Study On Dynamic Deformation Characteristics Of Loess Under Bidirectional Dynamic Loads

In China, loess is mainly distributed in arid or semi-arid regions where earthquake occurs frequently, so it has positive significance for anti-seismic design in loess areas to study dynamic characteristics of loess. When scholars simulate the seismic loads using dynamic triaxial apparatus, the majority follow the idea that the seismic loads can be simplified to be the dynamic shear load, and using stress on the surface of specimen at the angle of 45°, the seismic shear load is simulated. This simplified simulation method is reasonable when the earthquake origin is deep or the earthquake magnitude is small. However, when an earthquake with shallow source or large magnitude occurs, it is unreasonable to completely ignore the tensile & compressive load caused by the longitudinal seismic waves.At present, the research on dynamic characteristics of loess under bidirectional dynamic loads is uncommon, but the traditional research under unidirectional dynamic load have limitations. According to the research status above, in this paper, the dynamic deformation characteristics of loess under bidirectional dynamic loads are explored by using SDT-20 dynamic triaxixal apparatus, and how the initial shear stress, confining pressure, initial cyclic deviator stress, radial vibration amplitude and phase difference affect the dynamic shear modulus and dynamic shear strain is analyzed, and the Gd~γd curves of loess under bidirectional dynamic loads are described. Besides, by calculating transformation, the effects of tensile & compressive and shear dynamic loads on dynamic deformation of loess are analyzed. The conclusions obtained from the experimental research are showed below:(1)The initial cyclic deviator stress and radial vibration amplitude have no significant effect on Gd~γd curves of loess, but have obvious effects on Gd~N curves. With the rise of initial cyclic deviator stress and radial vibration amplitude, dynamic shear modulus at the same cycle gets smaller. The increase of initial shear stress and confining pressure makes dynamic shear modulus at the same shear strain lager. Under the bidirectional dynamic loads, dynamic shear modulus of loess reaches to the lowest level at the phase difference of 180°, and the dynamic shear modulus decreases with the phase difference rising from 0 to 180°, while the value of dynamic shear modulus increases moderately with the increasing of phase difference from 180° to 360°.(2) Loess has critical cyclic deviator stress under bidirectional dynamic loads and its value is 0kPa. When the cyclic deviator stress is less than the critical value, dynamic shear modulus increases with the rise of dynamic shear strain, while the cyclic deviator stress is larger than critical value, dynamic shear modulus decreases with the rise of dynamic shear strain, and the dynamic shear modulus reaches the highest level at the critical cyclic deviator stress. Gd~γd curves of loess under different initial cyclic deviator stress and different radial vibration amplitude can be normalized. Combining the method of reducing the dynamic shear modulus at the inflection point and the modified H-D model, the description of Gd~γd relationship of loess under bidirectional dynamic loads is achieved.(3)Both the increase of initial cyclic deviator stress and radial vibration amplitude accelerates the the development of dynamic shear strain, while with the rise of initial shear stress and confining pressure, dynamic shear strain at the same cycle becames smaller. The development of dynamic shear modulus along with the rise of cyclic number shows the adverse trends before and after the turning point of 180°. When the phase difference is less than 180°, increasing of phase difference accelerates the development of dynamic shear strain, while the developing speed of dynamic shear strain slows down with the increase of phase difference when the phase difference is greater than 180°. Under bidirectional dynamic loads, the stress combination with σdhm=60kPa and φ=180° is the most unfavorable combination for loess to resist the dynamic deformation.(4) By calculating transformation, it can be concluded that the shear and tensile & compressive dynamic loads both include two parts, constant load and cyclic load. The rise of shear and tensile & compressive dynamic loads caused by constant part decelerates the development of dynamic shear strain, but the rise caused by cyclic part accelerates the development of shear strain. When the shear and tensile & compressive dynamic loads increase simultaneously, the development of dynamic shear strain gets faster. When the shear dynamic load remains constant, increase of tensile & compressive dynamic load accelerates the development of loess dynamic shear strain, which means that tensile & compressive dynamic load has the similar effect on loess dynamic shear strain with shear dynamic load.