Study On The Relationship Between Shear Wave Velocity And Depth Of Stratum In Xi'an Area

Shear wave velocity,which has a wide applications in geotechnical investigation and earthquake engineering,can not only reflect the dynamic characteristic,but also reflect the rigidity characteristic of rock and soil.In practical projects,the shear wave velocity of the soil layer is usually measured by in-situ test.However,during the measurement of shear wave velocity,there are not the ideal velocity values because of the limitations of in-situ testing methods and site conditions.Many studies have shown that there is a strong correlation between shear wave velocity and depth,and the distribution of shear wave velocity has obvious region characteristic.Therefore,using existing shear wave data and combining with local geological conditions,the relationship between shear wave velocity and depth analysed by statistical method will provide an important reference for geotechnical investigation and geotechnical seismic design of the local area.Using the in-situ data from 108 deep holes(90 meters)and 402 shallow holes(30 meters)from Xi'an Seismic mico-zoing and Xi'an Metro Line 3 seismic safety evaluation,the empirical correlation of shear wave velocity and depth is established considering the engineering geological zoning and rock and soil types respectively by means of direct method.In addition,using the shear wave velocity data within a depth of 30 meters,the paper analyses the relationship of velocity and depth by indirect method,and then compares the indirect method results with the corresponding direct method results.Through statistical analysis,the following conclusions have been acquired:1.According to the engineering geological zoning of Xi'an sesmic micro-zoning,the relationship of the shear wave velocity and depth of each geological zoning with similar soil structure is analysed by direct method,and the empirical formulas are gived out in 30-meter and 90-meter statistical intervals respectively.In the depth range of 30 meters,the variation trend of the shear wave velocity with depth satisfies the power-law function,complex power-law function or quadratic polynomial function.In the depth range of 90 meters,the variation trend of the shear wave velocity with depth satisfies the quartic polynomial,quantic polynomial or precewise function relations.2.For the common rock and soil types of Xi'an area,the relationship of shear wave velocity and depth of the difference soils in three major geological zoning is classified to analyse by direct method.It obtained the optimal models,parameters and range of depth appliance,and the empirical formulas are established in 30-meter and 90-meter statistical intervals respectively.In the depth range of 30 meters,the variation trend of the shear wavevelocity with depth satisfies the liner,power-law,complex power-law or quadratic polynomial function.In the depth range of 90 meters,the variation trend of the shear wave velocity with depth satisfies the cubic polynomial,quartic polynomial,quintic polynomial or precewise function relations.3.The statistical results of indirect method show that travel time and depth have a strong correlation which can be described by a power-law and logarithmic model,and the power function'fitting accuracy is higher than the logarithmic function model.And the corresponding relationship of depth-shear wave velocity is that the power function model is superior to the linear function model.Through the comparison between the indirect method and the direct method,it's found that there is a certain gap between the two methods,regardless of the power function or the linear function.And the gap of linear model is larger than the power function model.For the power-law model,in the shallow surface,the result of the two methods basically coincide;after 10 to 15 m,the direct method result is larger than the indirect method.For linear model,the direct method result is larger than the indirect method at shallow surface,and after 10 to 15 meters,the result of indirect method is larger than the direct method.