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Wellbore instability problems mainly contribute to increased nonproductive time (NPT) through drilling operation developments in the Southern Iraq oilfield. This work aims to study the impact of drilling fluid chemicals on shale formation based on lab tests. At the same time, a mechanical earth model (MEM) constructs for the wellbore failure analysis, which simulates the mechanical effect on the shale formation. The Lab tests showed that the shale formation is of many types, including illite, kaolinite, palygorskite clay mineral, montmorillonite, and non-clay minerals quartz and calcite, where kaolinite was a dominant clay mineral of the shale. The shale samples swelled slightly when sodium silicate was used with polymer mud, but they had significant swelling when potassium chloride was applied with polymer mud. The MEM showed that the leading cause of those problems is using unsuitable mud weight (10.4 ppg) during the drilling of the Tanuma formation. Thus, the model indicates that an equivalent mud weight of 11 ppg for vertical wells will be required to stabilize the Mishrif and Tanuma shale formations. This work was unique in studying chemical and mechanical effects to analyze the wellbore stability and develop the optimum solutions. [ABSTRACT FROM AUTHOR]

If, as previously hypothesized, the effective elastic response of the lithosphere is sensitive to the imposed stress regime, then it may vary in time and produce distinctive geomorphic responses. Such effects will be at their most crucial in landscapes of low relief. Motivated by the existence of numerous small endorheic (internally‐drained) basins in central Australia, we examine the influence of changing elastic response in the presence of large embedded loads in the lithosphere underlying stable continental interiors. Focusing on the western Lake Eyre Basin and adjoining Lake Lewis basin—an area with a close correlation between drainage pattern and extreme Bouguer gravity anomalies—we devise a set of numerical simulations that incorporate the flexural response to time‐transient horizontal stresses. The simulations demonstrate that transient changes in the effective elastic thickness can drive topographic changes in low‐relief landscapes, including drainage capture and the development of endorheic basins, consistent with field observations. Plain Language Summary: Extreme density anomalies in central Australia suggest the presence of significant stress within the lithosphere, even though the region lies far from any tectonic plate boundaries. This in situ stress probably dates back to a mountain‐building period in the Paleozoic. The density anomalies correlate closely with drainage patterns and a set of internally‐drained catchments, suggesting an important relationship exists between the deep‐earth and landscape‐forming processes. We propose the driving mechanism is the result of changes in the rigidity of the lithosphere, which in the presence of in situ stress lead to surface uplift or subsidence. We use a landscape evolution model to show that topography similar to field observations can be simulated by imposing cycles of uplift and subsidence brought about by these changes in lithospheric rigidity over tens of millions of years. In consequence, the lithosphere must have fairly low rigidity counter to previously held ideas about continental interiors. Key Points: Loads embedded within the central Australian lithosphere produce large flexural responses that may evolve cyclically over timeSurface deflections induced by transient lithospheric rigidity can explain endorheic basins formed at wavelengths of order 100 kmCyclical behavior in lithospheric rigidity is consistent with the observed geomorphic record of erosion and deposition [ABSTRACT FROM AUTHOR]

Wellbore instability occurs when an open well cannot maintain its size, shape, or structural stability. It is a common issue especially in shale sections and can be caused by both mechanical factors, such as poor drilling technique or weak rock, and chemical factors, such as interactions between the rock and drilling fluid. This problem can lead to costly and dangerous complications. In this study data from three wells in Nahr Umr Oil Field (NR-10, NR-12, and NR-14) in southern Iraq were analyzed to determine pore pressure and rock strength parameters, as well as in situ horizontal stresses. The results showed an increase in pore pressure and horizontal stress in shale units, and various geomechanical parameters were also estimated. The study showed the fault regime in the area is a strike-slip fault and suggested the estimated pore pressure values while drilling hole sections to prevent problems at abnormal and subnormal formations. [ABSTRACT FROM AUTHOR]

