Influence of Ground Stress on Coal Seam Gas Pressure and Gas Content

The influence of ground stress was quantitatively analyzed on coal seam gas pressure and gas content in this paper.Mining activities in coal mine can result in stress concentration in the coal(rock)body around the mining space,but porosity of the coal seam would not change too much.Therefore,gas pressure and gas content in the coal seam are slightly affected.Studies showed that the free gas was gradually transformed into adsorbed gas,and the gas adsorption volume was small,and then gas pressure increases roughly linearly when the porosity decreased because of stress influence.Additionaly,when porosity of coal seam reduced to 40%,the amount of adsorbed gas accounted for no more than 10%of coal seam gas content,and the increase of gas pressure did not exceed 15%of the original gas pressure.

Numerical simulation study on the influence of the ground stress field on the stability of roadways

We adopt the concept of generalized plane strain to model a roadway in a stress field.This can avoid limitations caused by simplifying the stress analysis as plane strain.FLAC3D was used to investigate the maximum tensile stress and displacement of a roadway in a known stress field for angles,α,between the roadway axial direction and the maximum principal stress of 0°,30°,45°,60° and 90°.This theory was applied to the analysis of an engineering case.The results indicate that stress and displacement of the surrounding rock increase as the angle,α,increases.This provides some significant guidance for a reasonable layout of roadways in a known stress field.

Ground stress and its impact on the stability of the surrounding rock in the Lüliang mining area

Ground stress is the fundamental cause of deformation and failure during underground structural engi- neering. Field stress measurements in the main coal bed in the Lüliang mining area were made by the bore hole, stress relief method. From these data the ground stress distribution of the mining area was obtained. The relationship between the horizontal principal stress and the deformation and failure of a roadway is discussed with an engineering example. The results indicate that horizontal stress dominates in the shallow crust in the Lüliang mining area. Roadways at different angles to the maximum principal stress have different levels of stress concentration. This leads to a significant difference in stability of the corresponding roadways. These research results provide an important criterion for determining roadway position and direction, stope layout, and roadway support design.

Effect of ground stress on hydraulic fracturing of methane well

Most of the coal reservoirs in China are of low-permeability, so hydraulic fracturing is widely used to improve the per- meability in the extraction of gas by ground drilling. The ground stress around the well was analyzed by using theory of elasticity. The pressure when the well fractured is formulated and the effect of ground stress on pressure is discussed. The effect of ground-stress-differences on hydraulic fracturing was analyzed by using the numerical software RFPA2D-Flow in reference to the tectonic stress in Jincheng coal area. The results show that: 1) the position where initial fracture appears is random and fracture branches emerge when the fractures expand if ground stresses in any two directions within a horizontal plane are equal; 2) other- wise, the fractures expand in general along the direction of maximum ground stress and the critical pressure decreases with in- creasing ground-stress-differences and 3) the preferred well-disposition pattern is diamond shaped. The preferred well spacing is 250 m×300 m. This study can provide a reference for the design of wells.

Combined back-analysis method of ground stress based on refined geological modeling

A new back-analysis method of ground stress is proposed with comprehensive consideration of influence of topography, geology and nonlinear physical mechanical properties of rock on ground stress. This method based on non-uniform rational B-spline (NURBS) technology provides the means to build a refined three-dimensional finite element model with more accurate meshing under complex terrain and geological conditions. Meanwhile, this method is a back-analysis of ground stress with combination of multivariable linear regression model and neural network (ANN) model. Firstly, the regression model is used to fit approximately boundary loads. Regarding the regressed loads as mean value, some sets of boundary loads with the same interval are constructed according to the principle of orthogonal design, to calculate the corresponding ground stress at the observation positions using finite element method. The results (boundary loads and the corresponding ground stress) are added to the samples for ANN training. And on this basis, an ANN model is established to implement higher precise back-analysis of initial ground stress. A practical application case shows that the relative error between the inversed ground stress and observed value is mostly less than 10 %, which can meet the need of engineering design and construction requirements.

