中国岩溶

Research on hydrogeomorphologic characteristics and evolution of the watershed on the Gaopo karst tableland in Huaxi of Guizhou

LUO Shuwen, MAO Yongqin, WU Kehua, GAO Zhandong, WANG Huicheng, WANG Deyuan, SUN Yan, DENG Yadong, ZHANG Hongzhi

2024, 43(5): 991-1006. doi: 10.11932/karst20240501

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In order to explore the hydrogeomorphologic characteristics and formation mechanism of the watershed on the Gaopo karst tableland in Huaxi, Guizhou, this study investigates the spatial distribution of geological structure, lithology and geomorphology, in conjunction with the hydrological characteristics of the study area. It employs the theory of karst hydrogeomorphology to discuss the development of these characteristics. The results indicate: (1) The karst landforms of the Gaopo karst tableland, including poljes, edge poljes, peak-forest valleys, island peak clusters and karst caves, are well developed and exhibit layered structural characteristics. (2) The peak-forest valley is mainly developed in the watershed area between the Dumu river and the Baijin river. (3) The landform transitions from the watershed to the drainage base, displaying a spatial distribution pattern of peak-forest valley→peak-cluster depression→valley (or isolated peak).The analysis shows that fractures, joints and faults are the dominant factors controlling the northeastward and southeastward trend of the valley in the study area. Spatial distributions of carbonate rocks and non-carbonate strata play a certain decisive role in karst geomorphology. On the Gaopo karst tableland in Huaxi, there is an erosion base at Grade-5 in the valley and at the transition end of the broad anticline, but not in the watershed area. This phenomenon is caused by the fact that the crust began to rise again before the river tractive erosion in a certain period spread to the study area. In the broad anticline area, the tractive erosion path of the previous period spread to the center area, so only a Grade-4 erosion base was formed in the study area. Based on the layered geomorphic structure, it can be seen that from the center to the edge of the tableland and then to the valley, there is a turning point in the erosion base at the edge of tableland. In addition, the erosion base of each grade from the anticlinal transition end to the anticlinal center (from the edge to the center of tableland) is relatively stable with small hydraulic gradients, but the hydraulic gradients in erosion bases from the transition end to the synclinal valley are much larger than those on the anticline. Therefore, in the process of drainage from the center to the edge of tableland and then to the valley, there is also a turning point for the groundwater level as well as the surface water level of Gaopo karst tableland at its edge. As a result, the flowing rates of both the surface runoff and groundwater runoff on the box anticlinal karst tableland are slow, or the runoffs are transformed into karst lakes and pools inside the box anticlinal karst tableland, whose side erosion is strong, forming karst landforms of peak-forest plain, peak-forest valley, etc. However, with the gradual strengthening of traceable erosion, the turning point also gradually retreated to the center of the tableland, and finally the groundwater level of the tableland disappeared. With the retreat of the turning point, the groundwater level gradually decreased, the saturation zone gradually thickened, and the movement of surface water and groundwater shifted from the horizontal to the vertical. As a result, a large number of depressions, funnels, sinkholes and vertical shafts were developed on the surface.Therefore, it is believed that the formation and evolution of karst geomorphology in the watershed area of the Gaopo karst tableland in Huaxi are closely linked to the development of platform hydrological structure. Throughout geological history, the platform water system has been affected by the tractive erosion caused by surrounding rivers, leading to the disintegration of the surface hydrological network from the edges toward the center of the platform. In this process, the karst geomorphology of the Gaopo karst tableland experienced four stages: stone clint-lake, peak-cluster depression, peak-forest valley and the degradation and extinction of platform geomorphology system.

Changes of air environmental factors in Zengpiyan cave site and its influence on the cultural layers

ZHANG Peng, GUO Fang, JIANG Guanghui, LIU Fan, JING Baokun

2024, 43(5): 1007-1019. doi: 10.11932/karst20240502

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The Zengpiyan cave site is a significant Neolithic cave site in Southern China, yielding complete human skeletal fossils as well as artifacts related to the diet and tools of ancient human inhabitants. This cave site provides essential evidence for understanding human activities in Southern China between 12,000 and 7,000 years ago. The cave system at the site consists of three distinct areas: the main cave, the water cave, and the low cave. The main cave retains an intact original cultural layer, featuring clear stratigraphic sequences. Cultural layers, representing the area where artifacts are preserved within the cave site, embody the most important cultural content of the site. Previous research on karst caves has shown that temperature, relative humidity, and carbon dioxide (CO₂) concentrations inside the cave exhibit markedly different temporal and spatial variations compared to the outside environment. These factors influence the formation and degradation of carbonate deposits such as stalactites.To understand the temporal and spatial variations of air environmental factors within the Zengpiyan cave site and their influence on the weathering of cultural layers, this study acts as a crucial supplement and extension to the environmental research on the Zengpiyan cave site, with a particular emphasis on the microenvironment of the cultural layers of the cave. The study specifically examines environmental factors such as cave temperature, relative humidity, and CO₂ concentrations. A hydrological year-long observation was conducted in Zengpiyan cave with the use of various air monitoring devices, along with the sampling and analysis of cultural layer deposits from different excavation units.The results reveal that the air environmental factors inside the cave exhibit three distinct seasonal stages: in winter, both temperature and humidity are low with large fluctuations, and CO₂ does not accumulate at the cave floor. In contrast, summer, brings high temperature and humidity with small and stable variations, resulting in CO₂ accumulation at the cave floor. Spring and autumn serve as transitional phases between winter and summer. Spatially, temperature and humidity show a gradual variation with the increase of cave depth. CO₂ concentrations exhibited a banded distribution with high-concentrations in the eastern section of the cave, suggesting that the water cave plays a significantly role in evevating the CO₂ levels within the main cave. The primary source of CO₂ in the cave is the overlying soil, with rainfall events identified as critical triggers for peak CO₂ levels within the cave. A comprehensive analysis suggests that ventilation effects are the main factors influencing the temporal and spatial variations of air within the cave. Additionally, cave morphology and tourism activities are also recognized as significant contributors to the air environment within the cave. Observing and combining X-ray diffraction analysis, it was found that there are significant differences in the degree of weathering of cultural layers near the entrance of the cave, water cave connections, cave middle sections, cave bottoms, and the upper and lower parts of the exploration area, which are related to the spatiotemporal changes of air environmental factors. For example, there are more dissolution cavities in the upper cultural layer at the cave floor, mainly due to the accumulation of water vapor in the upper part, which condenses and dissolves, transporting easily soluble components away from the cultural layer. In contrast, the cultural layer at the cave entrance is characterized by loose and powdery cementation, which is associated with the significant temperature fluctuations and frequent cycles of drying and wetting at the entrance.This study has documented the temporal and spatial variations of the main air environmental factors at the Zengpiyan cave site, which is characterized by a shallow, wide-open entrance. It has also explored the potential influencing factors and their impact on the weathering of cultural layers. These findings provide a scientific basis for the improved preservation and utilization of cave sites.

Study on hydrochemical characteristics and water inflow sources of typical karst water-filled mining areas under the influence of extended mining

YU Yang, JIN Xiaowen, XU Si, ZHANG Yu, CHEN Yanmei

2024, 43(5): 1020-1033. doi: 10.11932/karst20240503

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Mineral resources are an important pillar of China's vigorous economic development. With the rapid increase in the demand for metal minerals, the original metal mining areas continue to be mined. However, the specific geological conditions indicate that a significant number of ore bodies in China are closely associated with the limestone strata. As a result, the karst aquifer has emerged as one of the major hidden dangers threatening the deep mining of these ore deposits. Therefore, it is an important topic to study the prevention and control of pit water in mines of karst areas, but there is a lack of comprehensive case studies that have revealed the changes in water sources in pits due to deep mining on the basis of hydrochemical data from water inflow points at different mining levels. The Makeng mining area is a typical karst water-filled mining area in East China. Its hydrogeological conditions and pit water-filling conditions are complex, presenting significant challenges for water prevention and control efforts. In this study, 59 sets of hydrochemical data were selected from the water inflow points of horizontal tunnels at four mining levels (+420 m, +300 m, +200 m and +100 m) in the Makeng mining area from 2006 to 2016. Combined with the evolution law of the groundwater flow system in the mining area, various hydrochemical methods such as Piper three-line diagram, ion ratio, Gibbs diagram and chlor-alkali index were used to analyze the source of water inflow. The mixing ratios of different water sources were calculated, and a conceptual model of evolution of water sources at the water inflow points was generalized. Besides, the hydrochemical causes and recharge sources of water inflow points at different mining levels in the mining area were discussed.The results show that the decrease of groundwater levels gradually slowed down while and water inflow continuously increased during the mining process; therefore the recharge sources of pit water inflow points constantly changed. Secondly, the long-term water drainage and depressurization changed the direction of groundwater runoff in the mining area. Additionally, the water temperature, pH and TDS increased with the increase of mining depth, and the hydrochemical type gradually changed from HCO₃-Ca type to HCO₃·SO₄-Ca type and HCO₃-Na·Ca type. The dissolution of carbonate salt is the main controlling factor for the formation of groundwater in the mining area. Besides, controlled by faults and other geological structures, some water inflow points were influenced by different water sources in some degree. Moreover, different mining levels were also affected by pyrite oxidation, cation exchange, mixing action, etc. The sources of water inflow points at +420 m and +300 m were from karst water and P₁j¹ sandstone water, while the main sources at mining levels of +200 m and +100 m were also from karst water. The water inflow point at +100 m was recharged by low-temperature hot water along the water diversion fault from deep granite. The Ca²⁺ and Cl⁻ equilibrium models were used to calculate the respective ratios of mixing karst water with sandstone fracture water and of mixing karst water with hot water from deep granite. The recharge rates of fracture water at +420 m and +300 m were about 22.7%–82.8%, and the recharge rates of low-temperature hot water from deep granite at +100 m were 35.1%–38.3%. In the future, the monitoring of water quality in water inflow points at +100 m or above should be strengthened. If necessary, F1 and other faults exposed underground should be grouted so as to prevent the low-temperature hot water from the contact zone between the deep granite and the quartz sandstone of Lindi formation (C₁l) from entering the pit along the water diversion structure to recharge karst water. In this paper, the hydrochemical formation of water inflow source in Makeng mining area under the influence of extended mining has been discussed, and the dynamic characteristics of water inflow sources have been preliminarily analyzed, which can provide a certain reference for groundwater management and prevention and control of pit water in the same type of karst water-filled mining area.

