Model experiment on deep soft ground improvement of existing sand-filled subgrade with squeezed branch piles

There are limitations from the pavement structure layer and construction site space during the deep soft soil reinforcement of the existing sand-filled subgrade in expansion of highways. Firstly, a new method using squeezed branch pile (SBP) with one single plate for treatment of existing soft foundation was proposed in this paper. In this method a plate was set on the top of the piler to achieve the effect similar to a pile cap or support plate without breaking the road structure layer. Secondly, a physical model test of pile group foundation was performed, a pile-soil unit was simulated to study the bearing behavior of SBP. By analyzing the mechanical and deformation indicators of the pile, plate and soil, the bearing performance and soil deformation characteristics of the new reinforcement method were studied. Finally, a simplified two-dimensional model of the SBPs reinforcement method for the existing sand-filled embankment was derived by considering the soil parameters and pile size. The results indicated that the axial force of piles and soil pressure vary with time under constant load due to the compressibility of soft soil and the properties of loose sand; The total axial force at the top of the pile was approximately positively correlated with the magnitude of the constant load linearly. At the same time, the larger the diameter of the support plate was, the greater the total axial force at the top of the pile and the higher the proportion of pile bearing load. This study found that the pile bearing ratio of the SBPs reinforcement method was about 70%, proving effectiveness of this new reinforcement method. In addition, using the proposed two-dimensional simplified model for parameter analysis, it was found that the minimum pile spacing in the sand filled subgrade was related to the overlying load, the internal friction angle of the sand filled soil, the strength index of deep soft soil, and the geometric parameters of the support pile.