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Wulichong Reservoir, Located in Mengzi County of Yunnan Province, is the first undammed reservoir with a water depth over one hundred meters and a storage capacity over 50 million m3 successfully built in karst area in China. It was awarded silver prizes for national excellent engineering investigation and for excellent engineering design.
The catchment area is 310.6km2, the design annual runoff 14.43 million m3, and the mean annual discharge 0.457m3/s. The reservoir has a NPL of 1458m and a total storage of 79.49 million m3.
Main Technical Features and Key Technical Issues:
Firstly, how to effectively block the leakage passage of Wulichong blind valley to form the reservoir and store water is critical to the project. The engineering geological and hydro-geological characteristics of the reservoir area, especially the law of karst development, were analyzed effectively through such approaches as geological investigation, mapping, hydro-geological testing, water chemistry and tritium content analysis, core drilling, tunnel exploration, geophysical prospecting, rock mechanics testing, borehole groundwater level observation, cave fillings dating and high-pressure curtain grouting test.
Secondly, the geological conditions were analyzed and demonstrated clearly. The reservoir area was classified into strong and weak karst zones based on the karst geological zoning; vertical stratification features of karst and lower development limit of deep karst (1200m) were demonstrated; it was ascertained that Xiaowozi Spring and Zuopo Spring were drain datum planes controlling Wulichong karst passage and thus proved the conduit-type karst development elevation was 1260m; the existence of high groundwater level zone in the west of reservoir area was proved based on the groundwater level observation and rock type analysis, and the permeability of strata along the curtain and fault structure were analyzed.
Thirdly, the curtain routing and curtain bottom contour line were reasonable and practical. The principles for curtain routing proposed by the designer were: the two ends of the curtain should inser_t into Cambrian slate; the south and north sections of the curtain should go through the highly karst developed zone with the minimum distance; the middle section of north-south curtain should be based on T2gc-2 calcite mudstone with strip containing carbon and undeveloped karst, make full use of the high groundwater level zone, and keep necessary distance from the karst passage in the reservoir area.
Fourthly, the layout and structure of the impervious curtain were reasonable. The grouting gallery was arranged in three layers, upper, middle, and lower. The layers were overlapped. The upper layer was in single row, and the middle and lower layers were in two rows in highly karst developed area and in one row in weakly karst developed area, hole spacing 2m.
Fifthly, the plug for the underground rivers was positioned properly. In accordance with the measured underground river data and curtain strike, it’s effective to set the concrete plug at the downstream of the confluence of underground rivers No. 2 and No. 3.