Field Of Rock Mechanics Research Articles

Estimating the three-dimensional joint roughness coefficient value of rock fractures

Measurement and estimation of the joint roughness coefficient (JRC) is a critical but also difficult challenge in the field of rock mechanics. Parameters for estimating JRC based on a profile derived from a fracture surface are generally two-dimensional (2D), where a single or multiple straight profiles derived from a surface cannot reflect the roughness of the entire surface. It is therefore necessary to derive the three-dimensional (3D) roughness parameters from the entire surface. In this article, a detailed review is made on 3D roughness parameters along with classification and discussion of their usability and limitations. Methods using Triangulated Irregular Network (TIN) and 3D wireframe to derive 3D roughness parameters are described. Thirty-eight sets of fresh rock blocks with fractures in the middle were prepared and tested in direct shear. Based on these, empirical equations for JRC estimation using 3D roughness parameters have been derived. Nine parameters (θs, θg, θ2s, SsT, SsF, Van, Zsa, Zrms, and Zrange) are found to have close correlations with JRC and are capable of estimating JRC of rock fracture surfaces. Other parameters (Zss, Zsk, Vsvi, Vsci, Sdr and Sts) show no good correlations with JRC. The sampling interval has little influence when using volume and amplitude parameters (Van, Zsa, Zrms, and Zrange) for JRC estimation, while it influences to some extent when other parameters (θs, θg, θ2s, SsT and SsF) are used. For their easy calculation, the equations with amplitude parameters are recommended to facilitate rapid estimation of JRC in engineering practice.

In plane loaded masonry walls: DEM and FEM/DEM models. A critical review

This work is dedicated to the assessment of the nonlinear behaviour of masonry panels with regular texture and subject to in-plane loads, by means of numerical pushover analysis and an analytical homogenized model. Two numerical models are considered and adopted for performing a set of numerical tests: a discrete model developed by authors and a discrete/finite element model frequently adopted in rock mechanics field and effectively extended to masonry structures. In both models the hypotheses of rigid blocks and elastic–plastic joints following a Mohr–Coulomb yield criterion are adopted. The aim of this work is twofold: (1) a comparison and a calibration of the numerical models, evaluating their effectiveness in determining ultimate loads and collapse mechanisms of masonry panels, by assuming a nonlinear homogenized model for regular masonry as reference solution; (2) the evaluation of sensitivity of masonry behaviour and numerical models to panel dimension ratio and to varying masonry texture. In a first case study, sliding collapse mechanisms changing to overturning collapse mechanisms for increasing panel and block height-to-width ratio are obtained and the results given by the numerical models turn out to be in good agreement. Furthermore, a second case study, dedicated to square panels supported at base ends and vertically loaded, shows different ‘arch mechanisms’ depending on block height-to-width ratio.

Coupling between the statistical damage model and permeability variation in reservoir sandstone: Theoretical analysis and verification

Reservoir sandstone contains pre-existing weaknesses, and its fluid flow properties may be altered when subjected to mechanical loading. Simulation of the complete deformation process of reservoir sandstone and its fluid transportation is important in the fields of rock mechanics and gas production. Based on the statistical damage theory with consideration of void volume changes and fluid pressure, a new constitutive model was developed to describe the mechanical characteristics of saturated porous media in geo-materials. On the basis of the Weibull distribution for rock micro-strength, the damage variable was defined and a semi-analytical permeability variation model was established. Experimental results demonstrated that the theoretical model was in good agreement with the test data during rock deformation, and the permeability variation is a function of the stress-induced damage. Furthermore, the applicability of the rock failure criterion based on the Drucker-Prager criterion and the energy release principle is discussed. Visible influences of void volume change and fluid pressure in the damage variable calculation were also detected.

