| The natural rock mass generally contains many defects,such as joints,cracks and flaws.These defects make the rock mass show obvious non-uniformity,discontinuity and anisotropy,which brings immeasurable potential challenges and threats to the excavation and support during engineering construction.This article selects two common materials of coarse-grained sandstone and fine-grained sandstone as the research object,different types of pre-existing flaws were set up,and their mechanical properties and failure mechanisms under uniaxial,biaxial,conventional triaxial and true triaxial stress conditions are studied in depth.The main works and research findings are written as following:(1).The uniaxial and biaxial compression characteristics of coarse&fine sandstone are studied.Under uniaxial compression conditions of 30°,60°,90° angles and equal length single fractures and complete rock samples,the peak strength value ?p,peak axial strain ?p and elastic modulus E of the fractured coarse&fine sandstone all show a consistent size relationship,that is 90° fractured rock sample>60°fractured rock sample>30°fractured rock sample.The peak strength value of flaws and intact coarse&fine sandstone under biaxial stress is higher than that under uniaxial stress.Under uniaxial and biaxial compression conditions,the peak strength value ?p and elastic modulus E of cracks and intact fine sandstone are higher than those of coarse sandstone under the same conditions.(2).The mechanical properties and damage evolution law of fractured coarse&fine sandstone under conventional triaxial conditions are studied.The peak strength value?p,damage stress value ?cd,elastic modulus value E and peak axial strain ?1 of cracks and intact coarse&fine sandstone are all increase obviously with the increase of confining pressure stress(10MPa?20MPa?30MPa),while the peak hoop strain ?h generally decreases with the increase of confining pressure.It shows that the confining pressure has a significant strengthening effect on the ultimate bearing capacity of the rock sample,but of an obvious restraining effect on the lateral expansion and deformation.The peak strength value of the coarse&fine sandstone sample decreases first and then increases with the increase of the fracture angle(30°?60°?90°),and the peak strength value of the 60° fractured coarse&fine sandstone is the smallest.In addition,the effective cohesive force of the coarse&fine sandstone samples increases with the increase of the fracture angle,and the effective internal friction angle first decreases and then increases with the increase of the fracture angle,and the 60°fractured rock sample has the smallest effective internal friction angle.The effective cohesion values of the cracks and intact fine sandstone samples are greater than the coarse sandstone samples under the same conditions,and the effective internal friction angles of the cracks and the intact coarse sandstone samples are greater than the fine sandstone samples under the same conditions.The initiation angle of the main crack at the tip of the fissure coarse&fine sandstone is less affected by the confining stress conditions,and the inclination angle of the prefabricated fissure plays a decisive role.(3)The mechanical properties and fracture damage laws of fractured coarse&fine sandstone under true triaxial conditions are studied.When the fracture surface is loaded parallel to the maximum principal stress ?1,the peak strength value of coarse&fine sandstone is the highest,.when the crack surface is loaded parallel to the intermediate principal stress ?2,its peak strength value is the smallest.The peak strength of the fractured coarse sandstone sample shows strengthening and promoting effect within the range of true triaxial intermediate principal stress ratio ??[2,10].However,under the condition of higher intermediate principal stress(?>6),the fractured fine sandstone decreases in strength as the intermediate principal stress ratio increases.The peak axial strain ?1 and the peak minimum principal stress strain ?3 of the fractured coarse&fine sandstone generally decrease with the increase of the intermediate principal stress value(=2?4?6?10),while the elastic modulus E is positively correlated and increases simultaneously.The rock bridge dip angle has a significant effect on the strength and deformation characteristics of coarse&fine sandstone.The peak strength value of the fractured rock sample with the rock bridge dip angle of 135° is the highest relative to the 45° and 90° samples.Under true triaxial compression,the macroscopic cracks of the rock sample are mainly distributed on the intermediate principal stress loading surface,while the maximum principal stress and the minimum principal stress loading surface are relatively complete.When the fracture surface is loaded parallel to the intermediate principal stress,the structural characteristics of the pre-existing flaws play a decisive role in the fracture of the rock sample,the rupture zone is generally an anti-wing-shaped crack that starts and expands from the tip of the pre-existing flaws,and connects and penetrates the upper and lower ends of the rock sample to form an unstable fracture surface.(4)The deformation and fracture mechanism of coarse&fine sandstone are studied.When the confining pressure is relatively small or zero(such as uniaxial or biaxial stress conditions),the shear strength on the fracture surface of the rock sample is mainly contributed by cohesion c,while the internal friction angle ? has little effect on it.When the confining pressure is large(such as conventional triaxial or true triaxial stress conditions),the coarse sandstone sample needs to overcome the interaction of coarse grains or even fracture before sliding failure,so the internal friction angle plays a decisive role in the strength of coarse sandstone.For fine sandstone,due to the smaller grains and better roundness,the mechanical interaction between the grains is relatively weak(even with the limitation of confining pressure),so the contribution of the internal friction angle to the strength of the fine sandstone is limited.In addition,the small-grain sandstone,which roundness of the grains is better,and the strength is higher.Under the action of confining pressure,the sliding between particles after the initial closure is accompanied by creeping and dilatation of the particle unevenness before reaching the maximum shear strength,so tensile microcracks are generated.For large-grain sandstone,the shape is more irregular and rough,and the grain strength is relatively weak.Under the action of the confining pressure,the creeping tendency of the grain bumps is suppressed,and the micro bumps are sheared,which leads to the cracking of the grain crystals,so the failure process is dominated by shear microcracks.(5)Based on the generalized particle dynamics numerical method,the mechanism of the crack initiation,propagation and connection process of coarse&fine sandstone is revealed.The crack tip of coarse&fine sandstone is the first to crack and propagate due to the concentration of tensile stress,and the microcracks quickly connect and penetrate along the direction of the maximum principal stress under the mixed action of tension and shear,which generate the main rupture zone,while the far-field rupture zone is mainly formed in the post-peak stage under tension.The stress amplification effect is the main mechanical mechanism that drives the connection and penetration of the crack tip rock bridge:For collinear fractures(Type ?),the rock bridge area is mainly affected by the overlap of the shear stress field to form a shear stress amplification effect.The fracture connection mode is shear connection(S mode),and the connection cracks are secondary coplanar cracks.For stepped cracks(Type ?),the rock bridge area is mainly affected by the overlap of tensile stress fields to form a tensile stress amplification effect.The connection mode of the cracks is tensile connection(T mode),and the connection cracks are wing cracks.For coincident cracks(Type ?),the main compressive stress fields in the rock bridge area overlap to form a compressive stress amplification effect.The crack connection mode is compression connection(C mode),and the connection cracks are secondary inclined cracks. |
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