Karst hydrogeological problems and countermeasures of a proposed railway in plateau slope area of southeast Yunnan

The study area is located in the plateau slope area of southeast Yunnan at the southern margin of Yunnan-Guizhou Plateau, which is the surface watershed zone between Nanpanjiang River Basin and Honghe River Basin at the source of the Pearl River. There exist intense erosion and cutting, complicated topographic and geomorphic conditions and geological structure. The study area belongs to Gejiu sub-region in the stratigraphic region of South China. The widely exposed middle Triassic Gejiu formation and other aquifer groups are rich in water. Due to the complicated hydrogeological conditions, the study area is divided into 2 first-level water systems, 7 second-level water systems, 26 third-level water systems, and 12 fourth-level water systems. Karst is strongly developed with abundant precipitation and strong hydrodynamic effect. Underground river pipelines, depressions and sinkholes are also widely distributed. There are 9 large springs and 20 underground rivers (underflows). Through field investigation, observation, tracer tests and experiments, the influence of karst hydrogeological conditions and problems on the route selection of a proposed railway in the study area are analyzed and assessed under the guidance of geological and karstological theories. The study shows that 80% of the aquifers are the carbonate aquifer group with uneven water abundance from the intermediate to extremely strong degrees. Groundwater mainly exists in karst pipeline. Karst is intensively developed in the study area where the tertiary dissolution process creates the vertical distribution of multi-layer karst caves. At the depth of 100 m, strong karst caves are developed with at least 1 layer or at most 3-4 layers. Large springs, underground rivers and pipelines are also widely distributed. The length of karst pipelines ranges from 6 km to37.75 km. The maximum 6 m³s⁻¹. tracer test results show that there exist underground dissolved pools, or lakes in the main pipeline of underground rivers. It is predicted that the middle line of the tunnel will suffer the largest water inflow, followed in turns by the south line, the north line and the through-line. The through-line is expected to experience relatively small risk of water inflow in tunnel excavation, and hence it is recommended in route selection. Geothermal resources are distributed near the contact zone between the Bozhushan granite body and the Cambrian strata in the west of Wenshan. The temperature of hot springs generally ranges from 45° to 50°. In the belt-shaped geothermal reservoir, temperatures are 70-110 ℃ by analogy of those in the neighboring area. The gray and thin medium dolomite intercalated with calcareous dolomite in the middle Cambrian Longha formation (Є₂l) shows serious desertification. The surface sanding dolomite is broken with poor stability, and can be broken by hand. Consequently, accidents such as surrounding rock collapse and roof falling are quite possibly to occur in tunnel excavation. In this study, suggestions to solve karst geological problems in the study area are put forward. Several areas should be avoided in tunnel excavation, such as the areas with rich groundwater, especially those with strong water abundance, the areas with intense Karst development, especially the vertical development areas of underground and surface karst, the runoff area of the main pipeline of Daquan underground river where the route should be planned as far as possible above the recharge area or water level of the main pipeline, the geothermal anomaly area exposed by granites, and the areas distributed with the karst sandstone stratum. These suggestions are expected to reduce the karst hydrogeological hazards and the damage to groundwater environment that the proposed railway construction may face, and to provide reference for the engineering construction of karst areas with intense development.