Borehole instability problems have caused non-productive time while drilling operations in the Nahr Umr shale formation. The drilling difficulties, including stuck pipe, caving, and tight holes, have been identified as significant problems in this formation. This study aims to comprehend the causes of wellbore failure and choose the most suitable drilling strategy. In this regard, a mechanical earth model (MEM) was performed using open-hole wireline logging measurements from the offset wells data. Those data included gamma ray, density log, compressional wave velocity, shear wave velocity, neutrons porosity, and image log. Moreover, there were also measured data such as leak-off tests and modular formation dynamics testers that were applied to validate the model's accuracy. The analysis exhibited that the leading cause of wellbore instability problems was improper mud weight (10.6 ppg). In addition, inappropriate drilling practices and the heterogeneity of the Nahr Umr formation have magnified the risk of drilling problems. The MEM outcome proved that the mud weight should be 11.6–13.5 ppg to safely drill a vertical well. Thus, this work can be applied as a cost-effective tool when designing future neighboring deviated wells. [ABSTRACT FROM AUTHOR]

The prediction of in situ stress based on azimuthal seismic data has extensive use in horizon crushability evaluation. Nonetheless, existing in situ stress seismic prediction models do not consider overburden pressure and tectonic strain, which limit the prediction accuracy. To this end, we propose a decoupled overburden pressure in situ stress prediction method under tectonic constraints. The key to this method is to consider that the overburden pressure could act on the rock skeleton and pore fluid, i.e. generating effective pressure and pore pressure; the pore pressure can be estimated using Eaton's method, and then the effective stress can be obtained. The relationship between tectonic strain and effective pressure is constructed based on Hooke's law, where tectonic strain can be calculated from curvature attributes extracted from seismic data. Introducing pore pressure and deriving a model for calculating the maximum and minimum horizontal stresses and the difference in horizontal stress ratio for orthotropic media (OA). When the Thomson anisotropy parameters and the pore pressure are neglected, the proposed model can be degraded to a conventional horizontal transverse isotropy medium in situ stress prediction model, which proves the validity of the model. The results of sensitivity analysis experiments affirm the need to decouple overburden pressure and account for tectonic strain when predicting in situ stress reasonably. Finally, single-well and azimuthal seismic prediction were carried out by using the a priori information from well logging and seismic data. [ABSTRACT FROM AUTHOR]

Abstract If, as previously hypothesized, the effective elastic response of the lithosphere is sensitive to the imposed stress regime, then it may vary in time and produce distinctive geomorphic responses. Such effects will be at their most crucial in landscapes of low relief. Motivated by the existence of numerous small endorheic (internally‐drained) basins in central Australia, we examine the influence of changing elastic response in the presence of large embedded loads in the lithosphere underlying stable continental interiors. Focusing on the western Lake Eyre Basin and adjoining Lake Lewis basin—an area with a close correlation between drainage pattern and extreme Bouguer gravity anomalies—we devise a set of numerical simulations that incorporate the flexural response to time‐transient horizontal stresses. The simulations demonstrate that transient changes in the effective elastic thickness can drive topographic changes in low‐relief landscapes, including drainage capture and the development of endorheic basins, consistent with field observations.

Copyright of Journal of Petroleum Research & Studies is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Hydraulic fracturing was the main technology to achieve the economic development of conglomerate reservoirs, knowing that the hydraulic fracture propagation mode was of great significance for improving the development of conglomerate reservoirs. This paper proposed a new method to understand the hydraulic fracture behavior based on a hybrid finite-discrete element method. The simulation indicated that a complex fracture network was created near the wellbore in the studied conglomerate reservoir, and hydraulic fracture propagation around the gravel layer was the main failure mode when the hydraulic fracture reached the gravel layer. From the simulations, it was shown that under small differences in horizontal stress and tensile strength, the hydraulic fracture propagated more easily around the gravel layer, while it could cross the gravel under large differences in horizontal stress and tensile strength. Greater tensile strength differences can reduce the complexity of the fracture network. In addition, higher pumping rates and viscosities of fracturing fluid contribute to the complex fracture network and also can produce more gravel crosses when the hydraulic fracture is met. The main reason was that a higher pumping rate and higher viscosity of fracturing fluid can obtain a higher net pressure, which can ensure the hydraulic fracture crosses the gravel layer. [ABSTRACT FROM AUTHOR]

