The Discrete Element Method (DEM) is increasingly used to simulate the behavior of rock. Despite their intrinsic capability to model fracture initiation and propagation starting from simple ...interaction laws, classical DEM formulations using spherical discrete elements suffer from an intrinsic limitation to properly simulate brittle rock behavior characterized by high values of UCS/TS ratio associated with non-linear failure envelopes, as observed for hard rock like granite. The present paper shows that the increase of the interaction range between the spherical discrete elements, which increases locally the density of interaction forces (or interparticle bonds), can overcome this limitation. It is argued that this solution represents a way to implicitly take into account the degree of interlocking associated to the microstructural complexity of rock. It is thus shown that increasing the degree of interlocking between the discrete elements which represent the rock medium, in addition to enhancing the UCS/TS ratio, results in a non-linear failure envelop characteristic of low porous rocks. This approach improves significantly the potential and predictive capabilities of the DEM for rock modeling purpose. A special emphasis is put on the model ability to capture the fundamental characteristics of brittle rocks in terms of fracture initiation and propagation. The model can reproduce an essential component of brittle rock failure, that is, cohesion weakening and frictional strengthening as a function of rock damage or plastic strain. Based on model predictions, it is finally discussed that frictional strengthening may be at the origin of the brittle ductile transition occurring at high confining pressures.
The investigation of rock damage behaviour is an important requirement for ensuring stability control and safety prediction in rock engineering. In this study, based on the linear energy dissipation ...law and energy dissipation coefficient, a theoretical method is introduced for characterising the damage of intact rocks under uniaxial compression conditions. The existing uniaxial compression test data of 14 kinds of rock materials (including 6 types of granite, 4 types of sandstone, 2 types of marble, 1 type of slate, and 1 type of limestone) were used to verify the effectiveness of the damage characterization method. The results indicate that the damage evolution process of rocks under uniaxial compression can be fully expressed by the proposed theoretical damage characterisation method, and the damage variable exhibits a quadratic functional relationship with the loading stress level. Moreover, with the linear energy storage law serving as a theoretical basis, the peak dissipated strain energy in a rock damage expression under uniaxial compression can be accurately calculated. The new theoretical damage characterisation method overcomes the shortcomings of conventional damage characterisation methods (e.g. estimating the peak dissipated strain energy through a hypothesis), and provides a new means for analysing rock damage from an energy viewpoint.
The constant
m
i
is a fundamental parameter required in the Hoek–Brown (H–B) failure criterion for estimating rock strength. Triaxial tests for the calculation of this constant are time-consuming and ...expensive. In this paper, a new perspective is presented for the physical meaning of the parameter
m
i
of the H–B model in consideration of the contact friction coefficient. A correlation between the contact friction coefficient and
m
i
is established using published data. A practical approach is proposed to determine the value of the parameter
m
i
. In addition, previously proposed methods of estimating the residual strength of rock are reviewed. A novel method based on the H–B failure criterion is established to predict the residual strength of rock. The single model parameter used in the new model controls the residual strength nonlinearity. The strengths predicted by the proposed method for limestone, granite, slate, and sandstone are in good agreement with those measured during laboratory tests. The corresponding errors are within a range of 15%. This method is applied to an actual rock mass around a tunnel in the Hanjiang to Weihe River Project of China.
•Advance in the rock porosity estimation by Infrared Thermography is presented.•Differently shaped and sizes specimens are tested to bring statistics to IRTest.•The rock volume affects the Cooling ...Rate Index.•Porosity prediction equations for each considered rock shape and size are provided.
The prediction of rock porosity through infrared thermography holds a high potential for non-destructive testing procedures for natural stones, but at the same time there are still few scientific experiences on its applicability on different rock types. In fact, although international standards on laboratory rock characterization allow the use of differently shaped and sized specimens, data available in the literature concern the application of this novel procedure only on cubical rock specimens, with promising results even in the perspective of a potential future standardization of the test. In this paper, motivated by the need for non-destructive analysis of construction and cultural heritage materials, 143 rocks were analyzed to study the reliability of infrared thermography on specimens with different geometries. Results demonstrate that the cooling rate within the first 10 min of test remains the most suitable index for the prediction of rock porosity. This physical property was further analyzed by separately considering the normal and the effective porosities against the rock cooling speed. The best statistical correlations were found for the prediction of total porosity, although satisfactory trends were achieved also for indirectly estimating effective porosity. According to four different sets of specimens, prediction equations were developed from statistical analysis. Achieved outcomes argue strongly for additional scientific research on this prospective test, with the goal to define a standardized, non-destructive, and quick alternative to the common procedures currently used in laboratory for measuring porosity.
