关键词: 上海地区, 典型土层, 剪切波速, 经验模型, 拟合外推

Abstract:

To provide a reference for predicting shear-wave velocity in the Shanghai area, this study compiled a large amount of borehole data from the region, classified and organized the major soil layers and their spatial distributions, analyzed the relationship between shear-wave velocity and depth, and established depth-dependent relationships and extrapolation models for different soil types.

After excluding records with abnormal waveform signals and low signal-to-noise ratios from the shear-wave velocity tests, 318 valid boreholes were retained, comprising a total of 30, 846 test points distributed within a depth range of 3~100m. The measured shear-wave velocities generally range from 100 to 500m/s. The data distribution is relatively concentrated and exhibits clear regularity, with shear-wave velocity showing a positive correlation with depth. At depths of approximately 50m or shallower, the variation of shear-wave velocity with depth displays a pronounced nonlinear trend. At depths of approximately 15m or shallower, shear-wave velocity increases relatively slowly with depth, after which the rate of increase gradually accelerates. Around 20m, the increase in shear-wave velocity with depth reaches its maximum rate and then gradually slows down. From approximately 50 to 100m, the increase in shear-wave velocity becomes more gradual and generally follows a linear trend.

In the Shanghai area, strata within 100m depth consist of unconsolidated Quaternary deposits ranging from the Holocene to the Middle Pleistocene, mainly composed of silty clay, clay, silt, and sand. Based on the lithology, consistency, and density of the soil layers, 46 soil layers were classified and simplified stepwise from subcategories to broader categories. According to the distribution of field-measured shear-wave velocity errors, soil types were merged into a single category when the difference in shear-wave velocity distribution between two soil types was within ±5%. Statistical analysis indicates that the differences in measured shear-wave velocity are relatively small between mucky silty clay and silty clay, between clayey silt and sandy silt, and among silty sand, fine sand, and medium-to-coarse sand, with average deviations of less than 5%. These soil types can therefore be grouped into silty clay, silty soil, and sandy soil, respectively. However, the deviations among fluid-plastic clay, soft-plastic clay, and stiff-plastic clay, as well as those between silty soil and sandy soil and among silty soil, sandy soil, and clay, are relatively large, with average deviations exceeding 5%; therefore, these soil types are not suitable for merging. Overall, shear-wave velocity follows the order of stiff-plastic clay>soft-plastic clay>fluid-plastic clay, sandy soil>silty soil, and silty sand>clay. Accordingly, the typical soil layers within 100m depth in the Shanghai area were grouped into five categories: Silty clay, soft-plastic clay, stiff-plastic clay, silty soil, and sandy soil.

Linear, quadratic polynomial, and power-function models were used to fit the depth-velocity relationships for the five soil types, and the goodness of fit was evaluated using the coefficient of determination, R². The quadratic polynomial and power-function models showed the best fitting performance, whereas the linear model performed relatively poorly. Compared with the power-function model, the quadratic polynomial model performed less satisfactorily in fitting deep soil layers, particularly for soft-plastic clay and silty soil, where substantial deviations from the mean values occurred at greater depths and unreasonable trends were observed. In contrast, the power-function model demonstrated better overall fitting performance, effectively constraining the fitted shear-wave velocities in deep soil layers and producing a relatively stable increasing trend, thereby showing advantages for velocity extrapolation. By extrapolating the shear-wave velocity characteristics of soil layers deeper than 100m using the power-function model and comparing the results with those reported by the Shanghai Earthquake Agency, it was found that the deep shear-wave velocities extrapolated from measured data at depths shallower than 100m increase gradually with depth, while the rate of increase progressively decreases. The variation trend obtained from the model extrapolation is generally consistent with the fitting results of the Shanghai Earthquake Agency, although the extrapolated values in this study are approximately 70m/s higher overall. However, considering the measured velocity data from deep boreholes, which exhibit substantial variability, the extrapolation curve obtained in this study generally falls within the range of the measured data distribution, and its trend is consistent with the measured data. This indicates that extrapolating shear-wave velocities for soil layers deeper than 100m using measured data from similar soil layers at depths shallower than 100m is relatively reliable. The empirical fitting formula for shear-wave velocity in soil layers deeper than 100m obtained in this study is V_S=-3.2+76.2×H^0.382, which can be used as a reference in practical applications.

In summary, this study conducted a detailed investigation of soil-layer distributions and the variation of shear-wave velocity with depth in the Shanghai area. A power-function model was used to fit the relationship between shear-wave velocity and depth for five typical soil layers, and shear-wave velocities were extrapolated for depths shallower than 3m and deeper than 100m. The depth-dependent shear-wave velocity relationships obtained in this study can provide a reference for predicting shear-wave velocities in the Shanghai region.

Key words: Shanghai area, typical soil layer, shear wave velocity, empirical model, fitting extrapolation