mechanism of exfoliation joint formation in granitic rocks yosemite national park abstract fractographic techniques reveal mechanical aspects of exfoliation in granitic rocks at yosemite national park and electronic surveying provides information on their attitudes and dimensions in the middle elevations of the cliff of el capitan exfoliation consists of early fractures a few meters to tens of meters in size which are fan shaped fans at upper elevations point upwards whereas fans at lower elevations point downwards parallel to the cliff the fans interact with each other and merge into composite joints hundreds of meters in size normal to the minimum compressive principal stress this polarity indicates exfoliation by longitudinal splitting and buckling the palaeostress causing the exfoliation on the southwestern side of half dome is estimated to range between 0 01 mpa and 0 94 mpa differences in fracture paleostresses of joints may be used in comparing their relative propagation velocities it is suggested that the large exfoliation on the southwestern side of half dome had undergone a prolonged sub critical growth before attaining rapid fracture velocities under post critical conditions

paleostress determination in a rock by a fractographic method abstract a fractographic technique hitherto used in ceramics for the evaluation of fracture stress is adapted here to calculate fracture paleostresses in rocks an example from a granite in east sinai is analyzed the results obtained for the ranges of local paleofracture stress the severest flaw size and the initial stress intensity factor agree reasonably well with previous evaluations of fracture stresses in granite and with direct observations of the rock s grain size

preuplift joints in granites evidence for subcritical and postcritical fracture growth this study relates to jointing in granite that took place in the crust before uplift thus offering an insight into paleo fracture processes our investigation reveals a unique set of joints within granite of the south bohemian pluton exposed in the borsov quarry southeast of prague in the czech republic these joints occur in close proximity and exhibit a great variety of fracture surface morphologies the joints in the borsov granite may be divided into four groups based on fractographic features 1 partial mirror planes without fringes 2 mirror planes rimmed by en echelon fringes 3 complex en echelon fringes and 4 hackled fringes on the basis of theoretical experimental and fractographic data obtained in outcrops we have constructed a curve for the change in crack velocity v as a function of the stress intensity factor k i and we have plotted the various joints in the borsov granite on this curve the spread of the joints along the curve indicates that the joints terminated under widely different physical conditions possibly responding to repeated extensional events and different fracture mechanisms hence a new quantitative method of joint classification is introduced such a great variety of fractographic features is unknown from joint sets that occur in a single quarry in sedimentary rock because their formation always relates to a particular fracture mechanism we argue that the joints in the borsov granite formed before uplift in a fluid enriched environment while the granite responded to diverse local effective stress conditions

three zone characterisation of coupled strata and gas behaviour in multi seam mining abstract we propose here a three zone conceptual model in overlying strata of a longwall panel that accounts for the coupled behaviour of strata deformation and gas flow the model comprises a fractured gas interflow zone a de stressed gas desorption zone and a confined gas adsorption zone the fractured gas interflow zone represents the area where mining induced cross strata fractures and bedding separations are well developed with high permeability in both the vertical and horizontal directions coal seam gas can easily be released from this lower zone to flow down into the mine workings the de stressed gas desorption zone which lies above the fractured gas interflow zone is another significant gas producing zone in which strata are highly de stressed however mining induced fractures in this zone are mainly created in the form of bedding separations which only increase horizontal permeability and thus the gas cannot easily flow vertically down to the mine workings in the upper confined gas adsorption zone strata depressurisation is limited the major proportion of coal seam gas in this zone remains adsorbed and cannot be effectively captured while both lower zones are the targets of gas drainage the fractured gas interflow zone is the main source of ventilation gas emission and the prime area of gas control we have developed an approach to determine the height of these three zones based on the hypothesis of key stratum in strata movement and verified the approach using gas drainage experience at a chinese coal mine the applications of the three zone concept in selecting appropriate gas drainage methods for varied mining conditions assessment of methane recovery efficiency and gas drainage optimisation and maximisation in a mining district of china are also discussed

evolution of polygonal patterns in stratified mud during desiccation the role of flaw distribution and layer boundaries detailed analysis of diagnostic surface morphologies of fractures in natural mud indicates that mud cracks systematically nucleate at the bottom of the mud and propagate vertically upward toward the free surface and laterally outward toward adjacent cracks earlier generations of mud cracks rupture the set of desiccated layers altogether forming polygonal patterns that are similar throughout the mud sequence later generations of mud cracks subdivide each layer separately forming markedly different polygonal patterns within individual mud layers a simple mechanical model draws an analog between cooling of granular materials and drying of mud it shows that subfact defects at grain boundaries near the base may become critical before defects associated with fine particles at the top even though the stress profile due to drying is usually more tensile at the top thin section analysis and sieving method indicate that grain boundaries at the bottom are several times longer than at the top of the mud due to the natural sorting of grains illustrating the validity of the model this study suggests that stress variations with depth are less important for pattern evolution of mud cracks than previously theorized however nonuniform flaw distribution and surface discontinuities play a fundamental role during mud crack nucleation and growth these results have significance for several subjects including experimental studies of cracking induced by desiccation the previously suggested analogy between natural mud fracturing and basalt fracturing and the use of mud cracks in stratigraphic interpretation

