Analysis on the hydraulic connection and medium characteristics between tunnels and karst springs by tracer tests:A case study of Guanshan tunnel
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.
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