# Natural Para Rubber in Road Embarnment Stabilization
## 1 Introduction  
A road embankment, which is the soil layer, can withstand strong compression but the tension property is very weak. The reinforcement material, which is stiffer than the road material, has become one alternative to apply for increasing stability, bearing capacity, and strength of soil, especially in road construction [1, 2]. Geogrid, which is made from polymer materials, such as polypropylene, polyester, high-density polyethylene, or other polymers with high elastic properties, have been widely applied and placed in road embankments to improve the performance of weak subgrades and the base course for both paved and unpaved roads [3]. The biaxial geogrid is generally used for road reinforcement to distribute a load in two directions, which are the machine direction (MD) and cross direction (CD). The geogrid produces interlocking between aggregates and the geogrid [4], which creates an aggregate-geogrid composite [5] to form a confined zone with three functions (lateral restraint, bearing capacity increment, and membrane support). With the presence of the interlocking mechanisms, the geogrid can improve the lateral restraint at the interface and prevent both lateral and vertical movements. The higher shear strength surface is developed by using geogrid reinforcement to increase the bearing capacity and support the wheel load as a membrane support [4, 6]. The biaxial geogrid can be applied to reduce the base course thickness by 30% [7]. The thickness of a road embankment for both paved and unpaved surfaces can be reduced by 20% to 70% depending on the strength of the subgrade and the type of geogrid [8]. Therefore, geogrid reinforcement can increase the bearing capacity, reduce rutting, and decrease the settlement [9]. The performance of geogrid reinforcement depends on the reinforcing layer and the reinforcing position. The position of the geogrid reinforcement depends on various parameters, which play an important role, such as the distance of the topmost layer from the loading bed (u), the distance between geogrid layers (h), the loading area width (B), the width of the geogrid layers (b), the number of geogrid layers (N), the loading distance from the edge of slope (D), and the slope angle (\(\beta\)). The bearing capacity increases with an increase in the reinforcing layers within the road embankment [10]. In some cases, geogrid reinforcement with a single layer is sufficient if the depth of the reinforcing position is appropriate [11]. [PERSON] [12] found that the optimal amount of the reinforcing position for u/b and h/b were 0.6 and 0.5, respectively. [PERSON] and [PERSON] [13] proposed that having two layers of a geogrid-reinforced slope is the optimal number of geogrid reinforcements based on the bearing capacity. [PERSON] et al. [14] investigated the optimal position for geogrid reinforcement, which led to the best performance of the geogrid reinforcement by considering the maximum bearing capacity of the railway embankment; the optimized results were the normalized distances u/B = 0.3 and h/B = 0.4 with three layers of the geogrid reinforcement, based on B, u, and h being 4.8 m, 1.44 m, and 1.92 m, respectively. Moreover, four layers of the geogrid reinforcement did not increase the bearing capacity of the railway embankment. In recent years, biodegradable- or natural-reinforced materials have been investigated to increase their applications in geotechnical engineering [15; 16]. Currently, para rubber prices are experiencing a problem in Thailand. Moreover, the natural para rubber is interesting not only because of its superior properties but also to overcome the problem of supply and price in Thailand. Natural rubber is a biopolymer consisting of isoprene units (C\({}_{5}\)H\({}_{8}\))\({}_{n}\) linked together in a 1/4 cis-configuration. Not only poly cis-isoprene molecules but also other cellular components are included in the natural rubber, while synthetic rubber is almost completely composed of polyisoprene with not more than 90% of cis-bond.