permeability variations within mining induced fractured rock mass and its influence on groundwater inrush this paper is concerned with the evaluation of permeability of fractured rock mass due to the cover stress re establishment which is a major factor in controlling water and gas flow rate induced by mining operations in fractured rock the case study considered in this paper is based on the results of observations of groundwater inrush and a decrease in water inflow from the fractured roof strata due to mining advancing in the taiping coalmine shandong province china a conceptual model of an effective porous media was used to assess the permeability distribution in the fractured zone induced by coal mining the cover stress re establishment in gob fractured rock mass was evaluated using an empirical formula based on the surface subsidence a simplified conceptual model of the fractured zone was used to evaluate the deformation of fractured zone along with the evaluation of changes in the rock permeability above the gob due to the cover stress re establishment these data were then used to calculate the water inflow rate into the panel predicted water inflow rates have been found to be in good agreement with those from monitoring data this study improved the understanding of the mechanisms of the post mining cover stress re establishment on permeability change of the overburden fracture rock strata these results can then be applied for numerical simulations of the process of overburden failure and consequent groundwater inrush due to coal mining

observations and classification of roof strata behaviour over longwall coal mining panels in india abstract a study has been made of a number of mechanized longwall panels in various coal mines in india concentrating on the geology physico mechanical rock properties and the behaviour of the coal measure roof rocks during mining the research has highlighted the splitting and caving characteristics of the strata rocks and enabled the development of a roof rock classification system roof rocks are classified into six categories based on the a mean weighted uniaxial compressive strength b rqd c i type of rocks ii presence of cracks fissile beds splitting etc iii presence of water and d thickness of bed layer the behaviour of layers and composite layers in the immediate roof rock mass is also enumerated relations between the rock strength properties and values for the bulking factor of the failed rock are included the concepts of a weighting zone and caving zone are proposed the paper also clarifies the occurrence of different types of failure such as periodic falls all these findings should be helpful during calculation of the powered support capacity

evolution of the stress field near the arava basin located along the dead sea fault system as revealed by joint sets abstract to reveal the development of the stress field near the arava basin located along the dead sea fault system we characterized joint sets that were exposed mainly in the santonian and eocene chalk rock outcropping in the study area in total 2600 strike orientations of single layer joints slj and multilayer joints mlj mirrored four main stress fields 1 hnw j1 326 2 hnnw j2 343 3 hnne j3 033 and 4 hn j4 360 the results showed that the relatively lower values of the fracture spacing ratio fsr and their standard deviations in j1 implied that j1 was formed during the burial stage of the eocene chalk formations the inconsistency between the statistical parameters of the log normal distributions of joint spacing and fsr raised doubt about the applicability of the saturation joint density model in our studied area the joint sets characteristics suggest that the regional hnw operated during the eocene during the miocene the regional stress field changed clockwise to hnnw probably in association with the early stage of transform motion along the dead sea fault in the pliocene pleistocene the hnne and hn local stress fields both operated and were accompanied by a tensional stress field that can be identified in the intensive normal faulting evolved along the western arava basin margins

columnar jointing in vapor phase altered non welded cerro galan ignimbrite paycuqui argentina columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits however the non welded cerro galan ignimbrite at paycuqui argentina contains well developed columnar joints that are instead due to high temperature vapor phase alteration of the deposit where devitrification and vapor phase crystallization have increased the density and cohesion of the upper half of the section thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures above 630 c but the lack of welding textures indicates temperatures below the glass transition temperature in order to remain below the glass transition at 630 c the minimum cooling rate prior to deposition was 3 0 10 3 8 5 10 2 c min depending on the experimental data used for comparison alternatively if the deposit was emplaced above the glass transition temperature conductive cooling alone was insufficient to prevent welding crack patterns average 4 5 sides to each polygon and column diameters average 75 cm are consistent with relatively rapid cooling where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer the presence of regularly spaced complex radiating joint patterns is consistent with fumarolic gas rise where volatiles originated in the valley confined drainage system below joint spacing is a proxy for cooling rates and is controlled by depositional thickness valley width we suggest that the formation of joints in high temperature non welded deposits is aided by the presence of underlying external water where vapor transfer causes crystallization in pore spaces densifies the deposit and helps prevent welding

stress field fluctuations along the dead sea rift since the middle miocene the dead sea transform dst is the suture between the african and the arabian plates both of which subduct northward under the eurasian plate the collision of these plates with eurasia and their relative motion are the source for their internal deformation especially in the vicinity of the dead sea transform geological observations indicate that two distinct paleostrain regimes operated adjacent to the dead sea rift and in the sinai israel subplate 1 wnw shortening less than 10 and nne extension beginning in the turonian is associated with the development of the syrian arc fold belt and attributed to the syrian arc stress field sas and 2 middle miocene to recent nnw shortening and ene extension associated with the 105 km sinistral displacement along the dead sea transform and the opening of the red sea is attributed to the dead sea stress field dss it was originally suggested that the sas terminated during the middle miocene with initiation of the dss however trend and age analysis of many structures shows that formation of sas compatible structures continued after the middle miocene up to the recent in some middle miocene and younger rocks structures such as faults dikes volcanic lineaments and tectonic stylolites compatible with both stress fields were reported in a few locations structures compatible with both stress fields crosscut each other it is suggested that the movements which resulted in the development of the syrian arc and other sas compatible structures are continuing to the recent and that dss movements are superimposed upon them the overlapping of sas and dss structures adjacent to the dst results from spatial and temporal fluctuations in the overall stress state we propose that such variations could be caused by the superposition of large earthquake stress drops associated with movements along the dst on a steady background plate scale stress regime associated with the sas thus dss compatible structures should form in preseismic activity periods when dst related elastic strain is high on the other hand the sas compatible structures should form during interseismic activity times subsequent to large local stress release along the dst

