Brazilian Splitting Research Articles

Experimental Study on the Damage and Failure Characteristics of High-Temperature Granite after Liquid-Nitrogen Cooling

To analyze the influence of liquid-nitrogen cooling on the damage and failure of high-temperature granite, granite samples were heated to 150~600 °C for natural cooling and liquid-nitrogen cooling treatment. Brazilian splitting tests were carried out as the samples returned to room temperature, and basic tensile and energy evolution parameters were obtained. Acoustic emission signal parameters during loading were recorded. The experimental results showed that the heating process caused damage to the granite samples, and liquid-nitrogen cooling further increased the degree of damage. Specifically, the ultrasonic velocity of liquid-nitrogen-cooled samples was lower than that of naturally cooled samples at each heating temperature. With an increase in heating temperature, the AE ring-down counts of liquid-nitrogen-cooled samples were higher than that of naturally cooled samples. At the same heating temperature, the dissipated energy of naturally cooled samples was greater than that of liquid-nitrogen-cooled samples. Liquid-nitrogen cooling could effectively promote the propagation of microcracks inside high-temperature granite and result in a reduction in the mechanical strength of granite, which could be conducive to the efficient fracture of high-temperature rock during fracturing.

Particle flow simulation of Brazilian splitting failure characteristics of layered shale

Shale reservoir is the research hotspot in the development of unconventional oil and gas resources. The properties of bedding plane have an important influence on the mechanical behavior of shale. According to the fracture characteristics of layered shale under Brazilian splitting load, the effects of bedding angle, bond strength ratio and natural fractures on the tensile strength and fracture pattern of layered shale in Brazilian splitting test were studied by particle flow code. The evolution mechanism of meso–macro fracture of shale is analyzed by combining the dynamic change process of particle mesoscopic force chain and crack. The results show that three types of fracture patterns are observed: splitting tensile failure of shale matrix, splitting tensile failure along bedding plane and tensile–shear composite failure along shale matrix and bedding plane. Mesoscopic cracks initiate at the top and bottom loading points of the disk. After the peak load, mesoscopic cracks propagate through and appear dense force chains until macroscopic failure occurs. The bond strength ratio of bedding plane affects the failure pattern and peak strength of the specimen. With the increase in bedding angle, the tensile strength decreases gradually. Considering the influence of natural fractures, the tensile strength of shale is lower, and the crack propagation is more complex. The research model and results provide theoretical basis for the mechanical properties evaluation and fracture pattern analysis of shale.

Time-dependent virtual crack model of rock with application to slope stability

Time-dependent deformation of rock involves the continuous to discontinuous deformation of joints, cracks, holes, and other defects under long-term stress conditions. Thus, it is of great significance to investigate the correlation between internal crack propagation in rock and external time-dependent damage and failure of rock to better understand the long-term stability of engineering rock mass. Here, we put forward a time-dependent virtual crack model to achieve the entire continuous-discontinuous time-dependent deformation progress of rock from the microscopic to the macroscopic scale. Simulated stress-strain curves for granite, including the mode of failure (axial splitting and Brazilian splitting), agree well with experimental data. We also used the virtual crack model to study the stability of one of the slopes of the Xiaowan Hydropower Station (China). The simulation results indicate that the weakly disturbed zone is the potential sliding surface, and that the deformation of the slope is similar to in-situ measurements. In the model, the vertical displacement decreased by 500 mm in 180 days and the axial strain rate decreased from 25 mm/d to zero, suggested that the slope is currently stable.