Temporal and spatial variations of soil CO₂ and δ¹³${\mathrm{C}}_{{\mathrm{CO}}_2} $ from different land uses in typical dolomite areas of Southwest China

WANG Xiaoduo, ZHOU Zhongfa, DONG Hui, DING Shengjun, GONG Xiaohuan, XIONG Yong, SU Dan, ZHANG Ye

2024, 43(5): 1034-1046. doi: 10.11932/karst20240504

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Shuanghedong national geopark is a prominent example of dolomite on a global scale. Soil CO₂ in the dolomite areas is an important driving force of karstification and has great significance for the global carbon cycle. However, the mechanism of soil carbon cycle in dolomite areas has yet to be clarified. In order to investigate the spatial and temporal variations and influencing factors of soil CO₂ and δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ in different land uses in typical dolomite areas and to clarify the mechanism of soil carbon cycle in these areas, this study selected Shuanghedong National Geopark, a typical dolomite area in Southwest China, as the study area. Soil CO₂ and δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ were sampled from six typical land use types in Shuanghedong national geopark. These samples were monitored outdoors and subjected to laboratory experiments over the course of one year. Additionally, the data were systematically analyzed with mathematical and statistical methods.The study results indicate as follows, (1) Soil CO₂ concentrations of different land use types were observed in the following order: cropland>irrigation grassland>abandoned land>scrubland>forested land>fallow land. Soil temperature and water content, overlying vegetation, organic carbon, and human activities were important factors influencing the changes of soil CO₂ in different land use types. (2) The values of soil δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ of different land use types were ranked as follows, scrubland>fallow land>cropland>irrigation grassland>abandoned land>forested land. Differences in diffusion rates of soil ¹²C and ¹³C, decompositional conversion of organic carbon and isotopic fractionation, plant photosynthesis and carbonate rock dissolution led to changes of soil δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ in different land use types. (3) In terms of temporal changes, soil CO₂ concentrations of different land use types were high in the rainy season and low in the dry season, while concentrations of soil δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ exhibited opposite seasonal patterns, showing small values in the rainy season and large in the dry season. Oxidative decomposition of soil organic matter, metabolic activities of microorganisms, respiration of plant roots and dissolution of carbonate rocks are the main sources of soil CO₂ in dolomite areas. Therefore, oxidation and decomposition of organic matter, dissolution of carbonate rocks, photosynthesis of plants, and metabolism of microorganisms store part of CO₂ in soil to form carbon sinks. The other part of soil CO₂ in the dolomite area are primarily attributed to the higher partial pressure of soil CO₂ compared to that in the atmosphere. This difference in pressure facilitates diffusion, allowing CO₂ to escape into the atmosphere as a carbon source. In addition, the study also indicates that different land use types, soil temperature, water content, overlying vegetation, human activities, and organic carbon all influence the intensity of carbon sources and carbon sinks in the soil carbon cycle in dolomite areas to varying degrees. The research findings have deepened the understanding of soil CO₂ in dolomite areas of northern Guizhou, revealed the spatiotemporal differences and influencing factors of soil CO₂ and δ¹³$ {\rm{C}}_{{\rm{CO}}_2} $ in different land use types, clarified the mechanism of soil carbon cycle in typical dolomite areas, and provided a reference for the study of soil carbon cycle in these areas.

Hyperspectral inversion of total nitrogen content in calcareous soil in karst areas

HE Wen, LI Yanqiong, YU Ling, WANG Jinye, NI Longkang, LI Ning

2024, 43(5): 1047-1056, 1064. doi: 10.11932/karst20240505

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Nitrogen is a component of many important compounds in plants, such as proteins, nucleic acids and enzymes, and hence is indispensable for the growth of plants. The nitrogen content in soil is one of the key indicators of soil fertility. Calcareous soil is one of the main soil types in karst areas. A rapid and accurate estimation of total nitrogen (TN) content in calcareous soil is an important guarantee for the scientific evaluation of soil environmental quality in karst areas. In recent years, the rapid development of hyperspectral remote sensing technology has brought new opportunities for a quick assessment of soil physical and chemical properties. However, it is still extremely challenging to rapidly assess soil nitrogen content in karst areas by hyperspectral remote sensing due to the weak optical signal of soil nitrogen and the interference of factors such as the complex ecological environment and the strong spatial heterogeneity of soil TN content in karst areas.Karst areas are extensively distributed in China, where calcareous soil is one of the main soil types, exerting a great influence on ecological protection and agricultural development. Therefore, it is of great theoretical and practical significance for us to develop hyperspectral inversion models suitable for the TN content of calcareous soil. Karst landforms are distributed across 70 counties/cities in Guangxi, with an area of 97,700 km², accounting for 41% of the total area of Guangxi and 10.8% of the total area of karst landforms in China. Taking calcareous soil in the karst areas of Guangxi as the research object, this study performed five mathematical transformations on soil spectra to improve the detection ability of spectral signals while eliminating spectral noise. Meanwhile, given the coexistence of linear and nonlinear relationships between soil TN content and spectra, the hyperspectral inversion capability of three models, namely partial least squares regression (PLSR), generalized neural network (GRNN) and PLSR_GRNN (a combined model of PLSR and GRNN), for soil TN content was compared and analyzed to establish a high-precision and rapid inversion model suitable for the TN content of calcareous soils in karst areas.The results showed as follows. (1) The TN content in calcareous soil was significantly correlated with various spectral bands from 400 to 2,500 nm. Among them, the TN content was more sensitive to the reflectance of the spectral bands near 600 nm, 1,300 nm, 1,600 nm, 1,900 nm and 2,300 nm. (2) The first-order differential transform (FDR), second-order differential transform (SDR), reciprocal logarithmic transform (lg(1/R)), reciprocal logarithmic first-order differential transform ((lg(1/R))') and envelope removal transform (CR) of the original soil spectra can improve the capability of inversion of TN content in calcareous soil to some extent. The transformation effects were roughly ordered by (lg(1/R))' > SDR > CR > FDR > lg(1/R). Overall, the spectral differential transform is superior to the envelope transform as well as the reciprocal logarithmic transform, and can better exploit the detection capability of the spectral signal for soil TN. (3) The PLSR algorithm had excellent predictive ability for the variation of TN content in calcareous soil. In the SDR transform case, the model had the highest accuracy and better model robustness without overfitting, with a coefficient of determination (R2) of 0.84 and root mean square error (RMSE) of 0.55 in the modeling set and R2 of 0.82 and RMSE of only 0.64 in the validation set. Compared with the PLSR algorithm, the GRNN model had greater prediction ability. However, the robustness of GRNN model was worse and the overfitting phenomenon was obvious. In the same SDR transformation case, the modeling set R2 of the GRNN model could reach 0.92, but the validation set R2 was only 0.59, so the overall performance was inferior to that of the PLSR model. (4) The PLSR_GRNN model can integrate the advantages of PLSR and GRNN model, maintaining the high predictability of GRNN model and avoiding the overfitting phenomenon. Among them, the best inversion model was established by SDR, with R2 of 0.92 and 0.90 for the modeling set and validation set, and RMSE of 0.43 and 0.51, respectively, which were suitable for hyperspectral inversion of TN content in calcareous soil in karst areas. In addition, the FDR, (lg(1/R))' and CR transformations also had excellent performance, with R2 above 0.80 for the modeling set and R2 above 0.75 for the validation set.Although the prediction accuracy of the GRNN model cannot be improved by combining the PLSR model with the GRNN model, the overfitting phenomenon can be effectively controlled. This modeling approach, which combines linear and nonlinear models, is more widely applicable than the PLSR model or GRNN model alone, and is more adaptable to more heterogeneous soil types, and will be more widely used. Rapid and high-precision prediction of TN content in calcareous soil in karst areas can be performed based on hyperspectral models. The results can provide a basis for regional soil remediation and utilization.