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

characteristics of stratum movement induced by downward longwall mining activities in middle distance multi seam abstract with the development of mining depth multi seam mining activities is becoming more and more frequent correspondingly the study on stratum movement characteristics induced by multi seam mining activities is of great significance for preventing water inrush disaster and forecasting support pressure surface deformation gas drainage and other mining problems according to the interaction among the caved zone the fractured zone and the continuous deformed zone caused by the multi seam downward mining activities multi seam can be divided into three types i e the short distance middle distance and long distance multi seam this paper carries out a case study on the middle distance multi seam mining activities in a coal mine in china to investigate the characteristics of stratum movement caused by downward longwall coal mining activities in the multi seam by means of borehole water level observation digital borehole camera test microseismic monitoring surface crack observation of the surface cracks and distinct element modelling by analyzing the in site monitoring results and the numerical simulation results it can be found that the failure of the key stratum between the lower and upper seams resulting in the connection between the failed zone in floor induced by the upper seam and the fractured zone induced by the lower seam after the failure of the key stratum the mining activities of the lower seam has a great influence on the secondary stratum movement in the overburden these effects are mainly reflected in the rapid increase of horizontal displacement and vertical displacement which caused the caved and fractured zones extend upward in the overburden and then surface cracks increase significantly the above research results are of great significance for the prevention and control of disasters induced by downward longwall mining activities in middle distance multi seam

new method for determining the elastic parameters of rock mass layers in the region of underground mining influence abstract the primary objective of the following paper is to determine the value of strata elastic parameters in a selected polish mining area using a new method based on finite element analysis gsi geotechnical classification and back analysis the individual rock layer strain parameters were estimated by means of comparing the measured subsidence trough with the result of numerical analysis carried out for the same geological and mining conditions as a result of such an analysis the values of the elastic modulus of the anisotropic model of strata were obtained in the region of underground mining influence

the stability of methane capture boreholes around a long wall coal panel this paper describes the application of computational modelling to the prediction of the stability of methane drainage boreholes during the extraction life of an active long wall coal panel within a united kingdom coal mine the modelling was undertaken to investigate the affects that changes in the roof geology and roadway support system may have on the deformation and closure of the gas drainage boreholes drilled from the tailgate across the goaf of an active long wall panel the paper also outlines the development of an analytical model to estimate the bending deformation axial strain and the rupture of borehole casing due to rock shear this model was used in conjunction with the geomechanical strata models to predict the distance behind the face line at which excessive borehole deformation and closure would occur this method was used to evaluate the optimum roadway support system and spacing between boreholes to maintain effective borehole drainage subject to the range of geological conditions encountered during the panel retreat a good correlation was obtained between the model predictions and the geomechanical and gas drainage data measured at a uk colliery

the role of the interbed thickness on the step over fracture under overburden pressure multilayer jointing in tight oil and gas reservoirs is a useful indicator for the prediction of fluid mobility since pore pressure balances the principal minor horizontal stress at depth joint nucleations considered as sub vertical are mainly driven by the overburden pressure we demonstrate with a mixed criterion combining necessary conditions of stress and energy how heterogeneity such as an interbed promotes initiation of a fracture by a step over mechanism referring to critical characteristics such as the interbed thickness a specific initiation criterion emerges giving rise to a size effect due to the interbed thickness and is compared to a typical homogeneous rock failure criterion jointing over an interbed is enhanced by large interbed thicknesses even for moderate effective confining stresses size effect is reversed according to the relative importance of singular effects effective minor stress normal to the fracture plane and structural effect parallel loading effective conditions

preliminary study on the feasibility of co exploitation of coal and uranium abstract both coal and uranium resources are found in the dongsheng area which is located in the northern part of ordos basin china with the coal seam below the uranium deposit in this paper we attempt to study the feasibility of co exploitation of the coal seam and the uranium deposit the permeability evolution in strata around the coal seam are selected as the key parameter indexing the separate and safe concurrent recovery of two deposits and a novel strain dependent permeability model is proposed covering the full deformation range of rock the proposed model is first verified with experimental data with good fitting of the results then an upscaled numerical model is established considering the geology conditions of dongsheng area this is implemented using comsol multiphysics and the interface of comsol with matlab is used to study the damage to the rock mass we show that a compared with the stress dependent permeability model the strain dependent permeability model proposed in this work has the advantage that each strain value is associated with a unique permeability value b the feasibility of co exploitation of the coal seam and the uranium deposit depends on the relative distance between two resources when the caved and fractured zone encroaches on the uranium deposit the coal seam should be abandoned conversely when the uranium deposit lays in the constrained zone the recovery of the coal seam would benefit recovery of the uranium deposit c the feasibility of the co recovery of the two resources can be optimized by the choice of mining approach the top caving mining approach and room and pillar mining approach can reduce the height of the caved and fractured zones and make the co exploitation of the uranium and coal feasible this work provides a new approach to investigate the feasibility of co exploitation of uranium and coal

