Assessing compatibility of clayey soils using state variables for sustainable DMM in Saga Lowland, Kyushu, Japan

Abstract The deep mixing method (DMM) has become a leading ground improvement technique in Japan over 60 years. However, recent cases of construction defects suggest not only environmental factors but also human errors, such as poorly constructed improved columns. Engineers may struggle to visualize soil behavior from geotechnical borehole survey data from structural viewpoint. This study focuses on the unconfined compression characteristics, sensitivity, and compressibility of clayey soil samples collected from the Kyushu region of Japan. Statistical data, such as the mean values of state variables including failure strain ( ε f ), normalized deformation modulus ( E 50 / s u ), liquidity index ( I L ), and compression index ratio ( C c1 / C c2 ) were calculated. By incorporating soil structure considerations into the comparative analysis of these variables, the study aimed to identify thresholds that indicate the "compatibility" or "incompatibility" of clayey soils. Clayey soils with ε f < 2.8%, E 50 / s u ≥ 110, I L ≥ 1.11, and C c1 / C c2 ≥ 1.43 were classified as “compatible” with a bulky structure, while those with ε f ≥ 2.8%, E 50 / s u < 110, I L < 1.11, and C c1 / C c2 < 1.43 were “incompatible” with a dense structure. The findings provide a structural framework for engineers to interpret soil data, improving DMM quality, risk management, and sustainable construction.