Abstract: The vacuum preloading-combined surcharging method is an effective way to consolidate soft soils by using both vacuum preloading and the surcharging preloading methods. Based on the Darcy’s law, the seepage rate of the pore fluids in the porous media is proportional to the hydraulic gradient, so that the soil consolidation can be accelerated by increasing the head pressure or decreasing the drainage path. This paper presents a case history of the ground improvement of a railroad subgrade over soil soils. By monitoring the surface settlement and performing the laboratory testing on the mechanical properties of the subsurface soils, the effect of the ground improvement by using the vacuum preloading combined with surcharging method can be evaluated. It is concluded that the vacuum preloading method can reduce the pore water pressure of porous media and form a hydraulic gradient to accelerate the flow of pore fluid.  The surcharging can improve the shear strength of soft soils by dissipating pore pressure under the surcharging load. Under combination actions of two methods, the positive and negative excessive net pore water pressure will significantly improve the effect of soft improvement. 

Key words: Vacuum Preloading Combined Surcharging Method, Soft Slay, Subgrade Improvement, Field Monitoring, Surface Settlement