中国岩溶

MODFLOW-based analysis on seepage in discrete fissure networks

WANG Jinli, CHEN Xi, ZHANG Zhicai, KANG Jianrong, HU Jinshan

2025, 44(1): 1-14. doi: 10.11932/karst2024y043

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Studies on karst bedrock fissures face challenge in modeling due to the heterogeneity and anisotropy of fissures. Groundwater primarily flows through fissure and pipe networks within karst aquifer systems, while the bedrock pores and micro-fissures mainly serve as water storage. Therefore, the ideal hydrological model for karst areas is the continuum–fissure–pipe model. Fissures, as the main pathway for groundwater flow, significantly influence seepage in fissure media due to their connectivity. The fissure–continuum model (FC), which integrates the advantages of continuum models and discrete fissure network (DFN) models, can realistically characterize the dual texture of karst aquifers. The FC model can also account for matrix diffusion and water exchange between the matrix and fissures. However, this model remains challenging to accurately determine water exchange between rock matrix and fissure media.This study was based on the statistical distribution of fissure geometry and hydraulic parameters. It utilized the Monte Carlo stochastic simulation technique and MATLAB program to generate a two-dimensional fissure network diagram that mirrored the distribution of actual rock fissures. The adjacency matrix of an undirected graph of graph theory was employed to represent the intersection relationships between fissures in the fissure network. In addition, this study utilized percolation theory to eliminate isolated fissures and clusters of isolated fissures that cannot conduct fluid. This approach facilitated the generation of a first-level connectivity diagram. Fissures with single nodes were then eliminated to create a second-level connectivity diagram, and dead-ends in fissures were eliminated to generate a third-level connectivity diagram. Based on the connectivity diagrams of fissure networks, the cubic law and the continuity equation for seepage were applied to establish a single-phase, saturated, and stable DFN model. Each fissure in the connectivity diagram was mapped onto a finite difference grid to develop an FC model that combines DFN with MODFLOW. When eliminating the fissure dead-ends, computational rounding may result in the loss of some nodes. Therefore, the second-level and third-level connectivity diagrams were separately used for DFN seepage simulation and FC seepage simulation, respectively. Simulation programs were developed to reproduce cross-fissure test scenarios, and two multi-fissure cases were analyzed to investigate the effects of finite difference grid resolution and fissure dead-ends on the accuracy of stable seepage simulations. The DFN was used as a metric to evaluate the correctness and validity of FC model code writing. This study was based on the statistical distribution of fissure geometry and hydraulic parameters. It utilized the Monte Carlo stochastic simulation technique and MATLAB program to generate a two-dimensional fissure network diagram that mirrored the distribution of actual rock fissures. The adjacency matrix of an undirected graph of graph theory was employed to represent the intersection relationships between fissures in the fissure network. In addition, this study utilized percolation theory to eliminate isolated fissures and clusters of isolated fissures that cannot conduct fluid. This approach facilitated the generation of a first-level connectivity diagram. Fissures with single nodes were then eliminated to create a second-level connectivity diagram, and dead-ends in fissures were eliminated to generate a third-level connectivity diagram. Based on the connectivity diagrams of fissure networks, the cubic law and the continuity equation for seepage were applied to establish a single-phase, saturated, and stable DFN model. Each fissure in the connectivity diagram was mapped onto a finite difference grid to develop an FC model that combines DFN with MODFLOW. When eliminating the fissure dead-ends, computational rounding may result in the loss of some nodes. Therefore, the second-level and third-level connectivity diagrams were separately used for DFN seepage simulation and FC seepage simulation, respectively. Simulation programs were developed to reproduce cross-fissure test scenarios, and two multi-fissure cases were analyzed to investigate the effects of finite difference grid resolution and fissure dead-ends on the accuracy of stable seepage simulations. The DFN was used as a metric to evaluate the correctness and validity of FC model code writing. Results show that the total flow simulation values and head simulation of the fine grid under the two test conditions are in good agreement with the theoretical values of DFN. However, the node heads and total grid flow simulated by the FC model in two cases are either overestimated or underestimated in the coarse grid. Although elongated paths of grid flow Lg between cells are corrected by increasing the permeability coefficients of the cells, the coarse grid based on MODFLOW cannot accurately analyze the length of inclined fissures Lf on the grid. If the size of selected coarse grid is larger than the distance between two adjacent fissures, part of the two adjacent fissures will be merged in the grid, thereby shortening the actual flow path. Secondly, the length of inclined fissure Lf. is either overestimated or underestimated. The inaccurate analysis of flow paths will lead to an increase in the estimation error of flow rates and heads, especially in dense fissure zones near the center of the model domain. This effect can be greatly alleviated by refining the grid. The fissure dead-ends have non-negligible effects on MODFLOW grid flow. In this study, the MODFLOW-based FC model can be used to solve the pressure distributions between interconnected network of fissures and the rock matrix, overcoming the difficulty of determining water exchange between rock matrix and fissure media. This study can realize the expression of heterogeneity and anisotropy of fissure permeability in numerical computational units, which will promote the understanding of MODFLOW simulation of fissure flow based on connectivity.Furthermore, The size of the MODFLOW grid should be determined in combination with the size of the study area, which should be small enough to capture the detailed features in the flow, and large enough to reduce the computational time and cost. Generally, the grid size is determined by a trial algorithm. When FC approach is applied to the actual watershed, the spatial variability of topography and landform factors should also be considered, such as coupling surface elevation DEM data.

Experimental study on dissolution characteristics of gypsum intercalations in red layers under still and flowing water conditions

ZHONG Zhibin, FENG Jie, LYU Lei, ZHOU Qijian, LI Sijia, XUE Changrui

2025, 44(1): 15-23. doi: 10.11932/karst2024y046

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Gypsum intercalations with uneven thicknesses are widely distributed within sand and mudstone in red layers of the Sichuan Basin. In the process of groundwater infiltration or erosion, these gypsum intercalations inevitably undergo dissolution reactions, resulting in the formation of numerous pores. Additionally, the unique physical and mechanical properties of red layers can significantly reduce the bearing capacity of foundations, leading to uneven settlement deformation and even damage to engineering structures. Therefore, it is necessary to carry out more in-depth research on the dissolution characteristics of gypsum intercalations in red layers. Based on an avionics hub project in Chongqing, this study took the original rock containing gypsum intercalations in the dam foundation as a research object. A self-designed apparatus for flowing water dissolution was used to perform the dissolution test. By regularly monitoring the changes in ion concentrations of Ca²⁺ and SO₄^2- ions within the solution, the dissolution process of gypsum in the test can be characterized, thereby allowing for an investigation into the erosion characteristics of gypsum intercalations in red layers under different flow velocities.At the end of the test, the gypsum intercalations in different flow velocities exhibited erosion grooves to varying degrees. Notably, the depth of surface grooves in the sample exposed to dissolution under still water conditions was the smallest, measuring only 2–3 mm, while the deepest groove formed by dissolution under flowing water conditions reached 12 mm. This observation indicates that the dissolution rate of gypsum significantly increased under flowing water conditions. Following the test, the concentrations of Ca²⁺ and SO₄^2- in the solution were measured, revealing that the contents of both ions showed an increasing trend over time. Comparing the dissolution test under flowing water conditions to that under still water conditions, the average chemical dissolution rate of the samples in still water was lower, both in terms of numerical values and the magnitude of changes. Furthermore, the slope of Ca²⁺ content change curve for the experimental groups increased gradually with the rise in flow velocity, indicating that higher flow velocity has a promoting effect on the chemical dissolution of gypsum. Through the processing and conversion of the test data, it can be found that the average dissolution rate under different flow velocity conditions exhibited a rapid decline in a short time, followed by a gradual stabilization. At the initial time of the dissolution test, the average chemical dissolution rate of the samples indicated that the higher flow velocity corresponded to increased dissolution rates. With the experiment, the dissolution rate decreased rapidly and the decreasing trend under different flow velocity conditions was different to some extent. The greater the flow velocity was, the greater the dissolution rate became. The trend of curve for instantaneous chemical dissolution rate of each dissolution test group under flowing water conditions was generally consistent with the trend of curve for the average chemical dissolution rate. This trend exhibited a pattern of decline over time, with all curves demonstrating a sudden drop on the third day of the test, followed by a gradual decrease that eventually stabilized. According to the curve depicting the dissolution amount of each sample as a function of flow velocity, it is evident that both chemical dissolution and mechanical subsurface erosion in gypsum intercalations were enhanced with increasing flow velocity.

