Factors that influence the degree of hardening by air-drying of Maji-soils in the central region of Okinawa Island [Japan]

The degree of hardening by air-drying of 43 samples of Maji-soils (yellow soils, red soils and dark red soils) in the central region of Okinawa Island was determined by an unconfined compression test. Physical and chemical analysis was also conducted for the same samples, and the relationship between the degree of hardening and physical and chemical properties was investigated. The degree of hardening, i.e. unconfined compressive strength, in air-dried state varied among the examined soils. The maximum was 6.4 MPa and the minimum was 0.6 MPa. Clay content, pH and exchangeable calcium content were intimately related to the degree of hardening. Clay content showed a positive relationship to the degree of shrinkage of soil blocks by air-drying. It seems that clay content affects the degree of hardening through the degree of shrinkage. Soil pH and exchangeable calcium content were positively related mutually, and both showed a positive relationship to the degree of hardening. However pH was not related to the degree of shrinkage. Amendment of alkali solution (sodium hydroxide) or exchangeable cations (calcium, magnesium, potassium or sodium) increased the degree of hardening of an acidic soil. The degree of shrinkage was also increased by amendment of exchangeable cations, but it was not increased by alkali solution amendment. Fine particle size distribution examined by a laser scattering method and clay mineral composition showed no difference among the soils with different pH. Consequently, it is assumed that pH affects soil hardening through some physicochemical condition of the surface of soil particles, such as charge, that affects the microstructure of floe of soil particles, and exchangeable calcium fosters the shrinkage by air-drying and thus it increases the degree of hardening.