Acoustic Emission Characteristics of the Water Weakening Effect on Cretaceous Weakly Cemented Sandstone

Rock mass stability is often affected by water–rock interaction in underground engineering construction. Cretaceous sandstones often have weak cementation, low strength and strong water-holding capacity, and their rock mass strength is easily weakened by these activities. In this paper, the uniaxial compressive strength (UCS) and tensile strength (TS) of weakly cemented Cretaceous sandstones from different sedimentary facies under natural and saturated conditions were tested, and the loading process was monitored by the acoustic emission (AE) technique. The results show that the existence of water obviously weakened the mechanical properties of weakly cemented sandstone. The UCS and TS of saturated braided river facies sandstone decreased to 41.24% and 35.95% of their natural states, respectively, while those of desert facies sandstone decreased to 32.90% and 26.98% of their natural states, respectively. The AE characteristics of sandstone from different sedimentary facies were similar during loading due to weakening by water, including a decrease in cumulative AE energy, b-value fluctuation and reduction in the peak frequency distribution range. Fracture in the Brazilian splitting test was mainly due to the rapid initiation and coalescence of microcracks near the peak point. However, in the uniaxial compression test, the macro fractures were caused by many microcracks that occurred continuously during loading and finally connected. The high quartz and low feldspar contents strengthened the mechanical properties of braided fluvial facies sandstone compared to those of desert facies sandstone and lessened the effect of water weakening.

Experimental investigation of temperature effects on the physical and mechanical properties of red sandstone

With the rapid development of transportation in China, the number of tunnels is also increasing. However, tunnels may be affected by fire accidents during operation, and the surrounding rock will be irreparably damaged by high temperature. To investigate the influence of temperature on the physical and mechanical properties of red sandstone, physical properties were determined and the Brazilian splitting test was conducted on red sandstone after thermal treatment from 25 to 600°C. The results show that with increasing temperature, the apparent colour of the red sandstone gradually deepened. The P-wave velocity and Leeb hardness decreased with increasing temperature, and a typical temperature threshold (300°C) was identified. The tensile strength of red sandstone decreased quickly from 25 to 300°C and then decreased slowly from 300 to 600°C. The acoustic emission (AE) signal shows a high degree of consistency with the stress–time curve. As the temperature increased, the AE became more active, and when red sandstone was near failure, the AE signal increased sharply. The failure mode of sandstone mainly consists of a throughgoing main crack and a secondary crack, and the crack width also grows gradually. Moreover, we observe from polarized light microscopy images that the number of fractures in the sandstone also increases as the temperature rises. The variation of physical and mechanical properties of red sandstone is closely related to microstructure. These findings demonstrate that temperature has an obvious weakening effect on the physical and mechanical properties of sandstone, and provide theoretical guidance and significant engineering information for tunnel fire restoration. Thematic collection: This article is part of the Engineering Geology and Hydrogeology of the Anthropocene collection available at: https://www.lyellcollection.org/topic/collections/engineering-geology-and-hydrogeology-of-the-anthropocene

Nonlinear flow and permeability anisotropy characteristics in hydraulic fracturing-induced rock fractures subjected to various normal compressive stresses

Well productivity after hydraulic fracturing is strongly affected by the flow behaviors in created fracture complexity. In this study, gas flow tests were conducted in splitting fractures (induced by Brazilian splitting) and hydraulic fractures (created by hydraulic fracturing) to study the nonlinear fluid flow behaviors in rough fractures. 3D fracture surfaces were first digitally reproduced with a scanner and then the fracture geometry parameters (roughness profile index R_{p}, the root mean square of the fracture profile wall slope Z_{2}), aperture distributions were computed for both fractures. The influence of the fracture geometry characteristics on the nonlinear flow behaviors were systematically analyzed. The permeability anisotropy was numerically examined in detail. The results show that R_{p}, Z_{2} and mean aperture of hydraulic fractures are relatively smaller, indicating that the fracture surfaces are smoother. The transmissivity of splitting fracture is relatively higher than that of hydraulic ones, and the ratio of splitting to hydraulic one varies from 2.51 at sigma_{c} = 1 MPa to 60.95 at sigma_{c} = 40 MPa. By contrast, splitting fractures have larger critical Reynolds number and lower critical pressure gradient, implying that nonlinearity is easier to occur in splitting fractures. The average Forchheimer coefficient for splitting fractures is 0.00577, which is 1.5 times larger than that (0.0041) for hydraulic ones. In addition, the fractures exhibit strong permeability anisotropy and this anisotropic effect is significantly dependent on the loading stress. In general, the permeability anisotropy in hydraulic fracture is more significant, showing that their permeability anisotropy ratios K are larger and more sensitive to the loading. The obtained results confirm that there exist some discrepancies in flow behaviors between splitting and hydraulic fractures and for more accurate evaluation in engineering practice, hydraulic fractures should be fully considered.