Research on the mechanism and application of crystal deposition in corrugated drainage pipes

LYU Jianbing, CHEN Zuru, LI Jia, FENG LI, DONG Qinxi, WANG Hui, ZHANG Guocai

2025, 44(1): 24-37. doi: 10.11932/karst20250101

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The frequent clogging of drainage pipes of tunnel engineering in the limestone regions has significantly impacted the long-term operation and maintenance of tunnels. To better solve this problem, in this study, a multi-field coupled numerical model was constructed to thoroughly analyze the formation mechanisms and key influencing factors of drainage pipe clogging.In recent days, fluid simulation has become an important tool for solving such problems, thanks to the rapid development of computational power. At present, numerous simulation models have been developed to simulate pipeline scaling; however, the conditions they simulate are often far cry from the actual working conditions of tunnel drainage pipes. To be specific, most simulations of pipeline scaling are conducted under full-pipe flow conditions, while the actual tunnel drainage pipes are rarely operated under full-pipe conditions. Considering this situation, this study integrated Fick’s law of mass transfer with the Navier-Stokes equations, thereby coupling fluid dynamics with chemical reaction kinetics through flow velocity. This coupling was achieved by incorporating mass transfer processes, which allowed for the determination of flow velocity and the distribution of calcium carbonate content within the pipe. Subsequently, a computational and simulation model was established for the curved boundaries of corrugated pipe by integrating an equation for calculating deposition thickness. To further enhance the model’s accuracy, a combination of the phase-field method and dynamic mesh technology was employed. The phase-field method simulated the movement of the gas-liquid interface, while dynamic mesh technology simulated flow channel contraction changes due to deposition. Moreover, changes in flow channel contraction may alter the flow velocity and the distribution of calcium carbonate content within the pipe, which in turn affected the contraction of the flow channel. Additionally, to ensure smooth mesh movement, mesh smoothing conditions were set, and functions were applied at the inlet and outlet to transition the movement rate from zero displacement to a specified velocity. In terms of boundary conditions, this model adopted a turbulence model. The inner wall friction coefficient of the pipe was calculated by formulas from hydraulic design manuals. Subsequently, the inner wall friction coefficient obtained from experiments was converted into an equivalent sand-grain roughness height using the Nikuradse formula, which was then used to represent various wall conditions in the model. Given that the model involves corrugated pipes, further adjustments to the Nikuradse formula are required. From the perspective of energy conservation, the energy loss caused by corrugated pipe grooves in water flow can be divided into two parts: firstly, the energy dissipation caused by imparting rotation to the stagnant water within the grooves; secondly, similar to a smooth pipe, energy loss caused by friction along the pipe wall. This portion of energy loss is related to the length of the wall. In this model, the equivalent length of the corrugated pipe is 1.2 times that of a smooth pipe with the same length. In condition of energy dissipation, and based on a series of trial calculations, the logarithmic function with base 10 was adjusted to a logarithmic function with base 11.3. The dynamic impact of crystal formation on flow field changes under free surface conditions was successfully simulated using this method.Simulation results indicate that the model developed in this study exhibits high predictive accuracy when the inner wall friction coefficient is within the range of 0.2 to 0.3, with an overall deviation between 10% and 20%. However, when the friction coefficient is below 0.2, significant deviations occur in the simulation results, which are higher than the actual deposition results. This may be due to the need for further optimization of the coefficients in the wall functions and deposition formulas under low-friction wall conditions. Moreover, when the friction coefficient exceeds 0.3, this model becomes inapplicable due to the failure of the boundary layer, which is caused by excessively high height of equivalent sand-grain roughness.In conclusion, based on the above research findings, this study has further applied the model to simulate the actual working conditions of drainage pipes in a tunnel in northern Guangdong. By predicting the clogging time and formulating corresponding treatment plans, this study provides scientific basis and technical support for the optimal design and clogging prevention of tunnel drainage systems.

Research on groundwater renewal capacity in Jinan spring area based on tritium isotopes

MENG Qinghan, XING Liting, PENG Kai, ZHU Wenfeng, LIU Lian, HE Qiang, XU Bing, PAN Weiyan, SONG Qifeng

2025, 44(1): 38-47. doi: 10.11932/karst20250102

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The study area is located in Jinan City, Shandong Province, China, and belongs to the inland mid-latitude zone with a temperate monsoon climate. The average annual rainfall is 672 mm, and atmospheric precipitation is the main source of spring water recharge. The overall terrain of Jinan spring area is higher in the south and lower in the north, with the stratum of Archaean Taishan Group as the foundation, covered by the Cambrian and Ordovician strata. The strata incline to the north, forming a monoclinic structure. The karst development is weak. The water bearing medium is dominated by pores and fissures, supplemented by large fissures and pipelines. Groundwater tends to migrate northward along the strata and forms a karst spring group when encountering intrusion rocks from the Yanshan Period in the north. Under natural conditions, spring water is the main discharge form of karst water systems. Due to the intensification of human activities in recent years, the water levels of springs have decreased, and some springs have even cease to flow. In order to explore the groundwater renewal capacity of the Jinan karst spring area and to propose effective recommendations for spring water protection, this paper established a tritium isotope attenuation model for the study area. Previous experiments on mercury intrusion and karst aquifer seepage simulation in the study area have shown that the pore structure of the aquifer in the spring area and the hydraulic characteristics of different aquifer layers exhibit distinct spatial zoning. Moreover, drilling and hydraulic tests conducted in the direct and indirect recharge areas of the spring area have demonstrated that some areas are highly developed with karst fissures. These areas mainly consist of karst fissures and large fissures, which create advantageous seepage channels. Therefore, groundwater can be divided into karst water from karst fissures and large fissures and water from karst fissures and pores based on the characteristics of the medium in which groundwater is located. The piston flow model (PFM) is applicable to groundwater where the aquifer medium is predominantly composed of karst fissures and large fissures. In contrast, the entirely mixed flow model (EM) is suitable for groundwater where the aquifer medium is primarily characterized by a combination of fissures and pores. Since these two models cannot simultaneously account for the hydrogeological characteristics of the study area, this study proposes a new piston flow-entirely mixed flow model (PFM-EM) to facilitate a more accurate analysis of spring water renewal time. The model results show as follows: (1) The age of groundwater in the Cambrian Period ranges from 22 to 85 years, while the age of groundwater in the Ordovician Period ranges from 3 to 22 years. Additionally, and the annual water age of Baotu Spring is 15 years. When combined with previous EM model data, it is evident that the age of spring water gradually decreased during the abundant water period from 1989 to 2022. The decrease in the age of spring water indicates a gradual shift in the supply of spring water towards shallower strata. More importantly, it reflects an increased sensitivity in the response time of spring water to external influences. As urban built-up areas continue to expand, the hardening of urban surfaces has resulted in a reduction in the infiltration area of precipitation. To maintain a continuous flow of spring water, it is essential to limit the extraction of Ordovician karst water while simultaneously replenishing groundwater sources. (2) The age of groundwater increases with the depth of the aquifer, indicating that spring water is supplied by different water-bearing rock formations of the Cambrian and Ordovician strata. Given that the groundwater renewal capacity of the Cambrian strata is relatively limited and they exist within an open system, atmospheric precipitation and human activities directly or indirectly affect the deep aquifers. Therefore, it is also advisable to avoid intensive exploitation of the Cambrian aquifers to prevent a reduction in the proportion of deep Cambrian groundwater contributing to spring water replenishment. While the existing replenishment effects have yielded some positive results, it remains essential to continue replenishing groundwater. Furthermore, the regular monitoring of water sources should be conducted in the spring replenishment area to prevent contamination of groundwater and spring water by surface sewage.