Influence mechanism of calcium on soil organic nitrogen mineralization during calcareous soil evolvement in karst areas

XIE Yincai, YANG Hui, LI Jun, MIAO Xiongyi, YANG Lin, WU Shucheng

2024, 43(5): 1057-1064. doi: 10.11932/karst20240506

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Nitrogen (N) is an essential nutrient element for the normal growth and development of plants, and the major factor affecting the primary productivity of most terrestrial ecosystems. Studying the process of soil organic N mineralization on the basis of regional characteristics is of great significance for formulating reasonable fertilization measures, and optimizing soil environment and global N cycling. Karst is a unique ecological system restricted by N. In order to discuss the influence mechanism of calcium (Ca) in soil organic N mineralization during calcareous soil evolvement in karst areas, the typical black calcareous soil (BLCS), brown calcareous soil (BRCS) and red calcareous soil (RCS) at different soil evolvement stages in the karst areas in Guilin were selected as research objects. The BCR three-step sequential extraction procedure was used to determine Ca contents of different species, and the ¹⁵N tracing technology combined with the Markov Chain Monte Carlo (MCMC) algorithm-based numerical optimization model were adopted to investigate the mineralization of labile organic N (M_Nlab), recalcitrant organic N (M_Nrec) and organic N (M_Norg) to NH$_4^{+}$ in calcareous soil.The results showed as follows: (1) The orders of the contents of Ca species of BLCS and BRCS were both listed as exchangeable Ca (ECa) > acid soluble Ca (ASCa) > residual Ca (RCa) > water soluble Ca (WSCa) > organic compound Ca (OCCa); the order of the contents of Ca species of RCS was: ECa > RCa > ASCa > WSCa > OCCa. The contents of ECa were the highest among the different species of Ca in BLCS, BRCS and RCS in karst areas, repectively accounting for 80%, 64% and 48% of the total Ca contents of these three calcareous soils, showing that Ca in calcareous soil presents high activity. (2) M_Norg and M_Nrec of these three calcareous soils were listed as the following order: BLCS > BRCS > RCS; the order of M_Nlab: BLCS < BRCS < RCS. M_Norg was dominated by M_Nrec in BLCS, while M_Norg was mainly controlled by M_Nlab in both BRCS and RCS, showing that calcareous soil evolvement affected the mineralization process of soil organic N, and the soil inorganic N supply capacity decreased significantly during calcareous soil evolvement in karst areas. (3) Except the insignificant correlation between soil M_Norg, M_Nlab and M_Nrec, and OCCa and RCa, M_Norg and M_Nrec presented significantly positive correlation with the contents of ECa, ASCa and WSCa, respectively, while M_Nlab presented significantly negative correlation with these variables, which indicates that high contents of available Ca can promote M_Nrec but inhibit M_Nlab. As the main species of Ca in calcareous soil, ECa and ASCa were strongly leached during calcareous soil evolvement in karst areas, resulting in the significant decrease of soil M_Nrec, which was an important reason for the significant decrease of soil inorganic N supply capacity during calcareous soil evolvement. The study results will help us to clearly understand the characteristics of the distribution and migration of Ca, N mineralization and effect of Ca on N mineralization during calcareous soil evolvement in karst areas, and may provide the basic data for deep understanding of the influence mechanism of N mineralization process in calcareous soil. With the evolvement of calcareous soil, calcareous soil with high degree of evolvement in karst areas, such as BRCS and RCS, may be unfavorable for maintaining soil fertility and natural vegetation restoration due to low inorganic N supply capacity. Therefore, in the process of ecological restoration, for the karst areas with high degree of calcareous soil evolvement such as BRCS and RCS, improving the N content of calcareous soil can promote the effective supply of inorganic N in calcareous soil to create a soil environment conducive to vegetation restoration. This may be an effective strategy for rapid restoration of karst ecosystems.

Effects of land use patterns on the limitation of soil microbial resources in the karst areas of Southwest China

XIAO Shuangshuang, CHEN Wurong, FU Wei, ZHANG Jianbing

2024, 43(5): 1065-1075. doi: 10.11932/karst20240507

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The experimental area is situated in the Mulian comprehensive experimental demonstration zone of Karst Ecosystem Observation and Research Station of Chinese Academy of Sciences, located in Huanjiang Maonan Autonomous County, Guangxi Zhuang Autonomous Region. This region falls within the subtropical monsoon climate zone. The average annual temperature is 19.9℃, with an extreme high of 38.7℃ and an extreme low of -5.2℃. The average annual rainfall is 1,380 mm, characterized by abundant precipitation that is unevenly distributed throughout the seasons; the rainy season accounts for more than 70% of the total annual rainfall. The study area is characterized by a typical karst peak-cluster depression with an elevation of 272.0–647.2 m. The average bedrock exposure in the depression and the sloping land is 15% and 30%, respectively, and their soil depths are 20–160 cm and 10–50 cm, respectively. The soil primarily consists of yellow-brown lime soil and brown lime soil, which have been developed from dolomite. The volume fraction of topsoil gravel can reach 10%–40%. The soil texture is composed of clay loam and clay, with the mass fraction of silt ranging from 25% to 50% and that of clay from 30% to 60%, respectively.Soil microbial growth and activity are often limited by nutrient resources, and understanding which nutrients limit microbial activities is crucial for assessing ecosystem function and developing effective management strategies. Previous studies have shown that soil microorganisms are mainly limited by nutrient carbon (C) and phosphorus (P) during natural succession after the land has been left uncultivated in karst areas. However, measures such as fertilization in agricultural activities may change microbial resource limitations. In order to investigate nutrient resource limitations of soil microorganisms in agricultural land, this experiment selected four common land use patterns in Huanjiang, Guangxi, including enclosure, mowing, grass planting and corn planting, and analyzed enzyme activities related to C, nitrogen (N) and P in soil. Based on enzyme stoichiometry, the nutrient limitations of soil microorganisms were identified, and the influence of different land use patterns on the changes in microbial resource limitations in soil was clarified. This study could provide valuable guidance for the evaluation and management of different land use ecosystems.The results showed that soil enzyme activities changed significantly under different land use patterns, which was closely related to nutrients available in soil. Through enzyme stoichiometry, it was found that planting maize and grass significantly increased the nutrient C resource limitation of soil microorganisms, which was mainly caused by the reduction of soil organic carbon and microbial biomass carbon. The soil microorganisms of the four land use patterns were limited by N, but not by P. In addition, mowing and grass planting exacerbated the N limitation of microorganisms. Correlation analysis suggested that N resource limitation was associated with an increase in microbial biomass carbon, as microbial assimilation of carbon may require more nitrogen available in soil. In addition, microbial biomass C, N and P were significantly correlated with enzyme stoichiometry, which well explained the changes of microbial resource limitations under different land use patterns, and could be used as an effective index to assess microbial resource limitations in karst soil.In summary, this study indicates that changes in microbial resource limitations in soil should be fully considered in the evaluation and management of different ecosystems in karst areas, which was conducive to formulating more accurate and effective soil management strategies to promote the health and sustainable development of ecosystems. It is suggested to apply organic fertilizer such as manure or straw to alleviate the nutrient C resource limitation of soil microorganisms in karst areas where corn and forage are planted. In karst mowing areas, nitrogen fertilizer needs to be applied moderately to alleviate nutrient N resource limitation of microorganisms and to increase plant productivity.

Response relationships among CEC, mechanical compositions and mineral types in typical karst soil

XING Yuxin, YUAN Hong, JI Xiangtong, DUAN Chenglong, JIANG Jun, LIU Shanpeng, LIU Peng

