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Cracking of soils associated with subsidence is a complex and multiparametric problem. Local soil conditions could be responsible for the dramatic differential settlements and fissures manifest when the water pumping reduces the volume of the compressible strata. This situation is of extreme importance due to the level of damage to urban infrastructure and buried facilities (gas, water, and drainage) as well as to housing structures. In this research, using a simple geotechnical model of subsidence (finite element method, Mohr–Coulomb criterion) parametric combinations of materials and basement geometry are tested to define the geotechnical settings more susceptible to deformation and derived cracking. These approximations are compared with measurements and field surveys in Mexico City to validate the hypothesis. Defining the zones that are more susceptible to respond with cracking due to the phenomenon of subsidence can be especially important when designing urban development programs, restoration campaigns for buried pipes, even for construction and operation of new pumping wells. [ABSTRACT FROM AUTHOR]

Tension failure is a common failure mode for jointed rock mass, with the rock bridge as the main body subjected to the tensile load. To reveal the jointed rock mass stability, the influence of joint persistence (JP) on the tensile capacity and failure behavior of rock bridges is investigated. Considering different JP, the direct tensile test is performed on cubic jointed rock-like specimens using a newly developed direct tensile apparatus. During the failure, an AE detector and a thermal infrared imager are used to monitor the parameters (e.g., AE and temperature). Furthermore, an RFPA3D numerical program validated by the laboratory results is proposed to further investigate the tensile behavior of rock bridges. The results show crucial precursory properties of the AE parameters, indicating that they can be used to discriminate and monitor the failure process under various JP conditions. The temperature response to tension failure is not apparent. With increased JP, the equivalent tensile strength of rock bridges and the weakening degree of rock bridges exhibit a W-shaped variation trend due to the size effect and boundary effect. The load moment effect must be considered under a large JP, and the effect of JP on the rock bridge tensile strength needs to be considered when determining the safety factor for jointed rock mass stability. This study can contribute to a better understanding of JP, thus promoting the analysis of jointed rock mass stability. Highlights: Direct tensile tests on cubic rock-like specimens were conducted on a novel test apparatus. Effect of joint persistence on tension failure behavior was investigated. AE and thermal infrared imager were utilized to analyze the tension failure mechanism. The rock bridge's tensile strength exhibited a "W-shaped" trend with joint persistence increasing. [ABSTRACT FROM AUTHOR]

Quasi-static tension tests, quasi-static compression tests and dynamic compression tests were conducted to investigate the mechanical properties, constitutive behaviors and failure criteria of aluminum-polytetrafluoroethylene-tungsten (Al-PTFE-W) reactive materials with W content from 20% to 80%. The analysis of the quasi-static test results indicated that the strength of the materials may be independent of the stress state and W content. However, the compression plasticity of the materials is significantly superior to its tension plasticity. W content has no obvious influence on the compression plasticity, while tension plasticity is extremely sensitive to W content. Dynamic compression test results demonstrated the strain rate strengthening effect and the thermal softening effect of the materials, yet the dynamic compression strengths and the strain rate sensitivities of the materials with different W content show no obvious difference. Based on the experimental results and numerical iteration, the Johnson–Cook constitutive (A, B, n, C and m) and failure parameters (D1~D5) were well determined. The research results will be useful for the numerical studies, design and application of reactive materials. [ABSTRACT FROM AUTHOR]

This paper proposes the development of a 3D numerical model of the casing-cement-formation system to evaluate the mechanical tensile and shear failure of the cement at the casing-cement interface as the pressure and temperature conditions of the formation and borehole vary during production. The numerical model which includes the formation pressure, unlike others proposed, was designed by stages under finite element discretization and validated by analytical models. Results show that increasing the formation temperature increases the probability of tensile and shear failure in the cement, while increasing the wellbore temperature decreases these probabilities. On the other hand, the decrease in the well pressure reduces the probability of shear failure and increases the tensile failure. In the case of formation pressure, the opposite occurs. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model of an integrated 3D casing-cement-formation system, to evaluate the mechanical tensile and shear failure of the cement at the casing-cement interface as the pressure and temperature conditions of the formation and borehole vary during production. The model which includes the formation pressure, unlike others proposed, was developed by stages under finite element discretization and compared to analytical models. Results show that increasing the formation temperature increases the probability of tensile and shear failure in the cement, while increasing the wellbore temperature decreases these probabilities. On the other hand, the decrease in the well pressure reduces the probability of shear failure and increases the tensile failure. In the case of formation pressure, the opposite occurs.

