The Deterioration Of Mechanical Properties And Its Mechanism Of Cement-Improved Loess Under Freeze-Thaw Condition

With the further implementation of the strategy of building a strong transportation country and “The Belt and Road” initiative,the infrastructure construction in Northwest China has been further developed which represented by high-speed railways and highways.However,Northwest China is covered with loess,cement-improved loess(CIL)is used as a common filler for subgrade construction projects in there.And the area is also in the seasonal frozen area,the freeze-thaw(F-T)conditions in there has a significant effect on the stability of CIL subgrades.Therefore,it is of great theoretical significance to study the degradation principle of the mechanical properties of CIL under the F-T condition and its microscopic mechanism for calculating the additional settlement and evaluating the stability of CIL subgrade in the seasonal frozen area.From a combination of macro and micro perspective,in this paper,the mechanical characteristics of CIL under different number of F-T cycles,freezing temperatures and confining pressure were studied,its deterioration mechanism was revealed through microstructure testing.Substances of this paper include:(1)Triaxial tests were applied to CIL samples with different freezing temperatures,number of F-T cycles and confining pressure to analyse the influence of F-T condition on the stress-strain relationship and mechanical properties of CIL.The results show that: the stress-strain relationship curves of the CIL have an obvious strain-softening characteristic with strong strain-brittleness;the initial tangent modulus,strength and cohesion of CIL decrease with decreasing freezing temperature and increasing number of F-T cycles,and there is no clear relationship between its internal friction angle and the number of F-T cycles.(2)Scanning electron microscope tests and nuclear magnetic resonance tests were also applied to CIL samples with different cement admixture,freezing temperatures and number of F-T cycles to analyse the influence of F-T condition on the microstructure and deterioration mechanism of CIL.The results show that: when cement is added to the loess,the cement-hydrate wraps the loess particles and fills the pores between the loess particles,which significantly reduces the number of large pores,increases the number of small pores and improves the strength of the soil.Under the F-T condition,the average pore diameter of CIL increase with decreasing freezing temperature and increasing number of F-T cycles,the cement-hydrate wrap on the loess particles and filled into the pores of loess particles become more and more damaged as the increasing number of freeze-thaw cycles.So,it is considered that the damage to the cement-hydrate within CIL and the increasing pore space after F-T cycles are the main reasons for the deterioration of its mechanical properties.(3)The significance analysis of the effects of the number of F-T cycles,freezing temperature and surrounding pressure on the strength and initial tangent modulus of CIL,and the number of F-T cycles and freezing temperature on the cohesion,internal friction angle and pore characteristics of CIL were analysed.The results show that the above principles are generally consistent with the results of the single factor significance analysis,but when the interaction effects between two factors include confining pressure,freezing temperature and number of F-T cycles were considered,each of the effects of the interaction on the strength and initial tangent modulus is insignificant.(4)Basing the variation of initial tangent modulus and strength under F-T condition,the empirical formula for the deterioration of the strength,initial tangent modulus and internal friction angle of CIL was proposed.Meanwhile,a non-linear Mohr-Coulomb strength criterion for CIL was established by describing the relationship between large and small principal stresses when sample failures with a modified Weibull function.(5)To describe the stress and strain relationship of CIL more accurately,through introducing a breakage rate function and a local strain coefficient,a binary-medium constitutive model consisting of bonded elements(soil-particle cohesion)and frictional elements(soil particles or soil aggregations)was established to describe the stress-strain relationship of CIL under F-T condition.The test results indicate that the model can well describe the strain-softening phenomenon of the stress-strain curve of CIL and reflect the breakage mechanism of CIL.