2024, 43(5): 1076-1087. doi: 10.11932/karst2024y037

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In this study, a total of 146 profile samples were collected from 32 soil profile sampling sites in nine towns and cities in typical karst areas of Hunan Province. The physical and chemical properties of soil were analyzed through field investigation, excavation, and collection and description of soil profiles of limestone weathering parent materials. The relationships among soil CEC contents, mechanical compositions, and soil mineral types were also explored. The research findings laid a foundation for modification, fertilization and ecological restoration of karst soil.According to the Technical Specifications for Soil Analysis, the determination of soil physicochemical properties was conducted as follows: the soil pH values were measured by potentiometry. Both soil organic matters and total nitrogen contents in soil were determined by potassium dichromate heating method. Soil bulk density was measured by cutting ring method, while total phosphorus was assessed through the digestion-Mo-Sb anti-spectrophotometric method. Total potassium was analyzed via flame atomic absorption spectrophotometry. Exchangeable calcium and magnesium were quantified by EDTA titration method, and exchangeable sodium and potassium were also measued by flame atomic absorption spectrophotometry. The composition of soil particles was determined by pipette method, and the particle fraction was classified based on the system of United States (2.00–0.05 mm for sand, 0.050–0.002 mm for silt, and <0.002 mm for clay). Types of clay minerals were determined by X-Ray diffraction. The cation exchange capacity (CEC) was determined by ammonium acetate centrifugal exchange method. The effective cation exchange capacity (ECEC) was calculated in the following formula: [cmol·kg⁻¹]= H⁺+Al³⁺(⅓Al³⁺) + total extractability base.The results shows as follows. (1) The CEC contents of typical karst soil in Hunan ranged between 2.71–13.9 cmol·kg⁻¹, with an average value of 16.70±5.62 cmol·kg⁻¹. The sample values exhibited considerable variability. The average soil ECEC was 8.86±3.75 cmol·kg⁻¹, sigificantly lower than soil CEC. The particulate composition of soil was mainly silt and clay, resulting in a heavy texture and poor permeability in the study area. Based on the American grading method for soil fertilizer retention capacity combined with the measurement data, it was observed that the soil fertilizer retention capacity in the study area predominantly fell within the medium to strong levels, but the soil fertilizer capacity was inadequate. (2) The particulate compositions of soil followed a trend of clay>silt>sand, encompassing nine texture types. The soil samples with clay texture constituted the largest proportion of 31.50%. This was followed by silty loam and silty clay loam, which accounted for 30.14% and 27.40%, respectively. The soil samples from the study area included five distinct mineral types: siliceous hybrid, illite hybrid, kaolinite, kaolinite hybrid and hybrid. The hybrid soil exhibited the highest sand content, while the siliceous hybrid had the highest content of powder silt, and the kaolinite hybrid contained the most clay. The CEC and ECEC contents were the highest in illite hybrid and lowest in hybrid soil. In general, the soil texture of the 146 samples was primarily clay, silty loam and silty clay loam, resulting in heavy and compact soil that adversely affected the air permeability and drainage of soil. (3) CEC exhibited a highly significant correlation with both silt and clay content, while showing no significant correlation with sand. Soil ECEC was not significantly correlated with any of the soil particles; however, it demonstrated a highly significant positive correlation with soil pH, organic matter content and total phosphorus content (P<0.01). Additionally, there was a significant positive correlation between ECEC and total potassium content, indicating a relationship between ECEC and the primary physicochemical properties of soil. This suggests that ECEC may influence the soil characteristics more effectively than CEC and could have a greater impact on soil fertility. (4) The primary exchangeable ions were Ca²⁺ and Mg²⁺, with soil CEC showing an extremely significant positive correlation with K⁺, Ca²⁺, and Mg²⁺ (P<0.01). Furthermore, soil ECEC was significantly positively correlated with K⁺, and extremely significantly positively correlated with Ca²⁺ and Mg²⁺ (P<0.01). Soil CEC was not correlated with mineral type, while ECEC was highly significantly negatively correlated with mineral type. The contents of Ca²⁺, Mg²⁺ and K⁺ in soil exchangeable salt-based ions also had an effect on the contents of CEC and ECEC in soils of different mineral types.In this study, soil CEC was found to be extremely significantly negatively correlated with silt contents. Conversely, there was a highly significant positive correlation with clay contents. This indicates that in karst soil, finer soil particles contribute to a more compact structure, which enhances soil cation exchange and improves soil fertilizer retention quality. However, the fertilizer environment can limit the nutrient cycle between soil and crops. By adjusting the proportions of silt and clay in soil, it is possible to increase the CEC value and enhance soil fertility. Although no significant correlation was observed between ECEC and soil particle compositions, ECEC was found to be significantly positively correlated with soil pH, soil organic matter contents, exchangeable Ca²⁺ and exchangeable Mg²⁺. Additionally, it exhibited a highly significant negative correlation with soil mineral types, which better reflects the synergistic effects of soil fertility conservation and nutrient supply.

Analysis of karst leakage conditions and pipeline characteristics in the main blocked area of Yibasan reservoir, Yunnan

SONG Chen, PAN Xiaodong, LIU Tianyun, LUO Fei, ZENG Jie, PENG Cong, CHENG Ruirui, ZHANG Wenping

2024, 43(5): 1088-1098. doi: 10.11932/karst2024y033

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Reservoir leakage is a significant challenge in the construction of water conservancy projects in karst areas. Yibasan reservoir is a key water source project planned for Wenshan Prefecture in Yunnan Province. It is a storage reservoir composed of 11 surface seasonal karst lakes, underground river pipelines, karst caves and fissures. The proposed storage capacity is nearly 90 million m³. The main blocked area is planned to be built at the concentrated distribution line of large depressions. Many large depressions are located in the upper reaches, while the height differences in the lower reaches are relatively gentle, with an overall arc length of nearly 800 m. The project is planned to be constructed with the use curtain grouting, flood discharge tunnel and other methods, with a total storage capacity of 11,412,600 m³. The presence of leakage zones in the main blocked area and their characteristics are critical to the success of sealing process. Taking the main blocked area of Yibasan reservoir as an example, the hydrogeological survey, dynamic monitoring of groundwater levels, and geophysical exploration and drilling have been employed in conjunction to analyze the hydrogeological conditions in the main blocked area and to identify the location and development characteristics of karst leakage.The results show that the karst leakage zone in the main blocked area is developed in the pure carbonate rocks of the Devonian Donggangling Formation (D₂d). The lithology consists of thick layers of silty crystalline limestone, dolomitic limestone interbedded with thin layers of marl, and muddy limestone. The south-north water boundary is formed by the SW to NE compression and torsional reverse fault, Water permeable strata are located on both sides of east and west. After the confluence of the upper reaches of the underground river to the main blocked area, the river rapidly divides into southeast and northeast tributaries. The F6 water-guiding fault in the direction of NW to SE crosses the main blocked area and opens the F4 and F5 fault planes. The width of the fault influence is 80 meters, which provides the structural conditions necessary for the formation of karst leakage zone. This karst leakage zone encompasses both the main pipeline and the north branch pipeline, with the development location of the leakage pipeline being significantly controlled by faulting. The study revealed the seasonal variation of karst leakage, proposed the leakage conditions of the main pipeline and the northern branch pipeline, and analyzed the distribution characteristics, scale and filling properties of the leakage pipeline. Through the analysis of groundwater level and monitoring of groundwater flow, it has been observed that when the groundwater level in the reservoir area exceeds 1,330 meters, leakage occurs from the main pipeline to the northern branch pipeline. In the dry season, groundwater is mostly discharged downstream through independent pipelines. In contrast, during the wet season, about 1/3 to 1/2 of groundwater is discharged into the northern branch. Hydrogeological analysis, monitoring of groundwater dynamics, and results of geophysical exploration and drilling show that the development scale of the leakage zone of the main pipeline is larger than that of the northern branch pipeline. The drilling results also show that filling or exposed karst caves can be seen on the surface of the drilled core near the main pipeline and the northern branch pipeline, and there is an obvious "blowing phenomenon" after the formation of holes. There is no sediment filling in the karst cave with the northern branch pipeline above 40 m; however, there is an increase in sediment filling in the karst cave below this depth. Similarly, there is no sediment filling in the karst cave with the main pipeline above 50 m, while there is a notable increase in sediment filling in the karst cave below this depth. This suggests that the development depth of the underground river is below 40 m, and the drilling results are consistent with the geophysical findings. The karst leakage zone of the main pipeline exhibits a large leakage scale, characterized by a development width of about 30 m, a depth of about 120 m, and an elevation of about 1,290 m. The development location is identified between 940 m and 960 m along the first survey line and at the ZK08 borehole. In contrast, the karst leakage zone of the northern branch pipeline is smaller in scale, with its development situated along the second survey line at 785 m to 795 m, as well as at ZK04 and ZK05 boreholes. This zone has a development width of about 10 m, a depth of about 100 m, and an elevation of about 1,320 m. This study establishes a scientific basis for leakage prevention in the main blocked area of Yibasan reservoir, and offers a reference for the treatment of similar leakage issues in karst areas.

Impact of concealed karst caves on airport surface stability and treatment techniques