Numerical simulation method was used to explore the influence of in-situ stress on rock core discing under different stress combinations. The results showed that when the in-situ stress exceeds a certain value， rock core discing occurs. When the radial stress reaches 35MPa， rock core discing occurs， and the radial stress is the main factor that affects rock core discing， while the axial stress only leads to the partial failure on core surface. At the same radial stress， the core discing weakens with the increase of axial stress. The formation process of core discing is mainly dominated by shear failure， accompanied with a little tension failure. Rock core discing is a complex mechanical process. [ABSTRACT FROM AUTHOR]

This paper analyses the effects of vertical seismic force on initiation mechanism of near-fault large-scale landslides by using the Daguangbao landslide induced by the 2008 Wenchuan earthquake as an example. Field investigations showed that tension failure plays an important role in failure mechanism, and the tension failure is due to the effect of a large vertical seismic motion that occurred in the meizoseismal area during the earthquake. Firstly, two key issues in seismic analysis of near-fault large-scale landslide are proposed: i) how to select and correct the severe near-fault excitations with significant co-seismic displacements and ii) how to consider the effect of the landslide scale on estimation of material strength. Then, in order to investigate the effect of the severe vertical seismic force on the initiation of the huge Daguangbao landslide, five cases, 1) static; 2) pseudo-static only-horizontal; 3) pseudo-static horizontal-and-vertical; 4) dynamic only-horizontal; 5) dynamic horizontal-and-vertical, are performed using the finite difference program FLAC 3D . The simulation results are presented in terms of tension failure area, factor of safety, and displacement. The results confirm the significant effects of vertical seismic force on seismic slope failure mechanism as the tension failure areas and the displacements observed in the case of horizontal-and-vertical are larger than those observed in the case of only-horizontal. [ABSTRACT FROM AUTHOR]

How to quantitatively evaluate the permeability change of coalbed subjected to liquid nitrogen cooling is a key issue of enhanced-permeability technology of coalbed. To analyze the evolution process of permeability of coupled coal deformation, failure and liquid introgen cooling, the coal is supposed as elastic, brittle and plastic material. Its deformation process includes elastic deformation stage, brittle strength degradation stage and residual plastic flow stage. Combined with strength degradation index, dilatancy index of the element and Mohr Column strength criterion, the element scale constitutive model with the effects of confining pressure on peak-post mechanical behaviors is built. Based on the deformation process of coal rock, there exist two stages of permeability evolution of the element including decrease of permeability due to elastic contraction and increase due to coal rock element's failure. The relationships between the permeability and elastic deformation, shear failure and tension failure for coal are studied. The permeability will be influenced by the change of pore space due to elastic contraction or tension of element. Conjugate shear zones appear during the shear failure of the element, in which the flow follows so-called cubic law between smooth parallel plates. The calculation formulas of the permeability and the aperture of the fractures are given out based on the volumetric strain. When tension failure criterion is satisfied with the rock element fails and two orthogonal fractures appear. The calculation formulas of the permeability and the width of the fractures are given out based on the volumetric strain. Further, combined with the thermal conduction theory the permeability evolution model of coupled coal deformation, failure and liquid nitrogen cooling is presented. Then Fish function method in FLAC is employed to perform the model. The permeability's evolution process for coal bed cryogenically stimulated by flowing liquid nitrogen through gas production well in Wangyingzi mine, Liaoning province, is simulated and the results include; 1) When liquid nitrogen(LN2) is injected into a rock at warm reservoir temperature, heat from the rock will quickly transfer to the liquid nitrogen resulting in rapid cooling and con traction of coal bed. The nearer the position is to bore hole, the bigger the shrinkage deformation and thermal stress and coal fail when tension stress sufficiently built up. In this paper the tension failure band after 10 days' LN2 cooling is 0. 65 m. 2) In tension failure area the cracks from cooling stimulation lead to the significant growth of permeability. The maximum permeability for element is 1. 97 x 105 times more than that before cooling. 3) Apart from the bore hole, the thermal tensile stress leads to the growth of permeability at a rate of 1% ~14%, far less than that in the tension failure area. 4) With increasing time the failure area gradually slowly grows up. It indicates that the longer cooling time does not mean better effects. 5) The cooling fracturing area is found to have a 1. 0 m band. In practical engineering the pressure in hole bore and phase transition of water also influences the deformation and failure of coal, which leads to much more failure zone of cooling. 6) The evolution process of permeability of coupled coal deformation, failure and liquid introgen cooling can be better reflected by the model in this paper. This study is hoped to provide a simple but reasonable description of the permeability evolution of rocks subject to liquid nitrogen cooling. [ABSTRACT FROM AUTHOR]

