Study On Strength And Intermidiate Principal Stress Of Rockfill Material In Plain Strain State

Rockfill material is the main embankment material. Due to its wide grading distribution and different properties, it shows excellent engineering properties, such as high strength, small deformation and strong permeability. Therefore it has widely applied in engineering construction, especially with the expansion of height and scale of rockfill dam, study on strength and deformation characteristic of rocfill material in high confining pressure is increasingly important.Mechanical properties of rockfill material in high stress is the key factor to security and normal use of rockfill dam, thus it must make related test to study the strain-stress properties of rockfill material under plain strain condition. Arming to the problems in previous study, this paper conducts three groups of indoor tests of rockfill material which included general triaxial tests, square triaxial tests and plane strain tests. Through arrangement of the test results and theoretical analysis, the author studies its strength characteristic, intermediate principal stress and crushing properties under different experimental conditions. The results of this paper are as follows.(1)Significant differences between general triaxial tests, square triaxial tests and plain strain tests are proposed on experiment that for rockfill materials in this article, when under the same confining press, peak strength of the second is higher than first, while peak strength of the last is higher than second, especially when its density is large, the stress-strain curve of square triaxial tests shows softening characteristics and obvious peak bias stress. Based on those above, this paper gives the explanation in the view of degree of freedom to particle motion.(2)Each Mohr circle of square triaxial tests and plane strain tests has a common tangent, thus Mohr-Coulomb strength criterion is reasonable to reflect its strength characteristics. Besides, lode parameter of same kind of rockfill material approximately keeps constant in its failure state and their values are close to sine friction angle of square triaxial tests.(3)On the base of experiment, the relation between strength parameter of rockfill material in plane strain state and square triaxial tests is established using twin shear theory, and then the results of theoretical formula are compared with test results, it indicates that the formula of strength parameter proposed by this paper can mainly described the strength characteristic of rocfill material in plane strain test under constant minor principal stress. Through the analysis of influence parameter in plane strain state, it indicates that within the framework of this model, the friction angle of plan strain state is only related with triaxial state, while its cohesion mainly depends on the corresponding parameter of triaxial state and the friction effect is minor.(4)The change law of intermediate principal stress in entire deformation and damage process is revealed by plane strain test under constant minor principal stress. Based on the linear type Mohr-Coulomb strength law, Lode parameter and Load ratio of failure time are calculated through its equivalent generalized Mises failure law. On the hypothesis of linear law between Lode parameter and Load ratio in the loading process, its corresponding linear coefficients are determined from isobaric consolidation state and failure state, then the author deduce the formula of intermediate principal stress. On the bases of those above, the relationship between principal stresses is discussed in different value of strength parameter, and then the results of theoretical formula are compared with test results, it indicates that the formula of intermediate principal stress proposed by this paper can mainly described the relationship between principal stresses of rocfill material in plane strain test under constant minor principal stress.(5)Four groups of high confining pressure triaxial test of Qipei Hydropower bedding material are conducted. Based on the test results of particle size distributions, the fractal model is established, and its fractal behavior in crushing is studied. The results show that large fractal crushing dimension corresponds to high confining pressure in the same density. The fractal crushing dimension increases with density in the same confining pressure. The magnitude of fractal crushing dimension reflected the particle size of breakage, the uniformity of the particle size distributions. The larger fractal crushing dimension, the larger quantity of particle breakage is. Besides, there is a close relationship between fractal crushing dimension and Marsal index of particle breakage. Particle crushing results of rockfill material under different experiment conditions indicates that when under the same confining press, fractal crushing dimension in plain strain test is higher than it in square triaxial test, while fractal crushing dimension in square test is higher than it in general triaxial test.