YU Qian, WANG Chengcheng, ZHANG Shasha, KOU Jingyuan, WANG Zhifeng

2024, 43(5): 1099-1109. doi: 10.11932/karst20240508

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If an airport is built in a karst area, a concealed karst cave will pose a challenge to the airport construction because of its concealment. The adverse geological effects in the site area of the third phase expansion project of Guangzhou Baiyun international airport are significantly pronounced. The roof thickness of 132 exposed karst caves ranges from 0.1 m to 5.2 m, the height varies from 0.7 m to 13.5 m, the span measures between 0.5 m and 11.7 m, and the buried depth ranges from 13.2 m to 32.7 m. Most of the karst caves exhibit no filling or poor filling properties. The groundwater at the site consists of loose rock pore water, with a water level depth of 4.38 m. The bedrock is mainly composed of soluble limestone. Additionally, the rock mass in the engineering site exhibits significant folding. Affected by the Guangcong Fault, many secondary faults are developed, and the brittle rock mass is easily broken. Consequently, the action of external forces such as foundation construction and road rolling likely cause the breakage of cave roofs and uneven settlement of ground, which is not conducive to the safe construction of airport pavement. Therefore, the stability evaluation and treatment technique of karst foundation are very important to the smooth completeness of this project.Based on the FLAC3D numerical simulation method, a 100 m×50 m×50 m three-dimensional calculation model was established to analyze the influence of different thickness-span ratios and high-span ratios of concealed karst caves on the stability of the airport site under the filling load of the roadbed. A comparative analysis was conducted to evaluate the displacement, plastic zone, and stress of typical karst caves before and after treatment, providing a comprehensive assessment of their stability. Finally, a field test was carried out with the combination of high-pressure pouring of low-grade concrete and sleeve valve pipe grouting. The treatment effects of filling karst caves with self-compacting soil, foam concrete, and low-grade concrete were compared and analyzed through various methods, including by core drilling, borehole wave velocity logging, geophysical prospecting tests, standard penetration tests, compressibility tests, and wave velocity profiling.The results show that when the thickness-span ratio and height-span ratio of a concealed karst cave decrease and the filling height of the subgrade increases, the displacement and stress of the karst cave show an increasing trend and tend to be unstable as a whole. Compared with the span of the cave, the roof thickness is relatively insufficient, and the bearing capacity is inadequate to bear the upper load. This presents potential risks of roof breakage and cave collapse. It can be concluded that the cave will significantly affect the stability of the airport. When the thickness-span ratio (K_H) is greater than 1 or the height-span ratio (K_G) is greater than 2 for the concealed karst cave, the whole cave tends to be stable, indicating a high level of its stability. After addressing the typical working conditions of karst caves through a combination of high-pressure perfusion of low-grade concrete and sleeve valve tube grouting, it has been found that the displacement of the roof of the karst cave following the treatment of low-grade concrete grouting is only 0.6 mm. This represents a reduction of 99.11% compared to conditions without treatment. The phenomenon of stress concentration is significantly reduced, resulting in enhanced foundation strength and effective grouting outcomes. Based on the comparative analysis of numerical calculation and field test, it is proposed that low-grade concrete should be preferentially used as filling material in the treatment of concealed karst caves in the third phase expansion project of Guangzhou Baiyun airport, followed by foam concrete. This study can provide reference for the selection of karst treatment measures in South China.

Dynamic evolution characteristics of ground collapse of covered karst based on particle flow

SHI Hai, JIA Zhilei, BAI Mingzhou, ZHANG Ye, SUN Zibing

2024, 43(5): 1110-1120. doi: 10.11932/karst20240509

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Karst collapse represents a significant geological hazard, predominantly occurring in regions characterized by the presence of soluble rock formations, including carbonate rocks, calcareous clastic rocks and salt rocks, among others. The characteristics of karst collapse fall into three key attributes: a hidden spatial distribution, a sudden occurrence and a periodic recurrence over time. These attributes can collectively challenge the construction of major infrastructure in karst areas. A karst collapse disaster along the railway will potentially pose a significant threat to the safe construction and continued operation of the high-speed railways. During both the construction and operational phases of railways in karst areas, pumping and discharging groundwater, along with altering hydrodynamic conditions, have been identified as key factors contributing to karst collapse. The most widely used numerical simulation method is the finite element method (FEM), which is based on the assumption of a continuous medium. FEM simplifies a complex problem by breaking it down into more manageable components. It conceptualizes the solution domain as a collection of small interconnected sub-domains called finite elements. For each element, a suitable (simpler) approximate solution is assumed, and the conditions necessary for solving the overall domain are derived. However, soil is not a continuous medium; therefore, a model based on FEM is unable to simulate the local instability of a collapsed soil body, the damage process at a mesoscopic scale, and other phenomena. In light of the dearth of sufficient attention to the temporal effects and fine-scale mechanisms of the karst collapse process in current studies, this paper aims to elucidate the dynamic evolution laws and mesoscopic-scale collapse mechanisms of the expansion of overlying karst soil caves around the railway.A typical karst collapse site, namely the Beijing–Shanghai high-speed railway (Jiangxi section), was selected as the basis for calibrating the strength parameters of the collapsed soil body. This was achieved through a particle flow (PFC^2D) compression test, in which a contact bonding model was also introduced. This model assumed that the filler in the cave was entirely washed away due to the erosive effect of groundwater seepage. Additionally, the vacuum suction and erosion effect inside the cave were not taken into consideration during the simulation period. The effect of groundwater on soil strength was simulated by reducing the contact strength between particles below the modelled water level. In conclusion, a coupled flow-solid model of overlying karst collapse has been established, which can elucidate the dynamic evolution process and deformation characteristics of karst collapse from a mesoscopic view. Furthermore, the influence of various sizes of cave openings, thicknesses of overburden layers and groundwater levels on the deformation of the overlying karst collapse has been investigated. The migration laws of soil particles under the influence of different factors have been analyzed.The study demonstrates that during the evolution of overlying karst collapse, the contact force between particles undergoes a series of changes, which can be described approximately as follows: equilibrium of stress–formation of stress arch–destruction of stress arch–equilibrium of stress again–…– fracture of stress arch. The internal stress of the soil body demonstrates the following pattern: compressive stress gradually decreasing, tensile stress gradually increasing, and tensile stress disappearing. Additionally, the surface subsidence and porosity of the soil body tend to increase in conjunction with the collapse evolution process. It can be observed that the larger the opening of the cave is, the greater the depth and range of surface subsidence become, which in turn increases the likelihood of collapse. A reduction in the thickness of an overburden layer may cause a more pronounced surface subsidence, thereby increasing the likelihood of collapse. Similarly, an elevated water level may contribute to a more rapid expansion of the soil cave, which in turn will cause a more pronounced surface subsidence and an increased propensity for collapse. The relationship between surface subsidence and thickness of overburden layer is not significant when the thickness of the latter is large. The study provides a comprehensive account of the karst collapse evolution from a mesoscopic perspective, offering insights into prevention and mitigation of karst collapse during the construction and operation of high-speed railways.

Response of 3D high-density electrical method to the interface of rock (soil) layer and karst collapse

ZHANG Jiantai, ZONG Chuanpan, LI Fuquan, CHENG Long, JIAO Yongxin, HU Ziyuan, YUAN Liwei, ZHANG Laicheng

2024, 43(5): 1121-1132. doi: 10.11932/karst2024y039

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Karst ground collapse occurs frequently in karst areas in China, which can easily lead to a series of hazards such as road deformation, house destruction and pipeline rupture. Karst collapse is uncertain, hidden and sudden, which poses a great threat to people's lives and property. Moreover, karst collapse is potential to facilitate the connection between surface water and groundwater, which may lead to the intermingling of these two water sources and subsequently contribute to groundwater contamination. Therefore, karst survey is of great significance to urban planning and disaster prevention and reduction. However, within the complex urban environment, accurately depicting karst morphology presents a significant challenge for geophysical exploration. Linyi City of Shandong Province, located in the west of the Tanlu Fault Zone, is one of the important temperate areas of karst development in the world. This city is especially characterized by soluble carbonate strata of Cambrian system and Ordovician system. The survey area is located in the monoclinic hydrogeological unit of Linyi, where karst fissure water is developed. In the past, the disorderly exploitation of groundwater led to frequent flowing of groundwater and many occurrences of karst collapse. In this study, we carried out 3D high-density electrical surveys in the study area, and laid 14 lines for high-density electrical survey at intervals of 12 m between each measuring line and 2 m between each measuring point. We also adopted the Wenner device, setting 25 as the maximum isolation coefficient. In this study, we firstly analyzed the physical properties of rocks in the study area, the results of which provided a physical basis for us to utilize the high-density electrical method for dividing stratigraphic boundaries and detecting karst. Subsequently, we established a geoelectric model and a 3D geoelectric structure of the study area to analyze the response characteristics of 3D high-density electrical method. Afterward, we selected some engineering exploration holes for verification and repeated deduction to improve the accuracy of simulation results. Taking P10 section as an example, we drilled holes at Point 127 for verification. According to geological records, a karst cave with a thickness of 4.1 m was found at the position of 24–28.1 m, which was basically consistent with the predicted results.The results show as follows. (1) Based on the combination of forward and backward numerical simulation, the inversion results are believed to be reliable. (2) The 3D high-density resistivity method can provide a substantial amount of data and rich geoelectric information. It clearly reflects the interface between Quaternary strata and limestone, and offers an intuitive visualization of karst caves, making it highly effective in engineering site selection. (3) The analysis indicates that the karst drawdown funnel in Linyi City shifted from Lihang–Hougangtou in 2003 to Qijiazhuang village in 2007, during which karst collapse occurred frequently. Since 2012 after the nearby karst groundwater well that was overly exploited was closed, no karst collapse has occurred again. However, the early karst development left behind some caves, so thorough surveying should be conducted during urban construction. In conclusion, in cases where there are significant differences in physical properties, the high-density resistivity method can effectively characterize the interface and karst features in detail. It can also encompass extensive geoelectric information. Moreover, the 3D visualization of inversion results can enhance the clarity of the findings. This approach provides a solid foundation for subsequent engineering construction.