joint nucleation from cavity shaped flaws field observations probability of occurrence and 3 d mechanical analysis 1 we present field observations probability of occurrence and three dimensional mechanical analysis of joint nucleation from cavity shaped flaws a unique set of cavity shaped flaws and their associated joints were documented in cretaceous dolomite layers that crop out in central israel in this set cavities that initiate joints initiator cavities are commonly larger and located farther away from the bottom of the layer than non initiator cavities they also have significantly less neighboring cavities the probability of occurrence of initiator cavities is calculated by a logistic regression and compared to a mechanical analysis stress distribution around a cavity is calculated analytically using the solution for the elastic stress distribution around a spherical cavity in an infinite medium under triaxial loading and numerically using three dimensional simulations the analytical solution and numerical modeling show that a single cavity transforms the remote compression into local tension around its periphery the numerical simulations enable us to relate the magnitude of the tensile stress to the cavity size location within the bed and the relative orientation of the neighbor cavity our observations and analyses indicate the existence of effects arising from both the size of cavities and their distribution in the rock mass it is shown that the reduction of rock strength by increasing porosity cannot simply apply to heterogeneous media such as carbonatic rocks these results demonstrate the role played by large isolated and preferably located cavities in initiating joints in heterogeneous media

an experimental and numerical investigation on the deformation of overlying coal seams above double seam extraction for controlling coal mine methane emissions abstract this study investigates the deformation and the resultant pressure relief of highly gassy coal seams where a double seam mining operation takes place at lower depths at the wulan coal mine china in order to predict the depressurization effect in the overlying coal seams we simulate the extraction process by constructing a scaled model in the laboratory using similar rock materials analyses of experimental results concluded that due to the mining induced stress redistribution the pressure within target coal seams which were 109 m above the mined seams could be fully relieved to attain the statutory approval for gas drainage in addition to scaled model studies computational modeling studies were conducted using udec universal distinct element code which showed that the features of deformation resulting from the double seam extraction were different from that of in the case of single seam extraction the results of the numerical studies revealed that in addition to the panel margin on the air return side areas near the intake entry could also be considered as borehole drilling positions besides it was found that the gob of the extracted seams below the gassy one provided a buffering effect for the would mined protective coal seam and that the depressurization effect was largely weakened the laboratorial findings are instructive to the field practice of methane drainage during the mining operation a displacement comparison method was adopted to measure the dilated amount of protected coal seams and as expected the maximum dilation percentage was much more than 0 2 the critical value upon which the target coal seam is appropriately depressurized to allow gas drainage to be safely and effectively implemented

the dynamics of western north america stress magnitudes and the relative role of gravitational potential energy plate interaction at the boundary and basal tractions summary we investigate the forces involved in driving long term large scale continental deformation in western north america and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions in addition we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere we directly solve force balance equations for vertically averaged deviatoric stresses associated with differences in values of 1 lithospheric thickness times the gravitational potential energy per unit area gpe the gpe values are inferred using both the etopo5 topographic data set and the crust2 0 crustal thickness model deviatoric stresses associated with basal tractions are calculated globally with inputs determined from an isoviscous upper mantle 10 21 pa s 3 d large scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction in a 211parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators i e the directions and relative magnitudes of the principal axes of kinematic strain rates magnitudes of the total vertically averaged deviatoric stress field sum of gpe solution with the boundary condition solution range from 5 to 10 mpa within a 100 km thick lithosphere these magnitudes are calibrated by the gpe differences along with the spatial variation in deformation style there is a trade off between the scaling of the basal traction deviatoric stress field and the boundary condition solution however the combined boundary conditions plus basal traction solution is robust in both magnitude and style and when added to the contribution from gpe differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators gpe variations account for 50 per cent of the deviatoric stress magnitudes driving deformation while boundary condition stresses account for the remaining 50 per cent of deviatoric stress magnitude by comparing possible end member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western north america lies within the brittle seismogenic layer

effect of sand grain size on simulated mining induced overburden failure in physical model tests abstract to investigate the influence of sand grain size sgs on mining induced overburden failure in physical model tests uniaxial compressive experiments on rock like material with various sgss were performed and two sgss 0 0 5 and 0 5 1 mm were selected the young s modulus uniaxial compressive strength and tangent modulus were larger in rock like material samples with finer sgs and the failure mode changed from mostly shear failure to tensile failure with rising sgs as the mechanical characteristics of coarser sand samples are weaker the overburden breakage distance was shorter in the coarser sand model and the ratio of the average breakage distance between the two models was 1 688 the extent of the overburden fracture was heavier in the coarser sand model and closer pattern to that of the actual coal mine a large database of the heights of the overburden caved zone hcz and fractured zone hfz values with various mining height hm based on previous studies was compiled to obtain best fit parabolic empirical formulas the hcz hm and hfz hm ratios measured in the field were similar to those estimated by the proposed empirical formulas the discrepancy between the physical model test and field measurement may be explained by the fracture pattern in the overburden of the caved zone in the form of layered rectangles unlike the polygonal forms in the coal mine we propose four recommendations to optimize the physical model for such tests from the perspective of sgs the results and recommendations presented here can provide a useful guide for coal mining scientists and engineers designing models for testing of coal mining conditions