Experimental analysis and characterization of damage evolution in rock under cyclic loading

To investigate the influence of intermittent opening density on the mechanical behavior and fracture characteristics of rock, sand 3D-printed specimens containing different opening numbers are prepared using quartz silica sand and furan resin adhesive as printing substrates. Uniaxial compression testing is performed on these rock-like specimens. Based on digital image correlation (DIC) technology, a methodology for identifying crack types and evaluating rock damage is proposed to quantify the mechanical mechanism of crack evolution during loading. The experimental results are verified and explained using the rock failure process analysis (RFPA) code. The results show that with increasing intermittent opening density, the stress–strain curves of the specimens are similar. Meanwhile, the increase in intermittent opening density causes the stress field distribution of the specimen to change, which in turn leads to a decrease in the uniaxial compressive strength and elastic modulus. The types of cracks around the opening can be grouped into tensile cracks (T) and shear cracks (S), and the coalescence modes of the rock bridge between the openings can be grouped into shear coalescence (C1) and traction coalescence (C2). As the intermittent opening density increases, the coalescence type between openings changes from C1 to C2, and the overall failure pattern is converted from tensile-shear failure to tensile failure. The damage factor (Df) evolution is closely related to the intermittent opening density of the specimens. A one-way exponential decay correlation between the stress ratio and the damage factor is found. The above study is expected to deepen the understanding of the deformation and fracture mechanism of rock masses with openings and to provide a reference for the application of 3D printing technology in the rock mechanics field.

Measures for controlling large deformations of underground caverns under high in-situ stress condition – A case study of Jinping I hydropower station

The Jinping I hydropower station is a huge water conservancy project consisting of the highest concrete arch dam to date in the world and a highly complex and large underground powerhouse cavern. It is located on the right bank with extremely high in-situ stress and a few discontinuities observed in surrounding rock masses. The problems of rock mass deformation and failure result in considerable challenges related to project design and construction and have raised a wide range of concerns in the fields of rock mechanics and engineering. During the excavation of underground caverns, high in-situ stress and relatively low rock mass strength in combination with large excavation dimensions lead to large deformation of the surrounding rock mass and support. Existing experiences in excavation and support cannot deal with the large deformation of rock mass effectively, and further studies are needed. In this paper, the geological conditions, layout of caverns, and design of excavation and support are first introduced, and then detailed analyses of deformation and failure characteristics of rocks are presented. Based on this, the mechanisms of deformation and failure are discussed, and the support adjustments for controlling rock large deformation and subsequent excavation procedures are proposed. Finally, the effectiveness of support and excavation adjustments to maintain the stability of the rock mass is verified. The measures for controlling the large deformation of surrounding rocks enrich the practical experiences related to the design and construction of large underground openings, and the construction of caverns in the Jinping I hydropower station provides a good case study of large-scale excavation in highly stressed ground with complex geological structures, as well as a reference case for research on rock mechanics.

The Dynamic Fracture Process in Rocks Under High-Voltage Pulse Fragmentation

High-voltage pulse technology has been applied to rock excavation, liberation of microfossils, drilling of rocks, oil and water stimulation, cleaning castings, and recycling products like concrete and electrical appliances. In the field of rock mechanics, research interest has focused on the use of high-voltage pulse technology for drilling and cutting rocks over the past several decades. In the use of high-voltage pulse technology for drilling and cutting rocks, it is important to understand the fragmentation mechanism in rocks subjected to high-voltage discharge pulses to improve the effectiveness of drilling and cutting technologies. The process of drilling rocks using high-voltage discharge is employed because it generates electrical breakdown inside the rocks between the anode and cathode. In this study, seven rock types and a cement paste were electrically fractured using high-voltage pulse discharge to investigate their dielectric breakdown properties. The dielectric breakdown strengths of the samples were compared with their physical and mechanical properties. The samples with dielectric fractured were scanned using a high-resolution X-ray computed tomography system to observe the fracture formation associated with mineral constituents. The fracture patterns of the rock samples were analyzed using numerical simulation for high-voltage pulse-induced fragmentation that adopts the surface traction and internal body force conditions.