The seismic source release by fault slip under the influence of mining is an important factor to induce rock burst. In this study, the normal fault and reverse fault with different horizontal stress are constructed by numerical simulation and the fault slip law and stope dynamic response characteristics when mining the panel through the faults are compared and analyzed. Studies show that compared with mining the panel through the normal fault, the normal stress level of the reverse fault is higher, so the shear strength of the reverse fault is greater. When the fault coal pillar is small and the mining disturbance to the reverse fault is severe, the slip displacement of fault increases significantly, the shear stress drops suddenly, and the fault slip seismic source is greater. Under the influence of reverse fault slip dynamic load, the vibration velocity of coal and rock mass near the mining space, the reduction value of advanced abutment pressure and the affected range are larger, and fault slip rock burst is more likely to occur. The on-site statistical of fault slip rock burst cases are consistent with the numerical simulation results, verifying the reliability of the numerical simulation methods and conclusions.

Borehole instability problems lead to increased non-productive time, essentially during the drilling of shale formations. Past drilling experiences exhibited several problems: mud losses, tight hole, caving, mechanical stuck pipe, and extensive hole collapse. The study aims to construct a mechanical earth model (MEM) to analyze the wellbore stability. Thus, MEM was employed to define a safe mud weight window and improve the drilling performance on development wells. The specific analysis of wellbore instability showed that the primary cause of those instability problems was applying inadequate mud weight to drill the Tanuma shale formation. In addition, the minimum required drilling density to drill through the Tanuma formation is a strong function with a wellbore inclination. Thus, for the Tanuma formation, the model indicates that wellbore inclinations above 50° should be avoided. NW–SE azimuth provides less risk in terms of hole collapse in the Tanuma formation. Also, the sensitivity points showed that the appropriate mud weight for horizontal wells in Mishrif formation was 12.5 ppg. [ABSTRACT FROM AUTHOR]

To effectively control the deformation and instability failure of the mining gateway under the goaf in deep coal seams, taking the case of 209 working face transportation roadway in Longde Mine and adopting the mechanics of the roadway under goaf by field monitoring, theoretical analysis and numerical simulation, a combined support technology with grouting, full anchor cable and rack shed is proposed, and then the grouting parameters and support parameters are proposed. Research indicates that under the influence of great horizontal stress, the surrounding rock of the roadway was experienced bending and tensile failure, the roadway is deformed and unstable when tensile stress of surrounding rock is greater than the tensile strength of the coal rock; the area of the surrounding plastic zone was significantly reduced, and the stress concentration range of the roof and floor is decreased; the surface displacement of the roadway is effectively controlled after the support of the roadway, the deformation rate of the surrounding rock of the roadway tends to be stable after ten thousand steps, and the displacement of the roadway are basically stable at two thousand steps, the supporting effect is relatively efficiency.

Copyright of Chinese Journal of Computational Mechanics / Jisuan Lixue Xuebao is the property of Chinese Journal of Computational Mechanics Editorial Office, Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Isolation piles are widely used to increase the horizontal resistance of high‐speed railway under adjacent loading in soft soils. This study proposes a theoretical model for horizontal additional stress of pile‐reinforced area subjected to adjacent loading. The isolation piles and the soil between piles in the reinforcement area are equivalent to a uniform medium in the proposed model. The Heaviside function is introduced to avoid the partition solution problem of pile‐reinforced area and the foundation soils. A combined method with the inverse Fourier transform, Gauss integral, Lagrange quadratic interpolation, and Simpson quadrature formula is employed in the calculation process. The theoretical model is validated by the finite element method. According to the calculated results, the distribution of horizontal additional stress is obtained. The horizontal soil displacements at the location of the railway increases in the depth range of 0–2.5 m and then gradually decreases. The proposed model with engineering significance is available for preliminary design of isolation piles. [ABSTRACT FROM AUTHOR]

