Jianfeng Liu

Static and dynamic uniaxial compression tests on coal rock considering the bedding directivity

The mechanical properties and behavior of coal rock under both static and dynamic loading rates are of importance in the coal mining practices. In this study, both quasi-static and dynamic uniaxial compression tests are conducted on coal rock, considering the bedding directivity of coal rocks using a MTS hydraulic servo-control testing machine and split Hopkinson pressure bar (SHPB), respectively. The attained strain rates range from 10−5 to 10−2xa0s−1 for static tests and 20 to 100xa0s−1 for dynamic SHPB tests. For dynamic tests, pulse-shaping technique is utilized to achieve dynamic force balance and thus validate the quasi-static data reduction. A high-speed camera is used to capture the failure process in SHPB tests. The characteristics of failure mode, fracture strength, energy dissipation, and fractal dimension are investigated. A significant strain-rate-dependent behavior of coal rock is revealed, and the compressive strength, elasticity modulus and energy consumption increase with increasing strain rate. The bedding effect on the coal behavior at static strain rate is more prominent than that at dynamic strain rate. The measured strengths along different bedding directions exhibit distinct variations, featuring significant anisotropy. In addition, a sieving statistics analysis of the recovered fragments depicts obvious fractal; and the fracture dimension can be correlated to the fractal energy dissipation.

An improved Maxwell creep model for rock based on variable-order fractional derivatives

For describing the time-dependent mechanical property of rock during the creep, a new method of building creep model based on variable-order fractional derivatives is proposed. The order of the fractional derivative is allowed to be a function of the independent variable (time), rather than a constant of arbitrary order. Through the segmentation treatment, according to different creep stages of the experimental results, it is found that the improved creep model based on variable-order fractional derivatives agrees well with the experimental data. In addition, the fact is verified that variable order of fractional derivatives can be regarded as a step function, which is reasonable and reliable. In addition, through further piecewise fitting, the parameters in the model are determined on the basis of existing experimental results. All estimated results show that the theoretical model proposed in the paper properly depicts the creep properties, providing an excellent agreement with the experimental data.

Estimation of Biot’s effective stress coefficient from well logs

Effective stress governs the mechanical response of rock formations to variations in stress and pore pressure, which affect wellbore stability and reservoir integrity during drilling and production. Biot’s coefficient is employed to calculate effective stresses from total stress and pore pressure. Therefore, the measurement of Biot’s coefficient becomes crucial. However, the laboratory measurement of Biot’s coefficient is expensive and laborious. This paper presents three methods for computing Biot’s coefficient using logging data. The first method calculates Biot’s coefficient using the existing empirical correlations between porosity and Biot’s coefficient. The second and third methods calculate Biot’s coefficient using dynamic rock and solid bulk modulus, computed using rock and solid wave velocities, respectively. However, the second and third methods calculate the necessary solid wave velocities in different ways. The second method calculates solid shear and compressive velocities (Vs and Vp) using a newly developed correlation between the differential pressure, porosity and wave velocity of sandstone. The third method calculates solid wave velocities based on the significant finding that the Vp/Vs ratio with respect to the S-wave velocity is constant for sediments including highly compacted sand. Case studies were undertaken using logging data from the Gulf Coast Gas Wells. It was found that Biot’s coefficient calculated using the first method was highly dependent on the chosen relation, while the coefficients calculated using the second and third methods were related to well logs. Results from the third method show that Biot’s coefficient deflects to higher values in situations where gamma ray surveys read low API values. This is in agreement with the phenomenon that rocks with a smaller API should have lower a clay content and bigger value of Biot’s coefficient. Therefore, the third method is more reliable and also requires fewer input parameters.

Mechanical and permeability characteristics of rock under hydro-mechanical coupling conditions

A study on the mechanical characteristics and permeability properties of sandstone and limestone from a coal mine under 3D stresses has been carried out with the MTS815 rock mechanics test system, and the characteristics of mechanics and permeability before and after the rock failure under hydro-mechanical coupling conditions have been analyzed and discussed. The research revealed the difference of mechanics and permeability before and after the rock failure and obtained the relationship among the strength, deformation and differential water pressure. It also obtained the correlation among the permeability and stress, strain and revealed the features of fractures which influenced the correlation. The results show that there are obvious weakening effects of differential water pressures on the mechanical properties after the rock damages; the permeability of rock mass in practical engineering is mainly controlled by the features of fractures; the permeability has an negative index correlation with the axial strain ε1, and has a power function correlation with the mean stress σm(σmxa0=xa0(σ1xa0+xa02σ3)/3); for fractured limestone and coarse sandstone, the permeabiity will tend to be stable gradually when the mean stress reaches or exceeds a certain valuexa0σml.

Influence of water-insoluble content on the short-term strength of bedded rock salt from three locations in China

Unlike the presence of thick domes and thick layers of rock salt deposits in Europe and the USA, rock salt in China is mainly composed of thin layers (80–300xa0m), characterized by a high content of water-insoluble impurities. Thus, the influence of these impurities on the strength of rock salt is barely known and has become an important topic for basic research in the construction of underground salt caverns for energy storage in China. In this paper, the influence of water-insoluble content on the short-term strength of bedded rock salt, based on a series of combined laboratory tests including solubility, uniaxial and triaxial compression tests, on bedded rock salt samples from three locations in China, namely the Huai’an (HA) site in Jiangsu Province, the Pingdingshan (PDS) site in Henan Province and the Anning (AN) site in Yunnan Province, is presented. Samples in which the water-insoluble content was less than 50xa0% were defined as “rock salt” and the remainder as “impure rocks with salt”. Analysis of the experimental results for rock salt samples leads to the conclusion that the uniaxial strength, including the error in its measurement, increases with increase in the water-insoluble content in the sample. Meanwhile, the triaxial strength of rock salt will hardly vary with changes in the water-insoluble content. It can also be concluded that application of confining pressure will suppress the impact of the water-insoluble content on the short-term strength of rock salt, i.e. the inhibition will increase with increasing confining pressure. Application of Hou’s strength model to bedded rock salt from the three regions yields comparable results with the measured data, nevertheless with a bearable small degree of uncertainty. The modified Hou’s strength model has been proved to be more suitable for the determination of strength of bedded rock salt from the three study locations in China.

