| Abstrakt: |
The karst area in Southwest China is characterized by complex terrain and karst development, with a large area of exposed carbonate rocks and a wide distribution of karst depressions and valleys on the surface. Therefore, this area is highly subject to water inflow in tunnel construction. The Dapozi tunnel area of the central Yunnan Province is a deep-buried tunnel in the karst area, in which exist many faults and very complex hydrogeological conditions. Therefore, it is necessary to predict the tunnel inflow during construction by analyzing the water inflow conditions and identifying its sources, which can ensure the safety of tunnel construction. Currently, numerical and analytical methods are commonly used to predict water inflow. The analytical methods mainly calculate the water inflow of tunnel through empirical calculation formulas. However, most of these methods are based on some assumptions and specific boundary conditions, which may limit their applicability in complex cases. The numerical method can simulate complex geological conditions by obtaining accurate data about geological characteristic, but its accuracy is restricted by the accuracy of borehole data in numerical calculation. Therefore, the mutual supplementation of numerical and analytical methods can significantly improve the efficiency and accuracy of water inflow prediction. Besides, most of the previous studies only predicted the water inflow of tunnel in the survey and design stage. But they did not describe the actual construction progress in the model, and the accuracy of prediction results conducted separately with numerical or analytical methods were not verified by the actual water inflow data. By analyzing the stratigraphic lithology and geological structure of the strata in the study area, we preliminarily identified the construction sections with the risks of water inflow. Based on hydrogeological surveys, we collected water samples from tunnels, springs, and wells in the study area to measure the geochemical characteristics of groundwater. Furthermore, we analyzed the sources for tunnel inflow and degrees of karst development, using Piper trilinear diagrams and Gibbs diagrams. Based on the procedures above, we used analytic and numerical methods to calculate the maximum and average water inflow of each tunnel unit, and compared the predicted values with the actual ones. We have also built a numerical groundwater flow model based on FEFLOW, which couples the simulation of regional macroscopic groundwater distribution and employs a method that combines multiple time series with the comprehensive assignment of various internal boundaries to depict the dynamic construction process. The results show the study area is developed with three main faults in. Although the permeability of these faults is weak, the gullies formed by these faults will gather atmospheric precipitation and surface water, increasing water inflow risk during tunnel construction. The TDS of groundwater in the area gradually increases from south to north, indicating that groundwater flows from the south to the north of the study area. The δD and δ18O isotope results prove that the water inflow in tunnel is from the low altitude; the distance of the groundwater runoff path is medium; atmospheric precipitation is the primary supply source. Gibbs diagram shows that the ions in the groundwater are mainly from rock weathering, suggesting that there may be karst fissures in carbonate rocks due to water-rock interaction within the area. In addition, the analytical method can efficiently calculate the water inflow of tunnel in the preliminary survey and design stage of the project. However, this method cannot dynamically predict the change in the water inflow. In essence, it is an analytical formula derived from the theory on the stable movement of the phreatic aquifer to the complete well. Therefore, the groundwater table dramatically affects the result, and the prediction accuracy in the section of the high groundwater table of analytical method is lower than that of the numerical method. The numerical method focuses on the high-accuracy model of engineering scale in the karst system, which is controlled by the macroscopic groundwater distribution. It pays attention to the description of the dynamic process and the conditions of the project area. Specifically, it describes the dynamic construction process by multi-time series. The working conditions of the excavating and lining are described by "GAP" in FEFLOW, which can accurately predict the change of water inflow in construction. Therefore, based on the actual hydrogeological conditions, the analytical-numerical method can significantly improve the efficiency and accuracy of water inflow prediction. The methods and models used in this paper are significant for preventing and controlling water inflow disasters in high-risk tunnels. [ABSTRACT FROM AUTHOR] |
