# Calibration of Sliding Friction Coefficient in DEM between Different Particles by Experiment
## 1 Introduction  
Granular materials are widely prevalent in nature and industry, being found in weathered slopes, rockfill materials, railway slag, etc. The discrete element method (DEM) is the most effective method for the mechanical analysis of granular materials. DEM is a numerical approach proposed by [PERSON] and [PERSON] [1; 2; 3] based on [PERSON]'s second law. As a numerical method, DEM can be used for discontinuous media; therefore, this method is widely used in particle breakage [4; 5; 6], stress distribution [7; 8; 9], avalanching [10], stratification [11], and self-organization [12]. Because of the difficulty inherent in friction coefficient determination, the friction behavior of granular geomaterials has always been a problem in DEM. Present research methods of friction coefficient calibration in DEM mainly include the theoretical method, experimental method, and numerical simulation method. The linear model [3] is one of the most widely used theoretical models. Many scholars [13; 14; 15; 16] have conducted studies on the key parameters utilized in the linear model. Thecontact bond model is an optimized model based on the linear model. Several scholars [17; 18; 19] have used the model for simulation and applied different values of the friction coefficients according to uncoded values. Based on the theory of [PERSON] and [PERSON] [20], the Hertz contact model [21; 22; 23] is another widely used model. In order to study the mechanical behavior of soft particles interacting via frictional contacts, [PERSON] [24] combined the Hertz contact model and the contact dynamics method. [PERSON] [25] found, by comparing a variety of rolling resistance models, that the rolling friction coefficient is closely related to energy dissipation. Because of some assumptions made in DEM theory and the simplification of theoretical models, it is difficult to ensure the accuracy of associated research results. Compared with the theoretical method, the experimental method is more intuitive and can provide a basis for theoretical research and numerical simulation research. Therefore, the experimental method is more widely used to study the friction coefficient in DEM. The uniaxial compression test and shear test are methods used to effectively determine the friction coefficient in DEM through laboratory testing [26; 27; 28; 29]. [PERSON] and [PERSON] [30] obtained stiffness coefficient and friction coefficient groups by combining uniaxial compression tests and direct shear simulation tests and obtained relevant parameters through simulation. On the other hand, the sandpile experiment is the most commonly used method for the DEM parameter calibration [31; 32; 33; 34]. [PERSON] [35] studied the influence of particle diameter and particle material on the friction coefficient by using 1.5 mm and 3 mm glass balls and 3 mm nylon balls in sandpile formation and rotating drum experiments and carried out numerical simulations simultaneously. [PERSON] [36] found, through experiments with glass spheres, that the sliding friction and the rolling friction between particles and between particles and boundaries both affect the stability of the sandpile. Some scholars [37; 38] have also determined the friction coefficient with newer experimental methods. [PERSON] [39], through their experiment, determined the static friction coefficient and the dynamic friction coefficient. Because the experimental method has inevitable defects, and subsequent research usually adopts the method of numerical simulation, numerical simulation is a research method that has developed rapidly in recent years. With the development of DEM-related theory and computing technology, more and more scholars have begun to implement numerical simulation methods to study the friction coefficient in DEM.