To provide a compounding method of a solidification material by which an unconfined compressive strength taking a compaction energy into consideration, a cone index, an air space ratio, a coefficient of permeability, an effective cohesion, an effective angle of internal friction, etc. can be rationally set in relation to an addition rate of the solidification material in improved soil.

Specimens of an earth material and the solidification material compounded in different ratios are made, and a correlationship between the unconfined compressive strength and the addition rate of the solidification material is obtained. Each of the specimens which is raveled is compacted, and one or more of the cone index, the air space ratio, the coefficient of permeability, the effective cohesion and the effective angle of internal friction of each of the compacted specimens are obtained. The correlationship between the unconfined compressive strength and the compaction energy of each of the compacted specimens is also obtained. Referring to one or more of these, the unconfined compressive strength corresponding to the set compaction energy is selected from the correlationship between the unconfined compressive force and the compaction energy of the compacted specimens, and the addition rate of the solidification material corresponding to the confined compressive force is selected from the correlationship between the unconfined compressive force and the addition rate of the solidification material of the specimens.