Classification of groundwater systems and system characteristics in the Datang synclinal area of north Guizhou

HOU Jiangyong, XIANG Zhao, HU Yu

2025, 44(1): 48-56. doi: 10.11932/karst20250103

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In recent years, researchers have conducted various studies on the classification of groundwater systems and have summarized different methods of this classification. Investigations into groundwater system division and system characteristics have been carried out in several areas of karst mountainous areas in Southwest China. Notably, Guizhou Province has extensively conducted 1:50000 hydrogeological surveys; however, there has been no research focused on the classification of groundwater systems within the water-bearing system of the Datang Syncline Area in North Guizhou. This area is characterized by multiple bauxite exploration rights, and bauxite is known to be a karst water-filled deposit that is directly replenished by roof water. The karst hydrogeological conditions are extremely complex, which poses certain difficulties for the objective evaluation of the hydrogeological characteristics of bauxite in this area. This study provides an objective evaluation of the regional hydrogeological characteristics by examining the groundwater system in the Datang Syncline Area of North Guizhou. It mainly uses hydrogeological survey data collected from the study area, supplemented by on-site hydrogeological surveys, results of water quality analysis, and long-term observations of groundwater dynamics. This comprehensive analysis aims to delineate the boundaries and characteristics of the groundwater system in the area.The research findings indicate that the Datang Syncline Area in North Guizhou has a relatively independent groundwater system characterized by relatively complete recharge, runoff, and discharge conditions. This system mainly consists of underground river systems and dispersed discharge systems. The study area is divided into two groundwater systems, namely, the Datang synclinal underground river system and the Luolong anticline dispersed discharge system. The characteristics of each system are described. The water-bearing medium of the Datang synclinal underground river system consists of fissure–cave and cave–pipeline formations. The groundwater depth typically ranges from 50 m to 250 m. Except for the watershed in the southwest, the remaining boundaries are delineated by the aquitard of the Hanjiadian Group within the Zhiliu System. There are 392 surface karst features in the system, including sinkholes, inlets and outlets of underground rivers, and karst funnels. The system is mainly composed of underground river pipelines, with a total of seven main underground rivers. Among these, two larger underground rivers, S17 and S22, are located north of the Luolong River and are considered the main underground rivers in the system. The underground rivers S18, S19, S20, K1, and K2 are developed south of the Luolong River. The Datang synclinal underground river system characterized by a centralized injection supply, concentrated runoff through karst pipelines, and centralized discharge from underground river outlets. In the area north of the Luolong River, groundwater typically flows from north to south, whereas in the southern area, groundwater flows from southwest to northeast. The aquifer medium of dispersed drainage system of the Luolong Anticline is mainly composed of joint fissures. The boundary of the system is delineated as follows: the surface watershed to the northwest, the aquitard of Silurian Hanjiadian Group to the south and east, and the surface watershed to the west. This system is characterized by surface infiltration recharge, joint fissure runoff, and dispersed discharge through springs. The overall direction of groundwater runoff is from north to south. The dynamic changes in water flow are consistent with rainfall variations, and the dynamic changes of groundwater are relatively stable. The chemical type of groundwater is HCO$_3^{-}$- Ca²⁺. Compared with the dry season, the content of Ca²⁺ in surface water during the wet season decreases, primarily due to dilution by rainstorm. The increase of HCO$_3^{-}$- is relatively small, mainly because atmospheric precipitation during the rainstorm absorbs high concentration of CO₂ from soil and fallen leaves during the surface runoff process, which subsequently leads to chemical reactions. The research findings provide a basis for objectively evaluating the hydrogeological characteristics of bauxite and the regional groundwater resources in the study area.

Simulation study of groundwater flow and solute transport processes in karst underground rivers based on GMS

XIE Zixuan, JIANG Feng, WANG Ruofan, JIQIN Kebuzi, SHI Zheming, ZHAO Liangjie

2025, 44(1): 57-69. doi: 10.11932/karst20250104

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As the demand for the development and protection of groundwater resources in karst regions increases, accurately simulating the flow and solute transport characteristics of these waters becomes crucial. This study focuses on the Longdong underground river system in Zunyi City to construct a numercial model, utilizing the Groundwater Modeling System (GMS). By conceptualizing the water flow characteristics of the karst underground river with a high hydraulic conductivity coefficient (K), this study is aimed to achieve high simulation efficiency with fewer parameters. This approach is particularly advantageous in karst areas, where traditional modeling techniques may struggle to capture the complex interactions between water flow and geological features. Given the unique geological structures, karst conduits, and fractures present in these regions, a sophisticated model is necessary for proper conceptualization. The model considers the distinct heterogeneity and anisotropy of karst aquifers, with a particular focus on the complex flow patterns characteristic of conduit-dominated flow. Karst aquifers are known for their irregular and often unpredictable flow paths, which can significantly influence the movement of both water and solutes. This study underscores the importance of understanding these flow patterns, as they are critical for effective water resource management and pollution control in karst environments. To ensure the reliability of the simulation results, model identification and validation were conducted with the use of discharge data from the underground river's outlet from 2022 to 2023. This validation process is essential, as it not only confirms the model's accuracy but also enhances the credibility of its predictive capabilities for future scenarios. By utilizing measured data, the study improves the model's reliability, making it a valuable tool for researchers and pollution analysts. Subsequently, tracer tests were conducted to compare simulated and observed data, revealing temporal and spatial scale errors present in current solute transport simulations, particularly highlighting the limitations of using high hydraulic conductivity Darcy flow to characterize karst underground rivers. Tracer tests are vital in hydrological studies, as they provide insights into the movements of solutes within aquifers, enabling researchers to effectively assess of their models. The discrepancies observed in this study highlight the challenges faced in accurately modeling solute transport in environments with high hydraulic conductivity, where traditional assumptions may no longer hold true. This underscores the need for continuous refinement of modeling techniques to better align with the dynamic nature of karst systems. The findings indicate that GMS demonstrates good consistency in simulating karst groundwater flow; however, there is a need to enhance the accuracy of solute transport simulations, especially under high hydraulic conductivity (9000 m/d), where deviations between predicted results and actual observations were noted. These discrepancies emphasize the challenges of accurately modeling solute transport under high hydraulic conductivity conditions, suggesting that reliance on high hydraulic conductivity values may lead to oversimplifications that inadequately represent the complexities of solute movement in karst systems. By adjusting the hydraulic conductivity within the model and implementing buffer zones, simulation accuracy was improved, highlighting the significant impact on the range and velocity of solute transport. This indicates that parameter optimization is key to enhancing the predictive accuracy of the model. The introduction of buffer zones also underscores their potential to mitigate scale-dependent errors, providing a novel approach for managing uncertainty in karst system modeling. This innovative method not only enhances the reliability of the model but also serves as a transitional area that helps to smooth discrepancies between modeled and observed data, thereby facilitating model convergence. Finally, this study proposes several improvements for karst groundwater simulations, including optimizing model parameter settings, incorporating more complex hydrodynamic models (such as EPM, DC, and CDC models), and increasing the model's applicability under non-Darcy flow conditions. Furthermore, the study advocates the integration of other models to better represent the interactions between karst conduits and the surrounding matrix, which could lead to more accurate predictions of both flow and solute transport in complex karst terrains. Future research should prioritize the continued optimization of model parameters and enhance the model's predictive accuracy and applicability through extensive field validation. This will provide robust scientific support for decision-making in the management and conservation of water resources in karst regions. In summary, the development of more refined models to improve the resolution of both macroscopic conduit networks and microscopic fracture systems is essential for advancing karst hydrogeological research.

Study on karst development characteristics and water permeability of a pumped- storage upper reservoir in the Qinling Mountains, Southern Shaanxi

