| Loess,as one of the typical structural soil,is yellow or brownish yellow and mainly composed of silt and clay which characterized by some features,such as no bedding,vertical joint development,high porosity,metastable composition and weak cementation.Loess covers an area of about 640,000 square kilometers in China,accounting for about 6.7% of the land area.The geological environment of Loess Plateau of China(LPC)is fragile because of the steep slopes and deep ditches developed there.With the unprecedented development of social productivity and the increasingly strong intervention of human activities on the inherent behavior of the natural environment,geological disasters occur frequently in this area.Among them,loess slope geological disasters are typical,showing the characteristics of large scale and complex disaster mechanism,which seriously threatens the safety of people’s lives and property.Toppling failure,as one of the typical slope geological disasters in LPC,is closely related to the initiation and expansion of tensile cracks in soil,wherein the development and expansion of cracks are dominated by mode Ⅰ fracturing.In the previous numerical simulation of soil type I fracture,the soil is generally treated as a continuous medium,and the stress singularity at the end of the crack has not been fully considered,so it is difficult to obtain accurate results.Based on this,this subject takes numerical simulation as the research method,and uses the discrete element software PFC2 D to carry out the biaxial numerical simulation test of homogeneous soil.The simulation results are compared with the indoor triaxial compression test results,and the soil meso parameters that can be used for the simulation test of soil fracture behavior are obtained: particle size,effective modulus,stiffness ratio,tensile strength of parallel bond,cohesion of parallel bond and friction angle of parallel bond.At the same time,with the help of CT scanning technology,MATLAB and Vector Magic software,the digital image of Malan Loess macroporous structure is binarized and vectorized,which is introduced into the analysis of soil fracture characteristics.The numerical simulation of soil semicircular bending with prefabricated cracks is carried out,and the numerical simulation results of homogeneous soil and soil with macroporous structure are compared,The influence of macropore structure of Malan loess on mode I fracture characteristics of soil is obtained.In the process of numerical simulation,through the measurement circle function in PFC2 D software,the normal stress value of crack tip in the process of soil loading is obtained,so as to obtain the stress intensity factor of crack tip and the fracture toughness value of soil.This paper mainly obtains the following understandings:Based on the numerical simulation study of the fracturing behavior of Malan Loess,the following conclusions are drawn.(1)Through the comparison between numerical simulation and indoor test results,it can be seen that the discrete element method can accurately simulate the mode I fracture behavior of Malan loess.According to the variation law of the contact failure number of model particles,the mode I fracture process of soil can be divided into three stages,namely linear loading stage,stable expansion stage and unstable failure stage.(2)The existence of macropore structure will affect the distribution of stress in the soil during the fracture process.The stress distribution of homogeneous soil is more symmetrical during the loading process.The tensile stress concentration range at the crack tip of the model with macropore structure is larger than that of homogeneous soil,and there is also tensile stress concentration at both ends of independent pores in the specimen.(3)Macropore structure is the most prone part of tensile failure in Malan loess.Its existence will reduce the fracture toughness of Malan loess,especially when the pores exist near the prefabricated cracks of the model. |
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