Effect of the mineral spatial distribution heterogeneity on the tensile strength of granite: Insights from PFC3D-GBM numerical analysis

The mechanical characteristics of crystalline rocks are affected by the heterogeneity of the spatial distribution of minerals. In this paper, a novel three-dimensional (3D) grain-based model (GBM) based on particle flow code (PFC), i.e. PFC3D-GBM, is proposed. This model can accomplish the grouping of mineral grains at the 3D scale and then filling them. Then, the effect of the position distribution, geometric size, and volume composite of mineral grains on the cracking behaviour and macroscopic properties of granite are examined by conducting Brazilian splitting tests. The numerical results show that when an external load is applied to a sample, force chains will form around each contact, and the orientation distribution of the force chains is uniform, which is independent of the external load level. Furthermore, the number of high-strength force chains is proportional to the external load level, and the main orientation distribution is consistent with the external loading direction. The main orientation of the cracks is vertical to that of the high-strength force chains. The geometric size of the mineral grains controls the mechanical behaviours. As the average grain size increases, the number of transgranular contacts with higher bonding strength in the region connecting both loading points increases. The number of high-strength force chains increases, leading to an increase in the stress concentration value required for the macroscopic failure of the sample. Due to the highest bonding strength, the generation of transgranular cracks in quartz requires a higher concentrated stress value. With increasing volume composition of quartz, the number of transgranular cracks in quartz distributed in the region connecting both loading points increases, which requires many high-strength force chains. The load level rises, leading to an increase in the tensile strength of the numerical sample.

Dynamic constitutive model and ignition behavior of enhanced Al/PTFE

Al/PTFE reactive material is a kind of energetic material with direct damage ability. Numerical simulation is one of the most effective ways to investigate the internal damage and impact induced ignition mechanism. However, there is no suitable constitutive equation to describe the mechanical behavior of the reactive material under the impact load, therefore an appropriate constitutive model of the reactive material is of great significance. In order to obtain an appropriate constitutive model to describe the mechanical properties and impact induced ignition behavior of the material, the enhanced Al/PTFE reactive material was prepared by cold pressing and rapid cooling process. The dynamic mechanical properties of the enhanced Al/PTFE specimens under different strain rates were tested by using the SHPB/SHTB testing systems. And the parameters of the Zhu-Wang-Tang (ZWT) constitutive model were calibrated. Based on ZWT constitutive model, numerical simulation of Flattened Brazilian splitting was carried out. And dynamic compressive/tensile mechanical properties of enhanced Al/PTFE reactive material were obtained by combining theoretical derivation and numerical simulation of ABAQUS/Explicit. The effectiveness of this model is verified by comparing experimental and numerical simulation results. Futhermore, combined ZWT constitutive model with Equation of State (EOS) of ignition and growth, ANSYS/AUTODYN was used to simulate the process that Al/PTFE reactive material impacted the target. The destruction and ignition behavior of the projectile under the interaction of stress wave were explained.

Mode I fracture behavior of heterogeneous granite: Insights from grain-based FDEM modelling