DING Yu, GAN Weiqi, FENG Lei, YAN Yingjun, MA Feipeng

2025, 44(1): 70-78. doi: 10.11932/karst20250105

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China has a vast territory, with significant differences of internal and external forces such as lithology, geological structure, and climate in different regions, resulting in distinct regional characteristics in karst development. The Qinling Mountains is the north–south demarcation line in China, with complex topography, geological structure, and widespread distribution of soluble rocks. Consequently, karst in this region is an unavoidable problem in the construction of various projects. The development of karst may lead to reservoir leakage, posing a constraining influence on the construction of pumped-storage power station reservoirs. However, the current research on the characteristics of karst development at specific engineering sites in the Qinling area is relatively limited, and there is not much research on the impact of karst development characteristics on the permeability of the rock body in the reservoir areas. To provide a valuable supplement and reference for the study of karst development in the Qinling area, and to establish a reliable foundation for the design of pumped-storage reservoirs, leakage prevention and control, and risk assessment, this study takes limestone of a pumped-storage reservoir located in the Qinling Mountains of southern Shaanxi Province as the research object. Utilizing drilling, borehole TV imaging and in-situ pressure water tests, this study comprehensively evaluates the water permeability performance of the rock body. By integrating the characteristics of karst development, the research summarizes and analyzes the distribution patterns of limestone karst development in the study area. The permeability of the rock body is evaluated in conjunction with karst development characteristics, leading to the following conclusions:The linear karstification rate of three boreholes in the study area is greater than 10%, with an encounter rate of 47.1%. This indicates a strong degree of karst development. Using the contour line of 3% karstification rate as the boundary, the study area is divided into two karst zones: Zone I and Zone II which includes Zone II-1 and Zone II-2. The karst in Zone I is strongly to moderately developed, with a relatively high concentration of caves in the borehole area. In contrast, the karst in Zone II is weakly developed, and caves are seldom found in the borehole area. The depth of the karst caves is predominantly distributed between 10 m and 60 m underground, representing a typically shallow-buried karst system. The karst caves are distributed in a moniliform shape, with ZK107 and ZK113 being the most prominent, where seven caves are vertically distributed in a single borehole area. The gray oolitic limestone caves are the most developed, followed by brownish-red fine-crystalline limestone caves, while argillaceous limestone caves are not developed. The development of karst caves is consistent with the solubility pattern of limestone. In addition, the karst fissures are generally developed in all borehole areas, and can be categorized into two types: joint-solution fissures and solution fissure zones. These fissures are commonly filled with mud, calcium and calcite.Comprehensive analysis shows that the permeability performance of the rock body in the study area is jointly influenced by the karst development characteristics, the integrity of the rock body, and the filling characteristics. The logarithm of the average permeability (lg10q) exhibits a linear relationship with the linear karstification rate, which is expressed by the equation: lg(q)=A·Rlk+B, where Rlk denotes the linear karstification rate, and A and B are constants. The ratio coefficient A indicates the influence of karst development on the permeability of the rock body, with a calculated value equal to 3.39. Overall, the rock bodies in the study area demonstrate a pattern whereby the increase of karst development correlates with greater permeability of the rock body. A comparison shows that in moderately permeable rock bodies, karst development is primarily weak. In contrast, in strongly and very strongly permeable rock bodies, karst development is predominantly strong to moderate. The permeability of the rock body exhibits a clear linear trend, correlating with an increase in the linear karstification rate. Constant B reflects the effect of the integrity degree of the rock body on the permeability performance. Specifically, relatively intact rock bodies exhibit moderate to strong permeability (Constant B=1.58), while fractured and broken rock bodies indicate extremely strong permeability (Constant B=3.46). In addition, the filling characteristics of protogenesis fissures and karst caves can cause fluctuations in permeability. When the protogenesis fissures and karst caves are not filled, permeability tends to be high; however, when they are densely filled, the permeability will significantly decrease.

Study on ecological risk threshold of farmland soil in typical karst regions of Guangxi: A case study of Cadmium

HUANG Chunyang, HUANG Juan, ZHONG Xiaoyu, LI Jie, LI Mingyu

2025, 44(1): 79-88. doi: 10.11932/karst2024y032

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Cadmium (Cd) is a non-essential element harmful to human health and is a primary pollutant in agricultural soils. It is important to assess the ecological risk threshold of Cd in soil–crop systems. Previous studies have demonstrated that soils in the karst regions of Southwest China are anomalously enriched in Cd due to geogenic processes. Therefore, it is imperative to propose an effective method for accurately evaluating ecological risk threshold of Cd in agricultural soils within karst regions. Rice is the most widely cultivated cereal crop in Daxin County and is also the crop with the highest Cd exposure in the human diet. In this study, 102 sets of rice crops and their rhizosphere soils were systematically collected from typical karst regions in Guangxi Province. With the use of the Bur Ⅲ distribution, the species sensitivity distribution (SSD) curves for Cd in agricultural soils of typical karst regions in Guangxi were fitted, and a soil Cd safety threshold was established to protect 95% of rice crops.The results show that 90.2% of Cd in soil samples exceeded the screening value established by The National Environmental Quality Standard for Soil (GB 15618-2018). However, the Cd content in rice grains cultivated in those fields was quite low, and only 8.82% of the samples surpassing the Limit of Contaminants in Food (GB 2762-2022). This suggests a spatial mismatch in the Cd levels between the soil and the rice grains. Specifically, there are three primary scenarios: (i) the soil exceeds the Cd limit, but the rice remains safe; (ii) both the soil and the rice exceed the Cd limit; and (iii) the soil is within safe Cd levels, yet the rice exceeds the limit. In the study area, higher soil pH values correspond to reduced mobility of Cd, resulting in lower absorption and transfer of Cd by rice grains. Although the soil Cd form is mainly dominated by the content of bioavailable state, the soil pH is mainly weakly alkaline, which inhibits the absorption of soil Cd by plants. Under conditions where soil pH remains unchanged, changes in the soil will not pose a threat to food safety. This is one of the key reasons for the low Cd concentration in rice grown in high Cd soil. The existing standards of soil environmental quality are insufficient for the accurate evaluation of the level of Cd pollution in the soil of the study area. Given this, it is essential to establish the benchmark of soil environmental quality that can reflect the actual conditions. Therefore, using SSD, this study calculated the safety threshold for soil Cd to protect 95% of rice grains and evaluated the actual Cd pollution level in the study area. Based on SSD, the derived soil Cd safety thresholds for protecting 95% of rice grains under soil pH conditions of 5.5−6.5, 6.5−7.5 and 7.5−8.5 are 0.22 mg·kg⁻¹, 1.08 mg·kg⁻¹, and 6.4 mg·kg⁻¹, respectively. Compared with the limits of soil environmental quality stipulated by GB15618-2018, it is evident that the national standard is overly lenient for acidic soil (5.5 < pH ≤ 6.5) but overly stringent for neutral (6.5 < pH<7.5) and alkaline (pH > 7.5) soils. The derived value accuracy of evaluation results significantly increased from 23.5% to 92.1% with increasing pH values. In contrast, the screening value accuracy specified by national standards dropped from 47.1% to 6.3%. Therefore, the research results can be considered as more accurate evaluation thresholds, providing a scientific reference for safe production in Cd-contaminated rice paddies in typical karst regions of Guangxi.

Research on remote sensing identification of dolines with dense vegetation cover based on point cloud principal component analysis point cloud principal component analysis

FU Mingjun, HE Yang, DONG Xiujun, DENG Bo

2025, 44(1): 89-99. doi: 10.11932/karst20250106

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China's karst areas are widely distributed, with a total area of over 3.4 million square kilometers, accounting for one-third of the country's total land area. Karst engineering geological problems are widespread and difficult issues in the development of water conservancy and hydropower in Southwest China. In order to achieve the goals of "carbon peak" and "carbon neutrality", China is currently accelerating the construction of pumped storage power stations. However, karst leakage is one of the most important engineering geological problems faced by the construction of pumped storage power stations in areas with carbonate rock development. Therefore, in order to avoid reservoir leakage, it is of great significance to comprehensively, quickly, and accurately identify locations of dolines in reservoir and dam areas for water conservancy and hydropower engineering construction.The traditional investigation of dolines in reservoirs mainly uses the method of manual ground investigation and drilling. However, in the mountainous areas with dense vegetation cover in Southwest China, manual investigation is often very inefficient and limited by the strong concealment of dolines, making it difficult to achieve accurate and efficient investigation of large-scale dolines. To solve the problem of low efficiency in the investigation of dolines in dense vegetation areas, this article takes a reservoir area of pumped storage power station as a study area and proposes an automatic identification method for dolines based on point cloud principal component analysis to quickly identify and extract dolines in the reservoir area.The study area is located in the northeast of Sichuan Province, with an area of 10 km². It is situated at the junction of the Qianlongmen mountains and the northwest of the Sichuan basin, in the tilted core of the Yangtianwo area. The terrain slope is generally above 30°, and the maximum relative height difference in the area is 1,070 m. The main vegetation in the area is forest, consisting of evergreen broad-leaved secondary forests and shrubs. In the study area, carbonate rock formations, such as the Upper Devonian Maoba Formation (D₃m), Shawozi Formation (D₃s), and Middle Devonian Guanwushan Formation (D₂g), are mainly exposed with strong karst development.Firstly, airborne LiDAR technology was used to obtain ground 3D point cloud data in the study area after vegetation was filtered out. Then, in response to the strong directional characteristics of the concave shape of dolines, a preliminary extraction of dolines was achieved by the K-D tree nearest neighbor algorithm and principal component analysis, and an indicator eigenvalue ratio p was proposed. Finally, three filtering algorithms based on density clustering algorithm, namely, funnel frequency, length, and direction, were used to filter the background noise of the initial extraction results. The method used Receiver Operating Curve (ROC) to test the AUC value which was 0.854, and F-score is 0.859. This method is suitable for identifying and investigating dolines in karst areas with dense vegetation cover.

