Mechanisms of property changes of soft clays around deep mixing column

Abstract

Installation of displacement types of columns such as driven pile, sand compaction pile, lime compaction pile, and deep mixing column causes stress changes in the surrounding soil. The properties will change in surrounding soil. Strength of the surrounding soil will decrease, recover, and increase after column installation. The mechanisms of effects of deep mixing column installation on surrounding clay can be identified into seven categories: (1) disturbance or structure damage of soil surrounding the column due to blade mixing and expansion of admixture injection; (2) thixotropic behaviour of soft clay; (3) soil fracturing around column during mixing and expansion; (4) cement penetration and diffusion from column to surrounding soil through fractures and soil pores; (5) cementation due to increase of cation concentration in pore water; (6) consolidation effect due to increase of effective stress in surrounding soil after excess pore pressure dissipation; (7) heating caused by the chemical reaction of hydration and pozzolanic reaction. These influential factors were investigated through laboratory tests and theoretical analysis. Laboratory tests were performed to investigate the effects of soil thixotropy, soil fracturing, and cement penetration in soft Ariake clays. Theoretical analysis was employed to analyze the fracturing phenomenon. Fractures and cement penetration phenomenon were observed in laboratory test. For soft Ariake clay, test results show that strength due to thixotropic hardening regained completely within 70 days. Theoretical result shows that even in a very low admixture injection pressure, fracturing will occur in surrounding soil.