The failure of brittle rock has a significant influence on engineering stability. The purpose of this study is to understand the progressive fracture mechanism of heterogeneous granite from a mesoscopic perspective. In the present work, the fracture process zone (FPZ) model is used to characterize the constitutive relationship of rock cracking in the combined finite-discrete element method (FDEM). Moreover, the breakable grain-based model (bGBM) is established to study the fracture behavior of granite via the semi-circular bending (SCB) tests. To rebuild the meso-structure of granite specimens, an algorithm that considers the mesoscopic grain geometry characteristics and grain contact mechanical properties is utilized. Then, three classic examples, namely, a uniaxial compressive test, a Brazilian splitting test and a semi-circular bending test, are investigated with the numerical model. A good agreement of the load–displacement curves and crack propagation paths can be found between the simulation and laboratory tests. The meso-heterogeneity of granite cause the macroscopic fracture surface of the slit tip to be nearly upright, with local roughness and zigzag. The slit length has a significant effect on the damage distribution range of the SCB specimen, and the peak load and mode I fracture toughness decreases with increasing slit length and supporting span. The accurate measurement of the mode I fracture toughness of SCB specimens needs to consider the FPZ, and the average increment is 39% and 41% under different slit length and supporting span conditions, respectively.

Deterioration Effect of Sandstone Tensile Strength and Its Mesoscopic Mechanism under Dry-wet Cycles

The rock mass in the hydro-fluctuating zone of the reservoir bank slope is under dry-wet cycles for a long time, which will cause the deterioration of rock mass and induce geological disasters. In this study, a series of dry-wet cycle tests on the argillaceous quartz sandstone in the Three Gorges Reservoir area was carried out. Then, after different dry-wet cycles, the sandstone specimens were used to conduct the Brazilian splitting, scanning electron microscope, and 3D laser scanning tests. Herein, we provided detailed physical and microscopy image data to analyze the deterioration effect of tensile strength and mesostructure deterioration process of sandstone. With the increase of dry-wet cycles, the tensile strength of sandstone initially decreases rapidly, and then the decline rate tends to slow down. The evolution laws of fractal dimension and porosity are also significantly consistent with the deterioration of tensile strength. Moreover, further mesostructural analysis has revealed the repeated “absorption and swelling-dehydration and contraction” of clay minerals. This results in the breakage of framework mineral quartz and the expansion and connectivity of internal cracks, which ultimately deteriorates sandstone’s tensile strength.

Study of Mechanical Behavior and Deformation Field Characteristics of Sandstone under Cyclic Loading

The sandstone yield strength and tensile strength were increased under cyclic loading paths by using the Brazilian splitting method. The experiment shows that (1) the sandstone specimens exhibit multiple penetrating crack damage by cyclic loading, while the tensile strength slightly increases; (2) the sandstone horizontal strain (Exx) concentration area decreases, and the shear strain (Exy) concentration area increases under cyclic loading, and the damage form changes from completely brittle tensile damage to ductile mixed tensile-shear damage; (3) the mechanism of the increase in tensile strength of the specimen under cyclic loading was explained by digital image correlation (DIC) technique. The results indicate that a time lag occurs in the horizontal strain concentration zone, and the area of the penetration zone decreases from the cyclic loading effect, while the reduced stress concentration leads to increased tensile strength of the specimens.

Evaluation of Rock Brittleness Index under Dynamic Load

Rock is a typical brittle material, and the evaluation of its brittleness index has important guiding significance for hard rock resource exploitation, unconventional oil and gas resource exploitation, mechanical driving efficiency, rock burst prediction, and dynamic disaster prevention and control. At present, brittleness index often measures the brittleness of rock under static load; thus, whether it is applicable to dynamic load is worth exploring. In this study, static and dynamic uniaxial compression tests and Brazilian splitting tests were carried out on five kinds of rocks, including fine granite, coarse granite, shale, marble, and sandstone, using the INSTRON−1346 test system and split−Hopkinson pressure bar (SHPB), respectively. The brittleness index values of different rocks under static and dynamic load were determined, and the changes in the brittleness of rocks under different loading methods and different strain rates were studied. The definition of brittleness and the applicability of existing brittleness indices were also discussed. It was found that the loading rate amplified the variation of the brittleness characteristics of rock. When static load changes to dynamic load, the brittleness of rocks increases, and the brittleness relationship between different rocks remains unchanged. The more brittle the rock is under static load, the greater the range of brittleness enhancement is under dynamic load. It was also found that the brittleness of sandstone had an obvious effect on the strain rate. The brittleness of rock increases with the increase in strain rate, and the greater the strain rate, the greater the brittleness enhancement degree. These research results can provide reference values for dynamic disaster prevention and safe construction of deep rock projects such as mines and tunnels.

