• Geomechanics and Engineering
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• v.13 no.3
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• pp.505-515
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• 2017

Based on the collapse characteristics of a shallow rectangular cavity, a three-dimensional failure mechanism which can be used to study the collapsing region of the rock mass above a shallow cavity roof is constructed. Considering the effects of surcharge pressure and surface bolt on the collapsing block, the external rate of works produced by surcharge pressure and surface bolt are included in the energy dissipation calculation. Using variational approach, an analytic expression of surface equation for the collapsing block, which can be used to study the collapsing region of the rock mass above a shallow cavity roof, is derived in the framework of upper bound theorem. Based on the analytic expression of surface equation, the shape of the collapsing block for shallow cavity is drawn. Moreover, the changing law of the collapsing region for different parameters indicates that the collapsing region of rock mass decreases with the increase of the density of surface bolt. This conclusion can provide reference for practicing geotechnical engineers to achieve an optimal design of supporting structure for a shallow cavity.

• Structural Engineering and Mechanics
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• v.69 no.1
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• pp.43-50
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• 2019

In this research, the effect of bedding layer on the tensile failure mechanism of rocks has been investigated using PFC2D. For this purpose, firstly calibration of PFC2d was performed using Brazilian tensile strength. Secondly Brazilian test was performed on the bedding layer. Thickness of layers were 5 mm, 10 mm and 20 mm. in each thickness layer, layer angles changes from $0^{\circ}$ to $90^{\circ}$ with increment of $15^{\circ}$. Totally, 21 model were simulated and tested by loading rate of 0.016 mm/s. The results show that when layer angle is less than 15, tensile cracks initiates between the layers and propagate till coalesce with model boundary. Its trace is too high. With increasing the layer angle, less layer mobilizes in failure process. Also, the failure trace is very short. It's to be noted that number of cracks decrease with increasing the layer thickness. Also, Brazilian tensile strength is minimum when bedding layer angle is between $45^{\circ}$ and $75^{\circ}$. The maximum one is related to layer angle of $90^{\circ}$.

• Geomechanics and Engineering
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• v.31 no.1
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• pp.71-86
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• 2022

In Taiwan, an efficient approach for enhancing the stability of colluvium slopes is the drilled shaft method. For slopes with drilled shafts, the soil arching effect is one of the primary factors influencing slope stability and intertwines to the failure mechanism of the pile-soil system. In this study, the contribution of soil arching effect to slope stability is evaluated using the FEM software (Plaxis 3D) with the built-in strength reduction technique. The result indicates the depth of the failure surface is influenced by the S/D ratio (the distance to the diameter of piles), which can reflect the contribution of the soil arching effect to soil stability. When α (rock inclination angles)=β (slope angles) is considered and the S/D ratio=4, the failure surface of the slope is not significantly influenced by the piles. Overall, the soil arching effect is more significant on α=β, especially for the steep slopes. Additionally, the soil arching effect has been included in the proposed stability charts. The proposed charts were validated through two case studies, including that of the well-known Woo-Wan-Chai field in Taiwan. The differences in safety factor (FoS) values between the referenced literature and this study was approximately 4.9%.

• Smart Structures and Systems
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• v.22 no.4
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• pp.459-468
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• 2018

In this paper, the effect of non-persistent joints was determined on the behavior of concrete specimens subjected to biaxial loading through numerical modeling using particle flow code in two dimensions (PFC2D). Firstly, a numerical model was calibrated by uniaxial, Brazilian and triaxial experimental results to ensure the conformity of the simulated numerical model's response. Secondly, sixteen rectangular models with dimension of 100 mm by 100 mm were developed. Each model contains two non-persistent joints with lengths of 40 mm and 20 mm, respectively. The angularity of the larger joint changes from $30^{\circ}$ to $90^{\circ}$. In each configuration, the small joint angularity changes from $0^{\circ}$ to $90^{\circ}$ in $30^{\circ}$ increments. All of the models were under confining stress of 1 MPa. By using of the biaxial test configuration, the failure process was visually observed. Discrete element simulations demonstrated that macro shear fractures in models are because of microscopic tensile breakage of a large number of bonded discs. The failure pattern in Rock Bridge is mostly affected by joint overlapping whereas the biaxial strength is closely related to the failure pattern.

• Journal of Industrial Technology
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• v.26 no.A
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• pp.109-117
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• 2006

This study is an numerical study of predicting the behavior of anchor embedded in weathered rocks, subjected to uplift loads, about ultimate pullout capacity and the failure mechanism. Factors influencing the behavior of anchors were investigated by reviewing the data about in-situ anchor tests performing numerical modelling with changing the bondage length of anchor, diameter of anchor body and diameter of tenden, and by Correlations between those factors were evaluated to apply them to predict the behavior of anchors. As results of numerical analysis, a linear relationship between bondage length, diameter of anchor body and diameter of tenden with ultimate pullout capacity was obtained on the one hand, from the result of numerical analysis changing the Young's modulus of weathered rock, this parameter was found to inflence to load-displacement and ultimate pullout capacity within the range of 10%, which was mot so significant to affect.