A Modified Overstress Model to Simulate Dynamic Split Tensile Tests and Its Experimental Validation

The tensile strength of rock and rock-like materials is universally acknowledged to be much lower than their compressive strength and shearing strength (Meyers and Chawla 1999). Furthermore, the stability and reliability of rock structures are correlated remarkably well with the dynamic tensile strength of rock materials. Therefore, dynamic split tensile tests under high strain rate loads have been an active area of investigation in the field of rock mechanics. Carneiro (1943) first used Brazilian disc specimens to determine the tensile strength of brittle materials. Wang et al. (2004) improved these experiments by using flattened Brazilian discs (FBDs) instead to avoid stress concentration at the loading end. Wang’s method ensures that the initial crack only occurs in the central part of a specimen, which is vital for the validation of a split test. Recently, the split Hopkinson pressure bar (SHPB) has come into use as a general apparatus to determine the dynamic tensile strength of rock-like materials (Saksala et al. 2013; Xu et al. 2014). However, among the failure processes, crack propagation is difficult to observe, which makes it very difficult to judge whether or not the initial crack starts from the center. Finite element methods (FEMs) are widely used to simulate failure in brittle materials (e.g., Hang et al. 2015; Saksala et al. 2015). Compared with laboratory experiments, it is much easier and cost-effective to observe crack propagation in a simulation as long as an appropriate material model is built for the specimens. In this article, we build 3D models of FBD specimens in an SHPB apparatus and use these models to conduct numerical simulations of rock dynamic split tests. A modified overstress constitutive model is applied to describe the rock-like material. The failure patterns resulting from dynamic split tensile tests, as well as crack initialization and propagation, are studied with these simulations. The simulation results are verified by laboratory experiments using the SHPB and a high-speed camera.

The brittleness indices used in rock mechanics and their application in shale hydraulic fracturing: A review

Abstract Brittleness is commonly used to characterize the possible failure features in rocks, quantified by the brittleness index (BI(26)BIs, the applicability of each BI should be well understood to enable the selection of the most suitable for each application. This study reports on a detailed review of existing BI definitions in the rock mechanics field, the transition from brittle to ductile and the application of BIs to shale fracturing. The success of shale gas recovery using hydraulic fracking is greatly dependent on the shale's brittleness, since brittle shales have many pre-existing fractures and are easy to fracture in tensile and shear modes. A combination of laboratory and geophysical approaches are recommended for shale brittleness quantification. Precise quantification of brittleness is important, both in the laboratory and the field. Brittleness indices based on the elastic moduli (Young's modulus and Poisson's ratio) and mineral composition are common in field applications, and can be derived from both laboratory tests and field log data.

Numerical analysis on scale effect of elasticity, strength and failure patterns of jointed rock masses

It is of great importance to study the failure process and scale effect of jointed rock mass in the field of rock mechanics and mining engineering. In the present paper, initially the uniaxial compression test on granite was performed and acoustic emission (AE) sequence was acquired during the compression process in laboratory. Results from numerical simulations using the particle flow code in two dimensions (PFC2D) were presented, and compared with experimental measurements. It was observed that the approach was reasonably good in predicting the real response of granite rock samples. The mechanical parameter of joint model was then calibrated based on PFC2D model with experimental results. Finally the mechanical properties of complex rocks with discrete fracture network (DFN) were studied and scale effects on the elasticity and strength were then investigated. The result showed that the failure pattern was similar when the ratio of joint contact bond strength (both shear and normal) to rock contact bond strength was in the range of 3~9%. The elastic modulus and strength parameters were changed with the sizes of rock sample for DFN models. Moreover, the variation of rock failure pattern under different sizes was also studied and finally the representative elementary volume (REV) size of the considered rock mass was estimated to be 9 × 9 m. It is suggested that the failure pattern analysis should be considered in the REV study of jointed rock mass.