Artificial intelligence method for predicting the maximum stress of an off-center casing under non-uniform ground stress with support vector machine

The situation of an off-center casing under non-uniform ground stress can occur in the process of drilling a salt-gypsum formation,and the related casing stress calculation has not yet been solved analytically. In addition,the experimental equipment in many cases cannot meet the actual conditions and the experimental cost is very high. These comprehensive factors cause the existing casing design to not meet the actual conditions and cause casing deformation,affecting the drilling operation in Tarim oil field. The finite element method is the only effective method to solve this problem at present,but the re-modelling process is time-consuming because of the changes in the parameters,such as the cement properties,casing centrality,and the casing size. In this article,an artificial intelligence method based on support vector machine(SVM) to predict the maximum stress of an offcenter casing under non-uniform ground stress has been proposed. After a program based on a radial basis function(RBF)-support vector regression(SVR)(ε-SVR) model was established and validated,we constructed a data sample with a capacity of 120 by using the finite element method,which could meet the demand of the nine-factor ε-SVR model to predict the maximum stress of the casing. The results showed that the artificial intelligence prediction method proposed in this manuscript had satisfactory prediction accuracy and could be effectively used to predict the maximum stress of an off-center casing under complex downhole conditions.

Influence caused by initial tectonic stress on relative measurement of borehole groundstresinviscoelasticmedia

In this paper, we discussed the influence of the initial tectonic stress on the relative measurement value of the ground stress when a long cylindrical straingauge is used in the viscoelastic media. We simulate the whole system consisting of the stratum, cement ring and the straingauge cylinder with a three layer medium model and obtain the solution of radial displacement on the inner wall of the straingauge under the action of initial tectonic stress by using the correspondence principle. The result showes: 1) The influence of initial stress making of the radial displacement on the inner wall of the straingauge increases gradually with time at a gradually reduced rate and tends to a certain value eventually; 2) The principal stress with a magnitude of [C 0+C 1 exp (- ω 1t)+ C 2 exp(- ω 2t )] σ i0 in the opposite direction of initial stress will be superimposed to the measuring result of stress change without considering the initial stress. where C 0, C 1, C 2 and ω 1, ω 2 are related only to the mechanical property of the stratum, σ i0 (i=1, 2) are the two principal components of the initial tectonic stress. It is pointed out that the initial tectonic stress affects obviously on the measurement of the relative change of ground stress in the cases of soft strata and high initial stress.

Ground movement mechanism in tectonic stress metal mines with steep structure planes

When mining metal mines with steep structure planes by the caving method,there is a mechanical model in which the horizontal stress on the rock mass is simplified as a column before surface subsidence.The model is used to deduce critical support load and limiting column length for a given horizontal stress and support pressure.Considering the impact of the column effect,a method is proposed to determine the movement of the ground and caving area in a mine.After surface subsidence,the horizontal stress on a surrounding rock mass can be simplified to a cantilever beam mechanical model.Expressions for its bending fracture length are deduced,and a method is given to determine its stability.On this basis,an explanation for the large ground movement and subsidence scope was given.A case study shows that the damage effect of column and cantilever beam is significant for ground movement in metal-ore mine,and an appropriate correction value should be applied when designing for its angle of ground movements.

A time-domain nonlinear effective-stress non-Masing approach of ground response analysis of Guwahati city, India

The response of subsoil strata subjected to seismic excitations plays an important role in governing the response of the overlying superstructures at any site. Ground response analysis(GRA) helps to assess the influence of soil characteristics on the propagating seismic stress waves from the bedrock level to the ground surface during an earthquake. For the northeastern region of India, located in the highest seismic zone in the country, conducting an extensive GRA study is of prime importance. Conventionally, most of the GRA studies are carried out using the equivalent linear method, which, being a simplistic approach, cannot capture the nonlinear behavior of soil during seismic shaking. This paper presents the outcomes of a one-dimensional effective stress based nonlinear GRA conducted for Guwahati city(located in northeast India) incorporating the non-Masing load/unload/reload characteristics. The various ground response parameters evaluated from this study help in assessing the ground shaking, soil amplification, and site responses expected in this region. 2D contour maps, which are representative of the distribution of some of these parameters throughout Guwahati city, are also developed. The results presented herein can serve as guidelines for the design of foundations and superstructures in this region.