The weakening effect of hydrostatic pressure on rock mass of different lithology

AbstractnThe weakening effect of hydrostatic pressure (Pw) on rock mass is an important topic in the field of underground construction. In this article, a new technique is introduced to simulate the mechanical conditions and geological processes of rock mass formation using the MTS815 Rock Mechanics Test System. With this technique, intact rock specimens of marble, sandslate, greenschist, and lamprophyre acquired from rock masses are transformed into rock mass replica specimens with structural planes whose major mechanical characteristics resemble the target rock masses. These replica specimens are subjected to hydro-mechanical interacted tri-axial compression tests to reveal the weakening effect of the hydrostatic pressure on the rock mass. The experimental results show that hydrostatic pressures of 1–4xa0MPa have a significant effect on the mechanical properties of the rock masses. The internal cohesion c decreased dramatically when the hydrostatic pressure increased; however, the increased pressure had less effect on the internal friction coefficient μ. The deformation modulus decreased when the hydrostatic pressure increased under low confining pressure, but barely changed when the confining pressure rose above 20xa0MPa. These results have provided an important basis for engineering construction planning and have created a new effective way for laboratory studies of rock mass mechanical properties.

Research on Confining Pressure Effect on Mesoscopic Damage of Rock Salt Based on CT Scanning

The Rock salt is the optimal surrounding rock of underground energy storage, so its damage is directly related to the impermeability and stability of the storage. In this study, CT scanning technique was used to analyze the microscopic damage characteristics of rock salt, which damaged under the three-dimensional stress state. The distribution and evolution characteristics of porosity, crack area, crack volume and crack quantity of damaged rock salt under different confining pressures were obtained. The results revealed the confining pressure effect of the mesoscopic damage of rock salt. With the increase of confining pressure, the quantity and size of cracks in rock salt gradually decreased. The confining pressure of 10 MPa is the turning point where the porosity decreased from fast to slow and the failure characteristics of rock salt changed from the apparent shear failure surface to the surface micro cracks.

Experimental Investigations of Rock Dynamical Characteristics Under Cyclic Loading

Rock dynamic characteristics are important factors influencing the long-term stability of rock masses. In addition, they are important parameters for seismic response analysis and safety evaluation. In this study, a solution was discussed for the evaluation of the damping coefficient of rock masses under cyclic loading. Then, low cyclic loading tests with a frequency of 3 Hz were carried out on sandstone samples. Through these tests, the influences of the amplitude stress and cycle number on the dynamic elastic modulus and Poisson’s ratio, damping ratio and coefficient, and their relationships were evaluated. Under the cyclic loading condition, the dynamic elastic modulus and Poisson’s ratio increased parabolically and linearly with the increase in the amplitude stress, respectively; however, the damping ratio and coefficient decreased in accord with a power function. The damping ratio and coefficient increased linearly with the increase in the cycle number, and their increments for low-amplitude stresses were larger than those at high stresses. The results suggest that the damping parameters of rock masses could be obtained from the hysteresis loop measured from the intermediate cycle during a 31-cycle cyclic loading test.

Comparison of Triaxial Compression Short-Term Strength Tests and Data Processing Methods for Rock Salt

The strength and deformation characteristics of rock salt are extremely important parameters for the design, construction and operation of underground energy storage and waste repository. Obtaining the correct strength parameters for rock salt is of great significance to the design and safe operation of an underground repository. In this study, samples of rock salt from the Jintan site of Jiangsu Province and Jianghan site of Hubei Province were chosen for triaxial compression tests, to detect their short-term strength and deformation parameters. Comparing the two different test and data processing methods employed in this study, it is revealed that for rock salt with a large deformation, the strength data obtained by the Chinese test measurement is generally higher than the German test measurement. Thus it can be concluded that the German strength test and data processing method is more accurate.

Spatial Distribution of Acoustic Emissions of Rock Salt under Different Stress Conditions

In this paper, the study on the spatial distribution of acoustic emission (AE) events in rock salt under different stress conditions was reported. The study showed that under indirect tensile stress condition, the AE events were mainly induced by the stress concentration near the loading surface and by intergranular deformation before rock failure, while during failure and the post-failure process the AE events were essentially related to the damage development, especially near the tensile failure surface. Under uniaxial compressive stress condition, the closure of initial micro-cracks including those artificially produced during coring and sample preparation and intergranular deformation were the main mechanisms responsible for the occurrence of AE events at the initial deformation stage, and then transferred to damage development until the post-failure stage was met. In triaxial compression tests, the influence of the closure of initial micro-cracks and intergranular deformation was still the main cause for AE events at the confining pressure loading stage and the initial axial loading stage. However, the influence of plastic deformation becomes more pronounced at the plastic yield stage and the post-peak stage. Intensive occurrence and accumulation of AE events before rock failure was noticed under different stress conditions. In indirect tensile tests, the recorded AE events were mainly accumulated along the failure surface. However, this reasonable phenomenon was not evident in uni- and triaxial compression tests, where the spatial distribution is generally homogeneous.