Geohazard Mechanics最新文献

A new method of detecting stress change by temperature (DSCT), has been recently proposed on the basis of the experimental results in laboratory, and verified by field observation. In this paper, at first, physical background is concisely introduced, and experimental researches are followed. Then, the key techniques are reviewed, and the main results on in-situ observations are also given in detail. At last, we emphasize on the prospects of this method for being investigated further. The potential prospect includes six contents: (1) to observe the tidal force and its secondary fluid thermal effect; (2) to study temperature response to change in direction of the stress change; (3) to carry out practical engineering application; (4) to analyze the strong earthquake risk, based on bedrock temperature observation; (5) to conduct in situ experiment on DSCT; (6) to explain quantitatively the satellite thermal infrared anomaly. In short, considering that the dynamic change of the crustal stress is a key parameter of earthquake forecasting or engineering application, the method of DSCT has important practical significance for earthquake risk or engineering applications.

Comprehensive mechanized top-coal caving mining is one of the efficient mining methods in coal mines. However, the goaf formed by comprehensive mechanized top-coal caving mining is high, and the goaf roof collapse will cause strong dynamic pressure disturbance, especially the collapse of thick hard roof. Strong dynamic pressure disturbance has an influence on the stability of the roadway, which can lead to large deformation. In order to solve the above problem, a comprehensive pressure releasing and constant resistance energy absorbing control method is proposed. Comprehensive pressure releasing can change the roadway roof structure and cut off the stress transfer between goaf and roadway, which can improve the stress environment of the roadway. The constant resistance energy absorbing (CREA) anchor cable can absorb the energy of surrounding rock deformation and resist the impact load of gangue collapse, so as to ensure the stability of roadway disturbed by strong dynamic pressure. A three-dimensional geomechanics model test is carried out, based on the roadway disturbed by strong dynamic pressure of the extra-large coal mine in western China, to verify the control effect of the new control method. The stress and displacement evolution laws of the roadway with traditional control method and new control method are analyzed. The pressure releasing and energy absorbing control mechanism of the new control method is clarified. The geomechanics model test results show that the new control method can increase the range of low stress zone by 150% and reduce the average stress and the displacement by 34.7% and 67.8% respectively, compared with the traditional control method. The filed application results show that the new control method can reduce the roadway surrounding rock displacement by 67.4% compared with the traditional control method. It shows that the new control method can effectively control the displacement of the roadway disturbed by strong dynamic pressure and ensure that the roadway meets the safety requirements. On this basis, the engineering suggestions for large deformation control of this kind of roadway are put forward. The new control method can provide a control idea for the roadway disturbed by strong dynamic pressure.

The continuous collision of the Eurasian plate and the Indian plate has resulted in several earthquakes in the Himalayan region. The 6.9 Mw 2011 Sikkim earthquake, which caused immense damage to the built environment in Sikkim, was triggered by an intraplate source on the overriding Eurasian plate. Strong ground motions from the earthquake were recorded at stations established by IIT Roorkee as part of the PESMOS program. In this paper, near-field and far-field ground motions from this earthquake were analyzed to evaluate their key characteristics and examine their time-frequency features by employing Fast Fourier Transforms (FFTs) and Continuous Wavelet Transforms (CWTs). A comparison between the ground motion parameters of near-field and far-field seismic waves highlights the distinct characteristics of near-field ground motions. Additionally, the impact of near-field and far-field ground motions on the seismic response of a code-compliant RC building is investigated. The results from the non-linear time history analyses indicate that the roof displacements, drift ratio and strain induced in the frame elements are less than the code-prescribed maximum limits. Further, the demand and capacity levels for the RC frame elements were evaluated to compute the performance ratios. The results indicate that the extensive damage to reinforced concrete buildings in the 2011 Sikkim quake was primarily due to the non-engineered nature of the structures and also due to the non-compliance of the built structures to the seismic design code provisions.

