Loading Behavior and Soil-Structure Interaction for a Floating Stone Column under Rigid Foundation: A DEM Study

This paper investigates the loading behavior and soil-structure interaction associated with a floating stone column under rigid foundation by using the discrete element method (DEM). The aggregates and soft soil are simulated by particles with different sizes. The rigid foundation is simulated by two loading plates at the same position with the same velocity. The stress distributions and microscopic interaction between the column and soft soil are investigated. The vertical stress of the column increases with settlement and decreases with the depth. The position of the column with large radial stress also has large deformation, which decreases from top to bottom. The vertical and radial stresses of the soft soil increase with settlement, and the radial stress shows high value in the upper part of soft soil. The stress concentration ratio is obtained by two loading plates, which decreases from 2.5 to 1.55 during loading. The interaction between column and soft soil shows that the column does not penetrate into the underlying stratum but drags the surrounding soil down.

Systematic Review Based on the Study of Elastic-Plastic Transition Stresses

A systematic review based upon the study of elastic-plastic transition stresses. A worthwhile work about the analysis of elastic-plastic transition stresses in different rotating materials by varying different parameters is discussed. In the case of compressible material, the strain rates have a maximum value at the internal surface. It has been observed that radial stress has a higher value at the internal surface of the rotating disc made of incompressible material as compared to circumferential stress with thermal effect and this value of radial stress further increases. With the increase of angular speed, the value of radial stress further increases as compared to the case with no thermal effect. The magnitude of the stresses and pressure reduce with the variation of thickness needed for a fully plastic state. At the inner surface, the effect of heat increases stress for compressible material. The thickness and density parameters decrease the value of angular speed at the internal surface of the rotating disc of compressible material as well as incompressible materials. The radial and the hoop stress, both decrease with the increased value of temperature at the Elastic-Plastic stage, but with the reverse result obtained for a fully Plastic state.

Study on the Permeability Change Characteristic of Gas-Bearing Coal under Cyclic Loading and Unloading Path

Using the self-developed three-axis servo fluid-solid coupling system with gas-solid coupling of gas-bearing coal, the variation law of the permeability of gas coal under the stress cycle loading and unloading path was studied. The qualitative and quantitative relationships between permeability, axial force, and radial stress of gas-bearing coals were established, and the variation law of permeability of gas-bearing coals was discussed. The results show that (1) different cyclic loading and unloading stress paths correspond to the permeability characteristics of different gas-bearing coals. (2) Permeability of gas-bearing coal decreases with the increase of axial stress and radial stress, and it has a logarithmic function with axial stress and radial stress. This shows that axial stress and radial stress are important factors affecting the permeability characteristics of gas-bearing coal. (3) Under the same stress loading and unloading conditions, the axial stress is less than radial stress on the permeability of gas-bearing coal. In the cyclic loading and unloading axial stress process, the permeability of the gas-bearing coal varies by a smaller extent than the cyclically unloaded confining force. (4) The cumulative damage rate of gas-bearing coal under axial stress gradually increases with the increase of the number of cycles of loading and unloading, and the rate of the cumulative damage rate of permeability is less than the corresponding rate of radial stress.

Effect of magnetic field on the thermo-elastic response of a rotating FGM Circular disk with non-uniform thickness

In this study, the effect of the magnetic field on the thermo-elastic response of a rotating non-uniform circular disk of functionally graded material (FGM) is investigated for both steady and transient states of temperature. A single second-order ordinary differential equation of motion was developed for an FGM disk and solved along with the boundary conditions using the finite-difference method (FDM). The steady-state and transient heat conduction equations were also solved using the finite-difference method. Numerical results were presented and discussed for an Al/Al2O3 FGM disk of exponentially varying material properties keeping Poisson’s ratio and magnetic permeability uniform. Displacement and stress components were analyzed by increasing the intensity of the magnetic field for different cases of steady and transient states of temperature. The analysis suggests that the magnetic field has a remarkable effect on the displacement and stress distributions. It is also found that, high intensity of the magnetic field changes the nature and location of maximum stress. The transient analysis of magneto-thermo-elastic field suggests that the increase in the intensity of magnetic field results in the increase in stress intensity near the outer region of the disk and maximum radial stress always exceeds maximum circumferential stress. The effects of inner and outer surface radius, thermal gradient between inner and outer surface, and the outer surface thickness were also analyzed in detail. It was found that, with the decrease in outer surface radius and thermal gradient between inner and outer surface, maximum circumferential stress becomes higher than the maximum radial stress. In addition, the soundness and accuracy of the solutions were verified with the results from the standard computational method and analytical solution.

