| Jilin Dabusu National Nature Reserve(Jilin Qian ’an Mud Forest National Geopark)is located in Qian ’an County,Songyuan City,Jilin Province.There is the only “mud forest” sub-erosion geological landform group protected by the state at present.Mud forest is a peculiar geological landscape,but it is also a geohazard phenomenon.The formation and development of mud forest landform is the result of surface erosion and sub-erosion.Surface erosion has caused large-scale soil and water loss and destroyed mud forest landscape,while sub-erosion is the main cause of geomorphic evolution and slope dislocation hazard in the mud forest reserve,it also has posed a threat to roads,farmland,and residential buildings.Therefore,it is of great significance to protect the geological landscape and human settlements of mud forest reserve by strengthening the research on the sub-erosion landform,identifying the failure type of mud forest slope,carrying out in-depth research on the mechanism of sub-erosion,and taking effective governance measures timely.In this paper,the Jilin Qian ’an Mud Forest National Geopark is taken as the research object.According to the evolution stage of mud forest landscape,the mud forest was divided into formation area,mature area and extinction area.The mud forest landscape in the mature area was mainly affected by surface erosion,the soil was continuously lost and gradually went to extinction.Considering the characteristics of seasonal freeze-thaw of soil in the mud forest reserve,the erosion mechanism of loesslike soil affected by freeze-thaw action in the mud forest reserve was revealed from the perspective of soil erodibility.In the mud forest formation area,the main manifestation is dislocation disasters dominated by sub-erosion.Two main failure types of slope were proposed: cave piping failure and sub-erosion dislocation failure.The variation rules of porosity and concentration of liquefied particles during the sub-erosion process of slopes in mud forest area were analyzed based on numerical simulation.The loess-like soil loss prediction model was obtained based on laboratory tests and theoretical analysis.Finally,a scheme based on the principle of filtered siphon drainage was proposed to treat the control the sub-erosion dislocation hazard of slopes in mud forest.1.The formation and evolution of the mud forest is controlled by many factors:the shore of Dabu Su Lake is high in the east and low in the west,the hydrodynamics will be generated when the water converges into Dabusu Lake;the loess-like soil of mud forest is dominated by weakly hydrophilic powder particles with high porosity,and the clay minerals are dominated by illite and montmorillonite mixed layer,with strong dispersion;the climate,topography,and other factors also contribute to the formation of the evolution of the mud forest.Larger hydraulic gradient is the most important external factor for the formation of mud forest,while the special composition and nature of mud forest loess-like soil is the internal factor for the formation of mud forest.According to the characteristics of sub-erosion in the mud forest reserve,it is divided into formation area,mature area and extinction area,which shows the characteristics of headward erosion.The common erosion and sub-erosion landforms in the study area were classified and analyzed,and the evolutionary process of the formation of sinkhole,soil bridge,and soil column were analyzed.2.Surface erosion resulted in the reduction of mud forest landscape and soil and water loss.From the perspective of soil erodibility,shear properties,disintegration and microstructure of loess-like soil were selected as erodibility indexes,and different freeze-thaw cycles(0,1,3,6,10,15)were set as test variables.The unfreeze-thaw sample had the highest shear strength,the slowest disintegration rate,and the most complete structure.After 15 freeze-thaw cycles,the soil sample shear strength decreased by 7.1% to 20.9%,and the disintegration rate of the soil sample increased from 1.52 g/s to 2.51 g/s.The erodibility of loess-like soil under freeze-thaw action was evaluated by the improved mutation progression method.It was concluded that the process of loess-like soil from the first cycle to the third cycle had a great influence on the soil erodibility.After freeze-thaw,frost heave leads to cracks within soil particles and between structural units.After several freeze-thaw actions,cracks expand continuously,large aggregates and particles break into smaller structural units,and soil particles rearrange under the action of gravity to form a relatively stable structure.However,the connection between the reconstituted structural units is poor,so it is more vulnerable to external erosion,thus promoting soil erosion and accelerating the landscape degradation in mature areas of mud forest.3.According to the failure mechanism,the failure modes of soil slope in the mud forest formation area were divided into cave piping failure and sub-erosion dislocation failure.In order to deeply analyze the disaster-causing mechanism of the vertical dislocation failure of mud forest loess-like soil slopes,the multiphase coupled suberosion control equations were based on the continuum medium theory and continuity equations,and solved by PDE interface of COMSOL software.The porosity,concentration of liquefied particles,permeability and density of mixed fluid were calculated from the vertical dislocation failure slope in the mud forest reserve.The results showed that due to the hydraulic gradient at the seepage outlet was the largest,so the concentration of liquefied particles accumulated to the critical value the fastest here.Along with the increase of porosity and permeability,a mixture of fluids carrying liquefied particles flows out of the area,increasing the density of the fluid.After rainfall,the groundwater level increased and the hydraulic gradient increased,which greatly accelerated the process of sub-erosion.It is observed that the hydraulic gradient is the external inducement of sub-erosion in the study area.Therefore,the vertical dislocation failure of slope in mud forest can be controlled by reducing the hydraulic gradient.4.Slope failure in mud forest protection area is mainly caused by sub-erosion,so it is necessary to conduct a quantitative study on the erosion amount of mud forest loess like soil through erosion tests,in order to establish a prediction model for particle loss,which has guiding significance for the prevention and control of mud forest slope damage.The particle loss under different hydraulic gradients and initial pore ratio conditions was measured by sub-erosion test device,and the influence of both on suberosion was analyzed.The results showed that the larger the hydraulic gradient was or the larger the initial pore ratio was,the more favorable the occurrence of sub-erosion and particles loss.There were three stages in the sub-erosion process,which were sudden increase stage,slow increase stage and stable increase stage.The prediction equation of loess-like soil in mud forest particle loss considering both hydraulic gradient and initial pore ratio was established.The prediction model can well reflect the change law of cumulative particle loss with seepage time.5.A scheme based on the principle of filtered siphon drainage to control the dislocation disaster of sub-erosion in mud forest was proposed,and the concept of filter siphon drainage device is basically in line with the requirements of management,and can achieve the filtration mode of "water goes away,soil does not go away".PA tube with inner diameter of 4 mm was selected as the siphon,plastic blind tube with diameter of 50 mm was selected as the filter tube skeleton,and 200-mesh polyester spunbonded non-woven fabric was selected as the filter material.After assembly,the filter siphon drainage device was applied to the indoor model box.The combined filter siphon drainage device can effectively intercept loess-like soil particles while draining as expected,and can respond sensitively to simulated groundwater replenishment,effectively stabilizing the soil.Therefore,it is believed that the filter siphon drainage device can be applied to the treatment of the sub-erosion slope dislocation of mud forest area. |
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