The study of rock fracture under triaxial conditions captures the essential features. Conventional triaxialcompressivetestsunderconfiningpressurebeenusedasasimpleandeffectivewayto investigate theprogressivefailureprocessinrockmaterial. However, intermediate principal stresscan'tbeconsideredinconventionaltriaxialcompressiontests. The paper presents a numerical model to analyze three-dimensional rock failure process and fracture mechanism under multi-aixial stressloading. Six conventional triaxial compression tests are undertaken under different confining pressure to investigate the failure process of rock specimens and the confining pressureeffectisdiscussedintermsofstress-straincurves,fracturepatternsaswellaspeak strength. Thenanothersixtruetriaxialcompressiontestsareundertakenundertruetriaxial loadingstresstostudytheintermediateprincipalstresseffect. In conventional triaxial compression tests,the orientation ofthe shearfracture planes to the loading axis increases with the confining pressure and the peak strength increases with the increasing of confining pressure. Itcanbefoundthatthesignificanteffectofintermediateprincipalstressonthepeakhadtwo zonesintruetriaxial compressiontests, andthis phenomenon can be explained withthe Twins Shear Criterion. The numerical results also reveal that intermediate principal stress also influencesthefracturepatternsignificantly, whichhasbeenignoredby manyresearchers. The presented numerical model is approved to be a useful tool to investigate rock failure behaviors.


The combinations of statistical theory with numerical models such as the lattice model by Van Mier (1997) the bonded particle model by Cundal (1998), or the RFPA2D modelbasedonFEMbyTang(1998)arefoundtobe appropriate for modeling brittle materialssuchasrocks. However, due to high computational costs and the complicated fracturemechanism,mostofthesemodels have been generally developedin 2D. Few presentations can be found to explain the progressive failure of rock in 3D, including 3D crack initiation, propagation, and coalescence following3Dspatialfracturepathresulting from material heterogeneities.

On one hand, real fracture processes are 3D and not 2D, and the problems encountered in rock mechanics and engineering arealmostallthreedimensionaltosome extent. Rock and rock masses in nature, which consist of the earth'scrust,areunderthree-dimensionalstress condition,andfractureformationinrocks,including crack propagation,interaction and coalescence all shows three-dimensional features. As found by A. Caballero et al (2004), the complexity of crack patterns which emerge fromanevensimplegeometry,withprofusespatial bridging and branching until the final failure mechanisms are defined.

On theotherhand,thetwo-dimensionalanalysis

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