PURPOSE: To uniformly increase the strength of ground without use of large-scale machines as well as make the following excavating operation easier without changing the pH value of ground when forming a foundation in the cohesive soil ground having less than a given inner friction angle.

CONSTITUTION: A cohesive soil of an inner friction angle of 15° or less is mixed with a hydrated calcium silicate porous powder 2 in a proportion of 50-200kg/ m2/unit volume. A latticed net 1 having a tensile strength of 1-5tonf/m per unit width is placed horizontally with a placing width 5 of 1-3 times the foundation width 4 at an interval 6 of 0.5-1.0 times the width of foundation, and the foundation is formed on the nets 1.