The Effect Of The Vibration Frequencies And Sample Preparation Methods On Dynamic Behavior On Saturated Medium Sands

The liquefaction behavior of saturated sand has been the focus in the field of geotechnical engineering.The dynamic tri-axial test is still an effective method used to study the liquefaction behavior of saturated sand,because the specimen preparation is easier and the equipment is common.There are many factors affecting liquefaction featurec of soil, including the characteristics of soil itself,such as the soil type,the density,the structure,the graduation,permeability etc,and primary stress state and the characteristics of dynamic load such as the amplitude of vibrating load,the vibration frequency and so on.A series of dynamic tri-axial tests were done by "soil static and dynamic universal tri-axial and torsional shear apparatus" for saturated medium sand of the relative density D_r=28%,50%,70%,under consolidation ratio K_c=1.0,1.5 and confining pressure 100,200, 300kPa with vibration frequencies f=0.05,0.1,1.0Hz.The influence of vibration frequency were investigated for emphasis on the dynamic strength,peak dynamic pore water pressure, peak dynamic strain,dynamic strength index of saturated medium sand.The test results showed that the dynamic strength of saturated sand increased with the increasing vibration frequency.The cyclic numbers increased with the higher frequency to reach the same peak dynamic pore pressure and dynamic strain ratio for given load amplitude.The dynamic strength index valueφincreased with the increasing vibration frequency also.Specimen preparation is an essential step for indoor test.The different arrangement structure of internal soil grain were resulted from the different methods of preparation,and different of mechanical properties were made.For saturated medium sand with the relative density D_r=28%,50%,70%prepared by dry sample method knocking outside and wet sample method tamping inside,the dynamic strength tests were carried out under the isotropic consolidation with confeining pressure 100kPa and vibration frequency 0.1Hz,and the dynamic module and damping ratio tests were done under the isotropic consolidation with confining pressure 100,200,300kPa and the vibration frequency 0.1Hz.The influences of specimen preparation methods on dynamic strength,dynamic modulus and damping ratio were discussed.The tests results showed that the dynamic strength and dynamic module of saturated medium sand prepared by dry sample method were higher than the results of wet sample method without affecting the damping ratio.The commonly used sample saturation method is vacuum saturation method.Even if it is the best one,it can not make the saturation of sample higher than 95%sometimes.In such a case,usually back pressure saturation method is used which can improve the saturation degree to a certain exten,however it is a question whether the back pressure has an influence on dynamic behavior.Dynamic tri-axial tests were carried out with and without back pressure, respectively on medium sand with the relative density D_r=28%,50%,70%and standard sand with D_r=70%under isotropic consolidation with confining pressure 100kPa and vibration frequency 0.1Hz.The test results showed that the test with back pressure can improve the saturation of sample but had little effect on the dynamic characteristics of sand.