Fifteen-year investigation of volcanic cohesive clay improved by in-situ mechanical mixing with cement

Soil columns cemented by in-situ mechanical mixing were investigated for 15 years to demonstrate the long-term strength development and durability of cement-treated soil. The soil was a volcanic cohesive clay called Kanto-loam. The cement used was two special types of cement for general purposes. One type of cement was for soft soils and the other was for special soils (hexavalent chromium (Cr(VI)) elution reduced type). Both cement-treated soils increased in strength over the long term, maintaining more than doubling the 28-day strength after 15 years. The cement hydrate contributing to the strength development was identified as ettringite by powder X-ray diffraction (XRD). The ettringite was produced from an early age and was stable over the long term. C-S-H is also one of the major cement hydrates, but could not be identified by XRD due to its amorphous nature. The durability of the cement-treated soils was also verified by measuring changes in calcium content and pH. All measurements showed no tendency to decrease over time, confirming the long-term durability of the cement-treated soils. In addition, to investigate the impact of the cement-treated soils on the surrounding environment, the groundwater concentrations and surrounding soil elution of hexavalent chromium (Cr(VI)) were measured. Measurements were implemented several times from immediately after construction up to 15 years. None of the results exceeded the environmental standards, confirming that the cement-treated soil does not pollute the surrounding environment.