关键词: 复合岩样, 预制单裂隙, 围压, 裂隙位置, 破坏模式

Abstract:

Composite rocks are widely present in underground geotechnical engineering such as tunnel excavation，mining projects，and oil and gas resource extraction.Composite rocks are formed by the layered accumulation of rocks with disparate properties，and are rich in fissures.The intricate features of these fissures，combined with the distribution of stress，collectively influence the stability of composite rocks，making them prone to disasters such as roof collapse，expansion of surrounding rocks，and significant tunnel deformations.Therefore，investigating the mechanical failure mechanisms of composite rocks under different confining pressures and fissure positions is of paramount importance for effectively preventing disasters in underground engineering construction.Based on the above reasons，this study prepared composite rock samples with pre-existing fissures，conducted triaxial compression tests by varying the fissure positions，and analyzed and summarized the influence of fissure positions and confining pressure on the mechanical behavior and failure modes of composite rock.The main conclusions are as follows：（1）Limestone exhibits the highest strength，while sandstone undergoes the greatest deformation under compression.The mechanical deformation of the intact composite rock sample falls between sandstone and limestone，with its strength controlled by the sandstone portion and deformation restricted by the limestone portion.（2） Under lower confining pressures，the intact rock sample and the one with fissures in limestone exhibit brittle failure，while the one with fissures in the contact zone and sandstone exhibits ductile failure.With the increasing of confining pressure，rock samples generally exhibit ductile failure，and the dominant factor in the failure characteristics shifts from the fissure position to the confining pressure.The volumetric contraction of the rock sample increases with the rise in confining pressure，and the volumetric expansion is minimal when fissures are in sandstone.Volumetric contraction is influenced by confining pressure，while volumetric expansion is influenced by fissure position.（3）In uniaxial compression，unstable crack propagation occurs earliest，and at this point，fissures have the greatest impact on the mechanical properties of the rock sample.In triaxial compression，with a constant fissure position，higher confining pressures result in more stable crack propagation when fissures are in limestone.Simultaneously，the strength and elastic modulus of the rock sample show an increasing trend with the change in fissure position from limestone，through the contact zone，to sandstone.With constant confining pressure，crack propagation is most stable when fissures are in sandstone，and the degradation of the sample is minimal when fissures are in limestone.（4）When fissures are in limestone，tensile cracks dominate，whereas in sandstone，shear cracks predominant.With the increasing of confining pressure，the failure mode of the composite rock sample shifts from tensile failure to shear failure，gradually transitioning from fissure-dominated to confining pressure-dominated failure modes.

Key words: composite rocks, pre-existing single fissure, confining pressures, fissure position, failure modes