Reinforcement selection for deep and high-stress tunnels at preliminary design stages using ground demand and support capacity approach

Underground mining is going to be deeper gradually because near surface resources are going to be depleted. Therefore, risk of seismic events in underground mines is escalating. Additionally, existence of the large ratio of horizontal to vertical stress, could be a potential reason for high-stress condition and occurrence of dynamic activities. Depending on various parameters such as the level of induced stress, rock properties, etc., ground demand changes and it is difficult to estimate. On the other hand,under seismic condition, energy dissipation and deformation capacity of supports is the most important factors, however, rock support performance factors in dynamic conditions are still under investigation.Expanding the knowledge of reinforcement behaviour and capacity, specifically that of the rockbolt as a primary element in seismic conditions, would help to develop a suitable, safe and economic support design. This paper contains various methods to estimate ground demand including the intact rock properties approach, failure thickness and ejection velocity estimation, and rockburst damage potential method. It also covers measurement methods of rockbolts energy dissipation capacities such as drop test,blasting simulating, back calculation and momentum transfer measurement methods. A large-scale dynamic test rig is also explained. Based on the findings, a table and a graph to show the applicable range of each type of rockbolts were presented. Suitable rockbolt types for various ground energy demand and deformation capacity range were categorised in the table and the graph. The presented support selection method facilitates the selection of a suitable reinforcement system at the preliminary stages of design and guides the designer to adjust the support reinforcement system based on observed ground and support reaction.

Effects of gully terrain on stress field distribution and ground pressure behavior in shallow seam mining

This study proposes a novel approach to study stress field distribution and overlying ground pressure behavior in shallow seam mining in gully terrain.This approach combines numerical simulations and field tests based on the conditions of gully terrain in the Chuancao Gedan Mine.The effects of gully terrain on the in situ stress field of coal beds can be identified by the ratio of self-weight stress to vertical stress(η) at the location corresponding to the maximum vertical stress.Based on the function η =j(h),the effect of gully terrain on the stress field of overlying strata of the entire field can be characterized as a significantly affected area,moderately affected area,or non-affected area.Working face 6106 in the Chuancao Gedan Mine had a coal bed Jepth <80 m and was located in what was identified as a significantly affected area.Hence,mining may cause sliding of the gully slope and increased loading(including significant dynamic loading) on the roof strata.Field tests suggest that significant dynamic pressures were observed at the body and foot of the gully slope,and that dynamic loadings were observed upslope of the working face expansion,provided that the expanding direction of the working face is parallel to the gully.

Calculation of gas content in coal seam influenced by in-situ stress grads and ground temperature

On the basis of the analysis of coal bed gas pressure in deep mine, and the coal bed permeability ( k ) and the characteristic of adsorption parameter ( b ) changing with temperature, the author puts forward a new calculating method of gas content in coal seam influenced by in situ stress grads and ground temperature. At the same time, the contrast of the measuring results of coal bed gas pressure with the computing results of coal bed gas pressure and gas content in coal seam in theory indicate that the computing method can well reflect the authenticity of gas content in coal seam,and will further perfect the computing method of gas content in coal seam in theory,and have important value in theory on analyzing gas content in coal seam and forecasting distribution law of gas content in coal seam in deep mine.