Thermomechanical Analysis of Different Types of Sandstone at Elevated Temperature

Many centuries ago, humans used to build their houses using wood, but due to natural hazards such as fire, humans have now shifted to the concept of building their houses and monuments using rock materials such as concrete, sandstone, bricks, etc. Managing fires or extinguishing them in time, the change in the physical properties of stone building materials at elevated temperature, and the variation in the strength and deformation of stone building materials with change in temperature remain debatable issues. Understanding the effect of fire on stone building materials such as sandstones would be helpful to evaluate whether fire-damaged monuments can be strengthened or should be demolished, or whether damaged parts should be replaced. For centuries, sandstone has been used not only in India but all over the world for several purposes. Widescale application of sandstone can be seen in different monuments, temples, and buildings in India, i.e., the Red Forts of Delhi and Agra, places and buildings in Fatehpur Sikri, Kota, Jaipur, Jodhpur, Buddhist Rameswaram temples in south India, the Indian parliament house, the presidential house, the supreme court building, etc. Globally, the White House in the USA is made of Aquia Sandstone, and Ayres rock (Uluru) in Australia, the world’s largest monolithic sacred site, is made of kosic sandstone (Allison and Bristow 1999; Allison and Goudie 1994; BRE 1945; Hajpal 1999, 2002a, b; Chakrabarti 1993; Chakrabarti et al. 1996; Dorn 2003; Fitzner et al. 2003; Goudie et al. 1992; Alm 1985). In particular, India has witnessed several fire incidents recently (Gorton Castle, Shimla 2014; Connaught Place, Delhi 2014; Surat, Gujarat 2014). Hence, the effect of temperature on the strength and deformation behavior of stone building materials such as sandstones needs to be studied in detail before construction of any monuments using these materials. Under the influence of high temperatures but below the melting point of sandstone, its microstructure rearranges, new microcracks develop, and preexisting ones become widespread and widen. Meanwhile, various physical and mineralogical changes take place in the rock matrix. After cooling down to air-dried temperature, thermally induced changes become permanent in some of the sandstone matrix. The effect of temperature on the physical and mechanical properties of rock has become an important area of study in the rock mechanics field. Over the last few years, various investigations have been carried out in this field to determine the complex behavior of rock under thermomechanical (TM) coupling (Alshayea et al. 2000; Hudson et al. 2005; Cheng and Arson 2014; Heuze 1983; Jaeger et al. 2007; Tian et al. 2012). That work measured the variation of physicomechanical parameters such as the modulus of rock deformation, Poisson ratio, tensile strength, compressive strength, cohesion, internal friction angle, viscosity, thermal expansion coefficients, etc. with temperature (Brede 1993; Xu et al. 2008; Somerton 1992; Dwivedi et al. 2008). & P. K. Gautam pgautam.embedded@gmail.com; neurogeneticamit@gmail.com

Failure Characteristics of Rock-Like Material with Multi-Fissures under Uniaxial Compression

Mechanical behavior and failure mode of jointed rock is one of the significant researches in rock mechanics field. In this work, combined with similar material testing and discrete element numerical method(PFC) to investigate the mechanical behavior and failure mode of the rock-like materials with multi-fissures. The numerical analyses agree well with physical experimentation. It is found that, fissures will weaken the strength of the rock-like material, and when the angle of the fissures is about 25°, the strength of the material reaches a minimum value. The weakening effect of fissure on specimen strength would decrease gradually along with the increase of fissure angle. Compared with the effects of fissure angle, the influence of cracks number to the strength is relatively small. The fissure inclination angle was the main factor of the failure modes. With the different fissure inclination angles, the crack tip of Micro-cracks presents different developmental pattern. However, the influence of fissure distribution density on the failure mode mainly reflects at the fracture penetration mode.