Simulation of the failure process of non-intact rock containing initial cracks, voids, and other discontinuities is a research hotspot in rock mechanics and geotechnical engineering. The damage smear ...method has attracted wide attention due to its ability to realistically depict the complete failure processes of rock from the microscopic to the macroscopic scale. However, when portraying pre-existed discontinuities, the damage smear method suffers from major difficulties such as mesh dependency and stress singularity. To overcome the limitations of current methodology, this paper proposes a novel non-break modeling strategy inspired by the extended finite element method for dealing with embedded discontinuities, in which the discontinuity of the physical fields across crack faces or the void boundary, as well as the stress singularity at the crack tip, can be characterized by introducing appropriate enrichment functions, and the penalty method is used to impose frictional contact constraints. Applications of the combined method of the improved modeling strategy and damage smear analysis to laboratory-scale and engineering-scale numerical examples show a satisfactory approximation with the analytical solution or experimental observations, indicating that the current modeling strategy can further improve computational accuracy while retaining the advantages of the damage smear method.
In this paper, acoustic emission (AE) energies recorded during 73 uniaxial compression tests on weak to very strong rock specimens have been analyzed by looking at the variations in b-values, total ...recorded acoustic energy and the maximum recorded energy for each test.
Using 3D Particle Flow Code (PFC3D), uniaxial compression tests have been conducted on discrete element models of rocks with various strength and stiffness properties. An algorithm has also been used to record the AE data in PFC3D models based on the change in strain energy upon each bond breakage.
The relation between the total released acoustic energy and total consumed energy by the specimens has been studied both for the real data and numerical models and as a result, a linear correlation is suggested between the released AE energy per volume and consumed energy per volume of the intact rocks.
Comparing the recorded acoustic energies in numerical models with real data, suggestions are made for getting realistic AE magnitudes due to bond breakages (cracks) from PFC3D models by proposing a modification on Gutenberg–Richter formula that has been originally proposed for large scale shear induced earthquakes along faults.
Also, using the numerical model, an attempt has been made to quantify the damage to the intact rock by proposing a damage parameter defined as the total crack surface observed during the tests divided by the total crack surface possible based on size of particles.
The effect of overburden stress on the rock mass deformation modulus is a known issue. However, the effect of overburden stress has been studied less with empirical methods due to the lack of ...appropriate data. In this study, it is aimed to investigate the effect of overburden stress on rock mass deformation modulus using artificial neural network (ANN). Four ANN models have been developed in accordance with the purpose of the study. Two of these models do not contain the overburden stress parameter, but the other two models contain the overburden stress parameter. The prediction performance of the models containing the overburden stress parameter was obtained drastically higher than the others. In other words, the value account for (VAF) and root-mean-square error (RMSE) indices of the model having the inputs of rock mass rating (RMR) and elasticity modulus of intact rock (E
i
) are 73.3% and 462, respectively, while those of the model having the inputs of RMR, E
i
and overburden stress are 90% and 265. The other models developed in the present study yielded similar results. Consequently, with the ANN models developed in this study, the effect of overburden stress on E
m
is revealed, clearly.
As underground excavations are getting deeper and field stresses increase, the behavior of intact rock blocks plays an increasingly important role in understanding and estimating the overall rock ...mass strength. To model the brittle behavior of intact rock blocks, the stress–strain curve is usually idealized considering a linear strength mobilization approach (cohesion-weakening-friction-strengthening, CWFS), however, it is well recognized that rock presents a nonlinear behavior in terms of the confining stress. This study extends the strength mobilization in brittle failure of rock using nonlinear criteria. To determine the model parameters, a standard statistical method that uses the complete laboratory stress–strain curves of the intact rock is employed. Several hypotheses of linear and nonlinear models are statistically compared for different types of rock and confining stress levels. Results demonstrate that the best approach to model the brittle failure of rock is to consider a nonlinear strength envelope, such as the Hoek-Brown criterion assuming a residual uniaxial compressive strength different from zero and a mi parameter that increases, both with simultaneous mobilization. This model helps to recreate high-confining conditions and a more realistic transition between peak and post-peak strength. The obtained parameters are discussed and compared with literature values to verify the validity and to develop guidelines for the estimation of parameters, providing an objective mobilization criterion. Finally, the nonlinear model was applied to a finite element code and extended to a tunnel scale in the brittle rock under high-stress conditions. A reasonable fit between the simulations and the in-situ overbreak measurements was found.