To reflect the permeability change of loaded rock, the relationship between the permeability and stress-strain curve of rock was analyzed, and the process the deformation and failure of rock was supposed to be elastic, brittle and plastic. Based on the process the permeability evolution model for rock was presented. In this model the evolution of the permeability for the rock element include; (1) The permeability can be expressed by porosity during elastic deformation phase. (2) In the post-peak stage a shear failure rock element may be represented hydraulically as a unit of rock containing two oblique crossing conjugate fractures. (3) In the post-peak stage a tension failure rock element may be represented hydraulically as a unit of rock containing two orthogonal fractures. Then based on so-called cubic law between smooth parallel plates the permeability evolution model during dilation was presented. In the model the dilation effects of failed element on permeability was considered. Fish function method was adopted to implement the model in FLAC software. The permeability evolution process of a cubic loading rock sample under varied confined stress was simulated. The results show that the model can better reflect the permeability evolution due to elastic deformation or failure. Besides the effect of confined stress on permeability can also be better expressed. [ABSTRACT FROM AUTHOR]

The article presents a study which assessed the internal seismic stability of reinforced soil structures (RSS) subjected to earthquake loading using pseudodynamic method. The study used the horizontal and vertical sinusoidal accelerations, amplification of vibrations, shear wave and primary wave velocities and time period. It indicates that the approach is applied to measure the influence of the backfill properties, geosynthetic reinforcement and characteristics of earthquake ground motions.

This paper presents a method to evaluate reliability for internal stability of reinforced soil structures using reliability based design optimization. Using limit equilibrium method and assuming the failure surface to be logarithmic spiral, analysis is conducted to maintain internal stability against both tensile and pullout failure of the reinforcements. Properties of backfill soil and strength of the geosynthetic reinforcement are considered as random variables. For the seismic conditions, reliability indices of all the geosynthetic layers in relation to tension and pullout failure modes are determined for different magnitudes of seismic accelerations both in the horizontal and vertical directions, surcharge load and design strength of the reinforcement. The efforts have been made to obtain the number of layers, pullout length and total length of the reinforcement at each level for the desired target reliability index values against tension and pullout modes of failure. The influence of horizontal and vertical earthquake acceleration, surcharge load, design strength of the reinforcement, coefficient of variation of soil friction angle and design strength of the reinforcement on number of layers, pullout length and total length of the reinforcement needed for the stability at each level is discussed. [ABSTRACT FROM AUTHOR]

Abstract: The behavior of a specimen under uniaxial tension and the process of micro-fracture in that specimen are phenomena of considerable interest in understanding the strength characterization of brittle or quasi-brittle materials such as rock. In this study, numerical results based on a Rock Failure Process Analysis code (RFPA2D) are presented. They show the nucleation and growth of macro-cracks in relatively homogeneous and heterogeneous specimens under uniaxial tension. Although the details of macro-crack formation vary from specimen to specimen, the numerical simulations consistently display a number of features. In relatively homogeneous specimens, the macro-crack nucleates abruptly at a point in the specimen soon after reaching the peak stress. Before macro-crack nucleation, acoustic emission (AE) events, or micro-fractures occur at locations that are randomly distributed throughout the specimen. It is very difficult to predict where the macro-crack will begin. The failed specimen has no residual strength. However, relatively heterogeneous specimens show a somewhat different response, with more randomly distributed AE events appearing in the early stage of loading. In contrast to the homogeneous specimens, macro-crack nucleation in heterogeneous specimens starts well before the peak stress is reached, and the crack propagation and coalescence can be traced, which can then be used to predict the macro-fracturing of the specimen. For specimens with the same heterogeneity, however, the numerical simulations show that the failure modes depend greatly on the crack initiation location, which is found to be sensitive to the local disorder features within the specimen. [Copyright &y& Elsevier]