This study is devoted to amendment to some concepts related to the construction of Mohr's circles on the plane of variables “normal and tangential components of the stress vector on the elemental area”. As the tangential component is positive by definition (as a square root), we have to talk only about semicircles instead of Mohr's circles. To introduce negative values, we bring in the concept of the positive direction of the shear force connected with the projection on the first principal direction of the stress tensor. The considered approach allows us to determine the direction of the shear force (positive/negative) relatively to the principal axes of the stress tensor on any elemental area with known values of the principal stresses. The same approach is applied to the vector of deformations on the elemental area. To represent the application of these two vectors on the elemental area, we consider the work done by the forces acting (in the form of the Cauchy vector of stresses) on changes in the vector of strains. It is also shown that this work, even in the case of elasticity, does not always lead to an unambiguous result. It does not depend on the loading path only on octahedral elemental areas. The foregoing does not negate the existence of the elasticity potential as a whole (non-potency on one elemental area is annulled by the same non-potency on the other one). All this is important when, based on a set of slip areas, physical theories of plasticity and destruction (slip theories) are constructed.

In order to explore the effect of loading rate on physical and mechanical properties of dihydrate gypsum, cyclic loading and unloading mechanical tests were carried out at different loading rates. Test results were analyzed from the aspects of stress-strain curve, energy distribution mode, damage law and failure mode of specimen. The main research results obtained in the thesis are as follows: with the increase of the loading rate, the peak value of specimen damage first increases rapidly, and then in-creases slowly, and there is a damage threshold. In the early stage of loading, the dissipated energy of the specimen accounts for about 70% of the total energy, most of the total energy input is converted into dissipated energy. The elastic energy density shows an increasing trend with the increase of the loading rate. The elastic energy density is the highest when the loading rate is 400 ​N/s, and more elastic energy can be stored. The ratio of elastic energy u_e/u increases with the in-crease of loading rate and tends to be stable. The acoustic emission data show that the acoustic emission signals present a certain agglomeration phenomenon at the unloading point, and there is a “blank period” between the unloading point and the emergence of the next acoustic emission activity. In the early stage of specimen loading, friction-type acoustic emission is mainly generated. The cumulative ringing count when the load reaches the peak failure stress at low loading rate is more, indicating that low loading rate will produce more acoustic emission activities. With the increase of loading rate, the cumulative ringing number per unit time increases, indicating that the increase of loading rate accelerates the damage and failure of dihydrate gypsum near the peak value. The failure mode of gypsum specimens is shear failure, and the increase of loading rate of shear failure angle shows an increasing trend. The larger the loading rate is, the higher the strength of the specimen is. The more energy the press inputs during the loading process, the higher the energy absorbed by the unit volume specimen, which aggravates the development, expansion and penetration of the internal cracks of the specimen, resulting in the larger shear angle of the specimen. The test results provide a more comprehensive theoretical basis for the study of damage characteristics of dihydrate gypsum during cyclic loading and unloading.

The strain energy storage index W_ET was widely used to evaluate coal burst liability, but the scientific evidence for selecting the unloading stress level interval (around 80% of peak strength) remains lacking, and W_ET can not reflect the energy storage and dissipation ratio (ESD ratio) of the whole pre-peak stage for coal materials. In this study, these two key problems in W_ET calculation and application were solved based on the linear energy storage (LES) law. The LES law was defined as the linear relationship between the elastic strain energy and input strain energy for solid material during loading. Using the LES law, the elastic strain energy and dissipated strain energy of at 10 types of coals were calculated precisely, and ideal ESD ratio and general ESD ratio at any stress level will be obtained subsequently. The results also show that W_ET is extremely close to the ideal and general ESD ratio, which proves that the selecting stress level of W_ET calculation is scientific and reasonable. Furthermore, the general ESD ratio converges to the peak ESD ratio (namely peak strain energy storage index W_ET^P) as stress level increases. Compared with W_ET, W^p_ET not only reflects the ESD ratio of coal materials over the whole pre-peak loading stage, but also exhibits excellent stability. Consequently, W^p_ET is suggested as a new evaluation index of coal burst liability.

