Rock-like Samples Research Articles

Effect of particle size heterogeneity on fracture toughness and failure mechanism of rocks

Fracture toughness is a key parameter for studying of crack propagation and fragmentation processes in rock structures. It is a comparatively general phenomenon that rocks possess particle size heterogeneity. In the present study, an experimental investigation was made to evaluate the effect of grain size on failure mechanism and fracture toughness of rock and rock-like samples. For this purpose, some Single-Edge Cracked Brazilian Disks (SECBD) Granite specimen and Central Straight-through Crack Brazilian Disk (CSCBD) rock-like specimens were prepared. Diametrical compression load with a rate of 0.3mm/min was applied to the disks and AE sensors were used to monitor the fracturing process of the specimens. Result of laboratory tests indicates that particle size heterogeneity influence on the number of induced fractures, so that under loading sequences, more fractures were induced in the coarse grain specimens compared to the fine grain ones.

STRENGTH AND FRACTURE CHARACTERISTIC OF ROCK MASS CONTAINING PARALLEL FISSURES

In order to investigate the strength characteristic and crack extension law of rock mass containing parallel fissures, uniaxial compression tests of rock-like samples with different fissure angles were conducted combined with the digital photography measurement system. The results show that the degradation of peak strength and elastic modulus of samples is about 10.84%~35.62% and 9.79%~54.17% respectively with the effect of preset fissures. With the increase of the fissure angle, the peak strength of samples decreases first and then increases, forming a typical characteristic of “U” type, and it reaches the minimum value as θ is 45°. The failure form is closely related to the fissure angle. The extending types of secondary cracks are mainly inclined cracks and central cracks when the joint angle is 15° or 30°, and the final instability fracture mode is due to the inclined cracks. However, when the joint angle is 45°, 60° or 75°, the extending types of secondary cracks are mainly wing cracks and coplanar cracks, and the final instability fracture mode mainly depends on the coplanar cracks.

The role of flaws on crack growth in rock-like material assessed by AE technique

Experiments for the rectangular rock-like samples (made of high-strength gypsum and water–cement ratio is 1) with two parallel pre-existing flaws subjected to uniaxial compression were carried out to further investigate the influence of varying flaw geometries on mechanical properties and crack coalescence behaviors. According to the tests results, eight crack types were characterized on a basis of the mechanisms of crack nucleation, formation and propagation, and seven coalescence modes occurred through the ligament, including S-mode, M1-mode, M2-mode, M3-mode, T1-mode, T2-mode and T3-mode. The AE and photographic monitoring techniques were adopted to further clarify the procedure of the crack coalescence and failure during uniaxial compression tests and in consequence the whole process of crack emergence, growth, coalescence and failure was recorded in real time. The results of AE technique revealed that the characteristics of acoustic emission energy associate with crack coalescence modes, and AE location method can emphasize the moments of crack occurrences and follow the crack growth until final failure. This study put forward better understanding of the fracture and failure mechanism of underground rock engineering, like rock burst.

A coupled numerical–experimental study of the breakage process of brittle substances

Pre-existing cracks in brittle substances seem to be the main cause of their breakage under various loading conditions. In the present paper, a coupled numerical–experimental analysis of crack propagation, cracks coalescence, and breakage process of brittle substances such as rocks and rock-like samples have been studied. The numerical analyses are accomplished using a numerical code based on the Higher order Displacement Discontinuity Method for Crack (HDDMCR2D) analysis. A quadratic displacement discontinuity variation along each boundary element is assumed to evaluate the Mode I and Mode II stress intensity factors. Based on the linear elastic fracture mechanics theory, the maximum tangential stress criterion (i.e., a mixed mode fracture criterion) is implemented in the HDDMCR2D code for predicting the crack initiation and its direction of propagation (cracks propagation path). Some numerical and analytical problems in finite and infinite planes are solved numerically by the proposed numerical method, and the results are compared in different tables illustrating the accuracy and validity of the numerical results. Experimental tests are also being done to evaluate the final breakage path and cracks initiation and cracks coalescence stresses in rock-like specimens containing two random cracks. The numerical and experimental results obtained from the tested specimens show a good agreement between the corresponding values and demonstrate the accuracy and effectiveness of the approach.

