Analysis on the hydraulic connection and medium characteristics between tunnels and karst springs by tracer tests: A case study of Guanshan tunnel
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摘要: 文章以关山隧道入口周围的岩溶泉及落水洞为研究对象，以钼酸铵为示踪剂进行落水洞与下降泉之间的示踪试验。结果表明：投放点与两处接收点均存在水力联系，示踪试验浓度曲线显示1#取样点的通道比较单一且与投放点的水力联系较好，2#取样点的通道比较复杂，与投放点之间的渗流途径较长、水力梯度较大，可能存在小型水池或溶潭；分别利用经验公式和Qtracer2软件计算回收率，两处接收点的回收率结果均较低，投放点与接收点之间可能存在更大的支流或者示踪区存在双层岩溶含水系统，地下水流向了更深层的岩溶含水层；结合隧道高程，判断处于低位的隧道存在涌水风险并有可能对岩溶泉产生不良影响。对比两种回收率计算方法，经验公式简便易算，但在示踪剂浓度低、地下水流量小的情况下误差较大，而Qtracer2软件通过对浓度和体积进行积分，克服经验公式缺点，计算结果更精确可靠。Abstract:
In engineering construction, preventing geological disasters is crucial for protecting people's property and safety. Water inrush and mud gushing in tunnel construction are common engineering hazards in China, leading to severe consequences like construction suspension and spring disruption. Effectively preventing such incidents necessitates a thorough understanding of the hydraulic relationships and medium characteristics between surface and groundwater around tunnel sites. This study takes a detailed look at the karst springs and sinkholes surrounding the Guanshan tunnel, situated in a mid-low mountainous region in Funing county, Wenshan Prefecture, Yunnan Province. The area is characterized by its karst topography, where rock formations are exposed on mountain surfaces, coexisting with gullies. Beneath these mountains, the landscape features densely packed depressions filled with sinkholes and shafts, indicating a highly intricate karst hydrological system. This study primarily employs tracer tests to identify the dispersion of underground pipelines and the aqueous medium of groundwater. By analyzing concentration curves of tracer tests, the research aims to understand the distribution of pipelines between sinkholes and the paths of karst spring runoff. The tracer recovery rate is crucial for revealing the hydraulic connections around the Guanshan tunnel and offers a geological basis for assessing the potential risks of water inrush and mud gushing during tunnel construction. The final concentration curves indicate a clear hydraulic connection between the injection point and two receiving points. The concentration curve at the 1# sampling point shows a single peak, indicating a relatively straightforward channel with a strong hydraulic connection to the sinkhole. Conversely, the channel at the 2# sampling point demonstrates greater complexity, with concentration curves including plateau shapes. This suggests the presence of longer seepage paths and higher hydraulic gradients, potentially indicating small pools or solution ponds in the area. Calculated with both an empirical formula and the Qtracer2 software based on collected tracer concentrations, recovery rates at these two points are found to be low. The low recovery rates could imply the existence of large tributaries or a complex, dual-layer karst aquifer system, where groundwater flows to deeper levels. Given the tunnel altitude, it is inferred that tunnels at lower positions are at a heightened risk of water inrush, potentially affecting the karst springs in an adverse way. In this study, calculation methods of recovery rates are compared, which reveals that although the empirical formula is simple and easy to use, it may not be accurate under conditions of low tracer concentration and minimal groundwater flow. On the other hand, the Qtracer2 software, integrating concentration and volume, offers a more accurate and reliable method, especially in scenarios with sparse sampling intervals and limited data. The results of this study are significant in several ways. Firstly, they indicate that despite the low recovery rates at spring points 1# and 2#, there is a likelihood of deeper runoff channels existing between these points and the water point, which could pose risks of water accumulation. Although underground karst pipelines are relatively complex, actual engineering shows that large flows of groundwater will not be encountered during tunnel construction, and the possibility of water and mud inrush during tunnel construction or operation is low. However, the study recommends that construction processes should be managed carefully to prevent the discharge of wastewater and debris into groundwater, thus avoiding contamination of groundwater. Furthermore, this research provides valuable insights into the geological conditions surrounding the Guanshan tunnel. It highlights the importance of conducting detailed hydrogeological studies before commencement of any major construction project, particularly in karst regions known for their complex subterranean water systems. Such studies are not only crucial for the safety and success of the construction project but also for the protection of the surrounding environment and water resources. In conclusion, the findings of this study offer a comprehensive geological basis for assessing risks in water inrush and mud gushing in the Guanshan tunnel. They also serve as a useful reference for future projects in similar geological settings, particularly for choosing appropriate methods to calculate recovery rates of tracer tests. The study results may provide practical guidelines and methodologies for effective management of the risks associated with tunnel construction in karst terrains. -
Key words:
- tunnel /
- tracer test /
- ammonium molybdate /
- recovery rate /
- Qtracer2
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表 1 Mo6+背景值分析结果
Table 1. Analysis of Mo6+ background values
水点编号 名称 流量/L·s−1 Mo6+/μg·L−1 备注 DWJXSD 马鞍山落水洞 20 0.065 投放点 DWJ-01 大屋基1#泉点 10 0.110 接收点 DWJ-02 大屋基2#泉点 15 0.540 接收点 
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表 2 研究区地下水流速情况
Table 2. Flow velocity of groundwater in the study area
示踪剂投放点 接收点 距离/m 初现用时/h 峰值到达时间/h 平均流速/m·h−1 峰值流速/m·h−1 马鞍山落水洞 1#取样点 430 20 31 13.02 32.25 2#取样点 360 20 43 6.94 27.00
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