A series of laboratory experiments is conducted to study the changes in soil particles and pores under freezing and thawing cycles. The results show that during cyclic freezing-thawing processes

grain breakages occur within the soil particles

leading to a progressive increase in plastic limit

plasticity index and specific surface area of the soil. Meanwhile

the pore size distribution

tested by mercury intrusion porosimetry (MIP)

is also changed in the processes as macro pores increase

small pores reduce and micro pores almost remain unchanged. All the structural indexes

including particle composition

pore distribution specific surface area

tend towards stable after 15 freezing and thawing cycles. The intrinsic influence of soil’s three-phase components during freezing and thawing cycles is discussed; and it is considered as the fundamentals of the water phase change and moisture migration among soil particles. Correspondingly

the reaction of water phase change and migration on soil particles and pores is considered to be the primary cause of the change of soil structures.