Aseriesofpolyaxialcompressiontestsandindirecttensiontests withaxialconfinementsare performedtoassesstheeffectsoftheintermediateprincipalstressesontheelasticityand strengths of three types of sandstones. Over 50 rectangular shaped specimens (50x50x100 mm) are tested using a polyaxial load frame. Lateral stresses with different magnitudes are applied on two mutually perpendicular directions(σ2 ≠σ3),and varyingfrom 0 upto 17 MPa. The axial stress (σ1) is increased until failure. The results are presented in form of octahedral shear strength (τ oct) vs. mean stress (σ m) diagram, indicating that under the same σ m, σ2 can notably decrease the τ oct at failure. The failure envelope is lower for the greater σ23 ratio. The results also indicate that the sandstones are transversely isotropic. The effect of intermediate principal stress (σ2) on thetensilestrengthsisassessedbyconductingthe Brazilian(indirect)tensiontestunderaxial confinement. The σ2valuelinearlydecreasesthefailureloadonthedisksamples. A failure criterion (taken σ2 into consideration) can be developed using τ oct – σ m diagram and combining the tension test results with those of the true triaxial compression tests.


Rock deformation and strength are important parameters forthedesignandstabilityanalysisofgeological structures, e.g., foundations of dam, building and bridge, and hostrocksfortunnels and underground mines. The effects of the confining pressures at great depth on those properties arecommonlysimulatedinthelaboratoryby performing triaxialcompressiontestingofcylindrical rockcorespecimens. A significant limitation of such conventional methodisthattheintermediateand minimum principalstresseshavetobeidenticalduring the test while the actual in-situ rock is normally subjected toananisotropicstressstatewherethemaximum, intermediate and minimum principal stresses are different (σ1 ≠σ2 ≠σ3). It has been experimentally found that the intermediate principalstress(σ2)can notablyreducethe maximum principalstress(σ1)atfailureforintactrock specimens(Yangetal.,2007;Rajendraetal.,2006; Haimson,2006;TiwariandRao,2004;Haimsonand Chang, 1999). This suggests that the compressive strengths obtainedfromtheconventionaltriaxial compression test(under σ2 = σ3),asgivenbythe American Society for Testing and Materials – ASTM, may nottrulyrepresentthe actualin-situstrength w

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