Abstract:When an earthquake occurs,seismic waves propagating from the earthquake source to the ground include transverse waves (S-waves) and longitudinal waves (P-waves).Transverse waves produce dynamic shear loads whereas longitudinal waves produces dynamic tension and compression loads.Consequently,under the effects of these two dynamic loads,horizontal shear vibration and vertical tension and compression vibration occur simultaneously in loess.By simultaneously applying axial and radial dynamic loads on a triaxial specimen and using stress on the surface of the specimen at an angle of 45°,the effects of seismic tension and compression and shear dynamic loads are simulated and the characteristics of the dynamic shear modulus and dynamic strain of loess are analyzed under bidirectional cyclic load coupling.The test results indicate that phase difference has certain effects on the dynamic shear modulus of loess. With φ=180° as the turning point,the dynamic shear modulus of loess decreases first and then increases with increase in the phase difference, and when the phase difference is 180°,the loess dynamic shear modulus decreases to its lowest level.Analysis of the initial dynamic shear modulus of loess under the bidirectional cyclic loads shows that with increase in the phase difference, the initial dynamic shear modulus of loess follows the same development law as that of the dynamic shear modulus.When the phase difference is in the range of 0° to 90°,the initial dynamic shear modulus of loess decreases quickly,but when it is in the range of 90° to 180°,the decline velocity slows down. Correspondingly,the initial dynamic shear modulus increases slowly when the phase difference is in the range of 180° to 270°;however,the increase in the velocity of the initial dynamic shear modulus becomes higher when the phase difference is in the range of 270° to 360°.When the value of the confining pressure is 200 kPa and the radial vibration amplitude is 20 kPa,the initial dynamic shear modulus of loess under the phase difference of 180° accounts for only 10.77% of that under the phase difference of 0°,which is a decrease of approximately 90%,and the decline ratio shows an increasing trend with increase in the radial vibration amplitude.This indicates that the phase difference has significant effects on the initial dynamic shear modulus of loess,especially when the loess is exposed to bidirectional cyclic loads,with a phase difference of 180° between the axial and radial loads,the loess initial dynamic shear modulus drops dramatically and the ability of loess to resist the shear deformation becomes very weak or even lost.When the phase difference is less than 180°,its increase tends to accelerate the development of the loess' dynamic shear strain,and when the phase difference is greater than 180°,the development of the loess' dynamic shear strain reduces with its increase.The test results show that the load combination with a radial vibration amplitude of 60 kPa and phase difference of 180° has the most unfavorable effects on the deformation development of loess,and the dynamic shear strain of loess rises almost linearly with the increase of cycles under this combination.As a result,the destruction of loess occurs in a few cycles.Therefore,high radial vibration amplitudes should be avoided when the phase difference between the axial and radial dynamic load is 180° in practical projects.By calculating transformation,the manner in which the changes in the dynamic tension and compression and shear loads affect the development of the dynamic shear strain of loess is analyzed.The results show that the dynamic tension and compression load quickens the increase of the dynamic shear strain of loess,and when the dynamic tension and compression and shear loads change synchronically,the change of the dynamic shear load has more prominent impacts on the development of the dynamic shear strain of loess.