Study on Preapplied Annulus Backpressure Increasing the Sealing Ability of Cement Sheath in Shale Gas Wells

Summary Annulus pressure buildup (APB) problems in shale gas wells seriously affected on the safety and efficient exploitation of shale gas all around the world. The sealing failure of the cement sheath on interfaces caused by periodically changed fluid pressure in casing during hydraulic fracturing is treated as the main reason for APB in shale gas wells. Many methods are put forward to solve the APB problem in the field, and fortunately, the preapplied annulus backpressure (PABP) method shows an excellent utility. In this paper, an analytical model is established to explain the mechanism of the PABP method increasing the sealing ability of the cement sheath. The residual strain of the cement sheath and radial stress on interfaces are considered to analyze the factors that affect the effectiveness of the PABP method. In addition, based on the field data, an experimental device is established to test the validity of the PABP method and to certify the accuracy of the analytical model established in this paper. The analytical results show that the thickness of the casing has little effect on radial stress on interfaces. The outer diameter of the casing and the thickness of the cement sheath can temperately affect the radial stress. The elastic modulus of the cement sheath and the formation rock can significantly affect the radial stress. The higher elastic modulus of the cement sheath can dramatically increase the radial stress on interfaces. On the contrary, the higher elastic modulus of formation rock will induce smaller radial stress on the interfaces. In the field, the number of newly added shale gas wells with APB problems has dramatically decreased by using the PABP method. The work in this paper can be significantly useful for researchers and engineers to reduce the APB in shale gas wells.

A New Unified Solution for Deep Tunnels in Water-Rich Areas considering Pore Water Pressure

Pore water pressure has an important influence on the stresses and deformation of the surrounding rock of deep tunnels in water-rich areas. In this study, a mechanical model for deep tunnels subjected to a nonuniform stress field in water-rich areas is developed. Considering the pore water pressure, a new unified solution for the stresses, postpeak zone radii, and surface displacement is derived based on a strain-softening model and the Mogi-Coulomb criterion. Through a case study, the effects of pore water pressure, intermediate principal stress, and residual cohesion on the stress distribution, postpeak zone radii, and surface displacement are also discussed. Results show that the tangential stresses are always larger than the radial stress. The radial stress presents a gradually increasing trend, while the tangential stress presents a trend of first increasing and then decreasing, and the maximum tangential stress appears at the interface between the elastic and plastic zones. As the pore water pressure increases, the postpeak zone radii and surface displacement increase. Because of the neglect of the intermediate principal stress in the Mohr-Coulomb criterion, the postpeak zone radii, surface displacement, and maximum tangential stress solved by the Mohr-Coulomb criterion are all larger than those solved by the Mogi-Coulomb criterion. Tunnels surrounded by rock masses with a higher residual cohesion experience lower postpeak zone radii and surface displacement. Data presented in this study provide an important theoretical basis for supporting the tunnels in water-rich areas.

Axisymmetric contact analysis of piezoelectric materials with surface effect

Based on the surface piezoelectricity theory, this article predicts the axisymmetric smooth contact of a piezoelectric half-space indented by a rigid insulated punch. In this theory, the surface effect is characterized with four material parameters: the surface elastic constant, surface residual stress, surface piezoelectric constant and surface dielectric constant. By applying Hankel integral transform, the fundamental solution for this axisymmetric contact problem is derived with the surface effect. Then, the normal contact stress, radial electric displacement and radial stress of the contact surface are solved numerically. Finally, the surface effects on the normal contact stress, radial electric displacement and radial stress are analyzed. It is found that the surface effect is a significant influencing factor on the axisymmetric contact behaviors of micro-/nano-scale piezoelectric materials.

Drag of a cylindrical object in a two-dimensional granular environment

The drag of a cylindrical object in a two-dimensional granular environment is numerically studied. It is found that the drag law is fitted by the sum of the yield force and the dynamic force, the latter of which is reproduced by a simple collision model. The angular dependence of the radial stress on the surface of the object is given by the Gaussian below the yield force. The probability of the velocity drops of the object is investigated above the yield force, where this probability is independent of the packing fraction and the drag force.