| In the Three Gorges Reservoir area of the Yangtze River,multiple large-scale landslides,including the Huangtupo landslide,exhibit complex seepage-consolidation coupling systems under dynamic hydrological boundary conditions such as regional groundwater recharge,seasonal precipitation,and periodic reservoir level changes.These conditions often pose difficulties in accurate landslide deformation analysis and stability assessment.The formation and evolution of landslides are significantly constrained by hydrogeological conditions;thus,understanding the recharge sources,infiltration processes,and characteristics of landslide groundwater is crucial for landslide stability research and mitigation.In this study,we focus on the Huangtupo landslide in the Three Gorges Reservoir area,combining geological investigation,hydrogeochemical monitoring,and numerical simulation.Using the large-scale Badong landslide field experimental site constructed on the No.1 Riverside Sliding Mass of Huangtupo landslide,we investigate the hydrogeological conditions of the No.1 Riverside landslide and its hydraulic connection with the regional hydrological system to reveal the migration mechanism of landslide groundwater and the impact of groundwater recharge infiltration characteristics on landslide stability.In early studies of the Huangtupo landslide,hydrogeological conditions were primarily inferred and analyzed based on traditional surface geological surveys and borehole water level monitoring.Groundwater was generally believed to originate mainly from local atmospheric precipitation in the landslide area.Although the slope above the landslide contains limestone rich in karst water,previous research suggested that karst water in the mountain body(T1j)could not enter the third section of the Badong Formation(T2b3)due to the existence of siliceous sandstone and mudstone barriers in the second section of the Badong Formation.However,during the excavation of the experimental tunnel group at the Badong large-scale landslide field experimental site,groundwater in some tunnel sections exhibited continuous and stable large-scale recharge characteristics during the dry season without precipitation,inconsistent with local rainfall recharge volume and speed.This observation implies that,besides local precipitation,there exist stable and abundant alternative water sources for the landslide.This phenomenon is significantly different from previous understandings of the traditional hydrological boundary and recharge mechanism for such landslides.To more accurately assess landslide stability and develop effective mitigation measures,revealing the source and recharge mechanism of groundwater in the Huangtupo landslide is an urgent scientific issue to be addressed.This study focuses on the hydrogeological issues of the Huangtupo I landslide and its hydraulic connection with the regional hydrological system.We comprehensively applied major ions,trace elements,and stable isotopes of landslide groundwater as natural tracers to reveal the recharge sources and mechanisms of landslide groundwater,and further investigated the impact of groundwater recharge infiltration characteristics on landslide stability.The main research contents and results of this thesis are as follows:Ⅰ.Through the investigation of the regional geological environment background,we established a regional geological model,explored the applicability of defining the hydrogeological boundary of the landslide area based on lithology and structure,and studied the geological basis for the potential hydraulic connection between the karst water in the upper part of the slope and the landslide groundwater.Ⅱ.Through the study of the engineering geological and hydrogeological conditions of the Huangtupo I landslide,we revealed the anisotropic spatial characteristics of the landslide’s soil and rock mass,structural surfaces,and sliding surfaces.The groundwater flow in the landslide is controlled by the structural surfaces formed by tectonics and chemical weathering,exhibiting characteristics of conduit flow and vein-like flow.The seepage field shows strong heterogeneity and anisotropy,presenting preferential flow characteristics.A three-dimensional geological spatial structure model of the landslide was established,laying the foundation for subsequent research on the landslide groundwater recharge mechanism.Ⅲ.By studying the hydrochemical composition of groundwater in the landslide area and combining the analysis of the mineral composition of the rock and soil mass in the work area and the simulation of the water-rock interaction process,we revealed the recharge sources and pathways of the landslide groundwater.The research results show that the hydrochemical composition characteristics of the landslide groundwater indicate that,in addition to the recharge from local precipitation,the landslide groundwater is also subject to lateral recharge from the karst water in the upper part of the slope,and before the completion of the secondary relocation of the resettlement village in the Huangtupo landslide area,the landslide groundwater was also recharged by the infiltration of domestic sewage and irrigation water from the residents.Ⅵ.By studying the characteristics of hydrogen and oxygen stable isotopes in atmospheric precipitation,groundwater,and surface water in the work area,and combining the reference analysis of the stable isotope database of atmospheric precipitation in the surrounding area,we revealed the recharge elevation and mixing ratio of the landslide groundwater,and established a seepage conceptual model reflecting the recharge mechanism of the landslide groundwater.The research results show that the landslide groundwater mainly includes three recharge sources.The main source of groundwater is the karst water from the slope mountain behind the landslide,with a contribution ratio of 60% to 94%.The karst water from the Jialingjiang Formation limestone mainly recharges from the slope direction to the lateral rear.The karst water flows through a well-developed network of groundwater seepage,composed of joints,cleavage planes,faults,and shear fracture zones under the influence of regional tectonic stress and chemical weathering.After passing through the overlying strata of the Badong Formation,it reaches the rear of the landslide and forms a stable high water level.The second source is the infiltration of local precipitation in the landslide area.Under extreme meteorological conditions,concentrated and sustained rainfall causes a large amount of rainwater infiltration,which will raise the groundwater level of the landslide mass.Moreover,before the completion of the secondary relocation of the resettlement village in the Huangtupo landslide area,the direct discharge of domestic sewage and the infiltration of irrigation water from the residents living on the landslide were also important sources of recharge for the landslide groundwater.The higher concentration of ammonia nitrogen in the water samples collected from the tunnel of the large-scale outdoor experimental site of the Huangtupo landslide indicates that domestic sewage indeed infiltrates into the rock and soil mass of the landslide.