Study on spatial distribution of soft soil parameters and settlement deformation law of extension foundation based on CPTU

In view of the subsidence and distortion laws of the subgrades of expansion foundation in the renovation and extension project of old roads, this paper conducts in-situ tests on the soil of the new and old road foundations based on the piezocone penetration test (CPTU). Geostatistics was used to interpolate and predict the tip resistance of the cone penetrometer and friction sleeve resistance within this region; these interpolation results were subsequently verified independently. In line with the interpolation outcomes, the compression modulus and soil layers of the new and old subgrade were categorized. Meanwhile, by leveraging the refined stratification findings of subgrade soil acquired through geostatistics, a discretized element model covering the whole road section was established. The findings demonstrate that the established geostatistics model can accurately predict the tip resistance of the cone penetrometer and friction sleeve resistance of new and old subgrade soil. Moreover, the correlation coefficient R² of the independently verified results is greater than 0.87. The compression modulus (E_S) is primarily distributed between 0 and 20 MPa, and increases with increases in the depth. Overall, the E_S of the old subgrade exceeds that of the new subgrade. The differential settlement of new and old subgrades is large, and the maximum differential settlement is 727.17 mm. Based on the finite element model of the subgrades of both extension foundation created using geostatistical soil classification, the horizontal displacement and subsidence of the new and old subgrades were analysed. The maximum ground subsidence of the old subgrade is 280 mm, following the completion of the old road. After the old road has been in operation for 20 years, the maximum pavement settlement reached 680 mm. There is an evident differential subsidence of the extension foundation of the whole line, ranging between 234 mm and 529 mm in the 15 years after the renovation and extension project. In renovation and extension projects, it is of great significance for controlling the settlement of subgrades for both extension foundations to analyze and study their settlement and distortion laws by applying geostatistics based on limited exploration data.