting from the assumption of perfectly plastic behaviour. To this purpose, a series of petrophysical and mechanical tests were conducted on samples of soft calcarenite belonging to the Calcarenite di Gravina Fm. (Apulia, Southern Italy), focusing specific attention on the post-peak behaviour of the material under three types of loading (compression, indirect tension and shear). Typical geometrical features representative of real rock engineering problems observed in Southern Italy were assumed in the problems examined. The numerical results indicate the impact of soft rock brittleness in the assessment of stability and highlight the need for the adoption of innovative numerical techniques to analyse these types of problems properly.

Modulus Ratio and Joint Factor Concepts to Predict Rock Mass Response

The commonly adopted rock mass classifications, namely RMR, Q and GSI, are used to estimate compressive strength and modulus of rock masses. These values have been examined as per modulus ratio concept, M (rj) , for their reliability. The design parameters adopted in some of the recent case studies based on these classifications indicate that the M (rj) values for rock masses are higher than those of the corresponding intact rocks. The joint factor, J (f), which is defined as a weakness coefficient in rock mass suggests that modulus ratio of rock mass (M (rj) ) has to be less than the modulus ratio of the corresponding intact rock (M (ri) ), on the basis of extensive experimental evidence. With joint factor, compressive strength, elastic modulus, cohesion and friction angle were estimated and applied in the analyses of a few cases. The predictions of deformations with this approach agreed well with the field measurements by adapting equivalent continuum approach. The modulus ratio concept is considered to present a unified classification for intact rocks and rock masses. Soil-rock boundary, standup time in under ground excavations and also penetration rate of TBM estimates have been linked to M (rj) .

Analysing the modified Hoek–Brown failure criteria using Hungarian granitic rocks

The Hoek–Brown empirical failure envelope is widely used for determining the strength of brittle intact rock and rock masses. One of the two independent parameters in the formula is the Hoek–Brown’s material constant (mi), which has several uncertainties, since it relies on a set of triaxial tests. The present paper introduces a new approach to determine the material constant by using the uniaxial compressive strength, rather than the triaxial strength values. This newly calculated Hoek–Brown parameter was cross-checked by using published data and new experimental data obtained from granitic samples of Bataapati (Hungary). Based on these new equations new material constants are suggested to be used in calculations.

Distinct element modeling of strength variation in jointed rock masses under uniaxial compression

Rock masses exhibit strong anisotropy due to the structure of fracture networks embedded within the mass. We use a particulate discrete element method to quantitatively investigate the effect of spacing and inclination angle of the joints on anisotropic strength under uniaxial compression. In all of the numerical models, the intact rock masses are represented by the bonded particle model, and the joint planes are simulated by the smooth-joint model. Observations are made of the evolving stress–strain curves relative to the distribution and orientation of micro fractures. Apparent from this is that: (1) for the same fracture spacing, the strength and deformation parameters of jointed rock masses change in a “U” shaped curve when plotted against an increase in the inclination angle of the joints. The maximum value occurs at the inclination angles of 0° and 90°, and the minimum value occurs at an inclination angle of 40° or 50°; (2) for the same joint spacing, the failure mode of the jointed rock mass with different joint inclinations can be classified into three categories: splitting tensile failure, shear sliding failure along the joint surface and mixed failure of the two modes above; (3) for the same joint inclination angle, the normalized compressive strength and normalized elastic modulus both increase with an increase of the normalized joint plane layer spacing.

Crack Propagation of Jointed Rock and Application

The excavation engineering in jointed rock masses, due to the changes in its original stress state, the preexisting fissures will expand and new fissures may emerge, which will degrade its mechanical and strength properties. This paper uses the DDARF method to preinstall fissures of different numbers and spacing in the rock block, then studies the crack initiation, expansion, transfixion and the destruction process by numerical modeling experiment, and finds the relevant stress strain curve. It also studies the influence of the numbers of fissures and different spacing and the influence of lateral compression on the test specimen to find the strength envelope of the test specimen. The parameters are applied in a case study. The differences in the failure behaviors of the intact and jointed rock masses after cavern excavation are analyzed and compared.

Open pit slope deformation monitoring by fiber Bragg grating sensors

With microstrain resolution and the capability to sample at rates of 2000 Hz or higher, fiber Bragg grating (FBG) strain sensor offers exciting new possibilities for in situ deformation monitoring induced by blasting load in an open pit slope. Here, we are developing a new technology for measuring deformation in real time on the microstrain in an open pit slope during the blasting. A fiber optically instrumented rock mass strain sensor measured strain at 100-cm intervals along a two anchor rock bolt grouted in the slope intact rock mass. In field testing, a number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the slope stability and blasting cumulative effects. Therefore, FBG sensors are a useful tool for measuring in situ strain in intact rock masses.

Validation and implementation of a new method for monitoring in situ strain and temperature in rock masses using fiber-optically instrumented rock strain and temperature strips

Fiber-optically instrumented rock strain and temperature strips (FROSTS) are a new method for monitoring in situ strain and temperature in intact rock masses. FROSTS are an alternative to other techniques such as borehole extensometers or instrumented rock bolts for monitoring strain around underground openings. FROSTS have six fiber-optic strain and temperature gages installed at 30cm intervals along specially designed strips of 304-stainless steel and are embedded into the rock mass. FROSTS are modified Rock Strain Strips (ROSS). If pretensioned prior to installation, the FROSTS can measure both shortening and elongation. Two FROSTS were pretensioned and installed on the 4100 level (~1250m below the surface) of the Sanford Underground Research Facility (SURF) in Lead, SD. Data from the initial monitoring of these FROSTS record strain in the rock mass due to the closure of microcracks and fractures created during drilling for installation and then stabilization of the FROSTS sensors. The fiber-optic temperature sensors on the FROSTS also record a detailed thermal gradient of increasing temperature with depth into the rock mass. Laboratory experiments show that the FROSTS accurately measure strain, are more compliant than the rock mass they are embedded into, and thoroughly couple to the rock mass to ensure accurate readings. FROSTS provide an accurate and detailed record of in situ strain and temperature in rock masses and are useful for the structural health monitoring of underground spaces.