Ⅴ.By utilizing the variation characteristics of three groundwater chemical indicators,hydrogen and oxygen stable isotopes,trace element strontium,and groundwater mineral saturation index,on the temporal and spatial scale,we revealed the distribution characteristics of the dominant seepage channels in the landslide.By establishing a landslide numerical model considering the distribution characteristics of the dominant seepage channels,we carried out numerical simulation analysis to study the changes in the seepage field of the slope before and after rainfall,and the impact of direct discharge of domestic sewage from village residents on the landslide seepage field.Based on the seepage analysis data,we discussed the evolution law of slope stability before and after rainfall and before and after artificial sewage discharge and further studied the mechanism of the reactivation of the ancient landslide induced by the construction of resettlement villages on the ancient landslide.The research results show that:The variation characteristics of water chemical indicators on the temporal and spatial scale reveal that the dominant seepage channels in the landslide are mainly distributed along the sliding rupture surface.The continuous creep of the Huangtupo landslide leads to the development and enhancement of discontinuous surfaces such as cracks in the sliding surface area under mechanical action,thereby increasing the size,extension length,and quantity of these structural surfaces,as well as their connectivity.They have a more positive impact on the infiltration and transmission of rainfall.The water level monitoring data of the landslide mass and sliding bed strongly support the above conclusions.Numerical simulation analysis shows that the heterogeneous seepage field of the landslide responds unevenly in time and space to hydrological events such as rainfall and domestic sewage discharge on the slope,causing a significant decrease in the safety factor of the landslide.In particular,the concentrated and long-term discharge of domestic sewage from village residents into the landslide mass greatly weakens the safety factor of the landslide.Simulating the evolution of the landslide stability coefficient under the condition of domestic sewage discharge for one year,it is found that the stability coefficient of the landslide reaches the lowest on the98 th day,as low as 1.01,close to the critical state,and is extremely easy to be triggered.Subsequently,after the redistribution and adjustment of pore water pressure,the stability coefficient is maintained at 1.02-1.03,and the safety is still not optimistic,requiring urgent improvement.Combining the deformation development history of Huangtupo landslide,the mechanism of the reactivation of the ancient Huangtupo landslide is revealed: The landslide disaster records in the Huangtupo area show that after the construction of Huangtupo village and town and before the impoundment of the Three Gorges Reservoir,the Huangtupo landslide had already experienced deformation instability and local sliding disasters after rainfall.This indicates that human activities are the key factors for landslide instability and deformation,while rainfall serves as a triggering factor.The Huangtupo landslide is continuously and stably recharged by the karst water from the rear,resulting in a generally high normal groundwater level,which is unfavorable for landslide stability.The Huangtupo landslide’s riverbank collapse deposits have undergone multiple compressions and transformations,making the material structure relatively dense and permeability weak.Long-term deformation and multiple periods of sliding have led to the development of numerous cracks and fissures,which are interconnected to form dominant seepage channels,providing a material basis for the heterogeneous seepage field of groundwater.After a large number of residents moved in,the amount of domestic wastewater discharged in the Huangtupo area increased significantly.Due to the lack of protective measures in the channels,the concentrated discharge of domestic sewage infiltrated massively along the dominant seepage channels,leading to a significant uneven response in the landslide groundwater seepage field,reducing the overall safety factor of the landslide and inducing landslide deformation.The deformation further expands the crack size inside the landslide,enhances the connection and penetration between seepage channels,and increases the infiltration capacity and speed of local rainfall in the landslide.The precipitation in Badong County is large and has a distinct seasonality.Under extreme weather conditions,concentrated and continuous rainfall causes a large amount of rainwater infiltration,inducing local instability and even sliding of the landslide.The innovative aspects of this study are primarily reflected in the following three aspects:Ⅰ.The application of hydrogeochemical monitoring methods to large-scale landslide deposits,combined with geological surveys and numerical simulation techniques,has established a conceptual seepage model that reflects the groundwater recharge mechanism of Huangtupo landslides.This research has revealed the groundwater supply mechanism of landslides and proposed a novel approach for studying groundwater recharge mechanisms in landslides.Ⅱ.A method for determining the source of groundwater in landslides based on hydrochemical information has been proposed,elucidating the seepage characteristics of groundwater recharge in landslides.This method provides theoretical support for optimizing landslide drainage and prevention engineering.Ⅲ.Based on the spatial variation characteristics of groundwater chemistry indicators,this study has revealed the mechanisms through which domestic sewage and irrigation water supply affect the stability of ancient landslide-affected towns.This finding provides valuable insights for the prevention and control of similar water-related landslide disasters.For large-scale landslide deposits characterized by long-term creep in a complex geological background,this research undoubtedly represents a highly worthwhile direction for exploration.It is necessary to harness the potential of hydrogeochemical methods and delve into the value of hydrogeochemical information in landslide research,in order to provide more comprehensive and effective scientific basis for the prevention and control of large-scale landslide geological disasters with similar geological backgrounds. |
