MAIN SUBSIDENCE LAYERS AND DEFORMATION CHARACTERISTICS IN BEIJING PLAIN AT PRESENT
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摘要: 超量开采地下水引发的地面沉降已成为北京平原区最主要的地质灾害之一。精准识别现阶段地面沉降主要贡献层位,查明不同水位变化模式下土层变形特征,对实现地面沉降精准防控,建立合适的地下水-地面沉降模型具有重要意义。本文根据北京市7个地面沉降监测站内分层标和水位近十几年观测资料,对不同深度土层沉降变化特征和主要沉降层位进行了精准识别,系统分析了不同压缩层组与含水砂层在不同水位变化模式下的变形特征,探讨了黏性土层产生较大残余变形和滞后变形的原因。结果表明:(1)北京平原区现阶段主要沉降层位集中在第二压缩层组(中深部地层)和第三压缩层组(深部地层),平均沉降占比为31.01%和60.73%,且有增大的趋势。(2)不同深度土层变形量及其在总沉降量中的占比,不仅与相邻含水层水位下降幅度密切相关,而且与该土层的岩性和厚度有关。当可压缩土层厚度大,即使水位下降幅度较小,也可能会产生较大的变形量。(3)不同水位变化模式下,不同压缩层组和含水砂层的变形特征可概括为5类。含水砂层主要表现为弹性变形。不同深度的黏性土层表现出弹性、塑性和蠕变的变形特征,具有显著的黏弹塑性。(4)平原区地下水位总体以2017年为节点由降转升,土层变形特征前后差异性较大。第一压缩层组由弹塑性变形转变为弹性变形。第二和第三压缩层组以黏性土为主时,土层始终表现为塑性变形和蠕变变形。若以砂层为主时,2017年前为塑性变形和蠕变变形,2017年后则存在塑性变形、蠕变变形和弹性变形,黏弹塑性明显。(5)黏性土层存在较大残余变形和变形滞后主要由两种因素引起:其一,非弹性储水率大于弹性储水率。其二,黏性土层中超孔隙水压力消散较慢,存在释水滞后,进而导致土层变形滞后。Abstract: Land subsidence caused by over-exploitation of groundwater has become one of the most important geological disasters in the Beijing Plain. The important tasks are to accurately identify the main contributing layers of land subsidence and to analyze the characteristics of soil deformation under different water level change modes. They are of great significance for establishing a suitable groundwater-land subsidence model and achieving precise prevention and control of land subsidence. This paper uses the extensometer and corresponding groundwater level observation data at land subsidence monitoring stations in Beijing for the past ten years. It accurately identifies the main deformation layers and reveals the deformation characteristics at different depths of soil layers. It then analyzes the deformation characteristics of different compression layer groups and sand layers under different water level change modes. It discusses the reasons for the large residual deformation and the deformation lag of the clayey soil layers. The results show the following findings. (1)The main subsidence layers are the second compression layer group(middle-deep strata) and the third compression layer group(deep strata) in the Beijing land subsidence area. The average subsidence ratio is 31.01% and 60.73%. The proportion of subsidence is gradually increased. (2)The amount of soil deformation at different depths and its proportion in the total subsidence are not only closely related to variation of groundwater level, but also related to the lithology and thickness of the soil layer. When the thickness of the compressible soil layer is large, even if the groundwater level drops small, it can produce a large amount of deformation. (3)The deformation characteristics of different lithological soil layers under different water level change modes can be summarized into 5 categories. The sand layer is mainly characterized by elastic deformation. The cohesive soil layers of different depths have elastic, plastic and creep deformation. The soil layers have obvious characteristics of viscoelastic-plastic deformation. (4)The groundwater level in the plain has changed from falling to rising in 2017. The deformation characteristics of the soil layers are quite different. The first compression layer group is transformed from elastoplastic deformation to elastic deformation. When the second and third compression layer groups are mainly cohesive soil, the soil layer always exhibits plastic and creep deformation. If it is mainly sand layer, it can show the plastic and creep deformation before 2017, and the plasticity, creep and elastic deformation after 2017, with obvious viscoelastic-plastic features. (5)The large residual deformation and deformation lagging of the cohesive soil layer are mainly caused by two factors. First, the inelastic water storage rate is greater than the elastic water storage rate. Second, the excess pore water pressure in the cohesive soil layer dissipates slowly, and the soil layer exhibits delayed water release, which results in delayed soil deformation.
