# Mechanism and Application of Soilbags Filled with Excavated Soil in Soft Soil Subgrade Treatment
## 2 Principle of Soilbag Treatment for Soft Soil Subgrade
### Unconfined Compression Tests and Test Results  
[PERSON] and [PERSON] experimentally and theoretically investigated the mechanical performance of soilbags. The high compressive strength of soilbags can theoretically be explained by an additional cohesion that develops in soilbags resulting from the tensile forces in the bags under external loads [22]. The additional cohesion \(c_{T}\) is derived as follows:  
\[c_{T}=\frac{T}{B\sqrt{K_{p}}}\left(\frac{B}{H}K_{p}-1\right) \tag{1}\]  
where \(T\) represents the tension in the polypropylene bag: \(B\) and \(H\) represent the width and thickness of the soilbag, respectively: \(K_{p}\) is the passive earth pressure coefficients, equal to \(\frac{1+sin\varphi}{1-sin\varphi}\). \(\varphi\) is the internal friction angle of the soil inside the bag. Additionally, as seen in Equation (1), the additional cohesion \(c_{T}\) produced by the tensile force from the bag is a result of the collective effect of the tension, bag size, and internal friction angle of the soil inside the bag. The strength and deformation characteristics of soilbags filled with excavated clayey soil were evaluated using unconfined compression tests. Clayey soil, extracted from a field site of an ongoing rural road project where soilbags are being utilized for treatment, was used for this evaluation. The excavated clayey soil was air-dried naturally to a water content of 26.5% and a unit weight of 16.5 kN/m\({}^{3}\) on-site. The strength and deformation characteristics of soilbags are closely related to the cohesion and friction angle of the soil used. These two mechanical properties were determined via a drained direct shear test with a shear velocity of 0.8 mm/min on the air-dried clayey soil, followed by the Chinese Standard GB/T 50123-2019 (Standard for Geotechnical Testing Method) [28]. The detailed physical and mechanical properties of the soil are presented in Table 1. The bags used in  
Figure 1: Schematic view of granular soil replacement method and soilbag treatment method for soft soil subgrade. (**a**) granular soil replacement method. (**b**) soilbag treatment method. this study were constructed from polypropylene with a mass weight of 100 g per square meter. Tensile strength is another important index that affects the strength improvement of soilbags. The tensile strengths of these bags were determined to be 17.78 kN/m and 22.12 kN/m in the warp and werf directions, and the maximum extension strains in the warp and werf were identified as 18% and 24%, respectively, following the Chinese Standard GB/T 15788-2017 (Geosynthetics--Wide-Width Tensile Test) [29]. A soilbag measuring 40 cm in length, 40 cm in width, and 10 cm in height was used. To maintain the same initial density of the air-dried excavated soil on-site, these placement conditions were selected. Control over the packing state, and by extension, the achievement of the target unit weight, was meticulously managed through precise measurements of the soil weight and the known volume of the bags. While preparing the soilbags, each bag was filled with 26.4 kg of air-dried excavated soil considering the target unit weight of soilbags. Firstly, the clayey soil was pulverized and used to fill the polypropylene bags. The soilbags were sealed with a hand-held sewing machine. Vertical loading was applied continuously to each soilbag at a rate of 1 kN/s until a tear in the soilbag was observed, which was accompanied by a sudden drop in load. Throughout the compression phase, measurements of the soilbag's deformation and the vertical load were taken, and the results are displayed in Figure 2. Since the soilbags were not densely packed before the unconfined test and remained the same as they were sealed, the initial compression stress acting upon the soilbag increased at a gradual rate. As the compressive strain escalated, there was a corresponding rise in the stress exerted on the soilbag, leading to an increase in the compression strength of the soilbag, largely due to the internal tension of the bag and the compression of the encased soil.