Experimental Investigation On Dynamic Characteristics Of Sand-fines Mixtures Considering Plasticity State Of Fines

Sand-fines mixtures are widely distributed in China,especially in the developed regions,such as the coastal areas and river deltas.It is found that the sandy soil in the liquefiable sites is not only pure sand but also sand-fines mixtures.Investigations on the dynamic behaviors are focused more on pure sand and sand-fines mixtures are ignored.In addition,the site response study about sand-fines regions is barely reported.Therefore,a series of resonant column and undrained cyclic traxial tests were conducted to investigate the dynamic characteristics of sand-fines mixtures,including dynamic shear modulus,damping ratio and cyclic resistance ratio.The site response of the sand-fines regions was studied based on the test data via the finite element software,ABAQUS.The results can be used for improving the dynamic properties study of saturated sandy soil and the practical construction in the sandy sites.The whole study contents are as follows:(1)A series of resonant column tests were conducted to investigate the dynamic shear modulus and damping ratio of sand-fines mixtures.The plasticity of fines,fines content,saturation state,effective confining pressure,and relative density were taken into consideration.It is found that the disturbutions of the shear modulus-shear strain curves,damping ratio-shear strain curves and maximum shear modulus of the sand-fines mixtures appear nonlinear variation with the fines content increasing.The reason for the nonlinear variation is that the skeleton and mechanical behavior of mixtures change from being dominated by sand to being dominated by fines,and the mixtures change from coarse-grained soil to fine-grained soil with the fines content increasing.The threshold fines content of the samples is influenced by the host sand.The existence of the plastic fines lead to a decrease of the maximum shear modulus of mixture samples.And the influence of the nonplastic fines on the maximum shear modulus of mixture samples is dependent on the fines content.The existence of the fines lead to an increase of the damping ratio.The maximum shear modulus of various sand-fines mixtures was analyzed via the binary packing theory and fitted by the Hardin’s equation.It is found that the parameters of the Hardin’s equation increase with the increasing of the plastic index of the fines.(2)A series of undrained cyclic triaxial tests were conducted to investigate the cyclic resistance ratio of sand-fines mixtures.The plasticity of fines,fines content,void ratio,relative density,effective confining pressure and loading frequency were taken into consideration.The similarities and differences between the liquefaction process of sand-fines mixtures and pure sand were compared and analyzed.It is found that the liquefaction criterion of the pure sand samples can also be applied to the sand-fines mixtures.The liquefaction strength of the mixture samples increase with the increasing of the effective confining pressure,loading frequency and relative density.The influence of the fines on the liquefaction strength of the mixture samples cannot be described uniformly by a single dense state parameter relative density or void ratio.As a result,a dimensionless parameter was proposed to describe the dense state of the mixtures.In addition,a semi-empirical evaluation model on the liquefaction strength of sand-fines mixtures was proposed.The applicability of the model was verified via the test data of the author and other scholars.It is found that the model is in good applicability.(3)The finite element software,ABAQUS,was used to study the site response of sand-fines regions based on the test data.The depth of the sand-fines layer,the plasticity of fines,fines content,earthquake intensity and spectral characteristics were taken in consideration.The site characteristics of fundamental period,vertical distribution of peak acceleration,magnification times of the peak ground acceleration,response spectrum and response spectrum ratio were studied.The site response rules of the sand-fines regions were summarized.It is found that the existence of the fines will influence the site response.The main reason for the influence is that the existence of the fines lead to nonlinear changes of the dynamic characteristics of the site soil.