PURPOSE: To simply strengthen soft ground at low cost by a method in which hot water is used for a drilling auxiliary liquid and a pollutionless grout is diluted with hot water for use.

CONSTITUTION: Boring is made by using hot water as a drilling auxiliary liquid in soft ground by means of the bit 16 of a double-tube rod 8, and a pollutionless grout 17 is mixed with hot water. A small mount of a blowing agent 5 is added under pressures to different coagulants 17 to the grounds A and B 11 to be protected by an automatic apparatus 4 and a quick-setting type coagulant 17 mixed with a blowing agent is gelled in a soft sand state during the passage through a hose 6, a swivel 7, and the double-tube rod 8. lastly, the quick-setting type coagulant 17 with air bubbles is gelled in a branched sand gel state to strengthen the soft ground 10 by the improvement work.