Creep behavior and permeability evolution of coal pillar dam for underground water reservoir

Using goof as water storage space plays a vital role in the ecological environment and economic development of arid mining areas,while the rock strength and the stability of coal pillars in underground water reservoirs are closely related to creep process.In this work,triaxial creep-seepage tests were conducted for coal samples to develop new insights into the creep behavior and permeability evolution.The results showed that the creep deformation and permeability evolution of coal samples exhibit three stages,namely,the compaction hardening stage before the stress threshold,volumetric compaction stage,and volumetric dilatancy stage.The coal permeability decreases first and then increases with the creep strain and it is well correlated with the variation of volumetric strain.

Spatial correlation-based characterization of acoustic emission signal-cloud in a granite sample by a cube clustering approach

To extract more in-depth information of acoustic emission(AE)signal-cloud in rock failure under triaxial compression,the spatial correlation of scattering AE events in a granite sample is effectively described by the cube-cluster model.First,the complete connection of the fracture network is regarded as a critical state.Then,according to the Hoshen-Kopelman(HK)algorithm,the real-time estimation of fracture con-nection is effectively made and a dichotomy between cube size and pore fraction is suggested to solve such a challenge of the one-to-one match between complete connection and cluster size.After,the 3D cube clusters are decomposed into orthogonal layer clusters,which are then transformed into the ellip-soid models.Correspondingly,the anisotropy evolution of fracture network could be visualized by three orthogonal ellipsoids and quantitatively described by aspect ratio.Besides,the other three quantities of centroid axis length,porosity,and fracture angle are analyzed to evaluate the evolution of cube cluster.The result shows the sample dilatancy is strongly correlated to four quantities of aspect ratio,centroid axis length,and porosity as well as fracture angle.Besides,the cube cluster model shows a potential pos-sibility to predict the evolution of fracture angle.So,the cube cluster model provides an in-depth view of spatial correlation to describe the AE signal-cloud.

Evaluating the mechanical properties of anisotropic shale containing bedding and natural fractures with discrete element modeling

Natural fracture data from one of the Carboniferous shale masses in the eastern Qaidam Basin were used to establish a stochastic model of a discrete fracture network and to perform discrete element simulation research on the size efect and mechanical parameters of shale.Analytical solutions of fctitious joints in transversely isotropic media were derived,which made it possible for the proposed numerical model to simulate the bedding and natural fractures in shale masses.The results indicate that there are two main factors infuencing the representative elementary volume(REV)size of a shale mass.The frst and most decisive factor is the presence of natural fractures in the block itself.The second is the anisotropy ratio:the greater the anisotropy is,the larger the REV.The bedding angle has little infuence on the REV size,whereas it has a certain infuence on the mechanical parameters of the rock mass.When the bedding angle approaches the average orientation of the natural fractures,the mechanical parameters of the shale blocks decrease greatly.The REV representing the mechanical properties of the Carboniferous shale masses in the eastern Qaidam Basin were comprehensively identifed by considering the infuence of bedding and natural fractures.When the numerical model size is larger than the REV,the fractured rock mass discontinuities can be transformed into equivalent continuities,which provides a method for simulating shale with natural fractures and bedding to analyze the stability of a borehole wall in shale.

On the excavation-induced stress drop in damaged coal considering a coupled yield and failure criterion

Investigating the stress drop of abutment pressure is the key to a deep quantitative analysis of the discontinuous stress redistribution under mining.In the present study,uniaxial and triaxial compression tests are carried out separately to determine the bulk and shear moduli,the cohesion,and the internal friction angle of the coal samples.By extending the meaning of Mohr’s circle referring to yield stress instead of the maximum principal stress,a yield line is introduced to illustrate the stress drop of Mohr’s circle referring to yield stress instead of the maximum principal stress at the elastoplastic boundary.Furthermore,a theoretical solution of the stress drop as a function of the damage is proposed to investigate how the abutment pressure differs considering the yield line and failure line.In addition,applying the stress drop at the yield line in non-pillar mining,top coal mining,and protective coal mining shows that the damage has a nonlinearly positive influence on the stress drop.The results shows that the bulk modulus and internal friction angle have a more sensitive influence on the stress drop than do the shear modulus and cohesion.Finally,the stress drop is divided into a discontinuous stress drop at the yield line and a structural stress drop at the failure line.The stress drop is effective in describing the discontinuous stress redistribution and shows a clear difference in the movement direction of Mohr’s circle considering the unloading pressure.

Cluster modeling of the short-range correlation of acoustically emitted scattering signals

As a widely used measurement technique in rock mechanics,spatial correlation modeling of acoustic emission(AE)scattering signals is attracting increasing focus for describing mechanical behavior quantitatively.Unlike the statistical description of the spatial distribution of randomly generated AE signals,spatial correlation modeling is based mainly on short-range correlation considering the interrelationship of adjacent signals.As a new idea from percolation models,the covering strategy is used to build the most representative cube cluster,which corresponds to the critical scale at peak stress.Its modeling process of critical cube cluster depends strongly on the full connection of the main fracture network,and the corresponding cube for coverage is termed the critical cube.The criticality pertains to not only the transition of local-to-whole connection of the fracture network but also the increasing-to-decreasing transition of the deviatoric stress with an obvious stress drop in the brittle failure of granite.Determining a reasonable critical cube guarantees the best observation scale for investigating the failure process.Besides,the topological connection induces the geometric criticality of three descriptors,namely anisotropy,pore fraction,and specific surface area,which are evaluated separately and effectively.The results show that cluster modeling based on the critical cube is effective and has criticality in both topology and geometry,as well as the triaxial behavior.Furthermore,the critical cube length presents a high confidence probability of being correlated to the mineral particle size.Besides,its pore fraction of cube cluster is influenced strongly by the critical cube length and confining pressure.

Prediction of fracture and dilatancy in granite using acoustic emission signal cloud

The invisibility of fracture network evolution in the rock under triaxial compression seriously restricts the correlation modeling between dilatancy behavior and fracture interconnectivity.The key to solving such a challenge is strongly dependent on the accurate modeling of the spatial correlation in fracture network,which could be indirectly re-constructed by the acoustic emission(AE)signal cloud.Considering the interaction of local fractures,a cube cluster approach is established to describe the spatial correlation.The evolutional cube clusters effectively present the geometric characteristics induced by the increasing dilatancy of fracture.Two descriptors(i.e.three-axis length sum and pore fraction)are introduced to correlate cluster model with dilatancy behavior.Most fitting results support the linear correlation between two descriptors and volumetric strain,which verifies the sensitiveness of the cube cluster model to dilatancy.More importantly,by the statistical analysis of cluster structure,the cluster model shows the potential of calculating fracture angle.Moreover,a comparison between dilatancybased damage and porosity-based damage is made not to prove the best but provide an AE-based prediction of local damage evolution.Finally,four classical models for calculating fracture angle are compared.The deviations prove the huge difficulty of describing the development of the fracture network uniquely dependent on a fracture angle.The proximity of measured angle and cluster-based angle supports the effectiveness of predication by the cube cluster approach.