The harvest of hydrocarbon from the depleted reservoir is crucial during field development. Therefore, drilling operations in the depleted reservoir faced several problems like partial and total lost circulation. Continuing production without an active water drive or water injection to support reservoir pressure will decrease the pore and fracture pressure. Moreover, this depletion will affect the distribution of stress and change the mud weight window. This study focused on vertical stress, maximum and minimum horizontal stress redistributions in the depleted reservoirs due to decreases in pore pressure and, consequently, the effect on the mud weight window. 1D and 4D robust geomechanical models are built based on all available data in a mature oil field. The 1D model was used to estimate all mechanical rock properties, stress, and pore pressure. The minimum and maximum horizontal stress were determined using the poroelastic horizontal strain model. Furthermore, the mechanical properties were calibrated using drained triaxial and uniaxial compression tests. The pore pressure was tested using modular dynamic tester log MDT. The Mohr–Coulomb model was applied in the 4D model to calculate the stress distribution in the depleted reservoir. According to study wells, the target area has been classified into four main groups in Mishrif reservoir based on depletion: highly, moderately, slightly, and no depleted region. Also, the results showed that the units had been classified into three main categories based on depletion state (from above to low depleted): L1.1, L1.2, and M1. The mean average reduction in minimum horizontal stress magnitude was 322 psi for L1.1, 183.86 psi for L1.2, and 115.56 psi for M1. Thus, the lower limit of fracture pressure dropped to a high value in L1.1, which is considered a weak point. As a result of changing horizontal stress, the mud weight window became narrow. [ABSTRACT FROM AUTHOR]

The Subtropolis Mine is a room-and-pillar mine extracting the Vanport limestone near Petersburg, Ohio, U.S. In February of 2018, mine management began implementing a heading re-orientation to better control the negative effects of excessive levels of horizontal stress. The conditions in the headings improved, but as expected, stress-related damage concentrated within crosscuts. The mine operator has worked to lessen the impact of the instabilities in the outby crosscuts by implementing several engineering controls. With the implementation of each control, conditions were monitored and analyzed using observational and measurement techniques including 3D LiDAR surveys. Since the heading re-orientation, several 3D LiDAR surveys have been conducted and analyzed by researchers from the National Institute for Occupational Safety and Health (NIOSH). This study examines (1) the characteristics of each 3D LiDAR survey, (2) the change in the detailed strata conditions in response to stress concentrations, and (3) the change detection techniques between 3D LiDAR surveys to assess entry stability. Ultimately, the 3D LiDAR surveys proved to be a useful tool for characterizing ground instability and assessing the effectiveness of the engineering controls used in the heading re-orientation at the Subtropolis Mine.

Copyright of Fly Ash Comprehensive Utilization is the property of Hebei Fly Ash Comprehensive Utilization Magazine Co., Ltd. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To study the shape characteristics of the coal and rock under the horizontal stress, thoroughly understand the mechanism of coal and gas outburst, the analysis and comparison of cave shape characteristics and numerical simulation results in the outburst site of Pingdingshan No.8 coal mine are made to prove the accuracy and feasibility of the numerical simulation method, furthermore, the development characteristics of the shape of the caves can be obtained under the conditions of different conditions of horizontal stress σH. The experimental results show that trend length and inclination length of the outburst caves increase exponentially with the increase of horizontal stress σH, and the trend length increases faster, when the horizontal stress σH reaches 28 MPa, the growth rate of the curve is obviously improved; the hole length increases linearly with the increase of the horizontal stress σH, and the growth rate is slow.

