PURPOSE:To make execution of work easy by thrust a hollow cylindrical mandrel into weak subsoil with its openable end closed, and by discharging hardenable liquid substance into the subsoil as the mandrel is pulled out with the end opened. CONSTITUTION:A hollow cylindrical mandrel 10 is thrust into weak subsoil E roughly at perpendicular with its openable end closed. While the mandrel 10 is pulled out, a cone 12 attached to the end is detached allowing the end to open, and hardenable liquid substance 14 is injected into the inside. Then the liquid substance is allowed to discharge from the end of the mandrel 10 into the subsoil and a pilelike hardened body 16 is formed after hardening of the liquid substance 14. Injection pressure of the hardening liquid substance 14 is made changeable so that the pressure can be set to become lower in stages in proportion to the decrease of the depth. Thereby proof stress of the ground can be increased.
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