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

impact of mechanical heterogeneity on joint density in a welded ignimbrite joints are conduits for groundwater hydrocarbons and hydrothermal fluids robust fluid flow models rely on accurate characterisation of joint networks in particular joint density it is generally assumed that the predominant factor controlling joint density in layered stratigraphy is the thickness of the mechanical layer where the joints occur mechanical heterogeneity within the layer is considered a lesser influence on joint formation we analysed the frequency and distribution of joints within a single 12 m thick ignimbrite layer to identify the controls on joint geometry and distribution the observed joint distribution is not related to the thickness of the ignimbrite layer rather joint initiation propagation and termination are controlled by the shape spatial distribution and mechanical properties of fiamme which are present within the ignimbrite the observations and analysis presented here demonstrate that models of joint distribution particularly in thicker layers that do not fully account for mechanical heterogeneity are likely to underestimate joint density the spatial variability of joint distribution and the complex joint geometries that result consequently we recommend that characterisation of a layer s compositional and material properties improves predictions of subsurface joint density in rock layers that are mechanically heterogeneous

an estimation method for cover pressure re establishment distance and pressure distribution in the goaf of longwall coal mines measurement of deformations and stresses in the goaf of longwall panels is difficult due to the inaccessibility of the waste area this paper proposes a method for estimating the distance to return of the cover pressure and the stress distribution in the goaf of flat lying longwall panels where the caving is bulking controlled in the development of the method it has been considered that the final subsidence on the surface is a result of deformation of the caved material and deformation of the strata over the caved zone i e during both the caving and the compaction of the caved material in this respect the stress strain behaviour of the caved rock pile bending of the lowest uncaved strata depending on seam thickness and residual expansion of fractured strata vertically over the caved zone have been described by using previous field and laboratory investigations from the relations established between these parameters and surface subsidence the re establishment of cover pressure distance has been characterised with a three parameter power function in which the independent variables are depth excavation height bulking factor and compressive strength of the rock fragments it has been found that the cover pressure distance increases exponentially by increasing depth unlike previous findings of investigations in which the cover pressure distance increases as a linear function of depth on the other hand the cover pressure distance for a constant depth and mining height increases if the bulking factor of the rock pile increases and or the strength of rock fragments decreases when the mining height increases the bulking factor of the caved rock pile increases and hence this result with increase in cover pressure distance an estimation of the stress distribution curve for panels which are wide enough for the development of cover pressure has been expressed with a six parameter taylor series polynomial function the general form of the stress development curves found from the proposed method has been verified with curves obtained through numerical models

coupled disaster causing mechanisms of strata pressure behavior and abnormal gas emissions in underground coal extraction the generalization and application of fully mechanized caving with super great mining height sgmh face a series of new issues of which strata pressure behavior and abnormal gas emissions are the most prominent disasters and display a close relationship the tashan coalmine was chosen as the experiment base because of its sgmh and the overlying hard and thick main roof htmr this coalmine had experienced many damaging hydraulic support and abnormal gas emission accidents induced by strata pressure behavior the key strata theory and the fracture failure process analysis model established by 3dec were used to analyze the structural characteristics and fracture failure process of the htmr the geostress reduction and permeability enhancement redistribution of the adjacent coal measure strata were studied by the geostress and permeability distribution model established by flac 3d the htmr controlled the failure extent and the movement evolution of the entire overburden stratum the htmr acted as a large double sided embedded rock beam during primary fracturing and a cantilever articulated rock beam during periodic fracturing and it generated a huge hanging space above the gob the fully mechanized caving with sgmh caused large scale disturbances and permeability enhancements in the adjacent rock and coal strata then the substantial amounts of gas stored in the remaining coal surrounding rock strata and adjacent coal seam rushed out and aggregated in the caved and fissured zones of the gob forming a huge gas warehouse the strata pressure behavior and abnormal gas emission disasters primarily occurred after the primary and periodic fracturing of the htmr these disasters are predominantly caused by the escaping of the abundant gas that existed in the gob during mine pressure behavior processes when the ultimate broken span was reached as the htmr rotated and collapsed the substantial amounts of gas that accumulated in the gob escaped into the working face which easily caused abnormal gas emissions hydraulic presplitting and gas drainage technologies are proposed to solve these disasters

