Anti-floating failure mechanism of underground structures in expansive soil area and application of active anti-floating measures
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摘要:
膨胀土地层透水性弱、渗透性低，一般被视为相对隔水层或隔水层，其抗浮设防水位需综合考虑场地渗流特性、肥槽填料特性、水-结构相互作用而确定，建筑工程抗浮设计问题素来棘手难解。本文依托成都膨胀土地区某商住楼地下室局部抗浮失效案例，通过地下室渗漏水水源调查、基底地下水流量监测、排水系统排泄能力评价等方法，认为地下室上浮属肥槽施工控制不当使高压水渗入抗水板底部进而导致水力学条件失衡造成的。基于此，结合FLAC3D有限差分法，详细探讨了地下水浮力及膨胀土膨胀力作用下地下结构的力学行为特征。结合工程实际，提出一种以“卸压”为主的地下室抗浮方法，其理念是在底板开设卸压孔使地下水流动，通过主动式泄排适量地下水来减小或消除水浮力，并辅以观测及自动控制措施，实现抗浮目的。工程监测结果表明，流量及水头可控，运行效果良好，成本低廉。
Abstract:Expansive soil is a weakly permeable stratum with low permeability. It is generally regarded as a relative aquiclude or water resisting layer. Its anti-floating water level is needed to comprehensively consider the site seepage characteristics, fertilizer tank filler characteristics and water-structure interaction, which makes the anti-floating problem very prominent and difficult to deal with. Based on a building failure treatment case in the expansive soil area of Chengdu, through the investigation of the water leakage source of the basement, the monitoring of groundwater flow and the evaluation of the discharge capacity of the drainage system, it is considered that the basement floating is caused by the improper construction control of the fertilizer tank, which makes the high-pressure water penetrate into the bottom of the water resistant plate, resulting in the imbalance of hydraulic conditions. Combined with the FLAC3D finite difference method, the mechanical behavior characteristics of underground structures under the action of groundwater buoyancy and expansive force are discussed in detail. Combined with the engineering practice, an anti-floating method based on "pressure relief" is proposed. The idea is to open a pressure relief hole in the bottom plate to make the groundwater flow and convert the hydrostatic pressure into kinetic energy without changing the groundwater level, and supplement observation and automatic control measures is carried out to achieve the purpose of anti-floating. The monitoring results of the project show that the flow and head are controllable and the operation cost is low.
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表 1 研究区岩土体物理力学参数
Table 1. Physical and mechanical parameters of rock and soil mass
岩土体类型 重度/
（kN·m−3）内摩擦角/
（°）黏聚力/
kPa压缩模量/
MPa渗透系数/
（m·d−1）杂填土 19.0 8.0 8 0.2 黏土 20.0 6.0 15 5.0 2.5×10−3 含黏土卵石 19.5 20.0 8 1.5 全风化泥岩 19.5 15.0 15 6.5 1.2 
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表 2 现场渗透试验结果
Table 2. Field penetration test results
渗透点编号 Ki/（m·d−1） K均值/（m·d−1） 1 0.754 0.858 0.639 0.750 2 1.529 1.334 1.910 1.591 3 0.692 0.403 0.051 0.382 4 8.222 8.249 8.168 8.213 5 7.584 7.320 7.584 7.496 6 7.019 7.019 7.019 7.019 7 9.310 9.022 9.897 9.076 8 6.273 6.153 6.095 6.174 9 10.26 10.28 10.19 10.243 注：考虑到渗透试验初始状态对入渗影响较大，在处理时采用试验结束时段数据进行从后往前逐段累加处理，分别求得不同渗透系数Ki。
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表 3 肥槽积水补给来源水量计算汇总表
Table 3. Calculation summary of water supply source of fertilizer tank
补给来源 简化计算
补给量/m3对肥槽水位
的影响/m补给方式 降雨 12690 7.35 直接补给肥槽 膨胀土层地下水 微弱 微弱 远端地表水
（北干支渠）微弱 微弱 通过膨胀土层或填土层径流补给 上层滞水 600 0.3～0.4 直接补给肥槽
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表 4 结构体物理力学参数
Table 4. Physical and mechanical parameters of structure
内容 密度/
（kg·m−3）黏聚力/
kPa内摩擦角/
（°）体积模量/
MPa剪切模量/
MPa素混凝土垫层 1870 16.65 13.30 4.04 1.76 抗水板 1830 10.87 17.74 3.73 1.53 建筑体 1910 11.98 20.37 4.51 2.08
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表 5 排水卸压措施基本设计参数
Table 5. Basic design parameters of the drainage and pressure relief measures
参量名称 符号 建议取值 安全系数 k 1.4 潜水含水层厚度 H 8.5 m 卸压降水影响半径 R 210 m 卸压水位降深 h0 7 m 单个卸压点排水能力 q 100 m3/d 肥槽入渗系数 λy 0.2 地表径流系数 φ 0.5 设计暴雨强度 Pʹ 700 mm/d 汇水面积 S汇水 12690 m2 泄压孔半径 rw 0.053 m
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