Finite layer and triangular prism element method to subsidence prediction and stress analysis in underground mining

The application of the finite layer & triangular prism element method to the 3D ground subsidence and stress analysis caused by mining is presented. The layer elements and the triangular prism elements have been alternatively used in the numerical simulation system, the displacement pattern, strain matrix, elastic matrix, stiffness matrix, load matrix and the stress matrix of the layer element and triangular prism element have been presented. By means of the Fortran90 programming language, a numerical simulation system based on finite layer & triangular prism element have been built up, and this system is suitable for subsidence prediction and stress analysis of all mining condition and mining methods. Comparing with the infinite element method, this approach dramatically reduces the size of the set of equations that need to be solved, and greatly reduces the amount of data preparation required. It not only saves the internal storage, and the computation time, but also decreases the cost.

Modelling Propagation of Stress Waves through Soil Medium for Ground Response Analysis

buildings located at rock sites. Modelling wave propagation through soil medium helps to derive the primary and secondary wave velocities. Most of the time soil mediums are heterogeneous, layered and undergoes nonlinear strains even under weak excitation. The equivalent linear approximation with one dimensional wave propagation is widely adopted for modeling earthquake excitation for layered soil. In this paper, importance of local soil effects, the process of wave propagation through three dimensional elastic medium, layered medium situated on rigid rock, attenuation of stress waves due to material damping, equivalent linear approximation, the concept of one dimensional wave propagation, and a case study of one dimensional wave propagation as a part of site-specific ground response analyses for Delhi region are included. The case study brings out the importance of carrying out site-specific ground response analyses of buildings considering the scenario earthquakes and actual soil conditions for Delhi region.

高地应力-化学侵蚀耦合作用下炭质板岩蠕变试验及非线性蠕变损伤模型

为克服高程障碍并降低施工风险,可采用长大隧道穿越崇山峻岭,但这些隧道往往处于深埋高地应力环境,并受到化学侵蚀影响。为了解决此问题,以丽江—香格里拉炭质板岩大变形隧道为研究对象,采用室内试验和理论推导,研究深埋炭质板岩隧道受化学侵蚀作用下的围岩变形特性。在Poyting-Thomson体蠕变体的基础上,根据模型元件的力学特性,叠加了损伤元件、化学损伤元件和非线性元件,提出高地应力-化学侵蚀耦合作用下炭质板岩非线性蠕变损伤本构关系。研究结果表明:1)炭质板岩试样受化学侵蚀影响显著,侵蚀90 d试样所产生的轴向蠕变应变为侵蚀0 d试样的2.02倍,侵蚀60 d试样所产生的径向蠕变为侵蚀0 d试样的1.85倍;2)受侵蚀的炭质板岩试样在三轴压缩状态下破裂以斜向贯通裂隙为主,并产生一定的滑移错动裂隙,且沿轴线的拉伸劈裂破坏受围压作用抑制明显,未产生竖向贯通裂隙。

山东龙郓煤业10·20冲击地压事故区域应力背景与防控研究

矿山巷道、交通隧道等地下硐室围岩稳定与岩体所处区域地应力环境息息相关。分析区域深部地应力与地下硐室走向、形状等因素的关系,有助于提前规避硐室开挖风险。文章以山东龙郓煤业10·20冲击地压事故为背景,通过地应力测量与监测工作,初步揭示了山东西部地壳浅表层现今地应力环境,结合龙郓煤业矿区附近现今地应力场特征,探讨此次冲击地压事故产生的区域应力背景,并从地应力角度提出相应的防控建议。研究结果表明:测量深度范围内主应力大小总体上与深度成正比线性关系,最大水平主应力值为3.48~20.76 MPa,随深度增加梯度为0.0182 MPa/m;最小水平主应力值为3.44~14.95 MPa,随深度增加梯度为0.0130 MPa/m;区内最大水平主应力方位为北东43°~89°,平均方位为北东75°;地壳浅表层构造作用以水平构造作用为主,但随着深度的增加,逐渐向垂直构造作用转变;龙郓煤业10·20冲击地压事故的诱发机制主要是垂向应力大于水平主应力,现今处于拉张应力环境,尤其是巷道走向平行于最大水平主应力方向;建议龙郓煤业巷道轴线与最大水平主应力方向的夹角为60°~90°,同时巷道顶板可以采用拱形顶板,确保巷道岩体稳定。

