Since the great earthquake which hit the city of Kobe in 1995, seismic design code and criteria has been revised significantly so that structures can survive even after strong earthquake. Since then seismic reinformcent of existing facilities has also become urgent task among civil engineers. Soil liquefaction can be a critical cause for the damage of structures especially along the bay area of Japan. This paper presents an innovative construction method for ground improvement beneath existing structures. The method consists of the horizontal directional drilling method and the permeation grouting. Both the conventional technologies have been specially modified to enhance the work efficiency as well as the quality of the soil improvement. Then the field tests are shown that were conducted to see the good performance of the present method. The solidified bodies by the permeation grouting were exposed by excavation in the field tests to observe directly; the satisfied qualities were confirmed through the unconfined compressive strength tests using samples from the solidified bodies. Finally, one of the projects in which the present method was adopted is shown. The projects include total 4,800m length of drilling and 2,000 grouting points beneath oil tanks. The various aspects of the work is presented including drilling control, grout injection control and quality check on the solidified bodies. Also the monitoring system on the ground surface displacement is shown that effectively worked to avoid the serious damage on the tanks during the drilling and grouting work.