# Research on Mechanical Properties and Failure Mode of Conglomerate Based on Discrete Element Method
## 2 Gravel Model Based on Discrete Element Method
### Establishment of the Conglomerate Model and Parameter Calibration  
Taking the conglomerate as the research object, the discrete element numerical model is established to study the mechanical properties and failure morphology of the conglomerate. The conglomerate sample is taken from the depth of 3109\(\sim\)3259 m in Mahu Sag of Baikouquan Formation, Xinjiang. The gravel content is between 30\(\sim\)70%, the particle size is 5\(\sim\)70 mm, and the mineral composition is mainly compacted and cemented by quartz, feldspar and clay. Since gravel is a destructive material, this model uses particle clusters with strong strength to simulate gravel with high hardness. The model consists of matrix, cementation surface and gravel, as shown in Figure 2.  
The discrete element model reflects the adhesion between particles. Therefore, the meso-parameters assigned to the model need to reflect the mechanical properties of adhesion between particles, which cannot be obtained directly in laboratory tests. When assigning parameters, researchers usually take the stress-strain curve and failure mode of rock obtained by indoor experiments as relevance, and use the \"trial-and-error method\" to adjust the meso-parameters of the model. The results of the model are basically consistent with the stress-strain curve and failure mode of indoor tests [23; 24], and the meso-parameters obtained by trial-and-error are usually considered to be correct. Namely, the numerical model can reflect the mechanical properties and failure modes of indoor experimental samples. TCR-100 high temperature and high-pressure rock triaxial rheometer is used as the loading system for indoor uniaxial test, and the maximum axial load of the tester is 2000 kN. Figure 1: Parallel bonding model. The experiment works in displacement-control loading mode, and the loading rate is 1 mm/min. The stress-strain curve and the failure mode of the conglomerate are used as the comparison of the assignment of the meso-parameters. As shown in Figure 2, the standard size of model is \(\Phi\) 50 \(\times\) 100 mm, 34,142 particles in total, 91,239 parallel bonds in the model, and the loading speed is consistent with the indoor experiment. Meso-parameters are adjusted through continuous trial-and-error, and the relationship between meso-parameters and macro-parameters are analyzed according to Zhang [25]. Firstly, the Parallel bond tensile strength and Parallel bond cohesion in the meso-parameters of gravel, matrix and cementation region are adjusted to make the peak strength of the conglomerate numerical model consistent with the peak strength of the physical experiment. Secondly, the Parallel bond module is adjusted to make the elastic modulus of the conglomerate numerical model consistent with the elastic modulus of the physical experiment, and finally the Friction coefficient is adjusted to make the fracture propagation pattern of the conglomerate numerical model similar to that of the physical experiment. The mesoscopic parameters adjusted by the trial-and-error method are shown in Table 1. The meso-parameters of the numerical model are shown in Table 1, and the results of indoor experiments and numerical models are shown in Figure 3. The results show that the data of the indoor experiment are basically consistent with the data of the numerical model, and bypassing gravels mainly occurs in crack propagation; the errors of peak strength and elastic modulus are 2.08% and 3.11%, respectively, indicating that the meso-parameters are feasible.