Relationship between microstructure and degree of hardening of air-dried maji-soils under different pH conditions

Abstract

Some of the physical properties of the Maji-soils in Okinawa Island, Japan, cause problems, namely severe hardening by air-drying. Although the degree of hardening by air-drying is especially high under high-pH conditions, the mechanism of hardening enhancement associated with the pH has not been elucidated yet. In order to investigate the mechanism underlying hardening enhancement, observation of soil micromorphological characteristics and unconfined compressive strength measurement of air-dried soil blocks were conducted for three samples of Maji-soils: alkaline soil, acidic soil, and alkali-added acidic soil, with pH (H₂0) values of 7.6,4.9, and 7.8 respectively. Observation of micromorphological characteristics at a low and a high magnification revealed that there was no significant difference in the micromorphological characteristics among the three samples except for the planes that were observed only in the acidic soil and alkali-added acidic soil. The existence of planes can account for the lower strength of the acidic soil (2.6 MPa) than that of the alkaline soil (3.9 MPa). However, the strength of the alkali-added acidic soil ( 4.9 MPa) that also contained planes was higher than that of the alkaline soil. Therefore, it was considered that the existence of planes was not the sole cause of the pH-dependence of Maji-soils and that some other mechanisms were involved in the enhancement of hardening under high-pH conditions.

Key Words:

• air-dry
• hardening
• Maji-soils
• microstructure
• pH