| Loess is widely distributed in China, and it mainly lies in the central and western regions. With the implementation of the western development strategy, there will be more and more the project in loess area, so research on loess dynamics will help to promote economic development in these regions. Some aboard and domestic scholars have already proceeded some definite studies on dynamic property of loess, due to the limitation of experimental conditions, the conventional dynamic triaxial test apparatus and conventional dynamic torsional shear apparatus can not simulate the real stress condition of soil under foundation or sub-grade in real project. The initial stress conditions for soil element beneath the actual earth structure and foundation are usually rather complex and diverse, not only the principal stresses are not equal generally on three directions, but the initial direction angle of maximum principal stress changes. When it is forced by the cyclic loading for example earthquake, traffic, wave etc, dynamic properties of soils under complex initial stress condition are different form the general stress state. However, the research of dynamic properties of soils under complex stress conditions in china mainly focus on sandy soil, soft soil and other soil materials, dynamic characteristics of loess under initial complex stress conditions are seldom researched. So it has very significant academic meanings and engineering value to study the loess's dynamic deformation and dynamic strength under complex initial stress conditions, which can develop and prefect the soil dynamics theory, and guide the actual design and construction.This paper reformed the DTC-199 Type-torsional cyclic load triaxial apparatus, it can better simulate complex initial stress conditions of compacted loess foundation or sub-grade. The complex initial stress state mainly controlled by the average principal stress p , the initial angle of principal stressα0, the initial coefficient intermediate of principal stress b0 , the initial deviatoric stress ratioη0, by controlling these four stress parameters this article did a more systematic study on dynamic deformation and dynamic strength of compacted loess under complex initial stress state.Dynamic deformation of compacted loess include shear strain, axial strain, the dynamic stress - strain relationship,dynamic shear modulus and damping ratio. The results shows that:α0has an obvious influence on the dynamic shear strain and the hysteresis loop, whenα0=0o and 90o, the dynamic shear strain and hysteresis loop develop symmetrically; whenα0=30o, 45o and 60o, due to the role of pre-shear stress , shear strain one-dimensional cumulative effect and hysteresis loops one-way development are more obviously. Asα0increases, the axial strain from the compression state gradually transition to the elongation. p has a significant impact on the dynamic stress-strain curves, the dynamic shear stress increases with p increases in the same dynamic shear strain.α0 and b0 have definite influence on the stress - strain curve, and they are not obvious in the small shear strain amplitudes, Asα0 and b0 increases. The dynamic shear stress decreases to achieve the same shear strain. With p increases, the maximum dynamic shear modulus also increases and the maximum difference is about 90-130MPa.α0 and b0 have definite influence on the maximum dynamic shear modulus, but the discipline is not obvious, the maximum difference is about 2060MPa,α0and b0 have no effect to the normalized curve Gd /G0 -γd /γr. the damping ratio of different p,α0 and b0 can fit with straight line equation in theλ-lgγd semi-logarithmic plot. The degrees of dispersion show that p and b0 have no impact on the damping ratio.α0 has slightly influence on the damping ratio but which can be ignored.The Study on the dynamic strength of compacted loess try to keep other stress parameters constant and change only one stress parameters, research shows that: the dynamic strength of compacted loess increase with the average principal stress increasing. the initial angleα0 of principal stress has a considerable influence on the dynamic strength of compacted loess, dynamic strength reach to maximum whenα0 is 0o, Withα0 increases, dynamic strength reduces gradually and it reaches to minimum whenα0 is 90o, maximum reduction range is about 35% -40% . The initial coefficient b0 of intermediate principal stress has little influence on the dynamic strength of compacted loess, and has interval effects and amplification effects. When b0 increases from 0 to 0.5, the dynamic strength increases, but when b0 increases from 0.5 to 1, the dynamic strength decreases, the rate of increasing is less than rate reduction by 5 %. Degree of dynamic strength reduces as the number of cycles increase. The number of cycles influence on the dynamic strength is definite, which do not change when the initial stress conditions changes.
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