Abstract: The differences between granitic residual soil and its free iron oxide removed soil are studied from the aspects of mechanical, compressive properties and microstructure. The mechanism of the influence of free iron oxide on the above properties is also analyzed. The result show that the free iron oxide has a greater influence on the cohesion of the granitic residual soil and a smaller influence on its internal friction angle. The cohesion of granitic residual soil and its removing free iron oxide soil are 26.67KPa and 9.56KPa, respectively. The internal friction angle is 23.2°and 23.5°. The cohesion of the free iron oxide removed granite residual soil is lower than that of the original soil because the decrease of Fe³⁺ makes the colloidal electric double layer thicker, resulting in the increase of distance and decrease of the suction among colloidal particles. Compared with the free iron oxide removed soil, the granite residual soil has more small pores and the soil particles are more compact. With the increase of pressure, the compressive settlement of the free iron oxide removed granitic residual soil is first larger than that of its original soil, and then smaller than that of its original soil. More large pores and the soil agglomeration are the reasons that the compressive settlement of the free iron oxide removed granite residual soil is larger than that of the original soil at low pressure and smaller than that of the original soil at high pressure.

Key words: Free Iron Oxide, Mechanical Properties, Microstructure, Mechanism Analysis