Bolted joints are widely used to connect safety-critical parts in the aeronautical industry and require precise structural assessment tools. Focus of this paper is the development of an overall analytical calculation method to determine the stresses of a bolted joint with quasi-isotropic composite material, which is then used in a tension failure analysis. The stress solution is obtained by means of the Airy stress function. To predict crack initiation in the tension failure analysis use is made of the Theory of Critical Distances. Crack initiation occurs in the net section plane and its assessment requires precisely calculated net section stresses. For this purpose, the stress boundary conditions in load direction at the straight free edges must be fulfilled, which is reached by supplementing the field modelling the load introduction with auxiliary functions. This technique is a physically motivated means to deal with symmetric finite geometry problems involving straight free edges. However stress boundary conditions perpendicular to the load direction are not covered and some inaccuracies in the stress solution may arise. A failure analysis is conducted to investigate their impact on the predicted failure load using literature values based on Finite Fracture Mechanics as reference. [ABSTRACT FROM AUTHOR]

To investigate the dynamic characteristics and fracture mechanisms of gas-bearing coal samples, a split Hopkinson pressure bar experimental system (SHPB-GAS) was built, using which dynamic impact experiments of gas-bearing coal were performed. Stress wave signals were collected and processed to analyze changes in strain characteristics with time. Relationships between dynamic strength, failure strain, and various factors such as axial static load, confining pressure, gas pressure, and impact load were analyzed. Our results show that dynamic strength and failure strain increased with the increase of confining pressure and impact load, but decreased with the increase of axial static load and gas pressure. Furthermore, evolution of dynamic cracks and failure mode of gas-bearing coal were analyzed, with the results indicating that samples presented axial tension fracture under the combination loading (axial static load, confining pressure, gas pressure and impact load). The effects of these four factors on the fracture evolution in coal samples were then discussed. Under these conditions, the fracture mechanism of gas-bearing coal was investigated and the effects of shear stress and normal stress on the fracture surface were illustrated. Our findings show that gas flow over the coal surface will increase shear stress along the fracture surface, in turn inducing coal fracture. [ABSTRACT FROM AUTHOR]

How to quantitatively evaluate the permeability change of coalbed subjected to liquid nitrogen cooling is a key issue of enhanced-permeability technology of coalbed. To analyze the evolution process of permeability of coupled coal deformation, failure and liquid introgen cooling, the coal is supposed as elastic, brittle and plastic material. Its deformation process includes elastic deformation stage, brittle strength degradation stage and residual plastic flow stage. Combined with strength degradation index, dilatancy index of the element and Mohr-Column strength criterion, the element scale constitutive model with the effects of confining pressure on peak-post mechanical behaviors is built. Based on the deformation process of coal rock, there exist two stages of permeability evolution of the element including decrease of permeability due to elastic contraction and increase due to coal rock element's failure. The relationships between the permeability and elastic deformation, shear failure and tension failure for coal are studied. The permeability will be influenced by the change of pore space due to elastic contraction or tension of element. Conjugate shear zones appear during the shear failure of the element, in which the flow follows so-called cubic law between smooth parallel plates. The calculation formulas of the permeability and the aperture of the fractures are given out based on the volumetric strain. When tension failure criterion is satisfied with the rock element fails and two orthogonal fractures appear. The calculation formulas of the permeability and the width of the fractures are given out based on the volumetric strain. Further, combined with the thermal conduction theory the permeability evolution model of coupled coal deformation, failure and liquid nitrogen cooling is presented. Then Fish function method in FLAC is employed to perform the model. The permeability's evolution process for coal bed cryogenically stimulated by flowing liquid nitrogen through gas production well in Wangyingzi mine, Liaoning province, is simulated and the results include: 1) When liquid nitrogen(LN2) is injected into a rock at warm reservoir temperature, heat from the rock will quickly transfer to the liquid nitrogen resulting in rapid cooling and contraction of coal bed. The nearer the position is to bore hole, the bigger the shrinkage deformation and thermal stress and coal fail when tension stress sufficiently built up. In this paper the tension failure band after 10 days' LN2 cooling is 0.65 m. 2) In tension failure area the cracks from cooling stimulation lead to the significant growth of permeability. The maximum permeability for element is 1.97×105 times more than that before cooling. 3) Apart from the bore hole, the thermal tensile stress leads to the growth of permeability at a rate of 1%~14%, far less than that in the tension failure area. 4) With increasing time the failure area gradually slowly grows up. It indicates that the longer cooling time does not mean better effects. 5) The cooling fracturing area is found to have a 1.0 m band. In practical engineering the pressure in hole bore and phase transition of water also influences the deformation and failure of coal, which leads to much more failure zone of cooling. 6) The evolution process of permeability of coupled coal deformation, failure and liquid introgen cooling can be better reflected by the model in this paper. This study is hoped to provide a simple but reasonable description of the permeability evolution of rocks subject to liquid nitrogen cooling.