A method of soil improvement work drills the ground with a drilling head arranged at a lower end of a rotary shaft and sends the rotary shaft into the ground. The method mixes pressurized water with compressed air on the ground, to prepare a fluid mixture, feeds the fluid mixture through a feed path arranged along the rotary shaft into an ejector arranged on the drilling head, and ejects the fluid mixture from the ejector toward the ground to drill. An envelope defined by a front end of the drilling head has a conical shape.