Field Monitoring Test Analysis of Controlling Settlement for Expressway Soft Foundation by Combined Vacuum Surcharge Preloading Method

The combined vacuum surcharge preloading method has the double reinforcement effects of vacuum preloading method and surcharge preloading method. The lateral extrusion deformation due to surcharge preloading was counteracted by the lateral shrinkage induced by vacuum preioading, which ensures the stability of the upper embankment, accelerates the subsoil consolidation and enhances the construction speed. Based on the practical engineering of expressway soft soil treatment, the full monitoring was conducted for the complete and effective observation sections, according to four field test sections. The analyzed results show that the main settlement reinforced by the combined vacuum surcharge preloading method occurs in the early and middle phrase; the settlement and rate is large in the early stage,reaches maximum in the middle stage and becomes little in the last stage; the maximum settlement rate is 0. 09cm/d and less than the allowable value of 1cm/d required by of surcharge preloading method,which indicates that the reinforcement effect of combined vacuum surcharge preloading method is significant.