Study on the delimitation of affected zone of geological environment for karst underground engineering：taking Longgang district, Shenzhen City as an example
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摘要:
在大量建设城市地下轨道交通及城市更新工程的背景之下，我国城市岩溶地质灾害日趋严重。文章以深圳市3个岩溶地面塌陷事件为例，开展岩溶地下工程地质环境影响区的划定研究。首先运用高频岩溶地下水气压力监测技术对工程影响实际范围进行监测分析，然后结合工程施工参数、岩溶塌陷主要影响因素与水文地质试验参数，采用定性分析和量化计算的综合研究方法，推导出岩溶地下工程地质环境影响范围理论计算经验公式。结果表明：岩溶地下工程影响范围主要与渗透系数、工程深度成正比，与土层厚度成反比，推导的半定量理论公式适用于岩溶承压水条件下，可快速为缺乏地下水监测资料的岩溶地区地下工程安全建设及城市防灾减灾工作提供依据。
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关键词:
- 岩溶地下工程 /
- 岩溶地下水气压力监测 /
- 工程影响区 /
- 抽水试验 /
- 经验公式
Abstract:The construction of a large number of urban underground rail transit and urban renewal projects has intensified the urban karst geological disasters in our country. As one of the first demonstration areas of national urban construction, Shenzhen has developed rapidly in terms of underground rail transit and urban renewal projects over the years, hence leading to frequent karst collapse disasters due to its location in the karst area. Therefore, the summary of the experience and lessons from the development of karst underground space is of great significance for the engineering construction in karst areas. In this thesis, a preliminary study on the delimitation of the affected zone of geological environment for karst underground engineering is conducted based on three events of karst ground collapse in Shenzhen.
The affected zone of geological environment for karst underground engineering refers to the area where disasters are likely to happen due to the disturbance and damage of the rock and soil around the construction site during the construction process. The delimitation of the affected zone is not only conducive to the safe and smooth engineering construction, but also to the clear division of responsibility. The monitoring range of underground engineering construction in karst area usually reaches only tens of meters at the current stage. But when a disaster happens, the actual range influenced by engineering will often exceed hundreds of meters. Therefore, the current construction specification about the affected zone of geological environment for karst underground engineering is unreasonable and uncertain in some degree, and the relevant provisions are greatly challenged. For instance, it is stipulated that the engineering monitoring should be conducted within the plane range that is only 3 times as deep as the foundation pit during construction , and the description of the expansion of the monitoring range in the karst development area is not detailed. Therefore, the further research on the affected zone of geological environment for karst underground engineering is very necessary, so the actual affected range of the project can be effectively judged, and then the corresponding prevention and control measures can be taken.
On the basis of fully mastering the regional geological background and the geological conditions of site engineering, a preliminary study is conducted in this thesis. Firstly, the actual affected range of the project is monitored and analyzed by the high-frequency monitoring technology of karst groundwater pressure. The monitoring scheme should be formulated according to local conditions. The monitoring frequency should capture the disturbance changes of regional karst groundwater with more than 3-month monitoring cycle. The results of monitoring and data measurement of water levels indicate that the affected range of the project is closely connected with the karst groundwater drawdown funnel. The obvious anisotropy of karst aquifer medium at each site is indicated in the groundwater flow field, which is mainly controlled by karst development and structure. The maximum affected ranges of the three projects are 560 m, 820 m and 850 m respectively. Then, the analysis on the formation mechanism of karst collapse is conducted. Results indicate the collapse mechanism. The excavation and precipitation of foundation pit leads to the change of groundwater hydrodynamic internally caused of strong karst development, the change of hydrodynamic conditions disturbs karst groundwater or gas, and the force generated by karst water disturbance acts on the overburden floor through karst pipeline or crack. As a result, the overburden soil mass collapses and loses gradually, until the roof becomes unstable and damaged for the insufficient collapse resistance. Finally, the Gehart’s empirical formula of influence radius of confined water pumping hole is used for reference, combined with engineering construction parameters, main factors of karst collapse and hydrogeological test parameters. The mixed research method of qualitative analysis and quantitative calculation is adopted to theoretically deduce the empirical calculation formula of the affected zone of geological environment for karst underground engineering. Research results indicate that the affected range of karst underground engineering is mainly in direct proportion to the permeability coefficient and engineering depth. However, it is inversely proportional to the thickness of soil layer. The deduced semi-quantitative theoretical formula is suitable for the calculation of a relatively thick aquifer between the Quaternary and the karst aquifer. If the depth of foundation pit is greater than that of rock surface and of confined karst water in underground engineering, this fomular can be used to quickly provide the basis for safety construction of underground engineering as well as urban disaster prevention and reduction in the karst area lacking groundwater monitoring data.
