Mechanical Properties Of Thawed Clay And Permanent Deformation Of Subgrade In Seasonal Frozen Regions

Freeze-thaw cycles and heavy traffic loads are two main engineering hazards which are threatening the transportation infrastructure construction and operation in seasonal frozen region.Research on these topic have been lasted for decades but the engineering problems are still not settled.In view of the fact that the evaluation method of subgrade stability under effect of heavy traffic loads in seasonal frozen soil area is not perfect,this paper is closely related to the needs of road construction and maintenance in China.By means of combination of theoretical analysis,laboratory test and numerical simulation,the influence of freeze-thaw cycles on the evolution of subgrade soil structure is fully investigated.On this basis,the static and dynamic characteristics of the thawed subgrade soil are studied systematically,the dynamic permanent deformation prediction formula of the thawed subgrade soil is proposed.The water-thermal-dynamic coupling model of the subgrade is established and verified.The working state and dynamic response of the subgrade are studied.The calculation schedule of subgrade permanent deformation is proposed.Finally,the allowable deformation control standard of subgrade is determined according to the requirements of pavement layers.The main research contents and conclusions obtained are as follows,Based on the results of shear wave velocity test and industrial CT scan test,the effects of freeze-thaw cycles on the stiffness and density of subgrade clay are evaluated by using the shear wave velocity and gray value.By introducing the concept of anisotropic index,the evolutions of small strain shear modulus anisotropy and equivalent density anisotropy of subgrade clay with the number of freeze-thaw cycles are studied quantitatively.The influence mechanism of freeze-thaw cycles is revealed from a new point of view.It is considered that stiffness and density of subgrade clay are weakened by freeze-thaw cycles.The weakening effect is no longer obvious after 7 cycles.By means of isotropic compression-resilience test,triaxial static creep test,triaxial shear compression test and strain rate test,the compression-resilience characteristics,creep characteristics,stress-strain relationship and strain rate characteristics of subgrade clay under different freeze-thaw cycles are studied systematically.Relationships between the peak strength,slope of compression curve,slope of resilience curve and number of freeze-thaw cycles are established.A prediction model for triaxial creep of thawed subgrade clay is presented.Relationships between shear strength,secant modulus,shear strength index and strain rate of thawed subgrade clay are also studied.It is considered that the generalized hyperbolic model can describe the stress-strain curve of thawed subgrade clay under different strain rates.The evolutions of dynamic shear modulus,damping ratio,resilience modulus and cumulative plastic strain of thawed saturated clay during long-term cyclic loading were studied by a series of cyclic triaxial tests.It is verified that Martin-Davidenkov model and hyperbolic model can be used to predict the evolution of dynamic shear modulus and damping ratio of subgrade clay under long-term cyclic loading,respectively.A long-term modulus prediction model for subgrade clay considering freeze-thaw cycles,dynamic stress amplitude,confining pressure,loading frequency and vibration frequency is established.An empirical formula for predicting accumulative plastic strain of subgrade clay is presented,which can consider the factors of freeze-thaw cycles,dynamic stress amplitude,moisture content,initial stress ratio and vibration frequency.The thermal-hydro coupling model and the dynamic response model of subgradepavement are verified respectively by the water migration test of unsaturated subgrade soil and the field measured data of dynamic stress of subgrade.The thermal-hydrodynamic coupling model is composed of these two models in sequence.Using the thermal-hydro-dynamic coupling model to simulate and analyze the evolution of temperature and water field of subgrade in 20 years after built in seasonal frozen region.The working state and dynamic stress response of subgrade under different seasons and heavy traffic loads are studied systematically by establishing the calculation formulas of dynamic elastic modulus of subgrade soils in different seasons.The calculation method of uneven deformation of subgrade is put forward.The model of uneven deformation of subgrade and its quantitative function expression are determined.It is considered that the Gauss peak function can reasonably describe the uneven deformation curve of subgrade in seasonal frozen region.The formulas for predicting the uneven deformation of subgrade under different freeze-thaw cycles,heavy traffic loads and cyclic loadings are established.A gauss peak function parameter,which can describe the width and depth of uneven deformation curves,is selected as the control index for uneven deformation of subgrade.The bottom tensile stress of the subbase is selected as the research index to evaluate the safety performance of the pavement.The prediction formula of the bottom tensile stress of the subbase is established,which can consider the thickness,modulus of pavement structure layers together with the control index of uneven deformation of the subgrade.Taking the allowable value of bottom tensile stress of subbase as the control standard,the formula of control index of subgrade uneven deformation is put forward for the subgrade built in seasonal frozen region.A calculation example of determining the allowable value of subgrade uneven deformation is also given.