Analysis and prediction of spatial and temporal variation of carbon storage in limestone area in recent 30 years:A case study of the Hongshui River Basin

YAN Hongbo, ZENG Jinzhao, LU Xianjian, ZHAO Fengyang

2025, 44(1): 100-112, 123. doi: 10.11932/karst20250107

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The vulnerable ecology of karst areas is characterized by limited environmental capacity, poor soil quality, insufficient water resources, low land carrying capacity, and high yet vulnerable biodiversity. These factors contribute to low ecosystem productivity and a diminished ability to withstand disturbances. Therefore, it is crucial to understand the causes of changes in ecosystem carbon storage to prevent and mitigate ecosystem degradation and to support sustainable development in karst areas. The Hongshui River Basin is located in the northwest and central regions of Guangxi, characterized by high terrain in the northwest and low terrain in the southeast. It is the main tributary of the Pearl River Basin and encompasses the largest contiguous karst area in Guangxi, covering an area of 50,479.745 km². Of this, the karst landform area spans 33,942.048 km², accounting for 67% of the total area. The Hongshui River Basin is a typical karst basin located in Southwest China. It features widely distributed karst landforms, significant altitude variations, and generally thin soil with low fertility and poor water retention capacity. As a result, the ecological environment in this regions is extremely fragile. The vulnerability of natural attributes such as carbon storage function and soil erosion in karst basins, combined with human activities, has led to severe ecological degradation in karst areas.Based on the InVEST model, this study took the Hongshui River Basin as an example to evaluate changes in ecosystem carbon storage in the karst area for the years 1990, 2000, 2010 and 2020. At the same time, the PLUS model was employed to simulate the trend of carbon storage changes in the study area under three future scenarios: natural development, urban prioritization and ecological protection. A geographical detector was utilized to identify the main driving factors influencing land use, precipitation, temperature, population density, and other elements affecting the spatial heterogeneity of carbon storage in the karst region of the study area. The conclusions are as follows,(1) From 1990 to 2020, the spatial distribution characteristics of carbon storage in the Hongshui River Basin showed a gradual increase from the southeast to the northwest. Over the past 30 years, the total carbon storage in the basin gradually increased by 71.59×10⁶ t. The most significant increase in carbon storage occurred between 1990 and 2000, indicating that the carbon sink capacity of the Hongshui River Basin was stronger than its carbon source over this period. Overall, the carbon sink effect surpassed the carbon source effect in the basin.(2) In the year 2030, the carbon storage in the Hongshui River Basin is projected to be 927.67×10⁶ t, 914.84×10⁶ t and 930.71×10⁶ t under the scenarios of natural development, urban development and ecological protection, respectively. Compared with 2020, both the natural development scenario and ecological protection scenario for the Hongshui River Basin are expected to demonstrate a generally increasing trend in carbon storage. This indicates that the carbon sink capacity under these scenarios in the future will be stronger than that of the carbon source. In 2030, the carbon storage of the Hongshui River Basin will increase by 7.69×10⁶ t and 10.74×10⁶ t, respectively. Compared with the natural development scenario, the ecological protection scenario shows advantages in areas where the overall spatial distribution of carbon storage increases. Under the urban development scenario for the year 2030, the carbon storage in the Hongshui River Basin is projected to decrease by 5.14×10⁶ t. As urban socio-economic development necessitates the expansion of urban construction land, urban development will lead to a reduction in forest land, which has a significant capacity for carbon sequestration in karst areas. This reduction will disrupt the ecological balance in this regions, further diminishing the carbon sequestration capacity of the basin and gradually shifting its carbon source-sink effect from a carbon sink to a carbon source.(3) The single-factor detection results from the geographic detector indicate that land use is the main driving factor influencing the spatial heterogeneity of carbon storage in the Hongshui River Basin, with a q value of 0.833. Additionally, the average annual NDVI has been shown to explain the spatial heterogeneity of carbon storage, with a q value of 0.545. The interactive detection results show that the interaction between land use and the annual average NDVI factor have the most significant effect on the change in carbon storage within the Hongshui River Basin, with an explanatory power of 0.833. This indicates that the specific combination of the land use interactions, annual average NDVI and other factors—such as annual average temperature, annual average rainfall, digital elevation model, and population density—will influence the spatial distribution of carbon storage. The land use change factor is the main contributor to the increase of carbon storage in the Hongshui River Basin, followed by the annual average NDVI factor. The findings of this study may provide significant theoretical and data support for the sustainable development of carbon storage within ecosystem services in the Hongshui River Basin. Furthermore, they will assist in the formulation of more effective ecological protection and resource management policies aimed at enhancing the carbon sink capacity of the ecosystem and promoting environmental health and sustainable development.

Discussion on the formulas used to calculate the dissolution rate by the rock tablet method

PENG Guozhi, PU Junbing

2025, 44(1): 113-123. doi: 10.11932/karst20250108

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The dissolution rate of carbonate rocks is an important parameter for investigating the karst carbon cycle and evaluating the carbon sink effect; as well as a key indicator for studying karst development and karst hydrogeological problems. The rock tablet method is currently one of the main methods for quantitatively evaluating karstification and its carbon sink intensity, and has been widely recognized and used. The rock tablet method can not only analyze the factors affecting the dissolution rate of carbonate rocks (e.g., climate, lithology, soil properties), but also further explore the temporal and spatial variation characteristics of subsurface karstification by burying carbonate rock tablets at different depths. However, in current studies related to the calculation of carbonate dissolution rates using the rock tablet method, there are different versions of the dissolution rate calculation formula, and there are still some problems in its application, resulting in significant differences in the dissolution rate values calculated using different formulas for the same monitoring data, which adversely affects the accurate determination of the regional dissolution process and assessment of the karst carbon sink effect.This study firstly reviewed the origin, operational procedure, and basic principles of the carbonate rock tablet method and clarified the key links such as the preparation of the rock tablets (the standard rock tablets were uniformly made of limestone in the same stratum, usually the Rongxian formation of the upper Devonian collected from Guilin in Guangxi, or the upper Cretaceous collected from Slovenia, with a diameter of 4 cm and a thickness of 3mm to5 mm), burial layers (in the air, on the surface, and at the soil depths of 20 cm and 50 cm), burial duration (typically corresponding to one hydrological year), and weighing (using a balance with one-millionth precision). The advantages and limitations of commonly used carbonate dissolution rate calculation methods are summarized.Then, the paper analyzed some of the published formulas for calculating the carbonate dissolution rate using the rock tablet method and discovered that some of the formulas have the following issues,(1) The coefficient (method) from the daily dissolution rate to the annual dissolution rate is incorrect, further intensifying the uncertainty of the results; (2) The short-term test results were extrapolated over a long time scale, disregarding the influence of the dynamic changes in environmental conditions on the carbonate dissolution rate; (3) Unit conversion errors. These problems can result in significant calculation errors in the carbonate dissolution rate and make it difficult to conduct comparisons among regions.Finally, based on the fundamental principle of the rock tablet method, this study proposes a suggested formula for calculating the carbonate dissolution rate using the rock tablet method. The suggested formula can further enhance the standardization of the carbonate dissolution rate using the rock tablet method, providing data support for conducting global analysis and comparative research, revealing the laws of karst development, and improving the accuracy of karst carbon sink estimation. This study also proposes that methods should be selected based on the characteristics of the study area and research purpose. At the same time, it is of great necessity to strengthen the correlations between various test methods.