Applications of Discrete Element Method in Modeling of Grain Postharvest Operations

Grain kernels are finite and discrete materials. Although flowing grain can behave like a continuum fluid at times, the discontinuous behavior exhibited by grain kernels cannot be simulated solely with conventional continuum-based computer modeling such as finite-element or finite-difference methods. The discrete element method (DEM) is a proven numerical method that can model discrete particles like grain kernels by tracking the motion of individual particles. DEM has been used extensively in the field of rock mechanics. Its application is gaining popularity in grain postharvest operations, but it has not been applied widely. This paper reviews existing applications of DEM in grain postharvest operations. Published literature that uses DEM to simulate postharvest processing is reviewed, as are applications in handling and processing of grain such as soybean, corn, wheat, rice, rapeseed, and the grain coproduct distillers dried grains with solubles (DDGS). Simulations of grain drying that involve particles in both free-flowing and confined-flow conditions are also included. Review of the existing literature indicates that DEM is a promising approach in the study of the behavior of deformable soft particulates such as grain and coproducts, and it could benefit from the development of improved particle models for these complex-shaped particles.

Rheological behaviour and applicability of viscoelastic model for wave propagation in non-homogeneous polymer rods

Purpose – This purpose of this paper is to discuss certain aspects of five-parameter viscoelastic models to study harmonic waves in the non-homogeneous polymer rods of varying density. There are two sections of this paper, in first section, the rheological behaviour of the model is discussed numerically and then it is solved analytically with the help of Friedlander Series using Eikonal equation of optics. In another section, the applicability of the developed model is studied through harmonic wave propagation in polymer non-homogeneous rods. The authors have used linear partial differential equations for finding the dispersion equation of harmonic waves in the polymers. All the cases taken in this study are discussed analytically and numerically with MATLAB. Design/methodology/approach – A five-parameter viscoelastic model constituting of three dash-pots D 2(η 2), D 2′(η 2′), D 3(η 3) and two springs S 1(G 1), S 2(G 2) is considered. Where, G 1, G 2 are the modulli of elasticity and η 2, η 2′, η 3 are the Newtonian viscosity coefficients of the considered model, respectively. The parameters of the model are non-homogeneous in nature, i.e. module of elasticity and viscosity coefficients are space dependent. 1D problem is formed by taking the material in the form of rod of inhomogeneous polymer material by taking one end at x=0. The co-ordinate x is measured positive in the direction of the axis of the filament. Findings – When the density, rigidity and viscosity all are equal for the first material specimen, the sound speed is constant, i.e. non-homogeneous has no effect on speed and phase of the wave is given. So it becomes the case of semi non-homogeneous medium (a medium when characteristics are space dependent while the speed is independent of space variable). The use of five-parameter models is mostly restricted in the field of rock mechanics. Thus, these models can be used in determining the time-dependent behaviour of a polymer medium. Originality/value – The paper is original and it is not published elsewhere.

Estimation of the Rock Mechanical Properties Using Conventional Log Data in North Rumaila Field

Hydrocarbon production might cause changes in dynamic reservoir properties. Thus the consideration of the mechanical stability of a formation under different conditions of drilling or production is a very important issue, and basic mechanical properties of the formation should be determined.There is considerable evidence, gathered from laboratory measurements in the field of Rock Mechanics, showing a good correlation between intrinsic rock strength and the dynamic elastic constant determined from sonic-velocity and density measurements.The values of the mechanical properties determined from log data, such as the dynamic elastic constants derived from the measurement of the elastic wave velocities in the material, should be more accurate than that determined by direct strength tests with core samples. This can be attributed to the scale effect and sampling disturbances.The aim of this study was to present methods of determining measures of some mechanical properties, from available well log data (conventional sonic, density, and gamma ray) for a well in North Rumaila field.The mechanical properties include formation strength and Poisson’s ratio. For the formation strength, combined elastic modulus (Ec) and shear modulus (G) were determined. The Poisson’s ratio was determined by using three different techniques to permit the accuracy of their values. The elastic modulus, shear modulus, and Poisson’s ratio were then correlated with depth and effective stress.The results show that combined correlations are important source of the prediction of overpressure zones which represent a major problem encountered in drilling and production process.

