Assessment of seismic liquefaction hazard in Shanghai based on ground motion intensity and Standard Penetration Test
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摘要: 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。Abstract: 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.
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图 2 基于原位试验的液化判别方法示意图(Whitman, 1971)
Figure 2. Diagram of the liquefaction discriminant based on in-situ test(Whitman et al., 1971)
图 4 钻孔分布及砂层厚度分布情况
a—此次研究涉及的钻孔分布; b—上海市砂层厚度分布情况
Figure 4. Geo-engineering boreholes and thickness of shallow sand layers in Shanghai
(a) Geo-engineering boreholes for this study; (b) Thickness of shallow sand layers in Shanghai
图 5 典型地震液化易发区浅部砂层地质剖面
Figure 5. Geological profile of shallow sand layer in dypical seismic liquefaction prone zones
图 6 上海及邻区1970—2017年地震记录及潜在震源区分布图
a—上海及邻区1970—2017年地震记录; b—潜在震源区分布图(据上海市地震局和同济大学, 2004修改)
Figure 6. Earthquake events during 1970~2017 and potential earthquake source zones near Shanghai
(a) Earthquake events during 1970~2017; (b) Potential earthquake source zones in Shanghai and adjacent areas (modified after Shanghai Earthquake Agency and Tongji University, 2004)
图 7 上海市50年超越概率10%、2%峰值地面运动加速度
a—50年超越概率10%峰值地面运动加速度; b—50年超越概率2%峰值地面运动加速度(据上海市地震局, 1992)
Figure 7. Peak acceleration with 10% and 2% probability of exceedance in 50 years
(a) Peak acceleration with 10% probability of exceedance in 50 years; (b) Peak acceleration with 2% probability of exceedance in 50 years
图 8 50年超越概率10%地震液化指数分布及液化程度分级
a—50年超越概率10%地震液化指数分布; b—50年超越概率10%液化程度分级
Figure 8. Assessment of liquefaction risks with 10% probability of exceedance in 50 years
(a) Assessment of liquefaction risks; (b) Liquefaction zonation with 10% probability of exceedance in 50 years
图 9 50年超越概率2%地震液化指数分布及液化程度分级
a—50年超越概率2%地震液化指数分布; b—50年超越概率2%液化程度分级
Figure 9. Assessment of liquefaction risks with 2% probability of exceedance in 50 years
(a) Assessment of liquefaction risks; (b) Liquefaction zonation with 2% probability of exceedance in 50 years
表 1 上海市工程地质第四纪地层表
Table 1. Table showing the Quaternary strata of engineering geology in Shanghai
地质时代 工程地质编号 岩性 地质时代 工程地质编号 岩性 晚全新世 Q43 ① 填土 早全新世 Q41 ⑤3 粉质黏土 Q43 ②1 黏土 Q41 ⑤4 黏土 Q43 ②2 粉质黏土 晚更新世 Q32 ⑥1 黏土 Q43 ②3 粉砂 Q32 ⑥2 粉质黏土 中全新世 Q42 ③1 淤泥质粉质黏土 Q32 ⑦1 砂质黏土 Q42 ③2 粉砂 Q32 ⑦2 粉砂、细砂 Q42 ③3 淤泥质粉质黏土 Q32 ⑧1 黏土 Q42 ④ 淤泥质黏土 Q32 ⑧2 砂质粉土粉质黏土互层 早全新世 Q41 ⑤1 黏土、粉质黏土 Q31 ⑨1 粉细砂 Q41 ⑤2 粉砂 Q31 ⑨2 粉砂、细砂及中粗砂 
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表 2 地面峰值加速度对应液化判别标准贯入锤击数基准值
Table 2. Critical SPT blow count for varied PGA interval
地面峰值加速度/m/s2 0.9≤a < 1.4 1.4≤a < 1.9 1.9≤a < 2.9 液化判别标准贯入锤击数基准值 N0=7 N0=10 N0=12 据《建筑抗震设计规范》GB 50011-2010, 《中国地震动参数区划图》GB 18306-2015修改
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表 3 液化等级分级
Table 3. Classification of risk assessment based on liquefaction index
液化等级 轻微 中等 严重 地震液化指数IE 0＜IE≤6 6＜IE≤18 IE＞18 据《建筑抗震设计规范》(GB 50011-2010)
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表 4 上海市地震液化易发区域浅部砂层工程参数
Table 4. Description for major liquefaction susceptible zones in Shanghai
综合特征 苏州河、黄浦江沿岸 "冈身"沿线 崇明岛及长江沿岸 浦东临港地区 工程地层编号 ②3 ②3 ②3-1 ②3-2 ②3-3 ②3 地层层位 如东组 上海组 如东组、上海组 如东组 沉积环境 三角洲河流 滨海 河口 滨海 岩性特征 以砂质粉土为主, 黏粒含量高, 土质不均 大部分地区以砂质粉土为主, 部分地区为黏质粉土 上部为砂质粉土, 中部为黏质粉土, 下部为砂质粉土或粉砂 以砂质粉土为主, 颗粒较均匀, 具水平层理, 土质较均匀 含水量/% 34.4 30.3 30.1 33.1 31.2 29.6 孔隙比 0.98 0.79 0.87 1.01 0.89 0.84 黏粒含量/% 9.4 6.1 6.5 12.1 7.3 5.3 比贯入阻力/MPa 1.48 2.13 2.65 0.98 3.65 3.41 平均标准贯入击数 6.2 9.9 8.3 5.6 12.5 11.6 浅部砂层厚度/m 9~11 1~5 11~17 5~11
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表 5 上海市地震液化面积统计(50年超越概率10%)
Table 5. Statistics of liquefaction areas with 10% probability of exceedance in 50 years
液化等级 不液化 轻微液化 中等液化 严重液化 液化面积/km2 4524.73 1496.63 807.84 38.48 占上海市陆域面积比重/% 66.00 21.79 11.76 0.56
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表 6 上海市地震液化面积统计(50年超越概率2%)
Table 6. Statistics of liquefaction areas with 2% probability of exceedance in 50 years
液化等级 不液化 轻微液化 中等液化 严重液化 液化面积/km2 3961.40 638.00 1587.29 951.00 占上海市陆域面积比重/% 53.75 9.29 23.11 13.85
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