Study On The Dynamic Behaviors Test Of Loess In Li-Shan Piedmont Fault Site

With the development of modern city and the utilization of underground space,urban rail transit has become a key means to improve capacity,alleviate traffic and improve environment.However,city construction is affected by active faults,ground fissures,earthquakes and other disasters in Xi’an.The problem that the seismic dynamic response mechanism of subway tunnel in high intensity fault development area has been a serious research topic.Therefore,it is essential in carrying out experimental research for scientific and practical on the dynamic behaviors of loess around Li-Shan Piedmont fracture.The dynamic response field test and loess dynamic behaviors laboratory experiments were carried out based on the National Natural Science Foundation Project "Research on seismic dynamic response mechanism and safety design of metro tunnel across active fault(41977231)".The testes were focused on the field microseism and dynamic triaxial test.The research object is the loess in the piedmont fault section of Li-Shan.According to the test results,dynamic response characteristic spectrum curves,dynamic parameters and dynamic strength parameters were obtained.The results may supply useful data for shaking table model test of tunnel crossing fault,and determinate width of fault affected zone.The main research results are as following:(1)The spectrum characteristics of ground pulsation at the Li-Shan piedmont fault are as follows:the main peak is prominent,the secondary peak and harmonic are relatively rich.And the vibration frequencies are more concentrated.Few soft layers and hard layers on the site,stratigraphic is simple,wave velocity of the site has few sharply changes.The dominant frequency is between 2 Hz and 4 Hz.The average dominant frequency is 3.3 Hz,and the dominant period is 0.3 s in the site.(2)The maximum equivalent dynamic modulus of elasticity and the maximum equivalent dynamic shear modulus of unsaturated loess decrease gradually with the increasing of the saturation.The decrease rate decreases gradually with the increasing of saturation.The maximum dynamic stress and the maximum dynamic shear stress generally have a decreasing trend.With the vibration frequency increasing,the maximum equivalent dynamic elastic modulus and the maximum equivalent dynamic shear modulus of unsaturated loess have the increasing tendency,and the amplitude decrease gradually.It indicates an increasing trend in the maximum dynamic stress and the maximum dynamic shear stress.With the increase of confining pressure,the maximum equivalent dynamic elastic modulus and maximum equivalent dynamic shear modulus of unsaturated loess increase gradually.It shows an increasing trend in the maximum dynamic stress and the maximum dynamic.(3)The experimental constant K decreases with the increase of saturation.It gradually increases with the increase of vibration frequency.The experimental constant n is independent of saturation and vibration frequency.(4)The damping ratio of unsaturated loess increases with the dynamic strain,and close to a stable value.Confining pressure has little effect on damping ratio,the damping ratio value of remolded loess is between 0 and 0.159,and that of undisturbed loess is between 0 and 0.130.The damping ratio of unsaturated loess has not significant relationship with saturation and vibration frequency in Li-Shan piedmont fault site.(5)The maximum dynamic cohesion is 5.13 k Pa,and the dynamic internal friction angle is 10.92°,in unsaturated remolded loess.However the maximum dynamic cohesion is 5.97 k Pa,and the dynamic internal friction angle is 10.93 °,in unsaturated undisturbed loess.the dynamic cohesion of soil decreases and the dynamic internal friction angle decreases slightly,with the increase of vibration times.There is few influences for dynamic strength index of soil in soil disturbance.The dynamic cohesion difference between them is less than 1 k Pa,and the dynamic internal friction angle is about 10°.(6)The dynamic stress for unsaturated undisturbed loess is about 1～2 times of that for the unsaturated disturbed loess when the same dynamic strain condition is achieved.The maximum equivalent dynamic modulus of elasticity,the maximum equivalent dynamic shear modulus,the maximum dynamic stress and the maximum dynamic shear stress of the undisturbed soil are approximately 1～2 times of the disturbed soil.Soil disturbance has little effect on the test constant K,which is between 200 and 400.But soil disturbance has a significant effect on the test constant n.The n value of undisturbed soil is 1～2 times of that of disturbed soil.(7)The damping ratio of unsaturated undisturbed loess and unsaturated remolded loess are close.The overall trend of the two damping ratio curves is approximately coincident.The difference of damping ratio between them is less than 0.04.Therefore,it can be considered that soil disturbance has little influence on the Loess of the fault site in front of Li-Shan.