Multiphysics processes in partially saturated fractured rock: Experiments and models from Yucca Mountain

The site investigations at Yucca Mountain, Nevada, have provided us with an outstanding data set, one that has significantly advanced our knowledge of multiphysics processes in partially saturated fractured geological media. Such advancement was made possible, foremost, by substantial investments in multiyear field experiments that enabled the study of thermally driven multiphysics and testing of numerical models at a large spatial scale. The development of coupled‐process models within the project have resulted in a number of new, advanced multiphysics numerical models that are today applied over a wide range of geoscientific research and geoengineering applications. Using such models, the potential impact of thermal‐hydrological‐mechanical (THM) multiphysics processes over the long‐term (e.g., 10,000 years) could be predicted and bounded with some degree of confidence. The fact that the rock mass at Yucca Mountain is intensively fractured enabled continuum models to be used, although discontinuum models were also applied and are better suited for analyzing some issues, especially those related to predictions of rockfall within open excavations. The work showed that in situ tests (rather than small‐scale laboratory experiments alone) are essential for determining appropriate input parameters for multiphysics models of fractured rocks, especially related to parameters defining how permeability might evolve under changing stress and temperature. A significant laboratory test program at Yucca Mountain also made important contributions to the field of rock mechanics, showing a unique relation between porosity and mechanical properties, a time dependency of strength that is significant for long‐term excavation stability, a decreasing rock strength with sample size using very large core experiments, and a strong temperature dependency of the thermal expansion coefficient for temperatures up to 200°C. The analysis of in situ heater experiments showed that fracture closure/opening caused by changes in normal stress across fractures was the dominant mechanism for thermally induced changes in intrinsic fracture permeability during rock mass heating/cooling and that fracture shear dilation appears to be less significant. Significant effort was devoted to predicting the long‐term stability of underground excavations under (mechanical) strength degradation and seismic loading, perhaps one of the most challenging tasks within the project. We note that such long‐term strength degradation is actually an example of a chemically mediated process governed by underlying (microscopic) stress corrosion and chemical diffusion processes. In the Yucca Mountain Project, such chemically mediated mechanical changes were considered implicitly through model calibrations against laboratory and in situ heater experiments at temperatures anticipated to be experienced by the rock. A possible future research direction would be to simulate such processes mechanistically in a complete coupled THMC framework where C denotes chemical processes.

Triaxial creep tests and back analysis of time-dependent behavior of Siah Bisheh cavern by 3-Dimensional Distinct Element Method

Time dependent effects or creep behavior of rocks has great importance in further development of knowledge in the field of rock mechanics. An increase of pressure on support system due to creep behavior of rock is one of the most important issues in underground structure with weak surrounding rock mass. In this contribution a time-dependent behavior analysis of Siah Bisheh pumped storage powerhouse cavern with complex geometry being under construction on the Chalus River at the north of Iran were investigated. The cavern surrounded rocks containing Shale, Limestone, Sandstone and igneous rock in major parts is located in Alborz Structural Zone. The cavern is being built in a region that is highly prone to sheared and faulted zones. Therefore, it is essential to analyze and design underground structures to prevent any serious long-term damages in this region. The rock mass may exhibit continuous or discontinuous deformations due to excavation of large underground openings; therefore, deformation include shearing of joints and creep deformation of rock material. Because of the fractured and jointed rock mass, the Discrete Element method used to back analysis the time-dependent behavior of the Siah Bisheh cavern. In addition, triaxial creep tests were performed on rock specimens in order to estimate the time-dependent behavior of rock around the cavern. The creep tests and in situ measurements were employed to estimate parameters of power constitutive creep model being able to model the primary and secondary creep regions of rock masses implemented in the 3-Dimensional Distinct Element Code. Simulation results show good agreement with monitoring data. By excavating the lower stages of the cavern, some instantaneous deformation occurs in displacement–time curve of the crown.