The mechanism of the roadway rockburst remains unclear, lacking necessary theoretical model. To reveal the mechanism of the roadway rockburst, taking the "8.2" rockburst accident of the Tangshan Coal Mine of China as an example, the root of the accident was examined, and a mechanical model of the roadway side coal sliding was established. The sliding instability of the roadway side coal was driven by the horizontal stress. Results show that the damage incurred by the roadway rockburst is mainly the roadway side coal sliding, accompanied with floor heave. The combination of the coal with high bearing capacity near the working face and a cantilever beam develops due to the suspended hard roof forming a "seesaw" structure. Under the high abutment pressure of the working face, the roof above the working face rebounds upwards, and the coal below loses the clamping between the roof and the floor. Eventually, the rockburst occurs as a result of the instability of the "seesaw" structure under the horizontal stress. The conclusions obtained in this study provide the theoretical foundation for roadway rockburst prevention and control, and facilitates safe and efficient production. [ABSTRACT FROM AUTHOR]

In order to research influence of the maximum principal stress direction on stability of roadway surrounding rock, taking rectangular haulage roadway of 1101 working face in a mine in Guizhou as engineering background, deformation of roadway surrounding rock was researched when the maximum principal stress was horizontal or vertical stress by use of FLAC3D numerical simulation software. The research results show that when the maximum principal stress is horizontal stress, roof is greatly affected and prone to serious shear failure, and roof displacement is larger than that of two sides, so roof support should be strengthened. When the maximum principal stress is vertical stress, two sides are greatly affected and are prone to shear failure, and displacements of roof and two sides increase obviously with the increase of vertical stress, so two sides support should be strengthened. Based on numerical simulation results and the situation that horizontal stress is greater than vertical stress of the working face, the horizontal stress is approximately regarded as the maximum principal stress, and a supporting scheme was proposed which focused on roof support. The field monitoring results show that displacements of roadway roof and two sides are small after applying the support scheme, which verifies that the support scheme can maintain stability of roadway surrounding rock well.

The thickness of the reserved top pillar is one of the important factors to ensure the safety of the stage backfilling stope in metal ore. Based on the basic assumptions of plane strain in elastic mechanics，the mechanical model for reserved top pillar thickness of stage backfilling stope in metal ore is established and solved by the Galerkin method. Taking copper mine as an engineering example，the variation of horizontal stress of the reserved top pillar with thickness and upper load，and the variation between thickness and length of the room are studied. The research shows that： when the size of the room is fixed，and the thickness of the reserved top pillars remains unchanged，the horizontal stress increases linearly with the increase of the upper load. The upper load is constant，and the horizontal stress decreases in the binomial form with the increase of thickness. When the width，the horizontal stress and the upper load of the mine room are constant，the thickness of the reserved top pillars increases binomially with the room length increasing. The thickness of the reserved top pillars is determined combined with engineering examples，which can meet the safety requirements of mine，The research results have certain theoretical guiding significance for the selection of reserved top pillar thickness of stage backfilling stope in metal ore. [ABSTRACT FROM AUTHOR]

It is an important problem for the alternated strong and weak roof weighting to threat the safety of working face during shallow coal mining and the total thickness breaking of the thin bedrock to cause the serious ground subsidence. To reveal the mechanism of the abnormal mining damage, the pressure-arching rule in overlying strata was studied. Based on the monitoring data of the typical shallow coal working face, the mechanical models of the symmetrical stress arch, the squeezed arch and the hinged structure of the fractured strata were established, and the difference of the load bearing capacity between the structures and the influencing factors was analysed by the deduced formula calculation. Then the evolution characteristics of the pressure-arch in the fractured strata were revealed by the numerical simulation analysis. The results show that the global pressure-arch of multilayer strata always exits in the surrounding rock and moves forward with continuous mining. The single pressure-arch and hinged structure are formed in each stratum under the global pressure-arch. The pressure-arch enables the fractured strata to carry load efficiently, and the instability of the pressure-arch can cause strong roof weighting and ground subsidence. These conclusions provide a theoretical reference for the stability control of the overlying strata structure under shallow coal mining.