Dissolution process of carbonate rocks by different types of water in atmospheric environment and δ¹³C evolution of dissolved inorganic carbon

ZHAO Guangshuai, ZHU Yinian, XIE Yincai, SHEN Lina, WU Huaying, LI Tengfang, HUANG Qibo

2025, 44(1): 124-135, 146. doi: 10.11932/karst20250109

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Carbonate dissolution is a geological process that occurs in the shallow surface environment of the Earth. Due to its geochemical characteristics—such as low temperature, openness, sensitivity, and biological participation—it leads to variability in the concentration of dissolved inorganic carbon (DIC) and its δ¹³C (δ¹³C_DIC) values in karst water. In addition, although the global distribution of carbonate rocks (approximately 15.2%) is smaller than that of silicate rocks (approximately 41.8%), the rapid kinetics of carbonate weathering contributes to approximately 68% of DIC in large rivers all over the world. Therefore, exploring the effects of different types of water on the dissolution of carbonate rocks, changes in DIC concentration, and carbon isotope characteristics are of great significance. This study selects the Yaji Karst Experimental Site, a karst area in Southwest China characterized by acid rain, as the research object to conduct experimental investigations in the atmospheric environment. Additionally, this study examines the effects of atmospheric precipitation, soil water, and karst groundwater on this process. The effects of continuous soil CO₂ input and aquatic photosynthetic plants are excluded. This study also explores the influence of hydrochemical components in different types of water on the dissolution of carbonate rocks under acid rain conditions. Besides, it examines the control of CO₂ degassing and CO₂ exchange at the water–gas interface on the evolution of δ¹³C_DIC.The results indicate as follows, (1) Precipitation, along with its infiltration into the soil and subsequent exposure to the surface, still exert a significant dissolution effect on carbonate rocks, even in the absence of a continuous input of soil CO₂ and aquatic photosynthetic plants. However, under the same conditions, karst pipelines/fissure water exert weak or no dissolution effects on carbonate rocks after its exposure to the surface. (2) As the soaking time increases, the dissolution amount of carbonate rocks per unit time gradually decreases, and ultimately transforms into calcite crystals, with the highest dissolution rate of approximately 19.7 mg·(cm²·h)⁻¹. Except for the precipitation soaking group, the concentrations of DIC in the other soaking solutions gradually decrease and tend to stabilize. Furthermore, the decrease in DIC in the karst groundwater soaking solution is greater than that in the soil leaching soaking solution. The δ¹³C_DIC values increase progressively with longer soaking time, ranging from an initial value of −18.21‰ to −12.66‰, and reaching values between −5.79‰ and 3.11‰. Except for the precipitation soaking group, the CO₂ partial pressure (pCO₂) in the other soaking solutions rapidly decreases and tends to stabilize. The stable pCO₂ level in each soaking solution is mainly observed: the pCO₂ level in soil leaching soaking solution (1,000–3,000 ppmv) is higher than that in karst pipeline/fissure water soaking solution (1,000 ppmv) but is roughly comparable to that in precipitation soaking solution. The level of stable pCO₂ in all soaking solutions is higher than that of atmospheric pCO₂ (approximately 420 ppmv), with the former being roughly two to eight times higher than the latter. (3) If water is in an open atmospheric system without continuous input of soil CO₂ and without aquatic photosynthetic plants, when concentrations of SO$_4^{2-}$ exceed 29 mg∙L⁻¹ or concentrations of NO$_3^{-}$ exceed 50 mg∙L⁻¹, the salt effect and ion pair effect produced by SO$_4^{2-}$ and NO$_3^{-}$ can significantly increase the solubility of calcite and promote its dissolution. If the concentrations of SO$_4^{2-}$ and NO$_3^{-}$ in water are high, the saturation index of calcite, calculated based on Ca²⁺ and HCO$_3^{-}$ concentrations, may not accurately reflect the dissolution/crystallization state of calcite. Therefore, the simultaneous effects of various ions in water on calcite dissolution/crystallization should be considered, including ion effects, salt effects, and ion pair effects. (4) After soil vadose water or karst pipeline/fissure water is exposed to the surface, CO₂ in the water can complete the degassing process within a few hours. In the absence of a continuous input of soil CO₂ and aquatic photosynthetic plants, pCO₂ quickly reaches an equilibrium state and ceases to be the major factor controlling the dissolution/crystallization of calcite. The CO₂ degassing effect significantly increases the δ¹³C_DIC value, with an increase of up to +10.98 ‰. This results in a substantial discrepancy between the actual and theoretical contributions of soil or atmospheric CO₂ to DIC.

Numerical simulation of bearing mechanism of steel casing group in complex karst area

GUO Hui, MOU Yunzhen, ZHANG Wengang, CHEN Fei, ZHANG Yanmei, WANG Shuo, LIN Sicheng

2025, 44(1): 136-146. doi: 10.11932/karst2024y012

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The construction of pile foundations in karst areas presents unique challenges due to the unpredictable spatial distribution of underground cavities, which may significantly increase the associated risk of such projects. Karst topography is characterized by the presence of soluble rocks, such as limestone, which have been gradually dissolved by groundwater, resulting in the formation of crevices and caves. This geological feature poses significant difficulties for engineers, as the unpredictability of these subsurface formations can lead to complications during construction, especially when steel casings are installed for pile foundations. In karst areas, the risk to the construction process tends to be exacerbated by the possibility of encountering covered caves.When steel casings are built, the structural load may exceed the side friction of the casings, potentially causing them to sink. This situation escalates the construction risks, making the project more complex and challenging. The project for pile foundation construction in the complex karst area of Yinshawan, Jiujiang, Jiangxi (hereinafter referred to as the Yinshawan Project), exemplifies such challenges associated with this type of environment. This area is highly karstic, with caves constituting 76.2% of the karst topography and the largest cave reaching a height of 17.3 m. These characteristics require an improved approach to steel casing construction to prevent sinking and collapse. To solve these issues, this study introduces a three-dimensional numerical model that incorporates the presence of caves, informed by preliminary drilling data and exploration conditions specific to the Yinshawan Project. This model is a critical tool for understanding the subsidence behavior of steel casings in karst areas. This study classifies subsidence patterns into three distinct categories: single-row steel casing groups, triangular steel casing groups, and rectangular steel casing groups. It examines the mechanics of steel casing subsidence in areas affected by karst development. The method involves simulating specified displacements to retroactively analyze the sinking process of steel casing groups. Through numerical simulations, this study examined the load-bearing characteristics of steel casing groups in various sinking modes. The findings reveal that the sinking modes differed significantly in terms of the maximum longitudinal and transverse bending moments experienced by the casings. For instance, in the single-row steel casing group, the maximum longitudinal bending moment was found to be 1,620 kN·m, while the maximum transverse bending moment was 664.6 kN·m. In the sinking mode of the steel casing group within the rectangular area, the maximum transverse bending moment in the non-sinking steel casing was 637.8 kN·m, and the maximum longitudinal bending moment was 2,144 kN·m; both of these values were found in the same steel casing. In the sinking mode of the triangular steel casing group, the maximum longitudinal bending moment in the non-sinking steel casing was 2,090 kN·m, and the maximum transverse bending moment was 922.2 kN·m. The study offers detailed analysis of each sinking mode, highlighting the stress characteristics and potential risks of subsidence. Given these risks and the specific stress characteristics identified through simulations, a construction method in areas with significant covered caves, such as the project site in Yinshawan, should be prioritized. This method should focus on peripheral piling while selectively placing piles in the interior. This study provides valuable insights into the field of geotechnical engineering, offering guidance for future projects in similarly challenging environments.

Numerical simulation of deformation in the development strata of karst caves for slab foundations and the control of safe distance