Experimental and Numerical Research on 3D Crack Growth in Rocklike Material Subjected to Uniaxial Tension

AbstractUniaxial tension experiments on precracked rocklike samples and the related numerical simulations using FRANC3D were performed to investigate the growth mechanism of three-dimensional (3D) cracks in rocks under tension as well as the strength and failure behavior of samples. Experimental results indicate that the geometric characteristics of preexisting cracks, including crack dip angle, crack spacing, and crack intensity, have significant effects on the strength and failure modes of the samples. Failure of samples resulted from the propagation of a single crack and the coalescence of multiple cracks if the spacing between adjacent cracks is less than the length of the cracks. Distribution of mixed-mode stress-intensity factors (SIFs) and energy-release rates (ERRs) along the fronts of cracks with different geometric characteristics was investigated numerically, and the results can provide an interpretation for the experimental results. Based on numerical simulations of the propagation processes o...

Application of the expanded distinct element method for the study of crack growth in rock-like materials under uniaxial compression

The expanded distinct element method (EDEM) was used to investigate the crack growth in rock-like materials under uniaxial compression. The tensile-shear failure criterion and the Griffith failure criterion were implanted into the EDEM to determine the initiation and propagation of pre-existing cracks, respectively. Uniaxial compression experiments were also performed with the artificial rock-like samples to verify the validity of the EDEM. Simulation results indicated that the EDEM model with the tensile-shear failure criterion has strong capabilities for modeling the growth of pre-existing cracks, and model results have strong agreement with the failure and mechanical properties of experimental samples. The EDEM model with the Griffith failure criterion can only simulate the splitting failure of samples due to tensile stresses and is incapable of providing a comprehensive interpretation for the overall failure of rock masses. Research results demonstrated that sample failure primarily resulted from the growth of single cracks (in the form of tensile wing cracks and shear secondary cracks) and the coalescence of two cracks due to the growth of wing cracks in the rock bridge zone. Additionally, the inclination angle of the pre-existing crack clearly influences the final failure pattern of the samples.

Stress Analysis for Rock Mass Failure with Offset Joints

Two-dimensional finite element model was created in this work to investigate the stress distribution within rock-like samples with offset open non-persistent joints under uniaxial loading. The results of this study have explained the fracture mechanisms observed in tests on rock-like material with open non-persistent offset joints (Mughieda and Alzo’ubi, Geotech Geol Eng J 22:545–562, 2004). Finite element code SAP2000 was used to study the stresses distribution within the specimens. Four-noded isoperimetric plain strain element with two degrees of freedom per node, and the three-noded constant strain triangular element with two degree of freedom per node were used in the present study. The results of the present study showed that the tensile stress in the bridge area caused coalescence for specimens with overlapped preexisting cracks (joints) while the coalescence of the secondary cracks, due to shear stress, caused the failure of specimens with non-overlapping cracks.

Study of the remediation of a site contaminated with arsenic

Soil and rock-like material was collected from a former industrial site (arsenic factory) in order to study the degree to which the site is contaminated. Characterisation of the soil and rock-like samples indicated that the site is highly contaminated with arsenic: the soil material contains 2.5–3.5% (dry wt.%) of arsenic, the rock-like material up to 20%. According to Flemish norms ( VLAREBO, 1996) concerning the contamination of soils for industrial sites, remediation of the soil is required on a short-term basis when a value of 300 mg As/kg of dry solid is exceeded. In this paper, solidification/stabilisation with cement and/or lime for remediation of the site was studied. If later the solidified site material is landfilled, one of the criteria is that the arsenic concentration in the leachate, determined according to the DIN 38 414 S4 extraction test ( Deutsches Institut für Normung, 1984), must be lower than 1 mg/l ( VLAREM II, 1995). Solidification proved to be an efficient method to lower the amount of arsenic leached from the material to below the norm of 1 mg/l. The results of a semi-dynamic leach test indicated that arsenic was released very slowly from the solidified material: over the period of the leach test only 0.10% of the arsenic initially present was leached.