Discontinuous gradation effect on large-scale triaxial shear mechanical properties of soil-rock mixture

The geographical distribution of discontinuous graded soil-rock mixtures is extensive，and these materials are commonly used as backfill materials in subgrade and slope engineering. The discontinuous grading effect on their shear mechanical properties is a critical factor influencing compaction quality control. This study proposes a discontinuity index<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>D</mml:mi><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>that effectively quantifies the discontinuous grading characteristics of soil-rock mixtures. Large-scale triaxial standard specimens of soil-rock mixtures with different discontinuity indices were prepared，and triaxial shear tests were conducted using the DJSZ–150 large-scale triaxial testing apparatus. The influence of<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>D</mml:mi><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>and confining pressure<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>on the strength and deformation characteristics of soil-rock mixtures was analyzed. Subsequently，intrinsic relationships between the parameters of the composite power-exponential model and the deviatoric stress strength，<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>D</mml:mi><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>and<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>were investigated，leading to the development of the CPE model，which accounts for the effects of varying interrupted grain fractions and stress states. Finally，the impact of changes in discontinuous particle size fractions on the mesoscopic structural characteristics of soil-rock mixtures was explored，elucidating the discontinuous gradation effect on their shear mechanical properties. The results indicate that under triaxial compression，discontinuous graded soil-rock mixtures typically exhibit strain hardening characteristics. The deviatoric stress strength increases with the increase in<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>，while the internal friction angle initially increases and then decreases with the increase in<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>D</mml:mi><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>. In contrast，the cohesion exhibits an opposite trend. The parameters <italic>k</italic>，<italic>b</italic>，and <italic>n</italic> showing linear relationships with the<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula>. Both <italic>k</italic> and <italic>n</italic> show a linear relationship with the<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>D</mml:mi><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>，while <italic>b</italic> follows a quadratic function relationship with<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>D</mml:mi><mml:mtext>c</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>. The improved CPE model not only accurately predicts the strength and deformation characteristics of soil-rock mixtures，but also effectively reflects the influence of discontinuous grading features and stress state on their deformational mechanical behavior of soil-rock mixtures. Different discontinuous particle groups constitute distinct mesostructural characteristics of soil-rock mixtures. Under axial compression loading，an increase in the content of the largest particle group enhances the overall deformational mechanical properties of the soil-rock mixture. However，excessive amounts of large particles can increase the voids between coarse particles，leading to an initial increase in deviatoric stress strength followed by a decrease.