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表 1 地面沉降监测站地质条件和站内监测设施
Table 1. The geological conditions and monitoring facilities of the land subsidence monitoring stations
监测站 地质环境条件 站内监测设施 构造部位 地下水系统 第四系
厚度/m可压缩层
厚度/m地面沉降区 基岩标
/个分层标
/个地下水位
观测井/眼王四营 华北断坳—大兴隆起—黄村凸起 永定河冲洪积扇 185.00 118.00 大郊亭沉降区 1 7 5 望京 华北断坳—北京断坳—坨里—丰台凹陷 永定河冲洪积扇 250.00 170.00 来广营沉降区 1 7 5 天竺 华北断坳—北京断坳—顺义凹陷 潮白河冲洪积扇 500.00 350.00 后沙峪沉降区 1 10 6 八仙庄 燕山台褶带—密怀中隆断 温榆河冲洪积扇 413.00 248.00 八仙庄沉降区 1 10 6 平各庄 华北断坳—北京断坳—顺义凹陷 潮白河冲洪积扇 380.00 247.00 平各庄沉降区 1 7 5 张家湾 华北断坳—大兴隆起—牛堡屯凹陷 永定河冲洪积扇 460.00 294.00 通州沉降区 1 7 5 榆垡 华北断坳—固安—武清断陷—固安凹陷 永定河冲洪积扇 400.00 340.00 榆垡沉降区 1 7 5 表 2 地面沉降监测站各压缩层组沉降占比统计
Table 2. Statistics on the proportion of land subsidence in each compression layer group of land subsidence monitoring stations
压缩层组 天竺站 八仙庄站 王四营站 张家湾站 望京站 平各庄站 榆垡站 平均值 第一压缩层组 监测层位/m 2~35 2~59 2~24 2~49 2~29 2~32 2~53 8.28% 沉降占比 2.84% 11.08% 2.71% 8.96% 1.39% 3.16% 27.84% 第二压缩层组 监测层位/m 35~149 59~144 24~94 49~126 29~99 32~120 53~116 30.96% 沉降占比 62.67% 54.38% 28.29% -3.79% 31.65% 23.69% 19.81% 第三压缩层组 监测层位/m 149以下 144以下 94~182 126以下 99以下 120以下 116以下 60.76% 沉降占比 34.49% 34.54% 69% 94.84% 66.97% 73.15% 52.35% 表 3 不同压缩层组与含水砂层在不同水位变化模式下的变形特征
Table 3. Deformation characteristics of different compressed layer groups and sand layers under the change of groundwater level
不同压缩层组 地下水位变化模式 变形特征(2017年前) 变形特征(2017年后) 第一压缩层组
(浅部土层)(Ⅰ)地下水位总体较为稳定,以季节性反复升降为主 土层前期以较快速度压缩,后期压缩速度趋缓。包含塑性变形、蠕变变形和部分弹性变形 土层压缩与水位变化基本同步,主要表现为弹性变形 (Ⅱ)地下水位总体呈上升的趋势 土层持续压缩,伴随小幅回弹,残余变形量小。包含蠕变变形和弹性变形 土层压缩与水位变化基本同步,主要表现为弹性变形 第二压缩层组
(中深部土层)(Ⅲ)地下水位先持续下降,后逐步回升 水位持续下降阶段:土层以较快速度持续压缩,残余变形量较大。包含塑性变形和蠕变变形 水位逐步回升阶段:(Ⅲ-1)土层以黏性土为主,土层持续快速压缩,压缩速度未见减缓。包含塑性变形和蠕变变形。(Ⅲ-2)土层以砂层为主,压缩速度减缓。包含塑性变形、蠕变变形,后期出现弹性变形特征 第三压缩层组
(深部土层)(Ⅳ)地下水位先持续下降,后逐步回升 水位持续下降阶段:土层以较快速度持续压缩,残余变形量较大。包含塑性变形和蠕变变形 水位逐步回升阶段:(Ⅳ-1)土层以黏性土为主,土层持续压缩,压缩速度明显减缓。包含塑性变形和蠕变变形。(Ⅳ-2)砂层占比较大时,土层持续压缩,压缩速度减缓。包含塑性变形和蠕变变形,后期出现弹性变形特征 含水砂层 (Ⅴ)地下水位先小幅下降,后逐步回升 土层以弹性变形为主,残余变形量很小 表 4 各压缩层组和含水砂层的弹性和非弹性储水率
Table 4. Elastic and inelastic water storage rate of each compressed layer group and sand layer
不同压缩层组 分层标与水井 时间间隔 非弹性储水率/m-1 时间间隔 弹性储水率/m-1 第一压缩层组 F3-10、D3-6 2006.1~2012.12 2.79×10-4 2013.1~2019.12 6.60×10-6 F4-10、D4-6 — — 2009.1~2016.12 3.87×10-5 2017.1~2019.12 9.73×10-6 第二压缩层组 F3-5、D3-3 2006.1~2016.12 5.11×10-4 — — 2017.1~2019.12 3.58×10-4 F1-3、D1-3 2006.1~2016.12 3.13×10-4 2017.1~2019.12 1.12×10-6 第三压缩层组 F1-2、D1-2 2006.1~2016.12 2.88×10-4 — — 2017.1~2019.12 1.79×10-4 F6-3、D6-2 2009.1~2016.12 4.88×10-5 — — 2017.1~2019.12 3.10×10-4 含水砂层 F3-8、D3-5 — — 2006.1~2016.12 2.84×10-5 2017.1~2019.12 8.67×10-6 -
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