# Research on Mechanical Properties and Failure Mode of Conglomerate Based on Discrete Element Method
## 3 Research Process
### Gravel Content  
The complex sedimentary environment and tectonic background of the conglomerate make gravel particles appear in different shapes. According to the practice, the conglomerate models with 20%, 35%, 50%, 60% and 70% gravel content are constructed for the uniaxial compression test. The results show that with the increase in gravel content, the elastic modulus of the conglomerate also increases, and the fracture initiation time of rock is advanced. The reason for this is that with the increase in gravel content, the matrix and cementation area of the conglomerate are destroyed with the continuous compression in the axial direction, while the gravel is displaced without destruction and presents high strength. Due to the high gravel content, with the displacement caused by axial compression, gravels contact each other. The higher the gravel content, the more contact points between gravels [26,27], and the occlusion effect between gravels leads to an increase in the overall strength of gravel. The black scatters in Figure 7a-d show the effect of gravel content on the mechanical properties of the conglomerate. It can be seen that with the increase in gravel content, the elastic modulus and crack initiation strength both increase, and the fitting coefficient is greater than 0.8, which shows that the experiment has practical significance. However, there is no obvious feedback between uniaxial compressive strength, cumulative fracture number and gravel content, which is caused by too few data points and irregular gravel shape. In order to study the relationship between compressive strength and cumulative number of fractures and gravel content, ideal spherical gravel is used for numerical simulation. Figure 6: Influence of different cementation strengths on mechanical parameters and cumulative number of cracks. (**a**) Ratio of compressive strength to cementation strength; (**b**) Ratio of elastic modulus to bond strength; (**c**) Ratio of cumulative number of cracks to cementation strength; (**d**) Ratio of crack initiation strength to cementation strength. Uniaxial compression experiments were carried out on spherical gravel with different contents. As shown in Figure 7, with the increase in spherical gravel content, the elastic modulus of conglomerate increases, while the uniaxial compressive strength decreases when the spherical gravel content is 10-50% and increases when the spherical gravel content is greater than 50%. The reason is that gravel, as a heterogeneous body in the conglomerate, is prone to stress concentration under compression. The closer the distance between two gravels, the greater the additional stress, and the shorter the rock bridge, which is easier to break. Therefore, with the increase in gravel content, the uniaxial compressive strength decreases, and the cumulative number of cracks increases. However, when the gravel content increases to a critical point, the gravel contacts each other in the conglomerate, and the cementation between two contacting gravels is gravel-to-gravel cementation rather than weak gravel-to-matrix cementation, which will lead to an increase in the overall strength of the conglomerate as shown in the red box in Figure 8. In addition, when the conglomerate with high gravel content is compressed, the original non-contact gravel squeezes each other and bears a part of the pressure due to axial compression, which leads to the matrix compression of the conglomerate being converted into gravel compression. As the gravel content increases, the compressive strength of conglomerate increases, and the cumulative number of fractures decreases or tends to be flat. Figure 7: Effects of different gravel content on mechanical parameters and cumulative cracks. (**a**) Reaction between gravel content and ultimate; (**b**) Reaction between gravel content and cumulative; (**c**) Reaction between gravel content and crack initiation strength; (**d**) Reaction between gravel content and elasticity modulus.