Analysis of karst leakage conditions and pipeline characteristics in the main blocked area of Yibasan reservoir,Yunnan
Reservoir leakage is a significant challenge in the construction of water conservancy projects in karst areas. Yibasan reservoir is a key water source project planned for Wenshan Prefecture in Yunnan Province. It is a storage reservoir composed of 11 surface seasonal karst lakes,underground river pipelines,karst caves and fissures. The proposed storage capacity is nearly 90 million m3. The main blocked area is planned to be built at the concentrated distribution line of large depressions. Many large depressions are located in the upper reaches,while the height differences in the lower reaches are relatively gentle,with an overall arc length of nearly 800 m. The project is planned to be constructed with the use curtain grouting,flood discharge tunnel and other methods,with a total storage capacity of 11,412,600 m3. The presence of leakage zones in the main blocked area and their characteristics are critical to the success of sealing process. Taking the main blocked area of Yibasan reservoir as an example,the hydrogeological survey,dynamic monitoring of groundwater levels,and geophysical exploration and drilling have been employed in conjunction to analyze the hydrogeological conditions in the main blocked area and to identify the location and development characteristics of karst leakage.The results show that the karst leakage zone in the main blocked area is developed in the pure carbonate rocks of the Devonian Donggangling Formation (D2d). The lithology consists of thick layers of silty crystalline limestone,dolomitic limestone interbedded with thin layers of marl,and muddy limestone. The south-north water boundary is formed by the SW to NE compression and torsional reverse fault,Water permeable strata are located on both sides of east and west. After the confluence of the upper reaches of the underground river to the main blocked area,the river rapidly divides into southeast and northeast tributaries. The F6 water-guiding fault in the direction of NW to SE crosses the main blocked area and opens the F4 and F5 fault planes. The width of the fault influence is 80 meters,which provides the structural conditions necessary for the formation of karst leakage zone. This karst leakage zone encompasses both the main pipeline and the north branch pipeline,with the development location of the leakage pipeline being significantly controlled by faulting. The study revealed the seasonal variation of karst leakage,proposed the leakage conditions of the main pipeline and the northern branch pipeline,and analyzed the distribution characteristics,scale and filling properties of the leakage pipeline. Through the analysis of groundwater level and monitoring of groundwater flow,it has been observed that when the groundwater level in the reservoir area exceeds 1,330 meters,leakage occurs from the main pipeline to the northern branch pipeline. In the dry season,groundwater is mostly discharged downstream through independent pipelines. In contrast,during the wet season,about 1/3 to 1/2 of groundwater is discharged into the northern branch. Hydrogeological analysis,monitoring of groundwater dynamics,and results of geophysical exploration and drilling show that the development scale of the leakage zone of the main pipeline is larger than that of the northern branch pipeline. The drilling results also show that filling or exposed karst caves can be seen on the surface of the drilled core near the main pipeline and the northern branch pipeline,and there is an obvious "blowing phenomenon" after the formation of holes. There is no sediment filling in the karst cave with the northern branch pipeline above 40 m;however,there is an increase in sediment filling in the karst cave below this depth. Similarly,there is no sediment filling in the karst cave with the main pipeline above 50 m,while there is a notable increase in sediment filling in the karst cave below this depth. This suggests that the development depth of the underground river is below 40 m,and the drilling results are consistent with the geophysical findings. The karst leakage zone of the main pipeline exhibits a large leakage scale,characterized by a development width of about 30 m,a depth of about 120 m,and an elevation of about 1,290 m. The development location is identified between 940 m and 960 m along the first survey line and at the ZK08 borehole. In contrast,the karst leakage zone of the northern branch pipeline is smaller in scale,with its development situated along the second survey line at 785 m to 795 m,as well as at ZK04 and ZK05 boreholes. This zone has a development width of about 10 m,a depth of about 100 m,and an elevation of about 1,320 m. This study establishes a scientific basis for leakage prevention in the main blocked area of Yibasan reservoir,and offers a reference for the treatment of similar leakage issues in karst areas.