基于地面运动强度及标准贯入试验的上海地区砂土地震液化评价

Assessment of seismic liquefaction hazard in Shanghai based on ground motion intensity and Standard Penetration Test

上海市地处长江三角洲前缘,黄浦江和苏州河交汇区域,特殊的地理环境与沉积环境导致浅部砂层广泛发育。随着城市建设的不断推进,上海城市区域范围的砂土地震液化风险评价成为亟待研究的课题。文章基于上海市工程钻孔数据,结合地震地面运动加速度分布与标准贯入试验,建立区域性地震液化危险性评价模型,对上海市进行了地震液化危险性评价。研究认为当发生50年超越概率10%的地震条件下,上海市陆域面积的66.0%将不会产生地震砂土液化灾害,21.8%的陆域面积仅发生轻微液化,只有崇明、横沙、长兴三岛,黄浦江及苏州河两岸地震液化等级达到中等甚至严重,占全市陆域面积12.3%;50年超越概率2%的地震条件下,随着峰值地面运动加速度整体升高,全市范围内轻微—严重液化区域明显增多,可能发生地震液化的总面积达到全市陆域面积46.25%。上海市存在砂土地震液化的危险性,但是发生概率较低。研究认为,目前的抗震设计规范中上海市的设防烈度偏高,可能导致不必要的建设成本。同时研究中的不同超越概率下的地震液化危险性评价结果为上海市工程建设相关标准的合理化改进的提供了建议和参考。

Shanghai is located in the alluvial plain of the Yangtze River Delta, the merge area of the Huangpu River and Suzhou River. The unique geographical and sedimentary environment have formed the shallow sand layers in Shanghai. Due to the significant urbanization process in Shanghai, geological hazard analysis, particularly the assessment for seismic liquefaction hazard in the Shanghai urban area has become an subject to be studied urgently. In this paper, we presented a regional liquefaction hazard analysis model. Based on the borehole Standard Penetration Test(SPT) data and regional Peak Ground Acceleration(PGA) zonation of the Shanghai area, we analyzed liquefaction risks with different probability of exceedance in 50 years. As our results indicated, under the condition that earthquake with 10% probability of exceedance in 50 years happens, more than 66.0% of the land area in Shanghai will not be affected by earthquake-induced liquefaction, 21.8% will only surfer modest liquefaction, and only 12.3% has the risks of serious liquefaction. These places cover Chongming island, Hengsha island, Changxing island and the banks of the Huangpujiang River and the Suzhou River. Provided that earthquake with 2% probability of exceedance in 50 years happens, due to the overall increase of peak ground motion acceleration, not less than 46.25% of the land area may suffer from modest to serious liquefaction risks. Although the rare seismic liquefaction risks exist, the probability of that is quite low. The current high fortification intensity for Shanghai in Chinese Code for Seismic Design of Building may result in unnecessary cost of construction. Our study provides new ideas and suggestions for perfecting the Code for Seismic Design of Building for Shanghai.