relative fracture velocities based on fundamental characteristics of joint surface morphology two different joint surface morphologies plumes and rib marks characterize joint surfaces but the mechanical conditions that lead to the formation of either of the morphologies are not understood well we studied two orthogonal joint sets that cut the same santonian chalk beds in the judea desert israel joints of the j1 set are systematic relatively long characterized by almost exclusively by plumes and predate the shorter non systematic joints of the j2 set that are characterized by rib marks joints of the j1 set formed at high stress during deformation of the syrian arc folding in the late senonian joints of the j2 set formed at lower stress that occurred because of stress relaxation after the formation of the j1 joints a mechanical analysis indicates that the j1 joints propagated at subcritical velocities several orders of magnitude faster than the j2 joints based on previously published data of laboratory tests the plumes and the rib marks are semiquantitatively placed on the subcritical part of the fracture velocity vs stress intensity factor diagram

compaction characteristics of the caving zone in a longwall goaf a review broken rock and coal residual coal plus material from the immediate roof and overlying strata fill in the goaf in an area termed the caving zone due to its high porosity and permeability the caving zone contains gas and water which may have originated from the mined coal seam the adjacent unmined coal seam or from any aquifer or surface river thus studying the compaction characteristics of the caving zone can help understand gas and mine water drainage and identify steps to prevent spontaneous combustion of residual coal the stability characteristics of the caving zone after mining are affect surface subsidence as well as water and gas build up and use the caving zone is a potential underground storage of greenhouse gases therefore the time space relationship of caving zone compaction characteristics in the goaf has become an area for research focus in recent years in this study the formation height determination and compaction characteristics of a caving zone are examined reduction in block size and rearrangement of the fill are the main factors affecting the compaction process as re crushing and rearrangement of the broken coal and rock mass affect the secant modulus and pore size of the caving zone causing the secant modulus to gradually increase and pore size to decrease this in turn affects the macroscopic stress strain curve and seepage characteristics of the caving zone the strength and fracturing mode of the caving blocks are the main factors affecting the re crushing and rearrangement of the caving blocks the applicability and reliability of present research results and research methods are analyzed and the focus areas for future studies are identified using a combination of research methods including theoretical analysis laboratory testing numerical simulation and field measurement the compaction characteristics of a caving zone in longwall goaf can be accurately calculated

a conceptual model to characterize and model compaction behavior and permeability evolution of broken rock mass in coal mine gobs abstract this paper proposed a conceptual model of broken rock mass compaction based on elastic theory by simplifying the compression process this conceptual model assumes that the contact connection between two adjacent rock particles is similar to a cubic mass with this simplification the stress strain constitutive law is established the change in the secant modulus in the mechanical model derived from the variation of the connection coefficient agrees well with the reported experimental results the permeability of compacted rock mass evolution was modeled based on the cubic law the mechanical compression model was coupled with the permeability evolution model the modeled permeability evolution is consistent with reported simulation and experimental results the modeled permeability results were validated using karacan s data with broken shale rock properties compared to an intact rock mass we found that the stress strain curve of a compacted rock mass takes a longer compression path to reach linearity due to the void space compaction resulting from friction slipping and the re arrangement of particles it was also found that the particle elastic modulus does not contribute to the overall bulk compaction and permeability reduction at the initial compaction stage however the particle elastic modulus controls the permeability evolution for a fully compacted gob where the gob can be treated as an intact rock mass the proposed conceptual models will potentially lay the foundation for future permeability and caving behavior characterizations using numerical simulations for complex gob areas

tectonic analysis of the dead sea rift region since the late cretaceous based on mesostructures we have measured the orientations of small faults slickensides tectonic stylolites vein with secondary mineralization small folds and dikes in israel and sinai these structures are indicators of paleo stress or strain and provide the pattern of tectonic deformation data were collected at 130 stations most with tens of separate measurements most stations showed consistent deformation stations were located on exposures ranging from precambrian crystalline rocks to pleistocene sediments we have defined two tectonic stress fields each relatively uniform in both time and space one stress field with dominating maximum horizontal compression trending w to wnw in the late cretaceous to eocene rocks in the folds and plateaus west of the dead sea rift the second field with dominating horizontal extension trending e to ene in all rocks inside the rift and proximal thereto the first stress field is called the syrian arc stress and the second is called the dead sea stress a change in style of tectonic deformation which corresponds to the two stress fields is manifested also in the major structures in israel the late cretaceous to neogene deformation is characterized by long wavelength folds and monoclines whereas the neogene to recent deformation is characterized by normal and strike slip faults and volcanic activity

