FOUNDATION IMPROVEMENT METHOD

PURPOSE: To increase the strength of a ground by a method wherein water is infected into a sandy ground, an infiltrating flow is generated, water, in which grains smaller than sand grains in the sandy ground are mixed, is injected, water injection is stopped and water is absorbed from the sandy ground. CONSTITUTION: When a pipe member 20 is inserted into a sandy ground 10 and water is injected through holes 14 from a water supply source 12, infiltrating flows are generated in the ground 10, excess void water pressure is generated in a water injection region, and boiling or fluidization is generated. Water, in which the grains of cement, bentonite, etc., smaller than the sand grains of the ground 10 are mixed, is injected and dispersed into the water injection region of the ground 10 through the holes 14. When the supply of water is stopped and negative pressure is applied to holes 18 from a negative pressure source 16, water in the water injection region is sucked to the negative pressure source 16 while grains are left among sand grains. The boiling of the water injection region is stopped, and the spaces of the grains and sand grains are narrowed at peripheral soil pressure and brought to a consolidated state. Accordingly, ground density is increased, and strength can be improved.