基于MLR–ANN算法的地应力场反演与裂缝预测

中国页岩气储层埋藏深,受构造运动影响,地应力分布规律复杂,传统方法很难准确反演区域地应力大小和方向。提出多元线性回归和人工神经网络的耦合算法,对川南长宁—建武区块的页岩气储层及周边地应力场进行反演,并采用综合破裂系数法,对储层裂缝进行预测,划分裂缝发育区域。研究表明,基于多元回归和神经网络的耦合算法能准确反演区域的地应力场分布规律。研究区的地应力以挤压应力为主,方向在NE115°左右。受构造运动产生的断层周边应力较为集中,易发育剪切裂缝,裂缝以发育和较发育程度为主。研究区在邻近龙马溪组底部的五峰组上段和构造大断层部位裂缝发育程度较高。研究成果对该区块完善页岩气开采的井网布置、压裂优化设计和套管损坏防治等有一定的参考价值。

Pile-soil stress ratio in bidirectionally reinforced composite ground by considering soil arching effect

To discuss the soil arching effect on the load transferring model and sharing ratios by the piles and inter-pile subsoil in the bidirectionally reinforced composite ground, the forming mechanism, mechanical behavior and its effect factors were discussed in detail. Then, the unified strength theory was introduced to set up the elastoplastic equilibrium differential equation of the subsoil under the limit equilibrium state. And from the equation, the solutions were derived with the corresponding formulas presented to calculate the earth pressure over and beneath the horizontal reinforced cushion or pillow, the stress of inter-pile subsoil and the pile-soil stress ratio. Based on the obtained solutions and measured data from an engineering project, the influence rules by the soil property parameters (i.e., the cohesion c and internal friction angle φ) and pile spacing on the pile-soil stress ratio n were discussed respectively. The results show that to improve the load sharing ratio by the piles, the more effective means for filling materials with a larger value of φ is to increase the ratio of pile cap size to spacing, while to reduce the pile spacing properly and increase the value of cohesion c is advisable for those filling materials with a smaller value of φ.

西南山区铁路客运专线隧道底鼓病害变形监测研究

西南山区铁路客运专线具有高桥隧比、沿线地质条件复杂恶劣等特征,在建设和运营过程中易出现各类工程病害。文章基于某隧道底鼓变形病害,设计并开展包含围岩变形、仰拱受力和道床板沉降变形3个监测指标的隧道变形监测方案。通过对监测数据进行分析,获取隧道和围岩受力变形的演化规律,据此提出后续加密轨道几何形位静态、动态检测和衬砌结构贯通裂缝观测、实施列车降速运行等处理措施。相关研究可为该客运专线的安全运营提供有力保障,同时为类似工程隧道病害的监测和整治工作提供有益参考。

高地应力缓斜半煤岩巷道断面形状优化研究

为了优化高地应力缓斜半煤岩巷道的断面形状,以东欢坨矿3018工作面运道为工程背景展开研究。通过复变函数理论推导得出了倾斜煤层中不同断面形状的应力解析解,然后运用FLAC3D数值模拟求解了不同断面形状巷道的应力集中和塑性区情况,并以此为基础确定了巷道最优断面形状,最后结合现场工业性试验对研究结果的可靠性进行了验证。研究结果表明:高地应力作用下缓斜半煤岩巷道破坏呈现非对称形式;不管断面形状如何,巷道均在左、右两底角处呈现高应力集中,且应力集中程度矩形>梯形>直墙拱形;直墙拱形巷道稳定性最佳,梯形巷道次之,矩形稳定性最差,确定直墙拱形为最终巷道断面形状;现场巷道表面位移量监测结果表明,直墙拱形巷道更能满足现场安全生产要求。