# Research on Mechanical Properties and Failure Mode of Conglomerate Based on Discrete Element Method
## 3 Research Process
### Response of Cementation Strength between Gravel and Matrix  
Common cementation types in conglomerates are argillaceous cementation and calcareous cementation, and different cementation types lead to different cementation strength. When the conglomerate is subjected to external force, the strength of the cementation surface between gravel and matrix has obvious influence on the mechanical properties and fracture propagation of the conglomerate. In order to reduce the influence of gravel particle size on numerical simulation results, the numerical model selects particles with an approximately equal size to simulate, and studies the influence of cementation strength on mechanical properties and fracture propagation of the conglomerate by adjusting different mechanical parameters of the cementation surface between gravel and matrix. The deformation parameters, strength parameters and friction coefficient in the numerical model are set to 0.1-1.0 times that of gravel, while the other parameters remain unchanged. A uniaxial compression test was carried out to study the influence of different cementation strength on macroscopic mechanical properties of the conglomerate. Set the parameters in advance and specify the loading speed as 1 mm/min to ensure that the loading is quasi-static. The simulation results are shown in Figure 4. The uniaxial compressive strength and elastic modulus of conglomerate decrease significantly when the ratio of gravel-matrix strength to gravel strength decreases from 0.4 to 0.1. When the bond strength ratio is reduced from 1 to 0.4, the uniaxial compressive strength and elastic modulus bring no apparent change. It can be seen from the cumulative number of fractures that the lower the cementation strength ratio, the lower the corresponding pressure when fractures begin to occur in the rock. Due to the gravel, matrix and cementation area of the conglomerate bear axial loading pressure at the same time. When the cementation strength is low, the strength of the cementation area is obviously lower than that of the gravel and matrix area. With the increasing axial pressure, the cementation area will be destroyed, and cracks will occur when the axial pressure reaches the ultimate bearing capacity of the cementation area. Therefore, with the decrease in cementation strength ratio, the fracturing of rock will be lower when cracks begin to appear. As shown in Figure 4, when the cementation strength ratio is less than 0.4, the number of fractures witnesses a surge at the initial stage of loading. Combined with the fracture distribution map during conglomerate failure (Figure 5), it can be seen that the fractures generated at the initial stage of loading are caused by shear force of the cementation surface between gravel and matrix. When the cementation strength ratio is more than 0.4, the number of fractures in the conglomerate tends to be stable at the initial stage of loading, and most of the fractures occur at the peak strength of loading. Figure 3: Physical test and numerical model results. (**a**) Failure mode; (**b**) stress–strain curve. By comparing the influence of cementation strength ratio on mechanical properties of the conglomerate, the relationship between cementation strength ratio and uniaxial compressive strength, elastic modulus, cumulative fracture number and fracture initiation pressure is statistically analyzed. As shown in Figure 6, the uniaxial compressive strength, elastic modulus, cumulative number of cracks and crack initiation pressure vary with the cementation strength ratio. Fracture initiation pressure and the axial stress point of the first fracture in the corresponding rock sample are essential parameters to evaluate the occurrence of cracks in rock. When the cementation strength ratio is less than 0.4, the uniaxial compressive strength increases rapidly as shown in Figure 6a. The cumulative number of cracks corresponds to the total number of cracks produced in rock after compression.