## 1 Introduction  
The commonest geosynthetics-reinforced soil (GRS) structures that embed geosynthetics in the soil have been widely used in practical engineering. The backfill material is one of the main components of GRS structures and accounts for 30-40% of their cost [1]. The existing design specifications generally consider free-draining granular substances as the backfill material in the reinforced region [2, 3, 4, 5]. As clay is one of the most abundant and inexpensive building materials, if the modified clay available on-site can be used as the backfill material of GRS structures, then the cost can be reduced and the environmental impact associated with the disposal of the excavated soil can be lessened [6, 7].  
During the long-term operation of GRS structures, factors such as rainfall, the freeze-thaw cycle, and the change in the groundwater level can lead to a significant increase in the water content of fillers. When clay is used as the backfill material for the commonest GRS structures, the strength of the geosynthetics-clay interface decreases significantly with an increase in the water content of the clay [8], resulting in the instability of the structure. In order to solve this problem, the layered geosynthetics-granule-clay reinforced structure was proposed based on commonest geosynthetics-reinforced soil structures [9, 10],as shown in Figure 1. The proposed structures adopt a sandwich technology by laying a thin layer of sand around the geosynthetics and use a clay filler outside the thin layer of the sand. By using clay as the primary filler, some experimental studies conducted on the proposed structures [10; 11; 12; 13] demonstrate that they fully exploit the high-quality drainage performance of the granular material along with the superior friction between the geosynthetics and the granular material to ensure the long-term stability of structures.  
Based on the equivalent confining stress concept [14] and the pseudo-cohesion concept [15], the triaxial compression test is commonly used to study the mechanical responses of the geosynthetics-reinforced soil structure. To date, there is a lack of research on layered geosynthetics-granule-clay reinforced structures under triaxial compression. The simulation technique using three-dimensional particle flow software can accurately realize the triaxial compression test [16; 17; 18; 19]. This paper utilizes the three-dimensional particle flow software PFC\({}^{\text{3D}}\) to carry out the triaxial compression test of the layered geosynthetics-granule-clay reinforced structure. The geogrids and sand are used as the geosynthetics and granular materials of a layered geosynthetics-granule-clay reinforced structure. Based on PFC\({}^{\text{3D}}\), three types of cluster particles were secondarily developed to simulate sand and to construct the reinforced joints of the geogrid. Thus, the numerical triaxial compression test model of the layered geogrids-sand-clay reinforced (LGSCR) structure is established. Furthermore, the influences of the laying parameters of the sand layer and the geogrid-reinforced sand layer on the numerical triaxial compression test results of the samples are examined, which can provide a frame of reference for the study of the physical and mechanical properties of LGSCR structures.