# 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
The apparatus employed for the vane shear tests was the LFTD-1703 model, a specialized device designed to measure the undrained shear strength of soil. This apparatus is equipped with a maximum range of 20 kPa, making it well-suited for the precise measurement of soil shear strength in our experimental setup. In conducting the vane shear tests, its reading was first correctly calibrated and reset to zero. The cross-plate, equipped with teeth for better soil engagement, was then carefully pressed into the soil at the predetermined location within the subgrade model, reaching the depth required for accurate testing. To simulate the conditions that would lead to shear failure, we applied a constant vertical pressure and initiated rotation of the upper handle of the cross-plate shear apparatus at a controlled velocity of 8 s per rotation cycle. This rotation was continued until shear failure was observed in the soil, with the apparatus providing a real-time reading of the shear strength at the moment of failure. By maintaining this controlled testing regime, including a specific strain rate achieved through the rotational velocity, the undrained vane shear strength was accurately determined. Figure 4 shows the surface settlement over time of the model testing subgrades. From the data presented, it can be observed that the surface settlement of the soilbag-treated subgrade increased to 81.94 mm after stabilization under the first-stage load. With the application of the second-, third-, and fourth-stage loads, increments of 13.34 mm, 10.65 mm, and 4.37 mm in settlement were recorded, respectively. As a result, a final measured settlement of up to 110.35 mm was noted. For the subgrade treated with the sand cushion, the settlements recorded after stabilization under the first-, second-, third-, and the fourth-stage loads were 109.39 mm, 122.86 mm, 130.05 mm, and 137.86 mm, respectively, which were greater than those of the soilbag-treated subgrade under corresponding loads. Such differences in settlement are attributed to the staggered arrangement of the soilbags in the upper and lower layers, which have the effect of spreading the stresses, enabling the vertical stresses from the upper layers to be more uniformly distributed to the base [33]. From these observations, it is inferred that the soilbag-treated subgrade exhibited a superior bearing capacity compared to the sand cushion-treated subgrade. In Figure 5, the pore water pressure in the subgrades treated with soilbags and the sand cushion is depicted, measured at both the center and at 20 cm from the center, at depths of 0, 5 mm, and 15 mm, respectively. For both the sand cushion-treated and soilbag-treated soft soil subgrades, a sudden increase in the pore water pressure was witnessed once the vertical load was applied, reaching a maximum value, followed by a gradual decrease due to the dissipation of pore water. The pore water pressure in the subgrade treated with the sand cushion was found to dissipate faster than that treated with the soilbags due to the superior drainage effect of the sand compared to the soilbags. Nevertheless, under the subsequent second- to fourth-stage loads, a decline in the drainage capacity of the sand cushion layer occurred. This was caused by the compression of the sand particles into the underlying soft soil subgrade due to their minuscule size. The mix of soft soil and sand reduced the permeability ability of the sand cushion. Conversely, the soilbags, being resistant to compression into the underlying soft soil subgrade, mainly depended on the drainage through the contact interface and the gaps between the soilbags. Thus, their drainage capacity was generally unaffected by the vertical load. Consequently, under subsequent second- to the fourth-stage loads, a faster dissipation of the pore water pressure in the soilbag-treated subgrade was observed. after the fourth-stage load.