Yugui Yang

Researches on the Constitutive Models of Artificial Frozen Silt in Underground Engineering

The researches on the mechanical characteristic and constitutive models of frozen soil have important meanings in structural design of deep frozen soil wall. In the present study, the triaxial compression and creep tests have been carried out, and the mechanical characteristic of frozen silt is obtained. The experiment results show that the deformation characteristic of frozen silt is related to confining pressure under conventional triaxial compression condition. The frozen silt presents strain softening in shear process; with increase of confining pressure, the strain softening characteristic gradually decreases. The creep curves of frozen silt present the decaying and the stable creep stages under low stress level; however, under high stress level, once the strain increases to a critical value, the creep strain velocity gradually increases and the specimen quickly happens to destroy. To reproduce the deformation behavior, the disturbed state elastoplastic and new creep constitutive models of frozen silt are developed. The comparisons between experimental results and calculated results from constitutive models show that the proposed constitutive models could describe the conventional triaxial compression and creep deformation behaviors of frozen silt.

Effect of pore pressure distribution on fracture behavior of sandstone in nitrogen fracturing

To investigate the effect of pore pressure distribution on fracture behavior of rock in gas fracturing, nitrogen fracturing experiments on the sandstone are performed under different gas filtration conditions. Results indicate that the breakdown pressure of the sandstone reduces with the decreasing amount of gas filtration. In no gas filtration case, the breakdown pressure value of the sample decreases by 69.98 and 45.6% in comparison to no gas invasion case and gas filtration case, respectively. The failure pattern of the sandstone sample is determined by borehole pressure and pore pressure, and it is mainly controlled by borehole pressure at high injection rate and controlled by pore pressure at low injection rate. In addition, pore pressure can promote microcracks initiation and high pore pressure distribution contributes to generating complex multifractures network. Furthermore, in order to popularize the experimental results to the field application, a method of special graded particles is proposed to temporarily plug induced and natural fractures during fracturing process and then prop these fractures after fracturing. The method is expected to improve fracturing efficiency and increase production of oil and gas in practical application.

Compressive Mechanical Properties and Micromechanical Characteristics of Warm and Ice-Rich Frozen Silt

It is recognized experimentally that the compressibility of warm and ice-rich frozen soil is remarkable under loading, which will cause a significant deformation and affect the stability of infrastructure constructed in cold region. In this paper, the real-time computerized tomography tests of warm and ice-rich frozen silt were carried out. The microstructure characteristics in the process of loading were studied, and the macromechanical behaviors were obtained at the same time. The test results showed that the stress-strain curve of warm and ice-rich frozen silt is sensitive to temperature; the peak stress was greatly enhanced with the decrease of temperature, and the section area increases with the increase of axial strain; the water content mainly decreases with the increase of axial strain at −1°C; the change of water content is not obvious at −2°C in the loading process. The density damage changes little at first and then increases with the further increase of axial strain.

Anomalous Advection-Dispersion Equations within General Fractional-Order Derivatives: Models and Series Solutions

In this paper, an anomalous advection-dispersion model involving a new general Liouville–Caputo fractional-order derivative is addressed for the first time. The series solutions of the general fractional advection-dispersion equations are obtained with the aid of the Laplace transform. The results are given to demonstrate the efficiency of the proposed formulations to describe the anomalous advection dispersion processes.

The effect of liquid nitrogen cooling on coal cracking and mechanical properties

Liquid nitrogen is a type of super-cryogenic fluid, which can cause the reservoir temperature to decrease significantly and thereby induce formation rock damage and cracking when it is injected into the wellbore as fracturing fluid. An experimental set-up was designed to monitor the acoustic emission signals of coal during its contact with cryogenic liquid nitrogen. Ultrasonic and tensile strength tests were then performed to investigate the effect of liquid nitrogen cooling on coal cracking and the changes in mechanical properties thereof. The results showed that acoustic emission phenomena occurred immediately as the coal sample came into contact with liquid nitrogen. This indicated that evident damage and cracking were induced by liquid nitrogen cooling. During liquid nitrogen injection, the ring-down count rate was high, and the cumulative ring-down counts also increased rapidly. Both the ring-down count rate and the cumulative ring-down counts during liquid nitrogen injection were much greater than those in the post-injection period. Liquid nitrogen cooling caused the micro-fissures inside the coal to expand, leading to a decrease in wave velocity and the deterioration in mechanical strength. The wave velocity, which was measured as soon as the sample was removed from the liquid nitrogen (i.e. the wave velocity was recorded in the cooling state), decreased by 14.46% on average. As the cryogenic samples recovered to room temperature, this value increased to 18.69%. In tensile strength tests, the tensile strengths of samples in cooling and cool-treated states were (on average) 17.39 and 31.43% less than those in initial state. These indicated that both during the cooling and heating processes, damage and cracking were generated within these coal samples, resulting in the acoustic emission phenomenon as well as the decrease in wave velocity and tensile strength.

Researches on Damage Evolution and Acoustic Emission Characteristics of Rocks

Mechanical parameters of the rock are important for the design of geotechnical, mining engineering, and petroleum reservoir projects. Many researches have suggested that the mechanical variables of rock specimens, such as compressive strength and elastic modulus, do not have a single fixed value. Uncertainty in the basic mechanical variables of the rock material can significantly affect the structural performance and safety. In this study, a series of compression experiments with acoustic emission have been performed on rock specimens. The damage evolution characteristics of the rock in the process of loading were studied, and the macromechanical behaviors were obtained at the same time. Distribution characteristics of the strength and elastic modulus as random variables are illustrated, and the statistical damage model is presented by the authors to formulate analytical constitutive relations for deformation behavior. The comparisons between predicted results and experimental data show that the statistical damage constitutive model could well reproduce the deformation process of rock materials.