A Hybrid Anisotropic Elastoplastic Model for Layered Rock Mass and Numerical Implementation

An elastoplastic model that exhibits hybrid initial and stress-induced anisotropy is developed for layered rock masses. The formulation considers the thermodynamic aspect, and is consistent and rigorous. Both initial anisotropy and stress-induced deformation anisotropy are reflected by a hybrid anisotropic stiffness matrix influenced by the deterioration development degree (DDD) in different directions. By employing a combination of the strength criterion of rock material and rock bedding plane, an anisotropic failure formulation for layered rock mass has been established. The hybrid anisotropic model has been implemented in cellular automata software for the engineering rock mass fracturing process (CASRock). The performance of the anisotropic part is demonstrated by reproducing the deformation and failure characteristics of initial or stress-induced anisotropic behaviors for layered rocks under uniaxial, conventional triaxial, and true triaxial compression and Brazilian splitting conditions. Important features, such as the strength, mechanism, deformation, DDD, and fracturing process variation, can be captured by the proposed model. In addition, a numerical simulation of tunnel excavation in a layered rock mass is performed to study the anisotropic excavation-induced damage zone (EDZ) distribution in the field. The results indicate that the model is able to reproduce the observed failure mode satisfactorily.

Stress Relief and Permeability Enhancement with Hydraulic Fracturing in Overlying Key Strata of Deep and Soft Coal Seams.

The technical problems of high in situ stress, high gas pressure, high gas content, and low coal seam permeability are widespread in deep soft coal seam excavation, which leads to frequent occurrences of dynamic phenomena, such as coal cannons and blowout holes. Based on the high-pressure hydraulic fracturing technology and process, this study puts forward a new technology of gas drainage in deep and soft coal seams by fracturing the overlying key strata to cut off the stress transmission path among coal and rock strata. According to the theories of key layer and masonry beams, the distribution locations of the main and subkey strata are determined, and based on uniaxial compression and Brazilian splitting experiments, the mechanical parameters of key stratum were tested. Combined with the results of numerical simulation and field test, initial pressure and fracturing radius of hydraulic fracturing technology for overlying key strata were determined, the stress relief effect and permeability variation law of coal seam after hydraulic fracturing in the main and subkey strata were analyzed, and then technical schemes for simultaneous fracturing of the key layer were designed. Field application results showed that the stress concentration phenomenon in soft coal seam excavation had been alleviated, and the stress relief effect of coal seam and the permeability were increased obviously. The volume and concentration of gas drainage were increased by 10 and 11%, respectively, the gas amount by 1.22 times; the frequency of dynamic phenomena such as coal cannons decreased by 95%, the gas concentration in return air flow by 20% during mining processes. This paper provided an innovative technical idea and process for gas control in deep and soft coal seam excavation, which could effectively solve the common and difficult problems about frequent occurrences of excessive gas concentration and dynamic phenomena.

Tensile mechanical properties and fracture evolution characteristics of sandstone containing parallel pre-cracks under dynamic loading

Parallel cracks have a significant influence on the dynamic tensile failure behavior of rock. The systematic investigation of dynamic tensile mechanical properties and fracture evolution characteristics of fractured rocks are significant for guiding the safe excavation and stability sustaining of rock engineering. Two groups of dynamic Brazilian splitting tests were conducted for the sandstone specimens containing parallel pre-cracks. The influences of crack inclination angle (Group A) and rock bridge length (Group B) on the tensile mechanical properties and fracture evolution characteristics were analyzed. Results showed: the dynamic peak load of pre-cracked specimens is significantly lower than that of intact specimens. The peak load presents a V-shaped evolution trend with the growth of crack inclination angle. Differently, the dynamic peak load increased monotonically with the increasing of rock bridge length. In addition, the crack initiation points move from crack tips to crack walls when the crack inclination angle is increasing. When rock bridge length is large enough, the crack propagation paths are observed near the central axis of specimens. By utilizing the DIC method, six different failure modes are identified to explain the crack initiation and propagation mechanisms under dynamic loading. The tensile-shear fracture modes are mainly presented for specimens with different crack inclinations. Differently, the ultimate failure modes change from tensile-shear mode to tensile mode when the rock bridge length is increasing. In addition, numerical results (ANSYS/LS-DYNA) are in good agreement with the experimental results as a whole, which can verify and explain the experimental results. This study is expected to provide guidance for controlling fractured rock masses.

