# Forecasting Landslides via Three-Dimensional Discrete Element Modeling: Helong Landslide Case Study
## 1 Introduction  
Landslides are one of the most catastrophic geological disasters, and many landslides occur each year around the world, causing loss of life and property [1; 2; 3; 4; 5]. Many slopes are in a metastable state, which means they are currently stable but have a high possibility of failure under the heavy rainfall and some external loads. To mitigate the loss caused by potential landslide hazards, forecasting the areas that would potentially be affected is crucial, but this task is challenging.  
Previous studies regarding slope safety were mainly concerned about the stability of the slope, which is reflected in the calculation of the factor of safety (FOS). The calculation of the FOS first depends on the limit equilibrium method (LEM), which is based on statistics [6; 7; 8]. Then, the finite element method (FEM) is used to calculate the FOS to provide detailed stress and strain information [9; 10; 11; 12; 13; 14; 15]. However, both the LEM and FEM can only be used before failure, and cannot show the post-failure behavior of a sliding mass because LEM cannot calculate strain and displacement, and FEM has limitations when it comes to large-deformation analysis. Thus, assessing the dynamic movementand potential affected area of the post-failure landslide behavior is difficult using these traditional numerical methods.  
With the fast development of computational techniques, more numerical methods that can be used to analyze large-deformation problems are being applied to landslide simulations. These methods can be generally divided into two types: continuous and discontinuous methods. The continuous methods describe material behavior using macroscopic constitutive equations of mechanical, hydraulic, and thermal-mechanical behaviors. In addition to FEM, smoothed particle hydrodynamics (SPH) [16; 17], the material point method (MPM) [18; 19], and computational fluid dynamics (CFD) [20; 21; 22] are popular continuous methods used in landslide simulation. As for the discontinuous methods, the discrete element method (DEM) [23; 24; 25; 26; 27] and discontinuous deformation analysis (DDA) [28; 29] are most used. In the discontinuous methods, the material is formed by many discrete blocks, and interaction between the blocks satisfies certain physical laws. The macroscopic behavior of material is reflected by the interaction of the blocks. Although run-out behavior information, such as the run-out distance, velocity, and depth of the source, can be easily acquired from these continuous methods, they cannot simulate mass separation, which is important for the analysis of landslides [24].  
In this study, we used a DEM to forecast the failure potential of the Helong landslide, which was found to have a clear deformation in a field investigation in 2018. Before the forecasting work, the stability of the slope was first assessed using LEM. Then, reconnaissance, remote sensing, drilling, laboratory tests, and topographical analysis via electrical resistivity tomography (ERT) were used for the construction of a three-dimensional (3D) DEM model. Information about post-failure behavior was obtained from the simulation, which can be used for disaster prevention and mitigation.