Evolving neural network using a genetic algorithm for predicting the deformation modulus of rock masses

The determination of deformation modulus of rock masses is one of the most difficult tasks in the field of rock mechanics. Due to the high cost and measurement difficulties of in situ tests in modulus determination, the predictive models using regression based statistical methods, back propagation neural networks (BPNN) and fuzzy systems are recently employed for the indirect estimation of the modulus. Among these methods, the BPNN has been reported to be very useful in modeling the rock material behavior, such as deformation modulus, by many researchers. Despite its extensive applications, design and structural optimization of BPNN are still done via a time-consuming reiterative trial-and-error approach. This research focuses on the efficiency of the genetic algorithm (GA) in design and optimizing the BPNN structure and its application to predict the deformation modulus of rock masses. GA is utilized to find the optimal number of neurons in hidden layer, learning rates and momentum coefficients of hidden and output layers of network. Then the result is compared with that of trial-and-error procedure. For the purpose, a database including 120 data sets was employed from four dam sites and power house locations in Iran. Taking advantages of performance criteria such as MSE, MAE, r, proved that the GA-ANN model gives superior predictions over the trial-and-error model.

Numerical Models in Discontinuous Media: Review of Advances for Rock Mechanics Applications

The paper presents a description of the methods used to model rock as discontinuous media. The objective of the work is to bring to the geomechanics community recent advances in numerical modeling in the field of rock mechanics. The following methods are included: (1) the distinct element method; (2) the discontinuous deformation analysis method; and (3) the bonded particle method. A brief description of the fundamental algorithms that apply to each method is included, as well as a simple case to illustrate their use.

Verification of post failure behaviour of rock using closed-circuit servo-controlled testing machine

The analysis of the stress-strain curve is a fundamental aspect in the field of rock mechanics. Most rock masses are not intact but have discontinuities. It is important to study the stress-strain curve beyond the peak failure as to assess the characteristic of intact rock against non-intact rock. However, several difficulties arise in obtaining the complete stress-strain curve. Since most rocks exhibit brittle behaviour, they fail violently and uncontrollably when tested on conventional compression machines. In this study, two series of uniaxial compression test were performed on sandstone samples. The first test was conducted using a 2000 kN MaTest conventional compression testing machine and the results were compared with a 3000 kN Tinius Olsen servo-controlled testing machine. Based on the results obtained from sandstone samples, post-peak failure can be achieved by using the servo-controlled testing machine. Comparison between the modes of failure observed from the tests on both types of machines clearly show that violent fracture is not the intrinsic characteristic of the rock but due to the rapid release of strain energy from the loading machine.

The probabilistic distinct element method

AbstractThe distinct element method (DEM) that appeared in the 70s is now widely used in the field of Rock Mechanics to study the stability of fractured rock masses, the field of application it has been dedicated to. Until now, most of the engineers and researchers who have been using this method with a code such as Udec (Itasca Consulting Group), ignore in their model the uncertainties that come with the physical and mechanical properties of the studied rock masses. Nevertheless, because collected data on rock masses are usually very limited in number, and variable as well as imprecise in nature, investigation on such media is suffering from uncertainty. To take into account the uncertainties in data when carrying numerical models with the DEM and to analyse their effects on model results, we have developed a ‘probabilistic distinct element method’ based on the Udec code. This method allows entering most of the necessary physical and mechanical data under a probabilistic form and returns results also expressed in this form. Its main interest is then to provide a range of probable results instead of a non‐guaranteed unique solution. We have implemented this method by using the technique of approximation of the two first statistical moments to build a probabilistic calculation code named Pudec. It takes into consideration rigid or fully deformable block as well as joint mechanical behaviour and does not consider fluid and heat flow calculations. Copyright © 2007 John Wiley & Sons, Ltd.