Study on bearing characteristics of bridge piles and deformation mechanisms of karst caves in karst area

ZHAO Guiqing, ZHANG Jilu, ZHOU Xiaohan, LIU Xinrong, LIU Xinlin, GUO Hongqing

2024, 43(5): 1133-1143. doi: 10.11932/karst20240510

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Due to the highly undulating terrain, complex geological conditions, and the widespread distribution of karst topography developed with underground karst caves in Southwest China, the construction of bridges and tunnels will inevitably traverse areas that contain these karst caves. The karst effect changes the structure of rock masses, weakening the strength and increasing the permeability of the surrounding soil. In regions where caves are developed, the construction of pile foundations will not only encounter challenges in pile formation but also will experience impacts on the load-bearing capacity of foundations due to the presence of these caves. Prolonged loads can easily result in severe deformations, tilting, or even collapses of the superstructure. To elucidate the impact mechanisms of caves on the bearing characteristics of bridge piles during construction in karst regions, this study focused on viaduct piles from a highway project in Guizhou. Numerical simulations were employed to examine the settlement patterns of piles that completely traversed the cave, those that laterally intersected with the cave, and those that were adjacent to caves. Furthermore, the axial force distribution in the piles and the deformation characteristics of the caves were analyzed, along with the stability of the caves under various working conditions.The research findings indicate as follows, (1) Caves reduce the bearing capacity of bridge piles, although the degree of this effect varies by location. When bridge piles are situated near caves, the influence diminishes with increasing distance, becoming negligible when the horizontal distance exceeds 2D (where D represents the pile diameter). Conversely, when bridge piles pass through caves, their bearing capacity decreases as the burial depth of the caves increases. The maximum impact occurs when the cave is located directly beneath the rock-embedded section of the pile, resulting in a reduction of bearing capacity by approximately 16.5% to 17.5%. Engineering practices should give due consideration to caves near the pile tips. (2) The asymmetric distribution of caves contributes to differential settlement of the foundation. The shallower the cave or the closer it is to the bridge pile, the more significant the impact. Specifically, when the cave is located above the rock-embedded section of the pile or is less than 1D away from the bridge pile, it is necessary to implement reinforcement measures such as backfilling or grouting to prevent excessive differential settlement of the foundation. (3) For end-bearing piles, the influence of the cave on the side resistance of the pile mainly manifests as a loss of side resistance in the exposed section of the pile foundation. The impact of caves on end resistance is ranked as follows: piles fully traversing the cave>piles laterally crossing the cave>piles near the cave. (4) When bridge piles fully traverse the cave, the predominant failure mode of the cave is tensile failure of the roof. When bridge piles laterally cross the cave, shear failure primarily occurs near the pile foundation. When bridge piles are in close proximity to the cave, the potential failure mode is punching shear failure from the foundation. (5) When bridge piles cross the cave, it is crucial to consider the impact of the loads on the stability of the cave and to implement appropriate reinforcement measures. Additionally, when bridge piles are near the cave, caution must be exercised regarding the potential for localized instability of the cave if the distance is too short (<0.5D).

Mechanism analysis of karst ground collapse caused by the construction of punching piles in Fuwan, Foshan City

YI Shouyong, JIA Long, HAN Qingding, LUO Xiyi, ZOU Jie

2024, 43(5): 1144-1155. doi: 10.11932/karst20240511

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In recent years, the increasing intensity of human engineering activities—such as groundwater extraction, drainage for underground engineering, mine drainage, blasting, pile foundation construction and drilling engineering—has led to karst ground collapses. This phenomenon has become an important issue hindering the economic and social development of karst areas. If construction processes in these karst areas are not disposed properly, the punching pile construction commonly used in foundation construction can often cause severe ground collapse disasters. This article examines the karst ground collapse event triggered by the construction of punching piles in Fuwan, Foshan. Through field investigations and measurements, geological conditions contributing to the karst ground collapse were analyzed with drilling and geophysical exploration. A total of 37 boreholes were drilled in the vicinity the collapse, reaching a distance of 1,950 m. In around six of these boreholes (ZK6, ZK13, ZK15, ZK22, ZK18, and ZK31), a comprehensive exploration method was adopted to conduct three-dimensional infinite full-space exploration. This method utilized boreholes for deep underground exploration and high-precision detection of karst geological development within a 40-meter radiu. A total of 46 omnidirectional detection lines were laid out, covering a length of 2,990 m.Research has shown that the karst ground collapse group on Anhua road in Fuwan is primarily attributed to five factors: (1) The bedrock in this area is composed of high-purity gravel limestone, which is characterized by dense geological structures and numerous karst caves, creating conditions that are conducive to karst ground collapses. (2) The widely distributed muddy sand layers and residual soil layers at the bottom of the Quaternary system provide a favorable material foundation for the development of soil caves. (3) The sand layer is extentively distributed and exhibits a large thickness. Groundwater rescources are abundant and the hydrogeological conditions are complex. (4) The groundwater level is shallow and exhibits a strong hydraulic connection with the surface water. This area is located in a groundwater runoff channel, adjacent to the Xijiang river, and experiences frequent groundwater activities. (5) From the end of 2010 to January 2011, ground collapses occurred due to the vibration generated by punching pile construction.In addition, the formation of karst ground collapse group on Anhua road in Fuwan can be summarized in five stages: the fissure period, the karst cave period, the soil cave period, the collapse period, and the group collapse period. In the early stage, fissures formed in the bedrock as a result of tectonic action. Subsequently, under the combined influence of factors such as crustal uplift, meteorology, hydrology, structure, and marine transgression and regression, the small joint fissures in the bedrock were gradually eroded and expanded, leading to the formation of karst caves. These karst caves were then gradually developed toward the bedrock surface until the bedrock roof became exposed. This marked the beginning of the development period of soil cave. With the changing seasons and the dynamics of marine transgression and regression, groundwater levels fluctuated. The rise and fall of the groundwater erosion baseline were constantly circulating. When groundwater levels rose, soil that has remained dry for an extended time gradually softened, forming a soft layer. Conversely, as groundwater levels declined, soil lost its buoyancy due to the absence of water, resulting in a sharp increase in hydraulic gradient and an intensified erosive effect of pore water. Under the influence of vacuum negative pressure between pores and the erosion caused by groundwater, small particles within the sand layer were continuely displaced and transported to the underlying karst cave characterized by gravel limestone. Therefore, a soil hole was formed between the gravel limestone and the overlying sandy soil layer, and the hole developed and expanded in all directions. During the periods of collapse and group collapse, the construction of punching piles led to repeated fluctuations in the groundwater level. These fluctuations caused groundwater to flow to the surrounding areas through rock, soil and water, which damaged the original structural performance of the surrounding rock and soil, altered the original mechanical equilibrium limit, and triggered additional karst ground collapses in nearby areas.In summary, it is advisable to conduct a specialized karst exploration prior to the construction of punching piles in karst areas to identify the distribution characteristics of karst channels. At the same time, targeted construction plans should be developed. If karst channels are exposed during the construction process, it is essential to enhance the monitoring of karst groundwater to ensure the safety of both the engineering project and personnel involved.

Rapid assessment of water and mud inrush likelihood in karst tunnels based on decision tree model: A case in Siding tunnel of Guihe expressway

GUO Baode, ZHANG Yi, WANG Xinwen, CHEN Zhigao, DONG Zhiming, MENG Yan

2024, 43(5): 1156-1165. doi: 10.11932/karst20240512

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With the ongoing advancement of infrastructure projects, such as highways and railways, in China, karst tunnel construction is increasingly evolving towards longer, larger, and deeper dimensions. The problem of water and mud inrush in tunnels is becoming more and more severe. Effective measures for the advance prediction of water and mud inrush in tunnels have consistently been a pressing convern for management departments and engineering construction units. This issue also remains a significant challenge in related research fields. It is essential for us to assess the likelihood of tunnel water and mud inrush when determining whether to carry out advance prediction and identifying the appropriate mileage segment for conducting the prediction. This assessment is vital for targeted interventions, minimizing construction costs, and enhancing prediction accuracy. To some extent, the assessment of the likelihood of water inrush in tunnels falls under the category of advance prediction and serves as the foundational element for conducting such prediction.The rapid assessment of the likelihood of water inrush in karst tunnels is an important basis for deciding whether to carry out advance prediction and to take measures for emergency treatment. This rapid assessment was often absent in previous work. Even though assessments for the likelihood of water inrush were conducted occasionally in several tunnels, they were primarily based on qualitative methods of geological analysis. This often resulted in discrepancies from actual conditions, leading to misjudgments. In some tunnels, significant manpower and resources were sometimes expended on predicting water inrush, advance prediction, and tunnel waterproofing design, yet no water inrush occurred. Conversely, the sections of some tunnels that were neither predicted nor adequately protected experienced severe water and mud inrush. Therefore, the accuracy of assessments regarding the likelihood of water inrush in tunnels is crucial for determining whether to undertake subsequent prediction and prevention efforts. This study explored Siding tunnel on Guihe expressway as a case study. Utilizing karst hydrogeological surveys and research, in conjunction with geophysical exploration and drilling data, the study started with the degree of karst development and the relationship between the tunnel and karst groundwater, both of which are closely associated with water and mud inrush in the tunnel. Ten representative parameters for the decision tree model were identified and quantified, including karst rates in the transverse and longitudinal sections of the tunnel, as well as the vertical zonation relationship between the tunnel and groundwater. Based on this analysis, a rapid assessment method, along with indicator parameters and a predictive model for assessing the likelihood of water and mud inrush in the karst tunnel, was developed and practically validated. The results indicate that the highest probability of water inrush in each mileage of Siding tunnel is 88.1%, suggesting a very high probability of large-scale water and mud inrush. Notably, during the actual construction process, the occurrence of water and mud inrush was observed in the segment with the highest predicted likelihood.Using a decision tree model to analyze and assess the likelihood of water and mud inrush in the tunnel represents a valuable exploratory study. The decision tree model offers the advantage of selecting the optimal solution among multiple complex options, effectively mitigating errors arising from human experience-based judgments. This approach enhances the scientific rigor geological analysis related to assessments of water and mud inrush in tunnels. According to the principles of the decision tree method, this model can quickly assess both the likelihood and magnitude of water inrush based on various critical values for water inrush sizes and the probabilities associated with different branches. Furthermore, it can determine the probability of a specific scale of water inrush based on the rapid assessment of the likelihood of water inrush. In the future, it will be essential to refine the indicator parameters of scheme branches, state branches, and probability branches on different scenarios of water and mud inrush in tunnels. This refinement will provide targeted guidance for advance predictions in similar projects.