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

joint nucleation in layered rocks with non uniform distribution of cavities abstract characterization of critical flaws that initiate into systematic joints is essential for understanding the process of fracturing in sedimentary rocks in this study advantage is taken of the well developed surface morphology of joints in the dolomite layers of the judea group central israel and the role played by spherical cavity shaped flaws during nucleation and growth of joints is analyzed an analysis of cavity geometry and distribution reveals that joint initiation points are governed by such isolated relatively large cavities preferably located close to the bedding top and by stress gradients during joint nucleation in the absence of macroscopic cavities joint initiation points are located along bedding interfaces consequently joints typically show two forms of growth depending on the abundance and spatial distribution of cavities within the layers in layers with plentiful cavities joints preferably nucleate at critical cavities propagate vertically toward the bedding interfaces and horizontally toward adjacent joints and form elliptical fractures in layers free of cavities joints nucleate at bedding interfaces commonly propagate downward toward the layer base and adjacent joints and form semi elliptical fractures in both cases bedding interfaces between the dolomite layers impedes joint propagation fractographic methods based on joint surface morphology yield satisfactory first order estimates of the driving paleostress required to initiate joints in the judea group

loading configurations and driving mechanisms for joints based on the griffith energy balance concept abstract using the griffith energy balance concept to model joint propagation in the brittle crust two laboratory loading configurations serve as appropriate analogs for in situ conditions the dead weight load and the fixed grips load the distinction between these loading configurations is based largely on whether or not a loaded boundary moves as a joint grows during displacement of a loaded boundary the energy necessary for joint propagation comes from work by the dead weight i e a remote stress when the loaded boundary remains stationary as if held by rigid grips the energy for joint propagation develops upon release of elastic strain energy within the rock mass these two generic loading configurations serve as models for four common natural loading configurations a joint normal load a thermoelastic load a fluid load and an axial load each loading configuration triggers a different joint driving mechanism each of which is the release of energy through elastic strain and or work the four mechanisms for energy release are joint normal stretching elastic contraction poroelastic contraction under either a constant fluid drive or fluid decompression and axial shortening respectively geological circumstances favoring each of the joint driving mechanisms are as follows the release of work under joint normal stretching occurs whenever layer parallel extension keeps pace with slow or subcritical joint propagation under fixed grips a substantial crack normal tensile stress can accumulate by thermoelastic contraction until joint propagation is driven by the release of elastic strain energy within the earth the rate of joint propagation dictates which of these two driving mechanisms operates with faster propagation driven by release of strain energy like a dead weight load acting to separate the joint walls pore fluid exerts a traction on the interior of some joints joint propagation under fluid loading may be driven by a release of elastic strain energy and or work by the pore fluid during poroelastic contraction again propagation velocity dictates the driving mechanism with critical to supercritical joint propagation accompanying fluid decompression and subcritical crack propagation taking place under a constant fluid drive during axial shortening a jointing process called axial splitting may occur following wing crack growth from an initial crack tilted relative to the maximum compressive stress here the joint driving mechanism may be either the work of a remote stress i e a dead weight or the release of elastic strain energy i e fixed grips

deformation failure and permeability of coal bearing strata during longwall mining abstract to mitigate water shortage a few groundwater reservoirs are being built at the daliuta coal mine shaanxi province china it is essential to investigate the deformation failure and permeability of coal bearing strata during longwall mining and their influences on the construction of groundwater reservoirs in this paper the local geology was introduced briefly and the complete stress strain and permeability for different rock samples were investigated using laboratory experiments and then a three dimensional coupling model between stress and permeability was proposed after that a series of numerical simulation was performed to investigate the dynamics of stress permeability coefficient using flac3d it turns out that due to the relatively low mechanical strength of the coal bearing strata at the daliuta coal mine there are only two zones above the goaf that is caving zone and fracture zone and the fracture zone extends directly to the surface the rock mass permeability is closely related to the stress strain and the permeability decreases with the rise of stress in the stage of elastic deformation while it increases significantly in the stages of plastic deformation and failure the maximum value of permeability coefficient appears in the stage of strain softening and the value varies greatly with the rock lithology during the advancement of longwall face the redistribution range is much wider for permeability than for rock failure and the permeability is the largest on the periphery of goaf rock mass with increased permeability is horizontally oval shaped and vertically saddle shaped the permeability of coal seam roof and floor experiences three stages decrease increase and gradual recovery during coal mining the results are in good agreement with those of physical modeling and field observation the outcomes may contribute to the site selection and construction of the groundwater reservoir at the daliuta coal mine

investigating the effect of simultaneous extraction of two longwall panels on a maingate gateroad stability using numerical modeling abstract in longwall mining method the stability of the maingate and tailgate gateroads is of great importance because these excavations are used for essential services such as ventilation and transportation the aim of this paper is to develop a numerical model to investigate the stability of a maingate gateroad named p8 maingate during caving and extraction of p7 and p8 coal panels of tabas parvadeh underground mine stability analysis is carried out by critical shear strain criterion zone plasticity conditions support system efficiency and strength stress ratio conditions using flac3d a software based on finite difference numerical methods in the critical shear strain criterion method the stability analysis is conducted by comparing the shear strain increments of the element in the model and sakurai critical shear strain in the second stability analysis method based on zone plasticity the zones indicating shear plastic failures are the potential instability causing regions in the support system efficiency method the applied load and pressure are compared to the yielding strength of the support systems to check their stability in the strength stress ratio ssr method a safety factor is determined for the gates and the strength versus stress plots of the gates are assessed to control the stability condition the results of numerical modeling indicate the stability of the p8 maingate using these methods the modeling results are then compared to the instrumentation data to investigate the consistency where a good agreement was observed