Determination of Rock Tensile Strength Based on Arc Loading Splitting Test considering Modulus Effect

Tensile strength is an important control parameter for the design and stability analysis in rock engineering. In order to reveal the influence of elastic modulus on tensile strength test results, Brazilian splitting tests were carried out on specimens with different strengths, such as mudstone, white sandstone, green sandstone, red sandstone, and marble. Longitudinal and transverse strains were monitored to analyze the influence of elastic modulus on the maximum contact angle and peak contact load between head and disc. A numerical model of Brazilian splitting tests was built, and the contact load between the head and rock disc were detailed revealed. Based on the experimental and numerical results, a modified formula to calculate tensile strength considering the modulus effect was proposed theoretically. Results show that the maximum pressure between the head and disc increases and the contact angle decreases with the increasing of elastic modulus, which show great influence on the splitting failure; the distribution of contact load can be divided into three types: parabolic I, sinusoidal, and parabolic II, which are suitable for soft rock, medium-hard rock, and hard rock, respectively; the new models can improve the calculation accuracy of rock tensile strength.

An Innovative Method to Analyze the Hydraulic Fracture Reopening Pressure of Hot Dry Rock.

This paper focuses on a new test method and theoretical model for measuring and evaluating the reopening pressure during hot dry rock hydraulic fracturing. Firstly, rock blocks of four lithologies were collected from the hot dry rock strata. Hydraulic fracturing tests at high temperatures in real-time were conducted using drilled cubic specimens and drilled cubic specimens with a pre-crack. Breakdown pressure, reopening pressure, and fracture toughness were measured, respectively. In addition, Brazilian splitting tests at high temperatures in real-time were performed using Brazilian disc specimens to measure tensile strength. Secondly, an empirical equation for evaluating the reopening pressure during hot dry rock secondary fracturing was developed based on fracture mechanics and hydraulic fracturing theory. Third, the values calculated by the new equation, considering breakdown pressure, fracture toughness, and tensile strength, were compared to the values determined by the classical equation and to measurement results. It was found that the new equation predicted closer reopening pressure to the measurement results, regardless of the lithology of the hot dry rock. Moreover, with increasing temperature in the specimens, the error between the value calculated by the new equation and the measurement value remained low. In contrast, the difference between the classical equation predictions and the measurement results was widened. In addition, the reopening pressure was positively correlated with tensile strength and fracture toughness. Variations in lithology and temperature affected tensile strength and fracture toughness, which then changed the hot dry rock reopening pressure.

Master crack types and typical acoustic emission characteristics during rock failure

Acoustic emission (AE) signals contain substantial information about the internal fracture characteristics of rocks and are useful for revealing the laws governing the release of energy stored therein. Reported here is the evolution of rock failure with different master crack types as investigated using Brazilian splitting tests (BSTs), direct shear tests (DSTs), and uniaxial compression tests (UCTs). The AE parameters and typical modes of each fracture type were obtained, and the energy release characteristics of each fracture mechanism were discussed. From the observed changes in the AE parameters, the rock fracture process exhibits characteristics of staged intensification. The scale and energy level of crack activity in the BSTs were significantly lower than those in the DSTs and UCTs. The proportion of tensile cracks in the BSTs was 65%–75%, while the proportions of shear cracks in the DSTs and UCTs were 75%–85% and 70%–75%, respectively. During the rock loading process under different conditions, failure was accompanied by an increased number of shear cracks. The amplitude, duration, and rise time of the AE signal from rock failure were larger when the failure was dominated by shear cracks rather than tensile ones, and most of the medium- and high-energy signals had medium to low frequencies. After calculating the proposed energy amplitude ratio, the energy release of shear cracks was found to exceed that of tensile cracks at the same fracture scale.