The present article proposes an evolutionary development of the photoelasticity method for measuring stresses based on annular photoelastic sensors application along with stress pattern recording with the aid of a digital camera and its recognition using artificial neural networks. The analysis of the modern application of the photo-elasticity method for various problems within the theory of strength is presented. The principle of operation of photoelastic sensors based on the photoelasticity effect is considered. Optical patterns in an annular photoelastic sensor are presented for various values of the horizontal stress. The calculation of the stress state of the sensor for the following full-scale experiment has been performed, the estimate of the threshold conditions under which the sensor can be applied has been performed. As a result of a laboratory experiment, a dataset of 1500 isochromatic images has been assembled. A subspecies of a neural network, namely a convolutional neural network, has been applied as a machine learning algorithm. Different combination of models and optimizers have been employed. The application of downhole sensors for continuous monitoring of alterations in the rock mass stress state and the integration of this data into a digital field model based on Internet of Things technologies has been proposed.

Rock mass mechanics can be classified into engineering rock mass mechanics and disaster rock mass mechanics based on science and application. Their conception, object, scientific essence and application were elaborated. The connotation, studying method and theoretical framework of disaster rock mass mechanics were described. Disaster rock mass mechanics is a strongly nonlinear discipline which is a strong tool to study natural and artificially-induced disasters. The rock mass system where disasters happen exhibits extremely spatial-temporal nonlinearity in the critically unstable state. Hence, the potentially effective prediction and forecasting of disasters depends on statistical analysis of highly probable events. The direction of efforts for predicting and forecasting disasters could be to find the quantitative or semi-quantitative relationship between physical and biological information and instability of rock mass system.

Roof bolting has been used in underground entry (roadway) support in U.S. coal mines since the Coal Mine Health and Safety Act of 1969 (US Congress, 1977) recognized roof bolting as the only means of underground entry (roadway) support. For U.S. underground coal mines, roof bolting pattern is fixed at 4 ​× ​4 ​ft (1.2 ​× ​1.2 ​m), except in the Pittsburgh Seam where longwall mining is practiced, with occasional 3.6 ​× ​4 ​ft (1.1 ​× ​1.2 ​m) pattern. However, roof falls or roof failure often occurs in roof-bolted entries in U.S. coal mines. Roof falls can roughly be divided into four types: skin falls, large falls, cutter roofs, and massive falls. Based on this situation, the roof is initially strengthened by bolt based on suspension and friction mechanism. By comparing roof bolting patterns in different coal producing countries, bolt density in all other countries is much higher (except South Africa) than that used in the U.S. In spite of its long history of successful application with hundreds of millions of units installed, roof bolting design is still the lack of a commonly accepted method.

Low-frequency signals have been widely found in the conventional oil/gas field and volcanic region as well as during hydraulic fracturing of unconventional oil/gas reservoirs. Their generation mechanism has been ascribed to the flow of gas/fluid in the fractures, which can induce the Krauklis wave around fractures and can further excite low-frequency seismic body wave signals at diffraction points. Thus, it is theoretically feasible to determine the gas/fluid enrichment areas and migration pathways by locating the low-frequency signals. Here we have utilized a surface dense seismic array deployed above the Sijiazhuang coal mine in Shanxi province to detect and locate such low-frequency signals that are dominant in the frequency range of 1.5–4.0 ​Hz. Waveform migration-based location method is employed to locate these signals that have low signal to noise ratios. We further compare the distribution of low-frequency signals and coalbed methane concentrations that are estimated based on ambient noise tomography result with the same seismic array. The spatial consistency between low-frequency signals and coalbed methane enrichment areas suggests that detecting and locating low-frequency signals with a surface seismic array is an efficient way to identify gas enrichment areas and potential gas migration pathways.