Experiment on monitoring and early warning of karst collapses based on acoustic emission technology

PAN Zongyuan, DAI Jianling, WEN Rihai, MENG Yan, JIANG Xiaozhen, MA Xiao, BAI Bing, WU Yuanbin, ZHANG Xin

2024, 43(5): 1166-1178. doi: 10.11932/karst2024y020

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The karst collapse is a dynamic geological process in which the damage of soil mass and development of cavity ultimately result in the instability of the overburden layer. Therefore, identifying the characteristics and evolution of the damage of soil mass are the important prerequisite to develop effective monitoring and early warning methods for karst collapses. However, the commonly used early warning methods that utilize hydrodynamic conditions and optical fiber monitoring technique often overlook both the process of microscopic damage of the soil mass and the effect of damage on karst collapses. Previous studies on karst collapses induced by groundwater or rainfall typically employ qualitative or semi-quantitative methods. These approaches appear to be one of the reasons why karst collapses—major and prevalent geological disasters in karst regions—have seen limited advancement in monitoring and early warning system. In this study, acoustic emission (AE) and fiber grating technology have been firstly applied to conduct model tests on monitoring and early warning methods of karst collapses. The research on dynamic characteristics of AE has been conducted under different rainfall conditions. In addition, the characteristics of key time domains and frequency domains of AE have been identified and selected to establish a spatial-temporal responding mechanism between AE and karst collapses through model experiment.The results show that as follows, (1) The total ringing count, amplitude, and energy of AE in the shallow overburden layer ranged from 1 times to 348 times, 30.5 dB to 175.9 dB, and 0 PJ to 57×10⁻³, respectively, under heavy rainfall conditions. In contrast, the same parameters in deep overburden layer ranged from 1 times to 2,361 times, 30.5 dB to 179 dB, and 0 PJ to 946.4×10⁻³ PJ, respectively. In addition, under rainstorm conditions, these parameters in shallow overburden layer ranged from 1 times to 89 times, 30.5 dB to 140.9 dB, and 0 PJ to 22×10⁻³ PJ, respectively, while in deep overburden layer, they ranged between 1 times and 1,322 times, 30.5 dB and 213.1 dB, 0 PJ and 4,694.6×10⁻³ PJ, respectively. During the formation of karst collapses under heavy rain conditions, the ringing count of AE in the deep overburden layer increased by 6.78 times to 6.89 times compared to the shallow layer. Additionally, the amplitude of AE in deep overburden layer increased by 1.02 times to 1.12 times compared to the shallow layer. The energy of AE in deep overburden layer increased by 4.45 times to 16.6 times compared to the shallow layer. During the formation of karst collapses under rainstorm conditions, the ringing count, amplitude and energy of acoustic emissions in deep overburden layer increased by 14.85 times, 1.51 times and 213.39 times, respectively, compared to the shallow layer. (2) Under heavy rain conditions, the karst collapses were defined as creep-failure collapses which were caused by the expansion and instability of soil caves. But the energy emitted by the final collapse of the soil cave was larger than the energy emitted by creep-failure collapse, which indicated higher activity of AE signals. Under rainstorm conditions, the karst collapse was defined as compression-shear fracture collapse caused by the whole fracture and collapse of soil mass. In this process, the strain energy in the soil layer was completely released in a short time. Hence, the AE signals showed a sharp increase in dynamic curve in the second rainfall. This result indicated a strong relationship between AE signal characteristics and types of collapse. (3) There are four types of signal waveforms in the formation of karst collapse, which can be divided as slippage of soil mass, dislocation of soil layer, development of cavity and layer collapse. The irregular up-down fluctuation of signals represented the slippage of soil mass, while concave triangle signals indicated that rainfall dislocated stability of soil layer. Additionally, wedge-shaped signals indicated that a tiny cave was developed in the overburden layer. The signals which were showed as combined form of equilateral triangle and concave triangle implied the occurrence of karst collapse. Therefore, the signal energy, rising and falling time and duration of signal waveforms were closely related to soil deformation in the process of karst collapse. (4) In the formation of karst collapse, the spectrum signal waveforms of AE fell into high frequency narrow pulse form, in which spectrum energy of the four signal types were concentrated in the high frequency range of about 50 kHz and 20 kHz, respectively. (5) The cumulative ringing counts of AE were closely related to pore water pressure, soil pressure and displacement of soil mass. The ringing counts of AE increased in sudden during soil deformation or collapse. This observation indicated that AE technology is feasible for monitoring and early warning of karst collapses.

Early warning of settlement and deformation of high-rise buildings on soft foundations in karst areas

MA Lin

2024, 43(5): 1179-1186. doi: 10.11932/karst20240513

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The background of this article pertains to a high-rise building project, which encompasses a building land area of about 4,630 m² and features a roughly square shape in its plane. The safety classification of the building structure is designated as Level 2, while the foundation design is categorized as Level A. The foundation form is raft foundation. This building project is of high engineering importance. According to the survey results, underlying strata of the building foundation are mainly composed of clay and limestone layers, each possessing distinct engineering properties that are generally unfavorable. In addition, the average annual rainfall in the project area is 1,107.27 mm, and the dissolution effect of the bedrock will continue. Even worse, the building has already undergone unfavorable deformation. Therefore, to ensure the safe operation of the building, it is necessary to conduct research on early warning of its settlement and deformation.In this study, a total of three types of warning criteria were constructed, namely, stability criterion, ultimate deformation criterion, and deformation rate criterion. Among them, the implementation of stability criterion involves sharp point mutation analysis. This technique derived mutation characteristic values from sharp point mutation analysis of building settlement data and used these values to assess the stable state of building settlement. The ultimate deformation criterion was calculated with the use of the ultimate displacement formula to obtain the ultimate deformation value. This value was then combined with existing monitoring data to construct corresponding criterion indicators. The implementation of the deformation rate criterion was conducted with the rescaled range method, which calculated the Hurst index through rescaled range analysis of building settlement data. This analysis evaluated the development trend of building settlement and deformation. Giving the differing emphases of the three criteria, the early warning level of building settlement and deformation, determined based on principle of unfavorable conditions, is deemed reasonable and accurate through comparative analysis.According to the analysis results, there is a significant difference in settlement and deformation among the monitoring points of the building, with a range of 22.76−56.27 mm, indicating notable settlement characteristics. The primary causes of this settlement are the soil and groundwater conditions of foundation. In the process of early warning of building settlement and deformation, the analysis mainly focuses on monitoring points JC3−JC5. The stability criterion shows that the △ values of the three monitoring points are all greater than 0, indicating that they are all in a stable state, and their corresponding early warning levels of settlement and deformation fall into Levels I−III. The range of evaluation indicators for ultimate deformation criterion is from 0.61 to 0.74, corresponding to the early warning Levels II−III of settlement and deformation, that is, the early warning level of JC5 monitoring point falls into Level II, and the other two monitoring points are Level III. The deformation rate criterion shows that the Hurst index values of the three monitoring points are all less than 0.5, indicating a decreasing trend in the rate of building settlement and deformation. The trend level of JC3 monitoring point is Level 2, while that of the other two monitoring points is Level 1. By comparison, based on the principle of unfavorable conditions, the early warning level of the three monitoring points is determined to be Level III. Therefore, the degree of settlement hazard for this building is moderate. It is necessary to not only increase the monitoring frequency to observe subsequent changes but also to take reinforcement measures.Through this study, a novel approach has been developed for the early warning of building settlement and deformation. This method offers a theoretical basis for ensuring the safe operation of buildings in later stages and holds significant practical implications.