predicting of land surface uplift caused by the flooding of underground coal mines a case study abstract the liquidation of underground mines as in flooding of mining excavations and goafs often leads to movements of the rock mass and land surface as a result of pressure changes in the flooded zones the changes resulting from the rising water table caused by the changes in the stress and strain state as well as the physical and mechanical properties of rock layers can lead to damage to surface construction objects and environmental changes such as chemical pollution of the surface water this is why the ability to predict the movements of rock masses generated as a result of mine closure by flooding serves a key function in relation to the protection of the land surface and buildings present thereon currently the world literature presenting methods of forecasting ground surface uplift during mine flooding is very modest and is limited to three analytical methods pottgens fenk s and sroka s method however the numerical methods used in this subject area are limited only to modeling the phenomenon using the back analysis method and are not used for forecasting the author s methodology based on the finite element method presented in the article allows on after calibrating the 3d numerical model using surface subsidence measurements as well as geological and mining data to simulate the flooding process by changing the density of the medium according to classical soil and rock mechanics principles and then obtain prognostic values for ground surface uplift this methodology has been used in the example of a german underground coal mine which has been flooded in 1999 the authors have built a 3d numerical model and then performed simulations the results of calculations were compared with the results of measurements obtaining compliance at the level of 5

monitoring the strata behavior in the destressed zone of a shallow indian longwall panel with hard sandstone cover using mine microseismicity and borehole televiewer data abstract massive coal extraction from the longwall panel results in the formation large void space goaf which is followed by elevation in the stress level of the overlying strata the increasing stress leads to rock displacements in form of deformation fracturing and caving of the roof behind the powered supports for regaining the equilibrium state these rock displacements divided the longwall roof into three zones known as caved zone cz fractured zone fz and continuous deformation zone cd previous studies have shown that the overlying strata in the destressed zone behaves violently in presence of hard rock such as compact sandstone shale etc which is a major problem faced in the indian underground mines therefore the present study intends to monitor the roof behavior of a shallow longwall mine working depth 68 74m mining height 2 3m of central india which is overlain by a thick sandstone bed average thickness 37 48m using six months oct 01 mar 02 mine induced microseismicity containing event coordinates in 3d plane and event parameters such as magnitude energy and apparent stress along with borehole televiewer bhtv data the paper discusses the results of some important studies which are very essential for enhancing the safety in the mining area such as a the height of destressed zone was calculated through multiple approaches empirical relations mining height and bulking factor and found to be around eighteen times the mining height hc hf 18h b microseismicity study of the longwall roof showed that events corresponding to fractured zone had the lowest b value b 0 3301 and highest occurrence frequency a 2 3178 c borehole televiewer bhtv data showed the presence of massive fractures and water seepage level in fractured zone at depth of about 40 50m d the integrated approach of b value energy magnitude e m and gutenberg richter frequency magnitude relation showed that events corresponding fractured zone had the highest magnitude 1 2 and energy 106 6j and e the behavior of microseismic event properties such as event counts energy apparent stress and magnitude was also studied with respect to mine geology and abnormal changes were seen when the roof strata was encountered with hard rock mainly coarse grained sandstone with rqd 60 and fault ff the study successfully helped towards obtaining several useful results related to dynamic roof displacement and failures which very much enhanced the safety in the longwall mines

surface morphology of columnar joints and its significance to mechanics and direction of joint growth columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes new observations show that each band contains a single plumose structure and thus represents an individual crack or joint segment formed during a discrete growth event analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies columnar joints form by nucleation and growth of new cracks on the edges of older cracks each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face where thermal stress is concentrated inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle ductile transition of lava outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava cracks often diverge slightly from the planes of previous cracks probably because of spatial and temporal changes in directions of local principal stresses mechanical interaction causes a diverging crack to overlap curve toward and usually intersect the previous crack behind its edge leaving a blind tip that points in the overall growth direction of the columnar joints this and other directional criteria are applied to determine joint growth patterns in several lava flows of the western united states in two tiered and multi tiered flows downward growing columnar joints usually meet upward growing joints well below the middle of the flows which indicates very rapid cooling of upper portions relative to lower portions this supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows whereas conduction is probably the dominant cooling mechanism at the bases

bulking factors and extents of caved zones in weathered overburden of shallow abandoned underground workings abstract a comparison between the bulking factor and height of the caved zone for different weathered subsurface rocks over shallow abandoned mines and for strong rocks at intermediate and large depths of longwall mining is performed in this study vertical boreholes drilled from the ground surface have been used to detect the height of caving in weathered w and very weathered vw rock mass overlying abandoned underground workings at a shallow depth and then to calculate the bulking factor it is found that key parameters that control the bulking factor k and the ratio between the heights of caving and mining h h are different for w and vw rocks in w rocks such parameters are the rock strength of immediate roof and rock porosity whereas those for vw rocks are rock quality designation rqd and standard linear fracture intensity p10 it is argued that h h in vw rocks increases according to linear logarithmic law with an increasing ratio p10 rqd in the immediate roof and p10 in four rock layers over the immediate roof observations show that vw rocks exhibit smaller values of k 1 06 1 165 and larger h h 6 07 15 6 compared to w rocks having 1 09