Copyright of Mining & Metallurgy (10057854) is the property of Beijing Research Institute of Mining & Metallurgy Technology Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To investigate the mechanism responsible for mining-induced ground movement at the Jinshandian Iron Mine in China, the vertical and horizontal deformations of the ground-surface were monitored in situ over a period of 8 years. The results were analyzed and combined with subsidence results derived from laser-scanning measurements and collapse/cracking behavior of the ground. The results show that the horizontal displacement mainly occurs in the rock masses surrounding the mined voids. Moreover, several obvious onset points can be found in the displacement–time curves that are indicative of rapid increases in displacement. Currently, the scope of the ground movement of the footwall is larger than that of the hanging wall. This behavior is distinctive and distinguishes the Jinshandian Iron Mine from others. It can be attributed to the sliding–toppling failure of the column-shaped rock masses in the footwall under the combined effect of the in situ horizontal stress and high dip angles of the discontinuities. [ABSTRACT FROM AUTHOR]

In order to study deformation characteristics of roadway surrounding rock under effect of high horizontal stress in Gaojiapu Coal Mine, geostress of Gaojiapu Coal Mine was measured. The results indicate that Gaojiapu Coal Mine is dominated by horizontal stress. The maximum horizontal principal stress is 38.67 MPa and azimuth angle is 227.30°. Numerical simulation and field test of surrounding rock deformation of two return airways with different angles between roadway axis and the maximum horizontal principal stress direction were conducted. The results show that distribution of plastic zone is near the roadway surrounding and shows oval shaped when angle between the roadway axis and the maximum horizontal principal stress direction is 12°. Plastic zone of roof develops on top corners of the roadway and is similar to square when angle between the roadway axis and the maximum horizontal principal stress direction is 78°. The damage degree of roadway roof and floor is higher than that of two sides, and the larger the angle between the roadway axis and the maximum horizontal principal stress direction, the greater the deformation of surrounding rock.

Despite the various approaches in the literature adopted for analysing cone penetration test (CPT), determination of the initial horizontal stress and relative density of a sandy soil during this test has not been tackled yet. In order to propose a new method in this regard, a numerical study of CPT has been performed and the predicted results have been compared with several databases of comprehensive calibration chamber tests. The penetration mechanism has been then fully investigated by presenting different kinds of outputs for the surrounding soil. Finally, an innovative procedure has been suggested for estimating the initial horizontal stress and relative density during performing CPT in a sandy soil. The suggested method is based on recording the time of occurrence of a pre-specified displacement in the surrounding soil during the test. This recording can be carried out using high-precision digital inclinometer systems which are mounted in some boreholes in different directions and various distances around the location of CPT. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Mass changes and flow in the Earth's mantle causes the Earth's crust not only to move vertically, but also horizontally and to tilt, and produce a major stress in the lithosphere. Here we use a gravimetric approach to model sub-lithosphere horizontal stress in the Earth's mantle and its temporal changes caused by geodynamical movements like mantle convection in Fennoscandia. The flow in the mantle is inferred from tectonics and convection currents carrying heat from the interior of the Earth to the crust. The result is useful in studying how changes of the stress influence the stability of crust. The outcome of this study is an alternative approach to studying the stress and its change using forward modelling and the Earth's viscoelastic models. We show that the determined horizontal stress using a gravimetric method is consistent with tectonics and seismic activities. In addition, the secular rate of change of the horizontal stress, which is within 95 kPa/year, is larger outside the uplift dome than inside. [ABSTRACT FROM AUTHOR]

