# Mechanism and Application of Soilbags Filled with Excavated Soil in Soft Soil Subgrade Treatment
## 2 Principle of Soilbag Treatment for Soft Soil Subgrade
### Consolidation Model Tests and Tests Results  
Model tests on soilbag-treated soft soil subgrade were conducted in a transparent model box, measuring 80 cm in length, 50 cm in width, and 60 cm in height, to investigate the drainage consolidation properties of the soilbag-treated subgrade with excavated soil. For comparison, drainage consolidation tests were also conducted using the granular soil replacement method on the treated soft soil subgrade model. Considering the size of the model, a sand cushion that also has high strength and permeability properties was chosen to replace the granular soil cushion for the soft soil treatment in the model test. The geometry of the consolidation tests on the soft soil subgrades are shown in Figure 3. Within the model box, clayey soft soil, serving as the base of the subgrade model, was placed at a depth of 50 cm. The clayey soft soil used was the same as that in the unconfined compression tests. To replicate the natural conditions of the field in the model, the excavated clayey soil was dried, pulverized, and mixed with water, resulting in a reconstituted soil with a water content of 55% (the same as the soft soil on-site with natural water content). This reconstituted clayey soil was introduced into the model box and left to settle undisturbed for 24 h to achieve its initial state. For the soilbag-treated subgrade, two staggered layers of soilbags filled with clayey soil with a water content of 26.5% were laid on the soft soil base, each with dimensions of 30 cm \(\times\) 20 cm \(\times\) 5 cm, as shown in Figure 3a. It is worth noting that the polypropylene bags and the enclosed soil were the same as those used in the unconfined compression tests. For the sand cushion-treated subgrade, the height of the sand cushion was 10 cm--the same as the soilbag reinforcement cushion. Sand with grain sizes ranging from 0.06 mm to 5 mm was used. The cohesion and friction angle of the sand were 3.25 kPa and 35.4\({}^{\circ}\), respectively. On the top surface of the soilbag reinforcement cushion and sand cushion, four stages of vertical loads were applied to simulate the vertical stress imposed by the overlying fills during construction. These loads, measuring 6 kPa, 12 kPa, 20 kPa, and 28 kPa, were sustained for durations of 140 h, 140 h, 140 h, and 240 h, respectively. Figure 3: Schematic view of the consolidation tests on soft soil subgrades: (**a**) treated with soilbags and (**b**) treated with sand cushion. To evaluate the drainage consolidation behavior of the model-testing subgrades, a number of monitoring instruments were installed during and after the construction of the model, as shown in Figure 3. The instruments included pore water pressure transducers (denoted as A1-3 and B1-3 in the soilbag-treated subgrade, and a1-3 and b1-3 in the sand-treated subgrade), and two flexible displacement meters (denoted as D1, D2, and d1, d2). The pore water pressure transducers selected for the consolidation model tests were of the CJLY-350 type. Prior to installation, these transducers were meticulously soaked to ensure full saturation, a critical step to guarantee their operational accuracy and responsiveness to changes in the pore water pressure within the soil matrix. To further ensure the integrity and functionality of the transducers during the tests, each device was carefully wrapped in permeable gauge. This protective measure served to prevent the clogging of the transducer's sensitive measuring elements by fine soil particles, which could otherwise compromise the accuracy of the pore water pressure readings. At the end of the fourth-stage load application, the undrained shear strengths of the soil at different locations (E1-4, F1-4, G1-4, H1-4, H1-4 in the soilbag-treated subgrade, and e1-4, f1-4, g1-4, h1-4, i1-4 in the sand-treated subgrade) were determined by a vane shear test apparatus. This was performed to assess the drainage consolidation efficiency of both the soilbag-treated subgrade and the sand cushion-treated subgrade.