Abstract: Freeze-thaw cycles destroy the structure of soil, resulting in a reduction in mechanical properties of soil. A series of tests on lime-soil and fiber reinforced lime-soil is completed, including freeze-thaw test, the unconfined compressive test, scanning election microscopy (SEM) test, and nuclear magnetic resonance (NMR) test. Based on the test results, the effect of freeze-thaw cycles on soil strength and microstructure index is discussed. The results show that the compressive strength of lime-soil and fiber reinforced lime-soil decrease with the increase of freeze-thaw number, and the four stages for soil strength variation are identified in soil failure process, i.e., large reduction, small reduction, slow reduction and stability. In the case of high moisture content and frequent freezing and thawing, it is more conducive to the reinforcement of soil by fibers. The addition of fiber delays the formation and development of cracks in soil, reducing the connection of cracks. With the increase of freeze-thaw number, the porosity and the pore diameter increase, as a result, a part of small pores connect into medium pores and large pores. The porosity and pore distributions of soil vary slightly under freeze-thaw cycles because of the spatial restraint effect of fiber on soil and the friction between fiber and soil, which result in the strength and freeze-thaw resistance of fiber reinforced lime-soil is better than that of lime-soil.

Key words: freeze-thaw cycles, fiber reinforced soil, unconfined compressive strength, porosity, pore distribution