| The anti-seepage core wall of the rock-fill dam is usually filled with clay mixed with gravel,the core wall is easy to crack,because the deformation modulus of the clay mixed with gravel is much lower than that of the dam shell rockfill.If the cracks in the core wall expand,it will inevitably threaten the safety of the dam.Therefore,the crack of the core wall is one of the essential problems of the rock-fill dam construction.The stress state of core wall and its cracks is complex,to reveal the fracture and failure mechanism of the clay mixed with gravel under different stress states,and to propose corresponding fracture criterion are very important for solving problems of the crack of the core wall.Therefore,in this study,using experimental research and theoretical analysis,the test methods suitable for Mode-I and Mode-II fracture toughness of clay mixed with gravel were suggested,and indexes of assessing anti-fracture capacity are proposed.The fracture characteristics of clay mixed with gravel under different stress states were identified;the fracture failure mechanism was discovered;the fracture criterion under mixed loading conditions were established and a method for determining the fracture mode was proposed.The main work and innovative results are as follows:（1）The advanced test method suitable for Mode-I fracture of clay mixed with gravel was proposed;the fracture characteristics of Mode-I fracture resistance of clay mixed with gravel was achieved;the key factors for influencing the correlation between the fracture toughness and tensile strength of the soil were identified;the size of the micro-fracture zone of clay mixed with gravel was estimated.To overcome the shortcomings in the current Mode-I fracture soil test that are susceptible to interference and the effective fracture area is too small,the semicircular bending NSCB specimen was introduced,the preparation method and loading method of the NSCB compacted clay sample were established.The reasonable notch length and thickness of compacted soil samples were proposed.And the influencing factors of Mode-I fracture toughness of clay mixed with gravel were identified.The correlation between Mode-I fracture toughness and tensile strength was obtained.The reason for the difference in the linear proportional coefficient a of the fracture toughness and tensile strength of the soil was revealed.An empirical formula for the direct fracture compaction of the clay toughness and the tensile strength was established,and the size of the micro-fracture zone（critical size of the crack tip）of the clay mixed with gravel was estimated.（2）A Mode-II fracture tester for soil was developed.The Mode-II fracture toughness of the clay mixed with gravel,the loading conditions and sample conditions of the Mode-II fracture was proposed.The Mode-II fracture behavior was identified,and the reasons for the difference in Mode-II fracture toughness test results of different materials were analyzed.The evaluation indicators for Mode-II fracture of the clay mixed with gravel were suggested.According to the Melin shear fracture criterion,the loading conditions of Mode-II fracture were theoretically analyzed,and the Mode-II fracture testing instrument for soils was developed.The preparation method and prefabrication method of cube fracture samples of clay mixed with gravel were established.Though experimental trials and tests,the Mode-II fracture toughness test recommendations for clay mixed with gravel were identified.The maximum side length,test thickness,crack width ratio and maximum particle size of the reasonable sample were obtained.The mechanism of influence of the amount of gravel added on the shear fracture toughness was revealed.A theoretical analysis of the shear box fracture test method was carried out.The problem that the stress state and the theoretical stress state did not match during the loading process is pointed out.The Mode-II stress intensity factor formula under the actual stress conditions were built.The theoretical interpretation of Zhou Qunli’s empirical shear fracture criterion was carried out,and the reasons for different KIIC of different materials under different test methods were discussed.The unified graphical method to reveal the specific meaning of KIIC obtained by different test methods was applied.（3）The applicability of the generalized maximum circumferential stress GMTS criterion in describing the tensile and shear fracture behavior of soil is verified,and the cause of the differential composite envelope caused by the sample structure is investigated.The tensile-shear fracture failure mechanism of clay mixed with gravel was revealed.The semi-circular bending specimens with diagonal crack are introduced,and the I-II mixed fracture test of clay mixed with gravel was carried out.The empirical fracture criterion of the clay mixed with gravel was obtained.Considering that the critical size of the crack tip is large,the GMTS criterion was introduced,which is in good agreement with the I-II mixed fracture envelope of the clay mixed with gravel.The reason for the test results difference is found out,and the influence of crack dip angle and relative critical size on the stress field at the crack tip and the tensile cracking angle is revealed.The evaluation index and evaluation method for the cracking of clay mixed with gravel under tensile and shearing were suggested.（4）The fracture behavior of clay mixed with gravel with central fissure under uniaxial compression was found out,and the compressive shear-tension cracking criterion for non-closed cracks and closed cracks was established.Through the uniaxial compression test of the central crack,the compression and shear fracture characteristics of closed cracks and non-closed cracks were identified.Considering that the indoor fracture samples are mostly non-closed crack samples,the relative passivation coefficient and relative critical size were introduced;the geometric characteristics of cracks and the T-stress non-closed crack compressive-tensile initiation cracking criterion was established;the mechanism of the influence of relative passivation coefficient,relative critical size,and stress state on the non-closed crack compressive-tensile tensile initiation cracking was investigated.Some rock materials were compared and verified,and the scope of application of the criterion was suggested.A compression-shear-tension initiation criterion for closed cracks with full T stress components was established,and the relative critical dimensions,stress states,and T stress components were investigated for closed cracks.The mechanism of influence of compression-shear tensile cracking was verified by comparison with the experiments in this study to verify the reasonability of the compression-shear-tension cracking criterion for closed cracks in clay mixed with gravel.The reason for existence of cracks in compacted soil was explained.The differential fracture behavior of non-closed cracks and smooth closed cracks was identified.（5）The defects in the current shear fracture judgment criteria were analyzed.Based on the integrated tensile fracture and shear fracture modes,the compression compression-shear-shear cracking criterion for closed and non-closed fractures is established.The fracture mechanism of clay mixed with gravel under compression and shear was established.Theoretical analysis aiming at the shear fracture mode determination method proposed by Melin was carried out and its shortcomings were pointed out.Based on the comprehensive tensile and shear fracture criteria,using the stress-type criterion combined with the tensile and shear strength parameters of the material,the judgment method of initiation mode of non-closed cracks and closed cracks was established.The mechanism of influence of fracture geometric characteristics,stress state and material properties on the fracture failure mode of clay mixed with gravel was investigated.The internal mechanism of shear fracture of clay mixed with gravel was revealed,and the analysis method of cracks in the core wall of clay mixed with gravel was proposed. |