• Smart Structures and Systems
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• v.30 no.1
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• pp.49-62
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• 2022

Experimental and discrete element methods were used to investigate the effects of triple joints lengths and triple joint angle on the failure behavior of rock mass under uniaxial compressive test. Concrete samples with dimension of 20 cm × 20 cm × 5 cm were prepared. Within the specimen, three imbedded joint were provided. The joint lengths were 2 cm, 4cm and 6 cm. In constant joint lengths, the angle between middle joint and other joints were 30°, 60°, 90°, 120° and 150°. Totally 15 different models were tested under compression test. The axial load rate on the model was 0.05 mm/min. Concurrent with experimental tests, the models containing triple joints, length and joint angle are similar to the experiments, were numerical by Particle flow code in two dimensions (PFC2D). Loading rate in numerical modelling was 0.05 mm/min. Tensile strength of material was 1 MPa. The results show that the failure behaviors of rock samples containing triple joints were governed by both of the angle and the length of the triple joints. The uniaxial compressive strengths (UCS) of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. Furthermore, it was shown that the compressive behavior of discontinuities is related to the number of the induced tensile cracks which are increased by decreasing the joint length. Along with the damage failure of the samples, the acoustic emission (AE) activities are excited. There were only a few AE hits in the initial stage of loading, then AE hits rapidly grow before the applied stress reached its peak. In addition, every stress drop was accompanied by a large number of AE hits. Finally, the failure pattern and failure strength are similar in both methods i.e., the experimental testing and the numerical simulation methods.

• Journal of the Korean Geotechnical Society
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• v.33 no.4
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• pp.5-15
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• 2017

Three-hinge failure occurs in a jointed rock slope with a joint set parallel with slope and a conjugate joint set. Limit Equilibrium Method (LEM) and Finite Element Method (FEM) which are commonly used for slope design, are not suitable for evaluating stability against three-hinge failure, and this study performed parametric study to analyze the failure mechanism and to find influence factors causing three-hinge failure using UDEC which is a commercial two-dimensional DEM based numerical program. Numerical analyses were performed for various joint structural conditions and joint properties as well as ground water conditions. It was found that pore water pressure is the main factor triggering the three-hinge failure and the mode of failure depends on friction angle of basal joint and bedding joint set. The results obtained from this study can be used for adequate and economic footwall slope reinforcement design and construction.

• Geomechanics and Engineering
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• v.14 no.4
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• pp.345-354
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• 2018

Currently, layered geogrid method (LGM) is the commonly practiced technique for reinforcement of slopes. In this paper the geogrid-box method (GBM) is introduced as a new approach for reinforcement of rock-soil slopes. To achieve the objectives of this study, a laboratory setup was designed and the slopes without reinforcements and reinforced with LGM and GBM were tested under the loading of a circular footing. The effect of vertical spacing between geogrid layers and box thickness on normalized bearing capacity and failure mechanism of slopes was investigated. A series of 3D finite element analysis were also performed using ABAQUS software to supplement the results of the model tests. The results indicated that the load-settlement behavior and the ultimate bearing capacity of footing can be significantly improved by the inclusion of reinforcing geogrid in the soil. It was found that for the slopes reinforced with GBM, the displacement contours are widely distributed in the rock-soil mass underneath the footing in greater width and depth than that in the reinforced slope with LGM, which in turn results in higher bearing capacity. It was also established that by reducing the thickness of geogrid-boxes, the distribution and depth of displacement contours increases and a longer failure surface is developed, which suggests the enhanced bearing capacity of the slope. Based on the studied designs, the ultimate bearing capacity of the GBM-reinforced slope was found to be 11.16% higher than that of the slope reinforced with LGM. The results also indicated that, reinforcement of rock-soil slopes using GBM causes an improvement in the ultimate bearing capacity as high as 24.8 times more than that of the unreinforced slope.

• Geomechanics and Engineering
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• v.33 no.6
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• pp.597-609
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• 2023

The instability and failure of engineered rock masses are influenced by crack initiation and propagation. Uniaxial compression and acoustic emission (AE) experiments were conducted on cracked sandstone. The effect of the crack's dip on the crack initiation was investigated using fracture mechanics. The crack propagation was investigated based on stress-strain curves, AE multi-parameter characteristics, and failure modes. The results show that the crack initiation occurs at the tip of the pre-fabricated crack, and the crack initiation angle increases from 0° to 70° as the dip angle increases from 0° to 90°. The fracture strength k_cr is derived varies in a U-shaped pattern as β increased, and the superior crack angle β_m is between 36.2 and 36.6 and is influenced by the properties of the rock and the crack surface. Low-strength, large-scale tensile cracks form during the crack initiation in the cracked sandstone, corresponding to the start of the AE energy, the first decrease in the b-value, and a low r-value. When macroscopic surface cracks form in the cracked sandstone, high-strength, large-scale shear cracks form, resulting in a rapid increase in the AE energy, a second decrease in the b-value and an abrupt increase in the r-value. This research has significant theoretical implications for rock failure mechanisms and establishment of damage indicators in underground engineering.

• Tunnel and Underground Space
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• v.13 no.6
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• pp.444-454
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• 2003

Rock fragmentation technique by cutter penetration has widely been used in the mechanical tunnel excavation. Microcracks propagate and interact because of locally concentrated high stress induced by cutter penetration. which is caused by heterogeneity of rocks. In this study Weibull distribution function and degradation index are used to consider the strength heterogeneity of a rock and the degradation of rock properties after failure. Through the numerical analyses, it is shown that the lateral pressure has an important influence on the rock fragmentation. In the single cutter penetration, large chips are formed as lateral pressure increase. The cutter spacing is also an important factor that affects the rock fragmentation in the double cutter penetration. The fragmentation efficiency of the double cutter penetration is better when cutter spacing is 70 mm than 40 mm and 100 mm. From the results, it is expected that this study can be applied to a TBM tunnel design by understanding of chipping process and mechanism of rock due to cutter penetration.