| Seasonal frost soil is widely distributed in China, subgrade fillings exposed to natural environment will experience at least one freeze-thaw cycle every year. Under such strong freezing and thawing conditions, the diseases such as mud pumping, subsidence and strength reduction will arise, which would affect the normal operation and driving safety of railway. Meanwhile, along with the speedup of railway and construction of highspeed railway in seasonal frost area, the standard on overall stability of subgrade, smooth of subgade and anti-frozen ability continues to improve. Therefore, searching for good quality fillings, reducing the engineering costs and enlarge the applied range of modified soil has already become an important problem for solving.With the support of the Ministry Science & Technology Research Planning Project, in this dissertation, with the coming of Highspeed Railway construction in seasonal frost area, clay soil and modified soil was taken as the study object. Based on a review of status quo of research at home and abro, a series of tests were conducted, and the strength and deformation properties of modified-soil subjected to freezing and thawing were analyzed to investigate the freezing and thawing mechanism, to offer guidelines for the choice of improved method and blend ratio, to offer references for confirming the mechanical index of subgrade soil, to provide basis for freezing and thawing resistance index. Some achievements are as follows:(1) According to the result and shortage of previous research, the dynamical properties of modified soil subjected to freezing and thawing had been regarded to be short of systematic research, which had great meaning in theory and in practice.(2) Based on the frost damages investigation of Inner Monqolia railway, Northeast railway and Qin-Shen Passenger Line, the frost damage characteristic of subgrade in seasonally frost area was summarized. Meanwhile, the creative reason of frost damages was analyzed from the kind of soil, amount of water, temperature as well as the loading style.(3) The frost heave properties of clay soil were weak by means of frost heave tests, and the frost heave properties of modified soil were weaker, which had better anti-frost properties. The frost damage was not only frost heave deformation, but also the strength reduction of subgrade soil subjected to freezing and thawing. (4) By means of static triaxial test, the influences of freeze-thaw cycles, cooling temperature, and confining pressure on stress-strain relationship and strength properties of cement- and lime-modified soils were apprehended. The change rules of stress-strain, static strength, elastic modulus and shear strength index influenced by freezing and thawing were obtained. Moreover, several prediction models about the relationship between these indices and blend ratio and freeze-thaw cycles were established.(5) The cumulative plastic strain of modified soil, which mightbe change according to soil types, with varied cyclic loading and soil physical states were studied. The effect of dynamic stress amplitude, freeze-thaw cycles, cooling temperature, and confining pressure on cumulative plastic strain had been analyzed. Furthermore, based on the exponential model of Monismith, the line of normalization and modificatory was adopted to established the predition model about the cumulative plastic strain.(6) The critical dynamic stress of modified soils was analyzed, and the freeze-thaw attenuation coefficient was introduced and applied to the blend ratio design of modified soils for Highspeed Railway, which could better serve for the engineering practices.(7) Based on the previous research, the damage degree was introduced to establish the damage model about resilient modulus for cement- and lime-modified soil, which could better evaluate the service life of modified soils in seseanol frost area. Meanwhile, the relationship of between dynamic stress and strain was analyzed and studied by the hyperbola model of Hardin-Drnevich.
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