Abstract:Aimed at the existing inconsistent state—the whole ground is regarded as saturated soil and measured groundwater level falls in current research on soft ground improved with vacuum preloading,a series of indoor simulated experiments are conducted on vacuum pump dewater and several typical phenomena are analyzed in vacuum preloading project. Laboratory experiments illuminate that vacuum suction can induce three kinds of convections in different external conditions—water flow,air-water two-phase flow and air flow as follows:(1) the flow pattern of convection is air-water two-phase flow under the action of suction provided by jet pump in vacuum preloading project;(2) there is unsaturated zone in the soft ground improved with vacuum preloading and the sealed system isn¢t absolutely airproof in project practice,and there exist air supplied in the improved zone. Liquid-liquid and gas jet pumps draw groundwater by the convection of air-water two-phase flow,which results in groundwater level fall;(3) the unsaturated zone in the ground improved with vacuum preloading is quite similar to the ground of multi-row wells pumping water. The quantity of pumping water by jet pump is composed of the drained water by unsaturated gravity zone above groundwater level and elastic released water below groundwater level saturated soil induced by hydraulic head drop. It is the main reason to induce the contradiction or difference that the sealed boundary conditions of improved zone are different among current theory analyses,indoor model experiments and project practice in soft ground improved with vacuum preloading technique.