Root-pavement conflicts are a common challenge in urban forest management. Given the expense of pavement installation and the time needed to develop large trees, there is a preference for modelling design solutions to integrate trees and pavement with shared soil volumes. While Finite Element (FE) models are used in pavement analysis and design, and have been used to describe root growth in soils, there are few examples exploring of integration of the two applications with respect to designing urban root zones under pavements. Four series of FE models were developed to test the influence of an asphaltic concrete wearing surface layer thickness and base layer thickness (the top two layers in a pavement design) in surface crack formation from a growing tree root simulation in positions within the top zone of the base layer and below the base layer. The expected pavement damage was defined at places where horizontal tensile stresses exceeded 862 kPa. The testing series were developed from a common group of 18 FE pavement layer configurations using 3 asphalt concrete (AC) thicknesses and 3 granular base thicknesses with a growing root element at 2 root elevations. In the first series, an AC layer of a thickness of 7.6 cm did not exceed the horizontal stresses needed to develop a crack when a simulated root element increased from 3.6 to 5.1 cm when the root element was below the base layer at least 10.16 cm below the bottom of the AC layer. A second series of 77 model runs with a refined root simulation verified the impact of a changed root simulation (shift of element shape with a material modeling change that accounted for soil hardening during deformation). For the second series, a 2.54 cm diameter circular root was expanded to a final size ranging from 3.8 to 10.16 cm. Results of the second series confirmed and added detail to the first, but required a significant increase in computational time to accommodate the added model and data output details. The third series developed root simulations from an initial 0.508 cm diameter (the smallest diameter available in the PLAXIS FEM modeling platform) to a maximum 5.08 cm diameter in a series of 55 tests. The third set allowed a method to observe and compare the colonization step to a series of radial growth steps, which caused minimal influence to the stress state of the AC as a consequence of soil displacements from the root growth. Finally, in the fourth testing series of 58 simulations, we tested a cluster of three root elements at a 10.16 cm spacing 0.635 cm below the AC or the base layer, growing from an initial 2.54 cm to 6.35 cm diameter. Modeling three root elements at the 10.16 cm spacing generated tensile stress-induced cracks in all AC-base-root depth configurations. Displacements in those models exceeded the imposed upward displacement limit of 1.27 cm after doubling in size from an initial 2.54 cm diameter. [ABSTRACT FROM AUTHOR]

Abstract In-situ stress estimation is of great significance in deep coal mining. The vertical stress of the underground rock mass is generally estimated by the product of the upper cover average specific gravity and its depth, and it is difficult to estimate the horizontal stress. In this study, based on the Mohr-Coulomb criterion, the ultimate stable equilibrium conditions of the underground rock mass were applied to estimate the horizontal stress range. The estimated values agreed strongly with the in-situ measurement results, demonstrating the validity and feasibility of this estimation method. Meanwhile, this approach can explain the phenomenon whereby the ratio of the horizontal to vertical stress is distributed in a wide range of the shallow cruse zone but in a narrow range of the deep area. Furthermore, the influences of different parameters on the Mohr-Coulomb envelope were analyzed. [ABSTRACT FROM AUTHOR]

With the continuous increase in underground engineering depth, the risk of coal–rock dynamic disasters, such as rockburst, is becoming increasingly serious and complex threatening the safety of coal resources. To study the effect of horizontal stress on fractal characteristics of rockburst fragments in coal mining, this paper uses a digital drilling instrument to perform rockburst proneness tests on andesite and granite specimens with different horizontal stresses. Fractal theory is used to calculate the fractal dimension of the rockburst fragments for several combinations of parameters. The residual elastic energy index is combined with the fractal dimension of rockburst fragments to assess the propensity of rockbursts from macroscopic and microscopic perspectives. The experimental results show that the positive correlation between the fractal dimension and the A EF obtained under the two methods. The fractal dimensions of andesite and granite are 1.507–1.80 and 1.734–1.979, respectively. According to the rockburst proneness criterion, it is predicted that andesite and granite are all strong rockbursts under confining pressures of 20 MPa, 30 MPa and 50 MPa. Therefore, it is of some theoretical relevance to investigate the rockburst mechanism in terms of both the A EF and fractal dimensions of fragments under different unloading conditions. • Rockburst proneness is studied for rocks using digital drilling instrument with different horizontal stresses. • The residual elastic energy index is obtained from the drilling parameters to estimate rockburst proneness. • The residual elastic energy index is combined with the fractal dimension to assess the rockburst proneness. • The fractal dimensions of andesite and granite are 1.507–1.80 and 1.734–1.979, respectively. • The andesite and granite are all strong rockbursts under confining pressures of 20 MPa, 30 MPa and 50 MPa. [ABSTRACT FROM AUTHOR]