HUANG Yunlong, GAO Bin, ZHOU Hang, YU Zhe, HE Wen

2025, 44(1): 147-158. doi: 10.11932/karst20250110

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Karst areas are widely distributed throughout China, and karst caves are a common karst form in these regions. As China's economy continues to develop and the demand for rural revitalization intensifies, infrastructure, such as transmission lines, often has to traverse karst areas. In the entire process of infrastructure construction, ensuring the safety of foundations in karst strata is of utmost importance. The existence of karst caves significantly increases the complexity of the deformation of slab foundations. Unfortunately, the failure modes and deformation mechanisms of these foundations in such environments remain unclear. This lack of understanding makes it extremely difficult to propose targeted prevention and control measures in engineering practice. To address this issue, finite element software PLAXIS 2D is employed to establish a numerical model of the slab foundation of electric power towers in karst strata. This model is used to simulate the development of shear bands and load-displacement curves, thereby elucidating the failure mechanisms and conducting analysis on bearing characteristics of slab foundations in karst strata.Against the backdrop of real-world projects, finite element software is utilized to determine the most adverse simulation working conditions. Key factors including relative locations, sizes, and burial depths of karst caves, have been studied to determine their influence on the deformation of transmission tower foundations. Through this research, the deformation mechanism and bearing performance of slab foundations under various load combinations, different locations of karst cave, and varying sizes of karst caves have been examined. Based on the criterion—the limit value of foundation inclination in the serviceability limit state—the failure range of slab foundations in karst strata was delineated. This delineation may provide a crucial basis for the site selection of slab foundations in karst strata. The study results show that the locations, sizes, and load types of karst caves significantly influence the development of shear bands and the failure mechanisms of foundation soil. Affected by the locations of karst caves and the varying tension and compression loads on both sides, slab foundations exhibit diverse failure modes and bearing characteristics. Whether it involves a compression-compression combination or a tension-compression combination, the shear strain will pass from the compressed side of the foundation close to the karst cave into the karst cave. This may result in the formation of an arc-shaped shear band between the karst cave and the foundation. The most unfavorable situation occurs when a karst cave is developed directly beneath a compressed foundation. In terms of the bearing capacity of the foundation in karst strata, this capacity increases as the ratio of diameter (D) to width (B) decreases. Therefore, in practical engineering, the width of the slab should be maximized relative to the diameter of the karst cave. When the ratio D/B exceeds 1, further increase of the width of the slab foundation does not significantly enhance the bearing capacity of the foundation. Additionally, the larger the radius of the karst cave and the closer it is to the foundation, the more pronounced the impact on the inclination of the transmission tower. However, it is important to note that the influence of karst caves on transmission towers is limited to a specific range. Based on the numerical results, a safety design diagram of transmission tower foundations in karst strata is proposed, delineating the safety area of slab foundations. This design diagram aids to determine the construction locations of transmission towers, which can help reduce both line costs and expenses related to karst cave treatment, providing valuable reference for line site selection. This method can provide significant guidance for the site selection of slab foundations of electric power towers in similar karst strata and the management of karst caves.For the site selection of slab foundations for electric power towers, engineers can utilize the analysis results generated by this method to comprehensively consider various factors and select the most appropriate construction location. This approach can effectively minimize the risk of foundation deformation and enhance the stability and safety of electric power towers. At the same time, this method also helps to plan and design foundation structures in advance, allowing for better adaptation to the special geological conditions of karst strata. This approach also minimize uncertainties and difficulties during subsequent construction phrases. In terms of the management of karst caves, this method provides a basis for formulating scientific and reasonable treatment plans. According to the influence of karst caves on the inclination of transmission towers and the relationship between foundation bearing capacity and parameters related to karst caves, targeted measures such as filling and reinforcement can be implemented. In addition, by delineating the failure range of slab foundations in karst strata, the areas requiring focused attention and treatment can be identified more clearly, thereby enhancing the efficiency and effectiveness of karst cave treatment.

Distribution law and formation mechanism of karst collapses in karst plains of Mashan county, Guangxi

YAO Kezhui, HUANG Jiyou, XU Mo, WEN Zhenxing, LU Danmei, QIN Chang, KANG Xiaobing

2025, 44(1): 159-169, 198. doi: 10.11932/karst20250111

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Karst collapse disasters occur frequently and cause huge hazards in the karst dissolution plain of Guangxi. Research on the distribution law and formation mechanism of collapses is the basic premise for disaster early warning, prevention, and control. A total of 97 karst ground collapses have occurred in Xinshitun, Shifeng village, Guling town, Mashan county, Guangxi. The plane morphology of collapses is mostly approximately circular to elliptical, and the cross-sectional morphology is mostly funnel-shaped. The total area of collapse pits is 1,367 m², with a maximum of 254 m² and a minimum of 0.5 m². Ground collapses have damaged buildings, restricted traffic, and severely threatened the safety of nearly 500 people’s lives and property, resulting in direct economic exceeding 10 million yuan. In this paper, the Xinshitun karst collapse in Shifeng Village is taken as the research object. Through a comprehensive analysis of the natural conditions, including topography, geological structure, stratigraphic lithology, and meteorological conditions in the study area, along with the verification of geophysical prospecting and drilling methods, this study discusses the distribution law, influencing factors, and genetic mechanisms of karst collapses in the dissolution plain. Furthermore, it delves deeply into the genetic mechanisms of karst collapses in this area.The distribution of collapses in this area indicates as follows,(1) Ground collapses are mainly distributed in areas with strong karst development in fault tectonic belts. Of these collapses, 46 occur 0–50 m away from the fault, representing 47.4%; 32 are located 50–100 m away from the fault, accounting for 33%; and 19 are situated more than 100 m away from the fault, comprising 19.6%. Notably, as the distance from the fault increases, the frequency of collapses decreases significantly. (2) Ground collapses are distributed in areas with moderate thickness of overburden layers. Most of these collapses are concentrated in the areas where the thicknesses of overburden layers range from 10 m to 20 m, totaling 73 incidents, accounting for 75.3% of the cases. In contrast, collapses occurring in overburden layers with thicknesses of 0–10 m and 20–30 m are less frequent, representing 18.5% and 6.2% of the total, respectively. (3) Ground collapses are distributed in the areas where there are significant fluctuations in groundwater levels. A rapid decline of groundwater level can cause the roof of a karst cave, which is originally supported by groundwater, to lose its supporting force, thus ultimately leading to a collapse.In general, the karst ground collapses in the study area are mainly influenced by three factors: geological structure, overburden thickness, and groundwater level. Specifically, these collapses are mainly distributed in areas with strong karst development within fault tectonic zones, areas with moderate overburden thickness, and areas where groundwater levels exhibit substantial fluctuations. Furthermore, the collapses occurring in the area where the three control factors jointly affect are more concentrated. In the study area, collapses are mainly caused by vacuum suction and suffoion. Most of these karst collapses, approximately 88% of the investigated collapse points, are caused by vacuum suction, while a small proportion, around 12%, is attributed to suffosion. The collapse zones created by vacuum suction erosion are mainly found in shallow overburden area characterized by poor permeability. Significant fluctuations in groundwater levels can lead to a drop in water levels below the bedrock, resulting in the loss of buoyancy within the cavities and the generation of negative pressure suction erosion within them. When combined with gravitational effects, this phenomenon may cause the overlying soil above the cavities to exceed the critical support threshold, ultimately resulting in karst collapses at the ground surface. The collapse zones formed by suffosion exhibit a high degree of karst development, development of karst caves, weak adhesion of overburden layers, substantial fluctuations in groundwater levels, and susceptibility of soil particles to erosion. When the roof of an upper soil layer reaches its critical stability, the upper soil becomes unstable, leading to instantaneous collapses. Both factors are the main mechanisms responsible for the formation of karst collapses in karst plains. This study examines the spatial relationship between collapses and fault tectonic belts, fluctuations in groundwater levels, and thicknesses of overburden layers to summarize the distribution laws and formation mechanisms of karst collapses in the Guangxi karst plains. The research findings provide a reference for the prevention and investigation of karst ground collapses in similar karst plain areas.

Research on the characteristics and value of geological relics in the Enshi Grand Canyon-Tenglongdong Cave Global Geopark

LI Ting, LI Jiangfeng

2025, 44(1): 170-185. doi: 10.11932/karst20250112

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Enshi is situated in the northeastern extension of the Yunnan–Guizhou Plateau, at the juncture of the Daba Mountain and the Wuling Mountains. This area is characterized by extensive deposits of carbonate rocks, which have resulted in highly distinctive and rich karst geological relics. Nevertheless, due to its remote location and inconvenient transportation, there has been little scientific research on this region,as well as a lack of systematic and professional studies,This study aims at conducting a comprehensive assessment of the geological relics within the Enshi Grand Canyon-Tenglongdong Cave Global Geopark and determining its boundary scope. By analyzing the scientific, popular science, and tourism development values of the geological relics within the park, this study provides a scientific basis for the planning and management of the geological park. It also promotes the rational utilization and effective protection of these geological relics.Based on the field surveys, this study systematically analyzed the types and development characteristics of the geological relics within the park. In accordance with "Investigation Specifications for Geological Relics" and "Technical Requirements and Work Guidelines for the Construction of National Geological Parks in China", and with reference to the existing research results on the evaluation of geological relics, an evaluation index system for the geological relics of the Enshi Grand Canyon-Tenglongdong Cave Global Geopark was constructed. A comprehensive evaluation was conducted on 77 geological relics within the park, combining both qualitative and quantitative assessments.The study outcomes elucidate several key points,(1) The park is situated in the transitional slope area between the second and third steps of China’s giant geomorphological units, with a significant hydraulic gradient. The extensive carbonate rocks within the park have been subjected to the long-term and intense effects of surface water and groundwater, resulting in the unique karst landscape of the region. (2) Geological relics within the park can be categorized into three major classes, eight subclasses, and eleven minor classes based on their origins, conditions, and functional roles. The main types include karst landscapes such as peak clusters and depressions, peak forests, isolated peaks, gorges, stone pillar forests, stone buds, underground rivers, caves, and sinkholes, supplemented by water features, geological profiles, paleontological fossils, geological disasters, geological structures, etc. (3) The Tenglongdong Cave system in Lichuan and the stone pillar forest in Enshi are world-class geological formations that represent the most internationally significant geological relics within the park. They are characterized by their typicality, rarity, and scientific significance. The bio-reef limestone in Jiantianba in Lichuan, the Chaodongyan Gorge in Enshi, the karst cave system of the Banqiao Underground River, the fissure of the Yunlong River, and 14 other sites are national-level geological relics that possess exceptional scientific and aesthetic value, ranking among the best in China. Additionally, Daweng Sinkhole and Tuanbao Black Cave in Lichuan, Gaojiatang Cave in Enshi, the stone buds and solution doline groups in Shibanling, along with 59 other sites, are classified as provincial-level geological relics, which are important components of the park. (4) The synergistic evolution of the terrestrial Enshi Grand Canyon, the subterranean Tenglongdong Cave system, and the multi-layered karst landforms within the park represents one of the best and most complete regional responses to the intermittent crustal uplift movements resulting from the uplift of the Qinghai–Xizang Plateau in Central China. This phenomenon holds significant scientific value and international relevance for the study of the Qinghai–Xizang Plateau’s uplift and the geomorphological responses in Central China. The formation and evolution of the swallet stream in Qingjiang offer essential insights into the formation of the Three Gorges of the Yangtze River over the past million years. Additionally, they shed light on the geomorphology and the spatial distribution of surface water and groundwater in Central China, holding significant comparative value in the field of international geological sciences.The park boasts a diverse array of geological relics and numerous distinctive characteristics. Prominent among these are the geological relics associated with carbonate rocks, including the swallet stream in Qingjiang, the system of Tenglongdong Cave, the Enshi Grand Canyon, and the stone pillar forest in Enshi. These geological relics not only exemplify significant scientific research value but also holds great potential for popularizing geological science and enhancing tourism development. In conclusion, the Enshi Grand Canyon-Tenglongdong Cave Geopark possesses the requisite resources to be designated as a Global Geopark and meets the standards and requirements established by UNESCO Global Geoparks.

