Numerical Simulation of the Stress Field in Repeated Mining of Coal Seams Based on In Situ Stress Measurement

We established an evaluation index of the rock mass stress state for underground coal mines using the strength-stress ratio based on the measured in situ stress and the generalized Hoek–Brown strength criterion. Three in situ stress states, σcm/σ1m < 1.4 (high), 1.4 < σcm/σ1m < 3.6 (medium), and σcm/σ1m > 3.6 (low), were established based on the value of the unconfined compressive strength (σcm) and the maximum principal stress of the rock mass (σ1m). This index classifies the Burtai mine as a medium-high in situ stress field, which is in agreement with the on-site situation, establishing the reliability of the index. The working face was a three-dimensional geological model based on the log sheets. The initial conditions for the model were determined using the combined measurements of the in situ stress regression model. We performed numerical simulations of the roof stress field distribution under repeated mining. Mining the overlying coal seam leads to significant variation in the value and direction of the main roof, σ1, within the range of the front abutment pressure under the pillar and gob. Along the main roof strike direction, σ1 under the pillar is 1.5 times that under the gob, and the σ1 direction under the pillar is deflected by 5°, which is 30° smaller than that under the gob. This provides a reference for optimized underground coal mining.

Analysis of Surrounding Rock Creep Effect on the Long-Term Stability of Tunnel Secondary Lining

The secondary lining failure of deep buried soft rock tunnel often occurs, which is obviously related to the time factor. The formation mechanism of this phenomenon is studied in this paper. Therefore, the combination of in situ stress measurement and neural network inversion is used to analyze the distribution characteristics of in situ stress. At the same time, the creep characteristics of surrounding rock are tested in laboratory, and the key parameters are obtained. Combined with the characteristics of surrounding rock, the calculation model is established by using discrete element simulation technology and considering the joints of surrounding rock. According to the above multiple information, the stress characteristics of the secondary lining in different time periods are analyzed creatively. Finally, the method of setting arch and adding anchor bolt in key parts is proposed, and significant effect results are obtained.

Research on Fiber Bragg Grating Sensor Group for Three Dimensional In-situ Stress Measurement

The observation and estimation of deep crustal stress state is a key and difficult problem in in-situ stress measurement. The borehole wall strain gauge based on the overcoring stress relieving method is one of the main methods of in-situ stress measurement. In this paper, a strain sensing array based on FBG is designed by using the main structure of the classical hollow inclusion cell, and its layout scheme on the hollow inclusion is studied. According to the layout scheme, the in-situ stress inversion algorithm of hole-wall strain to stress is deduced; then, the triaxial loading and unloading experiment platform is built, and the calibration experiment of FBG strain sensor is designed; Finally, Abaqus finite element software is used to simulate the in-situ stress measurement process of the overcoring stress relieving. The FBG strain values of each measurement direction before and after the overcoring process are extracted, and the stress inversion equation is used to carry out the stress inversion. Through the comparison of the inversion results, it is proved that the FBG strain sensor group is feasible and reliable. The quasi-distributed FBG sensor module designed in this paper can invert the three-dimensional in-situ stress by measuring the hole-wall strain, which lays a theoretical and experimental foundation for the development and application of FBG hole wall strain gauge. It fairly makes up for the deficiency of the existing hole-wall strain gauge based on resistance strain gauge, provides direct and accurate observation way for hole wall strain measurement, and has important practical value for the development of in-situ stress measurement technology.

In situ stress measurement and analysis of the stress accumulation levels in coal mines in the northern Ordos Basin, China

For non-directional drilling cores, selection of samples and the test methods for in situ stress measurements to evaluate the Kaiser effect (KE) were proposed, and the magnitude and direction of the principal stresses were derived from first principles. Based on this approach, the KE for 423 samples in the Burtai and Baode coal mines in the northern Ordos Basin (NOB), China, have been investigated. The results show that the maximum horizontal principal stress (σH), the minimum horizontal principal stress (σh) and the vertical stress (σv) varied with depth and location, and the values increase with increasing depth. Generally, the horizontal stresses play a leading role. For the main stress regimes in the NOB, σH > σh > σv (Burtai Mine, < 172 m; Baode Mine, < 170 m) and σH > σv > σh (Burtai Mine, 170–800 m; Baode Mine, 170–400 m), and the σv > σH > σh stress regimes are mainly distributed in moderately deep to deep coal mines. For rock masses with a depth of 350 m, k ((σH + σh)/2σv) tends to 1, indicating that a deep critical state will gradually emerge. The test results are compared with those for the overcoring (OC) method, the anelastic strain recovery (ASR) method and micro-hydraulic fracturing (HF). The relative errors for σH, σh and σv were 14.90%, 19.67%, 15.47% (Burtai Mine) and 10.74%, 22.76%, 19.97% (Baode Mine), respectively, and the errors are all within an acceptable range, thus verifying the reliability of the KE method. The dominant orientation for the σH (Burtai mine, NE-NNE; Baode Mine, NEE) is obtained via paleomagnetic technology, and the data are consistent with those (NE-NEE) of the earthquake focal mechanism solutions for the area. Based on the Byerlee–Anderson theory, a discussion is given on the levels of stress accumulation in the rock mass of the mines. For dry rocks or hydrostatic pressure rocks, the friction coefficients of the faults are low for both locations, and the values are less than the lower limit (0.6) of the strike-slip faults slip, indicating that the stress fractures at a low level around the study areas are lower than the friction limit stress. The stress accumulation levels in the Baode Mine are slightly larger than those in the Burtai Mine.

Use of numerical modelling to determine optimum overcoring parameters in rock stress-strain state analysis

Before a mineral deposit can be developed deeper, a geomechanical survey should be carried out to determine the stress tensor of the rock mass. The most common in situ stress measurement techniques include the methods of hydraulic fracturing and overcoring. Each of them has its advantages and its drawbacks. The authors of this paper propose to conduct two stages of numerical modelling in order to enhance the efficiency of measurements performed by overcoring method. The first stage involves building a model of drilling to determine the optimum drilling depth. At the second stage, a core drilling model is built. Absolute transverse strains have been determined in reference points of a pilot borehole at each stage of core drilling. The created simulation models help choose the drilling depth of coaxial boreholes, estimate the potential growth of plastic strains that cause core destruction, define the amount of overdrilling to be done to coaxial boreholes and determine the optimum location for the measurement tools. The developed models can also be used to determine stresses by inverse problem solving.