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表 1 工程参数及岩溶发育指标一览表
Table 1. Engineering parameters and karst development indicators
基坑
工程基坑
长度/m基坑
宽度/m地面原
标高/m工程
深度/m土层
厚度/m岩溶发育
深度/m钻孔
/个见洞率/% 线岩
溶率/%数码城站 512 16~56 43.0 22.3 6~23 9~24 1876 42.9 24.8 5#地更新工程 200 150 35.0 19.8 7~25 10~25 1452 29.4 26.8 龙平站 191 28 35.8 27.0 7~24 9~22 1367 39.2 21.4 注：依《建筑地基基础设计规范》GB50007-2011表6.6.2划分标准，判定研究区岩溶强发育。 
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表 2 工地监测信息统计表
Table 2. Statistical table of site monitoring information
工程
场地监测点
数量/个监测点
平均间距/m监测面积/
km2监测
频率/min工程实际
影响半径/m地下水降落漏斗影响范围
形成日期/年.月.日数码城站基坑 19 200 0.52 2~5 850 2020.10.24 5#工地 74 100 0.85 2~5 560 2020.04.27 龙平站基坑 8 180 0.31 2~5 820 2021.02.02
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表 3 龙岗区塌陷点覆盖层厚度统计表
Table 3. Statistical table of overburden thickness on collapse point in Longgang area
塌陷编号 T01 T02 T03 T04 T05 T06 T07 T08 T09 覆盖层厚度/m 6 20.6 14 8 9.3 11.6 14.4 9.3 14.8
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表 4 场地抽水试验成果表
Table 4. Results of site pumping tests
试验场地 钻孔编号 试验段
埋深/m静止水位
埋深(h)/m水位降深
(s)/m涌水量
(Q)/m3·d−1影响半径
(R)/m平均渗透系数 
/
m·d−1数码城站 SMSW01 8.3~24.8 1.95 6.0 392.1 133.2 6.63 4.0 336.5 98.8 2.0 262.4 59.5 数码城站 SMSW02 4.0~28.0 1.90 6.0 442.5 502.3 67.87 4.0 302.2 330.9 2.0 156.8 161.3 数码城站 SMSW03 12.0~14.2 2.20 9.0 38.7 159.6 2.70 6.0 23.9 99.3 3.0 10.8 44.5 龙平站 LPSW01 4.0~29.5 4.51 6.6 527.2 167.8 6.33 5.6 450.3 140.8 4.7 375.6 115.9 龙平站 LPSW02 4.0~10.8 4.40 3.0 373.0 77.3 23.14 2.0 262.4 48.6 1.0 145.9 22.9 龙平站 LPSW03 15.0~20.0 4.00 10.2 5.3 32.6 0.09 6.8 3.3 20.4 3.4 1.5 8.8 龙平站 LPSW04 11.0~15.6 3.40 7.5 130.3 200.9 7.75 5.0 98.0 138.9 2.5 57.9 72.2 龙平站 LPSW05 13.6~17.0 3.10 10.5 590.9 510.4 28.30 7.0 496.5 375.4 3.5 302.2 199.6 5#地块 SW102 17.5~99.5 3.7.0 2.4 151.9 144.3 37.40
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表 5 经验系数α反演结果
Table 5. Inversion results of empirical coefficient α
区域 工程深度
H/m初始水位
埋深h/m土层平均
厚度d/m区域渗透
系数K/m·d−1实际影响
半径R/m理论影响
半径R′/m范围
误差/%经验
系数（α）平均经验
系数（
数码城站 22.27 1.9 12.97 37.25 850 848 0.2 5.9 5.9 龙平站 27.0 3.1 13.31 25.72 820 806 1.7 6.0 5#地块 19.84 2.38 16.91 37.4 550 559 1.6 5.8
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