Strategies for the sustainable tourism development of Guanyan cave based on SWOT-AHP analysis

DENG Yadong, LUO Weiqun, CHEN Weihai, MENG Qingxin, TU Chun, ZHANG Tanfeng, LUO Shuwen, PAN Tianwang, SHEN Lina

2025, 44(1): 186-198. doi: 10.11932/karst20250113

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Predominantly forming in the Devonian light gray, thick-bedded limestone, Guanyan cave is located in Caoping township, Yanshan district, Guilin. In 1985, Guanyan cave was explored by a joint Sino-British expedition team. After planned and designed in 1992, it was officially opened to the public on October 1, 1995. The designed tour path is approximately 2.8 km long and is renowned for its unique three-dimensional cave exploration experience of "sea, land and air". Over nearly three decades since its opening, Guanyan cave has achieved good economic benefits. However, with the construction of cave engineering and the influx of tourists, some of the cave's sedimentary landscapes have gradually turned black and dark, peeled off, and weathered, directly affecting the scientific and aesthetic value of Guanyan cave, thereby weakening its capacity for sustainable tourism development.To investigate the sustainability of tourism development in Guanyan cave, this study employs a perspective of sustainable development strategy and utilizes the SWOT analysis to assess the internal strengths and weaknesses, as well as the external opportunities and threats affecting the sustainable tourism development of Guanyan cave. The study identifies the key factors influencing sustainability and employs the Analytic Hierarchy Process (AHP) to determine the importance of these factors. Furthermore, a strategic model for sustainable development has been established through the SWOT quadrants. Based on the actual tourism conditions of Guanyan cave, targeted strategies and recommendations for sustainable development have been proposed.This study has obtained the following results,(1) According to the analysis of SWOT, the relationship of the related factors is as follow, strengths>weaknesses>opportunities>threats. The biggest strength of sustainable development of Guanyan cave lies in its great popularity of the scenic area, while its biggest weakness is the obvious degradation of landscape. The biggest opportunity of this cave roots in the tourism market while its biggest threat comes from homogeneous tourism competition. (2) Through the four-quadrant coordinate method, the strategic coordinate P (x, y) is determined to be P (0.0749, 0.0526), and P is located in the first quadrant. It is concluded that the SO strategic model should be selected for the sustainable tourism development of Guanyan cave. (3) Based on the SWOT analysis, the following strategies for sustainable tourism development of Guanyan cave are proposed: utilizing internal strengths to enhance the tourism image, managing the underground river to restore the unique characteristics of the tourism brand, leveraging policy opportunities to establish demonstration zones for sustainable development, taking advantage of market opportunities to create diversified tourism products, capitalizing on the opportunities presented by the establishment of demonstration zones to build an ecological tourism circle, using the advantages of the cooperations to improve corporate competitiveness, strengthening management and enforcing strict protection, cultivating tourism talent to improve the quality of tourism services, and encouraging local residents to participate the development to enhance their sense of belonging to the scenic area.Based on the actual tourism development of Guanyan cave, the application of the SWOT-AHP method to determine the strategic direction for sustainable development of cave tourism enables the precise formulation of sustainable development countermeasures. This approach holds significant reference to the sustainable tourism development of similar cave projects.

Research on the standardized system of protecting and utilizing karst cave tourism resources

LUO Shuwen, LUO Shiqin, WU Kehua, YANG Tao, DENG Yadong, LIU Yuxuan, MAO Yongqin, ZHANG Hongzhi

2025, 44(1): 199-212. doi: 10.11932/karst20250114

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Abstract:
Karst caves are among the most important geological tourism resources in the world, yet their delicate ecological environments present great challenges for both the protection and sustainable utilization. Once cave ecological environments are destroyed, restoring them is challenging. Therefore, the planning for the protection and utilization of cave tourism resources is an effective approach to the sustainable use of karst cave tourism. Although China has made great achievements in the theoretical research on the planning of karst cave tourism, there is insufficient systematization and standardization, which brings great challenges to the sustainable development of karst cave resources. Therefore, based on the current status of cave tourism development in China and existing research, it is essential to establish a standardized system for the protection and utilization of these resources, drawing on international researching findings. This study was conducted from the following aspects, (1) The fundamental characteristics of protecting and utilizing karst cave tourism resources were analyzed from the perspective of human-land relationship. This analysis revealed the internal relationship between the protection and utilization components of karst cave tourism resources, as well as the protection and utilization system of the resources. Additionally, it incorporated the technical requirements of planning protection and utilization of relevant geological heritage, and examined the influences and relationships affecting cave resources from four aspects: organization, leadership, planning and supervision. The goal was to develop a protection and utilization mode for karst cave tourism resources. (2) The core elements of planning and designing, along with their interrelationships were analyzed and summarized, based on the resource characteristics of protecting and utilizing karst cave tourism resources, and the social attributes of the tourism market. (3) Based on the protection and utilization mode for karst cave tourism resources, the basic components of the planning and designing karst cave tourism development were sorted out. The core elements and their interrelationships of the planning were examined, while its contents and characteristics were also elaborated. It is believed that the utilization of cave resources should be governed not only by its own characteristics, but also by their connection to relevant attributes of the surface. The detailed planning involves landscape design, which is also the core of planning and designing cave tourism. At the same time, the effective operation of cave tourism and the sustainable utilization of resources are the ultimate goals of planning and designing the protection and utilization of karst cave tourism. Relevant measures are necessary to ensure the healthy and orderly operation of these resources. (4) According to the thematic, professional, highly protective, regional coordination, and comprehensive characteristics of karst tourism resource utilization, and in conjunction with other geological heritage management experience, the protection and utilization mechanism for karst cave tourism resources has been developed based on four aspects: organization, leadership, planning and supervision.By analyzing the human-land relationship in the protection and utilization of karst cave tourism resources, this study summarizes the key elements involved in both protection and utilization. It establishes a model for the protection and utilization of karst cave resources, drawing on both domestic and foreign research findings and practical experience. Furthermore, this study analyzes the characteristics of karst cave planning through various frameworks, including SWOT analysis of resources, control planning, landscape planning, and support planning. This study aims to clarify the content of a planning system of protecting and utilizing karst cave resources. It illuminates the ideas and methods for planning the protection and utilization of karst cave tourism resources. It develops a protection and utilization mechanism, striving to enhance the practicability and feasibility of planning efforts. The goal is to ensure the healthy, orderly and sustainable utilization of cave tourism, while also guiding and supervising government actions. Besides, this study seeks to facilitate administrative approval, supervision, and operational management by both government entities and stakeholders involved in the protection and utilization of cave tourism resources. This study holds great practical significance in supporting the sustainable development of karst cave tourism resources.

2025 Vol. 44, No. 1

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