# DEM Simulation of the Load Transfer Mechanism of a GRPS Embankment with a Fixed Geogrid Technique
## 1 Introduction  
Soft soil has low strength and high compressibility, and it takes a long time to consolidate under a surcharge load due to its low permeability. Therefore, it is a great challenge to construct embankments on soft soil. Geogrid-reinforced pile-supported (GRPS) embankments have been widely used in highways, railways, airports, ports, and other infrastructure projects as a feasible soft-ground improvement technology [1, 2, 3, 4, 5, 6]. The combination of a geogrid and piles for embankments could reduce the total and differential settlements of embankments. Existing studies indicate that the tensile membrane effect [7, 8] of the reinforcement and the soil arching effect [9, 10, 11, 12] are the key mechanisms of load transfer inGRPS embankments. [PERSON] and [PERSON] [13] conducted a series of numerical studies on the geogrid-soil interaction mechanism under vertical cyclic loading and reported that when the geogrid was placed at a shallow depth, the deformation of the geogrid was significantly affected by the stiffness. There is a relationship between the laying position of the reinforcement and its tensile performance. [PERSON] et al. [14] deduced a simplified formula for the pile-subsoil stress ratio of the GRPS embankment based on [PERSON]'s planar semicircular soil arching model and reinforcement deformation arc model. [PERSON] [3; 12] used X-ray CT and visualization technology to observe the membrane action of the geogrid in the GRPS embankment and revealed the influence of a reinforcement layer on the load distribution of the embankment.  
To improve the performance of GRPS embankments, a fixed geogrid technique was proposed for GRPS embankments [15; 16]. As shown in Figure 1, compared with traditional GRPS embankments with unfixed geogrids, the fixed geogrid technique involves a mechanical connection between the reinforcement and the pile top so that the tensile effect of the reinforcement can transfer more of the embankment load to the pile. Although the fixed geogrid technique of a GRPS embankment was applied in engineering [16], the load transfer mechanism of this technique is not clear. Further research is needed to investigate the load transfer mechanism in the GRPS embankments using the fixed geogrid technique.  
The discrete element method (DEM), as a discontinuous numerical simulation method, was widely used to investigate the load transfer mechanism of the pile-supported embankments [17; 18]. [PERSON] et al. [8] suggested that the continuum-based numerical model cannot fully reflect the stress transfer mechanism of the embankment with granular fill, and established a three-dimensional discrete element numerical model of a GRPS embankment, which verified the rationality of the discrete element numerical method. [PERSON] et al. [17] used the FEM and the DEM to simulate a laboratory two-dimensional model test of a pile-supported embankment and reported that the discrete element method could better predict the load transfer and deformation of the embankment.  
In this study, based on the two-dimensional laboratory model tests conducted by [PERSON] et al. [19], three DEM models of pile-supported embankments were established to study the load transfer mechanism of GRPS embankments. The efficacy of the pile, the evolution law of the contact force chain, the axial force of the reinforcement, and the microscopic load-bearing structure of the soil were investigated. The research results are helpful for understanding the load transfer mechanism of GRPS embankments using the fixed geogrid technique.  
Figure 1: GRPS embankment with a fixed and an unfixed geogrid.