Numerical investigation on the effect of intergranular contact bonding strength on the mechanical properties of granite using PFC3D‐GBM

AbstractIn rock materials, the intergranular structure is more affected by various external conditions than the transgranular structure, and then dominates the macroscopic mechanical properties of rocks. In this paper, an innovative three‐dimensional grain‐based model based on Particle Flow Code (PFC3D‐GBM) for reproducing granite crystalline structures was developed. Based on this model, the Brazilian splitting test was conducted to investigate the effects of intergranular contact (IC) bonding strength on the mechanical properties and micro‐cracking behavior of the granite. Additionally, in this paper, the force chains and the orientation distribution of the force chains are shown in the form of three‐dimensional rose diagrams. The results are compared to the numerical results obtained by an initial particle‐based model to demonstrate the superiority of the proposed PFC3D‐GBM. The results indicate that the stress–strain curve of the sample in the Brazilian splitting test can be classified into four distinct stages: initial compaction, linear elasticity, damage, and splitting failure. ICs fracture at a higher rate than transgranular contacts (TCs) under static loading, as evidenced by experimental results of laboratory tests; during the entire loading process, all force chains uniformly distribute in all directions of a sample. As the external load increases, the number of total force chains decreases and the number of cracks increases. The main orientation of the high‐strength force chains is consistent with the external loading direction and is vertical with respect to the main orientation of the cracks. Furthermore, the number of high‐strength force chains increases with the level of external load. The sample's failure mechanism in the Brazilian splitting test is always a tensile failure, and this characteristic is unaffected by the modeling method or the IC bonding strength. Both the tensile strength and peak strain increase as the IC bonding strength increases. In this process, the number of total cracks in PFC3D‐GBM decreases, the proportion of transgranular cracks (Tcs) increases, and the proportion of intergranular cracks (Ics) decreases. The micro‐cracking behavior described above based on the PFC3D‐GBM can be explained more rationally. As the bonding strength of the IC increases, the particles involved in sample deformation are subjected to a greater stress concentration prior to the sample experiencing macro‐fracture, leading to an increase in the number of high‐strength force chains, which is the fundamental reason for the increase of the sample's tensile strength.

Development and Application of Similar Materials for Foundation Pit Excavation Model Test of Metro Station

Geomechanical model tests provide an intuitive and convenient method for observing physical phenomenon due to their easy implementation compared to in situ tests and prototype tests. The success of model tests depends heavily on the appropriate selection of model materials and proportions. Therefore, a new similar material is developed by utilizing the orthogonal experimental design method to conduct a series of proportioning tests. The new material is mixed with barite powder, iron ore powder, quartz sand, liquid paraffin, rosin, gypsum powder, and water. The physical and mechanical properties are studied through uniaxial compressive tests, Brazilian splitting tests, and direct shear tests. The influences of various raw material factors on the parameters of the similar material are systematically studied through range analysis. The results demonstrate that the mechanical parameters of similar materials have wide variation ranges; the adjustment range is 42.0–279.0 MPa for the elastic modulus, 0.37–5.37 MPa for the uniaxial compressive strength and 2.23–2.65 g/cm3 for the density. The new similar material has illustrated advantages in terms of performance stability, low price, and convenient production, which can simulate the similar relationship with different geomechanical model tests. The similar material is applied to a 3D geomechanical model test of the foundation pit excavation of Shenzhen metro station, which proves that the similar material can realistically reflect the change of earth pressure in the excavation process. With the deepening of excavation, the earth pressure curve shows significant fluctuations, and as the retaining structure is displaced, the parts with large earth pressure changes should be strengthened. The research methods and results can provide reference for further geological engineering research.