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Objective In the central Yunnan region, the geological structure is complex and karst formations are prevalent. Sudden water gushing and attenuation of spring flow are among the most difficult problems in tunnel construction. The Mengkuanggou karst water system exhibits a high degree of karst conduitization, with significant differences between karst fissures and karst conduits. Methods This study employs the MODFLOW-CFPv2 dual medium numerical model to simulate the karst water system of Mengkuanggou, providing a detailed description of karst pipeline and diversion tunnel to understand the influence of tunnel construction on groundwater flow and spring flow. Results The results show that (1) The MODFLOW-CFPv2 model can describe the complex geological structure of karst areas, and simulate the dynamic characteristics of the groundwater level and the response characteristics of karst spring flow in the study area. (2) Under the condition of strong discharge, the water inflow per unit length of the tunnel is 164 m3/d·m, with a stable water inflow of 69 m3/d·m, while the Mengkuanggou karst spring flow shows a significant downwards trend. During the simulation period, the average spring flow decreased from 1 578 L/s under natural conditions to 1 098 L/s, an overall decrease of 30.4%, and the peak flow rate decreased from 2 133 L/s to 1 375 L/s, a decrease of 35.5%. The construction of strong discharge conditions will have a significant impact on tunnel construction and the groundwater environment. Under the condition of limited discharge, the maximum water inflow of the tunnel is 39 m3/d·m, and the stable water inflow is 24 m3/d·m. The water inflow of the tunnel is significantly reduced, and the downwards trend of the karst spring flow in Mengkuanggou is also improved to a certain extent. During the simulation period, the average spring flow is reduced to 1 284 L/s, a reduction of 18.6%, and the peak flow is reduced to 1 617 L/s, a decrease of 22.1%. Conclusion The MODFLOW-CFPv2 dual-medium model used in this study accurately describes highly heterogeneous karst water systems with pipelines, karst caves and fissures in karst areas. It can be used to quantitatively evaluate the karst water of Mengkuanggou during Xianglushan Tunnel construction.

The Forchheimer equation is one of the basic equations widely used in non-Darcy seepage. The determination of coefficients A and B in the equation has always been a hotspot and difficulty in the field of porous media seepage. Different studies have proposed various empirical formulas for the coefficients A and B from seepage experiments. However, there are few studies evaluating the applicability of each empirical formula under homogeneous condition and heterogeneous with mixed particle size. In this study, to provide basic information for selecting the empirical formula of the Forchheimer equation under different porous media conditions, the normalized objective function (NOF) and linear regression method are used to evaluate the applicability of the empirical formula of the Forchheimer equation, on the basis of the seepage resistance experiment. The results show that: for homogeneous porous media, Sidiropoulou’s formula has a good prediction effect on hydraulic gradient. For the porous media mixed two kinds of particle size, the mass ratio and size factors of the mixed particle size should be considered based on the average particle size. The prediction effect of the Macdonald formula is slightly affected by the mass ratio and the mixed particle size; while the predicted hydraulic gradient from the Kadlecan and Knight formula is relatively stable. As to the porous media mixed five kinds of particle size, the predictive effect of using d60 as the characteristic particle size is fine.The Kadlecan and Knight formula is suitable to predict coefficient A, and the Ergun formula is effective to predict coefficient B. This study can provide a basis for selecting the Forchheimer equation for seepage of homogeneous and heterogeneous loose sand and gravel porous media in engineering.

Objective The Kuncheng Tunnel of the Central Yunnan Water Diversion Project crosses the Heilongtan and Bailongtan areas, the major headwater regions in the Chenggong District of Kunming City. Methods Tunnel water inrush may seriously threaten the safety of the urban water supply. In this paper, based on hydrodynamic data and hydrochemical data during tunnel construction, the characteristics of the karst water system, the origin of the springs, and the impact of tunnel construction were analysed. Then, a three-dimensional groundwater flow model for the Heilongtan-Bailongtan Section was developed and calibrated to simulate and predict groundwater level changes during the construction of the Kuncheng Tunnel, and the water environment effect of tunnel construction was evaluated. Results The results show that both the Heilongtan and Bailongtan springs are mainly recharged by the same karst P1q+m aquifer. However, controlled by aquitard P1d and the Hunshuitang fault downstream of the Sanjiacun Depression, the two springs actually belong to two relatively independent karst water systems. In addition, the excavation of the Kuncheng Tunnel has changed the regional groundwater flow field and cut off the flow of Bailongtan, while it has negligible impact on Heilongtan. Conclusion This study discusses the genetic relationship of karst spring, and quantitatively analyzes the impact of tunnel excavation on water environment, which has reference significance for preventing water inrush in tunnel construction.