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The deep mixing method was developed in Japan and Scandinavian countries in the 1970s. Subsequently, considerable research studies have been carried out on the strength characteristics of treated soil, the interaction of treated soil and surrounding soils, the development of new binders, the development of new execution machines and techniques, the establishment of design procedures and so on. However, long-term changes in the properties of treated soil have not yet been clarified. Two columns of in situ quicklime-treated soil in a soil tank for more than 37 years were analysed in this study. Laboratory tests were conducted on them at curing periods of 11, 27 and 37 years. This paper presents the change of wet density, water content, unconfined compressive strength and calcium content. Additionally, needle-penetration resistance, pH and electrical conductivity were measured in order to reveal the deterioration properties from the surface exposed to soil for more than 37 years. The results showed that the strength of the in situ quicklime-treated soil remained, even after 37 years. However, the deformation modulus tended to decrease and the treated soil deteriorated from the surface.

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