Study On The Dynamic Properties Of Lanzhou Loess And Associated Site Response Analysis

It is well known that China has the most widely distributed and deepest loess strata around the world and the Loess Plateau can be seen as the most representative loess landform in it.Due to the porous and metastable microstructure inside and the strong water sensitivity,loess always behaves high possibility of earthquake vulnerability and seismic subsidence.Historically,more than once disastrous earthquakes have been identified in loess area of China,causing plenty of fatalities and economic losses.Site response is influenced by many factors during an earthquake,and site condition plays a key role in it.Normally,the long-term component of earthquake motion can be magnified greatly by the deep and thick loess strata in the Loess Plateau,and this magnification effect will be more significant under the rainfall-earthquake coupling action,reducing the seismic stability of buildings seriously.As the essential input parameters in site response analysis,the study on the dynamic properties of soil has been the research hotspot in Soil Dynamics and Geotechnical Earthquake Engineering.In order to understand and evaluate the dynamic properties of loess more comprehensively and systematically,in this study particular attention was paid on Lanzhou loess,and a series of field and laboratory tests were conducted to obtain the dynamic properties of loess in a more wide strain range.Firstly,small-strain stiffness and damping ratio of Lanzhou loess were measured through resonant column tests.Comparisons between laboratory results and field data show that the initial shear modulus of loess in the laboratory was compatible to the ones measured in the field.After that,a strain-controlled cyclic triaxial device was used to collect the dynamic properties of loess in large strain area.Combining the laboratory test results,it is feasible to gain the dynamic properties of loess from small to large strain under various moistures and confining pressures.Lastly,1D equivalent linear and nonlinear site response analyses were conducted in a representative loess profile utilizing the research achievements in the laboratory as input parameters,an emphasis was made on the effect of surface water infiltration to the ground motion parameters(PGA,characteristic period and PSA).Additionally,comparations were also made between equivalent linear and nonlinear analysis results under different simulation conditions.The research findings would provide significant base and reference in investigating the ground motion characteristics of loess site more scientifically.The test results about small-strain dynamic properties of Lanzhou loess indicated that the initial shear modulus of loess(Gmax)was much more sensitive to water content and confining pressure than minimum damping ratio Dmin.Loess' s Gmax decreased with water content,and when water content approached the plastic limit(PL)the degradation rate was fattest.The water sensitivity of Gmax became more and more apparent with the increase of confining pressure.Dmin of loess decreased firstly and then increased again slightly with water content,but the effect of moisture on Dmin was very limited due to the small variation of Dmin.Similarly,there was also a power relationship between Gmax and confining pressure ?' for loess,while the fitting parameters A and n changed with water content in a power function way.A linear correlation could be found between Dmin and ?' in double logarithmic coordinates.Small-strain stiffness and damping ratio of loess were affected greatly by structure strength too.Actually,Gmax and Dmin of undisturbed loess were greater than the ones of remolded loess,but with water content increasing,the difference of Gmax and Dmin between undisturbed and remolded loess specimens reduced gradually.The dynamic shear modulus degradation curve(G/Gmax-?)and damping ratio curve(D-?)of Lanzhou loess tended to approach each other when water content and confining pressure increased.There was a linear relationship between reference strain ?0.85 and ?' for loess,and the correlations of G/Gmax-?/?0.85 could be normalized perfectly irrespective of water content and confining pressure.The test results of loess' s dynamic properties in large strain area showed that the dynamic shear modulus(G)of dry loess decreased with strain more dramatically than wetting specimens,and the relationship between G and ? could be fitted by exponent function.The large-strain G/Gmax-? curves of loess showed the normalization characteristic apparently under certain water content.With the increase of water content,large-strain damping ratio D(%)of loess decreased initially and then increased again,while D(%)of loess reduced continuously with confining pressure.When water content was a little high(W>14.11%),power function could be fitted to the normalized relationship of D/Dmax-? for loess.The difference of D(%)between undisturbed and remolded loess specimens also reduced gradually as water content increasing.Through combining the dynamic properties of Lanzhou loess in small and large strain areas,it could be found that the shear modulus degradation relationship(G/Gmax-?)and damping ratio curve(D-?)differs each other under different water content and confining pressure.The hypobolic model was feasible to depict the normalized relationship of G/Gmax-?/?0.5 from small to large strain area,irrespective of water content and confining pressure.Finally,a comparison between the measured data and previous research results indicated that G/Gmax-? relationship in the present study degraded more early and quickly while conventional D-? curves were enclosed fully by the curves of our study.Shear wave velocity of loess strata measured from the field down-hole test had suggested that the field Gmax accorded well with the laboratory Gmax obtained from resonant column test.Similarly,there was also a power relationship between shear wave velocity and burial depth in Lanzhou loess area.Simulation results of loess site response analyses declared that peak ground acceleration(PGA)values of simulation adopting the pressure-dependent dynamic constitutive relation were much lower than conventional results,indicating the influence of confining pressure on the ground motion of deep loess site was not ignored.The PGA data of equivalent linear analysis approached the values obtained from nonlinear analysis when the earthquake magnitude was small,while the equivalent linear analysis results exceeded the values of nonlinear analysis under medium or major earthquake.PGA values of site response analysis at surface reduced continuously as surface water infiltration depth increasing,and the reduction rate decreased gradually with infiltration depth of surface water.It could be concluded that surface water infiltration showed certain inhibition on the ground motion of loess site.In comparison with rock motion,the long-term component of ground motion was magnified significantly,enlarging the characteristic period of site meantime.Of course,this magnification effect of motion could also be further strengthened by the role of surface water continuous infiltration.