Design of foundation and karst treatment for a residential project in a karst development area

WEN Yage, LIU Jinshuan, YIN Zepeng, SHENG Jingyu

2024, 43(5): 1187-1195. doi: 10.11932/karst20240514

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With the rapid advancement of construction in China, the development of various buildings and structures in karst regions has been increasing. However, the challenges posed by karst geology cannot be overlooked, as it can lead to a series of serious engineering issues such as uneven foundation settlement, foundation collapse, and water leakage in basements. In extreme cases, karst may even result in the loss of foundation stability and structural damage. To effectively address these challenges, this study focused on a residential project in a karst development area within Zhejiang Province and conducted an in-depth discussion on the foundation design and karst treatment techniques of this project. The project comprises 8 high-rise residential buildings with 14 to 18 floors, a 2–3 story kindergarten, and single-story ancillary buildings, along with a one-level basement. Pile foundations were adopted for the main building sections, while a raft foundation was used for the single-level basement area. Considering the buoyancy issues in non-building areas of the basement, anti-floating anchors were employed, and the overall foundation design was rated as Class A. The geological investigation report revealed that the bedrock is soluble, with a cave occurrence rate of 16.6%, mostly in the form of point-like caves. Based on the relevant information above, this study elaborately discussed the foundation design schemes for different parts of the buildings and the basement. Based on the analysis of the base reaction forces under the buildings and the combination of engineering experience with geological conditions, bored piles with diameters of 800 mm and 1,000 mm were selected for the foundations beneath the buildings. After the solution of anti-floating pile was compared with the solution of rock anchor, anti-floating anchors for the conventional basement were planned to be used. For the construction of kindergarten, which required both load-bearing capacity and the ability to span caves, piles with the diameter of 600 mm were chosen as the anti-floating measure. Targeted research on treatment technologies for various types of caves was also conducted. Different karst treatment plans were formulated according to the height of the cave, whether it is ≤4 m for pile foundations or anchor traversals, or >4 m. This study provided construction considerations for cave treatment, including the adverse effects of caves on pile construction, the determination of the bearing stratum for engineering piles, and key matters such as pile testing. To ensure reliable karst treatment, quality inspections were equally important. These inspections confirmed that the cave treatment, integrity, and bearing capacity of the pile positions treated according to the principles outlined in this paper meet the code requirements, which can also validate the rationality of the results presented herein.Ultimately, this study has developed a comprehensive foundation design and karst treatment implementation plan for the project. Currently, there is a lack of discussion on the design of large-scale basements with flat rafts plus anti-floating anchors, and residential buildings with flat rafts plus bored piles in karst-developed areas. This study can provide certain reference for the design of related projects.

Geological conditions and geochemical characteristics of favorable shale gas zones in Danzhai of the Qiannan depression

LIU Haojie, ZHANG Qingyu, BA Junjie, JI Shaocong, NIE Guoquan, LI Zhen

2024, 43(5): 1196-1209. doi: 10.11932/karst20240515

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The Qiannan depression is located on the southwestern edge of the Yangtze block and is an important area of shale gas resources in South China. In recent years, significant progress has been made in shale gas exploration in this region. The shale in Cambrian Niutitang formation of the Qiannan depression is widely distributed, characterized by thick layers and abundant organic matter. Shale is developed in the mature stage and possesses significant potential for shale gas exploration and development. Although some progress has been made in the studies on shale in the Qiannan region, there is still a relative lack of in-depth research on the geological conditions, soluble organic matter, and reservoir formation mechanisms of the favorable zones. By measuring and collecting samples from typical profiles and survey wells of shale gas, this study systematically analyzed the geological conditions and organic geochemical characteristics of the favorable zones in Danzhai of the Qiannan depression. It also explored the distribution and development patterns of high-quality hydrocarbon source rocks in the Cambrian Niutitang formation.The results indicate that the Cambrian Niutitang formation in Danzhai is abundant in organic shale with a thickness of about 60 meters to 200 meters, showing an overall trend of gradually thickening from northwest to southeast. The total organic carbon content (TOC) of the Niutitang formation mainly ranges from 4.5% to 6.0%, while the organic matter maturity (Ro) predominantly varies from 2.5% to 2.7%, gradually decreasing from the northeast to the southwest. The mineral composition of shale in the Niutitang formation primarily consists of quartz, followed by clay minerals, and a small amount of carbonate minerals. The types of shale reservoir spaces include organic pores, inorganic pores, and microfractures, among which inorganic pores primarily consists of intergranular pores, intragranular pores, dissolution pores, etc. The shale matrix typically exhibits low porosity and permeability, categorizing the study area as a low porosity and low permeability shale reservoir. However, the development of local fractures can enhance both the porosity and permeability of shale. The tectonic activity has significantly influenced the preservation conditions of the strata and the degree of shale gas enrichment. Tectonic preservation continues to be a key factor affecting the shale gas enrichment of the Niutitang formation in Danzhai of the Qiannan depression. Due to relatively unfavorable enrichment conditions and the complex preservation of structures, the large-scale industrial development of shale gas in the Cambrian Niutitang formation faces significant challenges. However, the geological conditions of the shale in the Cambrian Niutitang formation in Danzhai are relatively favorable, indicating potential for further exploration of shale gas.

Development characteristics and genetic mechanism of carbonate minerals in shale member of the lower Carboniferous Luzhai formation in the depression of central Guangxi

DAN Yong, DENG Min, ZHANG Qingyu, YAN Jianfei, NIE Guoquan, DONG Hongqi, JI Shaocong, XIONG Guoqing, LU Bingxiong, MA Xiaolin

2024, 43(5): 1210-1222. doi: 10.11932/karst20240516

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The Carboniferous was significant for the development of "black strata" in the world. The exploration and development of shale gas in North America first began with the Carboniferous marine shale, and the Carboniferous (Mississippian) Barnett Shale has become one of the most important strata of shale gas production in the world. The distribution of Carboniferous marine shale in China is relatively limited, primarily developed in the Qian–Gui rift basin at the edge of the Yangtze plate. It is roughly distributed in depressions such as central Guangxi–Nanpanjiang in Guangxi, south Guizhou–southwest Guizhou, and central Hunan. In recent years, drilling activities in the Carboniferous depressions, such as Guirongye Well 1 and Qianziye Well 1 have yielded promising shale gas displays. In 2023, Guirongye Well 2 achieved industrial gas flow from the Carboniferous after the compression fracture was completed, indicating favorable exploration prospects. The Carboniferous strata in the Guizhou–Guangxi region are anticipated to become the next significant exploration target in China, following the shale gas of Wufeng–Longmaxi formation. However, drilling in this area revealed a rapid phase transition in the early Carboniferous, unstable distribution of shale, high levels of carbonate minerals within the shale, and the presence of common carbonate rock interlayers. On-site experiments on water invasion and gas logging suggest that shale gas production is influenced by carbonate minerals in the shale. The origins of these carbonate minerals in the shale of this region and their impact on gas production remain unclear.To this end, the Luzhai formation shale of the early Carboniferous in the depression of central Guangxi was selected to analyze the origin of carbonate minerals within the shale. This study aims to provide theoretical reference for predicting distribution of various types of carbonate minerals in the subsequent shale formation, as well as for gas content analysis and the optimization of the lower Carboniferous shale gas "sweet spot" in the depression of central Guangxi. The study area is situated in the Liucheng slope in the northern depression of central Guangxi. Field profiles and core samples from Rongye Well 1 were utilized as the research objects. Following the identification of carbonate mineral types, geochemical analyses of cathodoluminescence and carbon and oxygen isotopes were conducted. This was achieved through a comprehensive approach including X-ray whole-rock diffraction, thin section identification, and other analytical tests.Research findings indicate as follows, (1) The study found that the carbonate minerals in the Luzhai formation shale are primarily composed of calcite, with a content ranging from 5% to 80%, averaging at 30.33%. Additionally, there is a small amount of dolomite, varing from 0% to 9%, with an average of 2.1%. Carbonate minerals are mainly developed in carbonaceous shale, calcareous mudstone, silty calcareous mudstone, bioclastic carbonaceous mudstone, mud-microcrystalline limestone, and muddy limestone. They are also present in calcite veins distributed along the layers and in structural calcite veins that traverse the layers. (2) Cathodoluminescence analysis shows that massive calcareous mudstone and horizontally laminated micrite carbonate minerals are generally non-luminescent. Some carbonate minerals in silty calcareous mudstone emit weak red light, while others do not. The luminescent carbonate minerals include calcareous cements and certain calcareous bioclastics. The "floating" carbonate bioclastics found in carbonaceous shale typically emit weak red light. Lens-shaped and vein-shaped calcite exhibits weak cathode luminescence. (3) Analyses of carbon and oxygen isotopes show that the δ¹³C values of native carbonate minerals range from 2.70‰ to 5.81‰, with an average of 4.68‰. The δ¹⁸O‰ values range from –8.89‰ to –5.97‰, with an average of –7.54‰. For allochthonous carbonate minerals, the δ¹³C values range from 2.62‰ to 4.39‰, with an average of 3.30‰, and the δ¹⁸O‰ values range from –8.89‰ to –7.67‰, with an average of –8.04‰. The δ¹³C‰ values of diagenetic vein-shaped carbonate minerals range from –1.18‰ to 5.73‰, with an average of 2.78‰, while the δ¹⁸O‰ values range from –12.76‰ to –5.75‰, with an average of –9.77‰.Comprehensive exploration shows that the formation of carbonate minerals primarily results from normal sedimentation in seawater, followed by long-distance transport in seawater after being fragmented by waves and storm currents, and recrystallization during diagenesis. The formation of horizontal vein-shaped calcite may be attributed to supersaturated precipitation within differential compaction and contraction fractures filled by formation fluids during burial, while the vertical vein-shaped calcite is associated with multiple tectonic activities. Based on these findings, various models of carbonate mineral genesis have been established, providing theoretical reference for the optimal selection of sweet spots for shale gas in the lower Carboniferous depression of central Guangxi.

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2024, Volume 43, Issue 5

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Current Issue 2024, Volume 43, Issue 5

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Submission Guide

Current Issue
2024, Volume 43, Issue 5