| The western part of Jilin Province is an ecologically fragile region where distributes a large area of carbonate saline soil.Semiarid and seasonally freeze-thaw environment makes the soil mass here generally has significant fissures and dispersity.The environmental negative effects such as soil-water loss and secondary salinization are very serious.For the sake of improving the local ecological environment,and further answering the call of sustainable development in agriculture which is the local economic pillar,it is planned to construct artificial open canals in many counties and cities of this region for enhancing irrigation and gradually curbing and reducing desalinization.However,due to the existence of dispersity,the soils here show high water sensitivity,and are prone to structural collapse and strength loss in presence of liquid water.This property of soil is not conducive to the stability of canal slope.Therefore,it is necessary to investigate the water-soil interaction process of this dispersive soil and further understand the response mechanism of dispersive soil to different water conditions;On the one hand,it can provide experimental bases for the design of geotechnical and water conservancy projects,and on the other hand,it will benefit for the understanding of the engineering geological properties of this special soil,whose distribution is relatively limited and start time of study was somewhat late;hence,such researches will have both practical and theoretical significance.In order to achieve above purposes,the current study takes Qian'an county,where the soil features stronger dispersity in the project coverage area,as the research region.Through field investigation and lab experiments,the following aspects were carried out.Firstly,based on the multi-year field investigation results,the basic properties of dispersive soil and the main types and evolution laws of slope failure were given,and the main causes of failure were analyzed from perspectives of soil quality and environment.Secondly,since the vadose zone is with the most severe soil-water interaction and environmental impacts,considering the unsaturated conditions and the hydraulic processes related to various failure types,the hydro-physical properties of dispersive soil including water holding capacity,permeability and water stability were tested by filter paper suction test,constant-head permeation test and hydrostatic disintegration test,respectively.The test variables including initial water content,dry density and salt content were considered to reflect the influence of water-salt migration and structural variation of soil mass due to actual environmental changes.Finally,taking the cumulative soil-water loss phenomenon of canal slope subjected to rainfall as the background,a compacted dispersive soil slope with engineering characteristics was exposed to simulated rainfall for several times,and the progressive erosion-concomitant failure of the slope was studied.The main results obtained are summarized as following.(1)In the field,the dispersive soil slope can develop significant failure types such as surface salt crystallization,rill erosion,stress relaxation induced tension crack associated collapse,suffosion and gully erosion.It is found that the slope damage is the most serious at the beginning of spring and in July-August when rainy season comes.The main inducements are freeze-thaw associated cracks and scouring of concentrated rainfall.The enhancement of soil cracking makes the salt distribution on the slope surface change from uniform distribution after excavation to sporadic distribution.The main reason for the instability and disintegration of dispersive soil is that,the clay bound water film becomes thicker while meeting with water as greatly induced by sodium ion,and then the cementation strength of clay between themselves and towards the silt/sand particles reduces.Dispersity is the essence leading to damage,rainfall is the direct triggering factor,and the salt accumulation and fissure caused by evaporation and freeze-thaw cycle can aggravate the expression of dispersity.(2)The water holding properties of unsaturated dispersive soil under different dry densities and salt contents are characterized by matric suction.Greater dry density induces higher volumetric water content and more prominent water holding capacity.The effect of soluble salt on water holding capacity is suction-dependent,and overall the water holding capacity of soil firstly weakens and then improves as salt content increases.The soil water characteristic curve is established based on van Genuchten model.It is found that the dry density mainly affects the water holding capacity in the transformation stage.The air-entry value increases/decreases with the increase of dry density/salt content as a whole.The higher the dry density,the higher the threshold suction of the residual stage,and the lower the mass water content of the soil when entering the residual stage,which is not affected by the salt content in the test range.(3)Under constant-head permeation condition,the increase of dry density and salt content weakens/enhances the permeability of unsaturated dispersive soil,respectively.The moving distance of wetting front in unsaturated dispersive soil has a power function with time.The elapsed time when permeation terminates increases/decreases with the increase of dry density/salt content.Hydrus-1D is used to simulate the permeation process,and the temporal and spatial distributions of water,salt and hydraulic conductivity are obtained.The simulation results show that there will be a rapid salt accumulation at the arrival of the wetting front,but the subsequent leaching process is relatively slow.The salt content will reach its peak before the soil is saturated.When the matric suction is<2500 k Pa,the order of magnitude of the measured unsaturated hydraulic conductivity calculated based on the wetting front advancing method ranges between 10-10-10-6 m/s,and the main order of magnitude is?10-8 m/s,which is higher than the simulated value,so it is recommended to refer to the measured value in actual project to ensure safety.Seepage can force fine particles migrate downward,so as to simplify the pore structure of the soil near the soil column top.(4)The disintegration of unsaturated dispersive soil shows two different failure types due to varying mass water contents,namely weight increases-gentle disintegration-sudden disintegration,and continuous rapid disintegration;the mass water content at the inflection point is about 17%.The increase in dry density can enhance the water stability of soil.As salt content increases,the water stability first increases and then decreases,and the corresponding inflection point becomes higher along with increasing dry density,showing a density-dependent feature.The grey relation entropy analysis shows that the contributions of the three test variables to the duration of complete disintegration follow dry density>water content>salt content,but their contributions to the characteristic disintegration rate are close.(5)The effects of soluble salts on water holding capacity,permeability and disintegration of unsaturated dispersive soil shows different,which is related to the hydrodynamic conditions of soil under different test conditions.Therefore,the content of soil soluble salt needs to be concerned in relevant hydro-physical property studies.(6)Rill erosion,surface erosion,fine particle loss,surface sealing and its disassembly are observed on unsaturated dispersive soil slope during simulated rainfall,the cracking in superficial slope mass is significant during rainfall stop period,and the severity of above processes change dynamically with rainfall intensity and duration.It is found that 60 mm/h is the threshold rainfall intensity of rill erosion and surface erosion of compacted dispersive soil slope.The surface sealing phenomenon on the surface of the dispersive soil slope during rainfall can prevent rainwater infiltration.When the rainfall intensity reaches 60 mm/h,the surface sealing layer begins to be disassembled into clastic high-plasticity flakes.The 3D laser scanning results of the region of interest are calculated by Arc GIS,and the results show that the mean values of earth's surface incision and roughness increase to their maxima when rainfall intensity is about 60-80 mm/h.After one rainfall event,the cracks emerged on the slope surface due to water loss will heal during the next rainfall event,which is the result of the dispersion of the soil on the crack wall and the filling by eroded soil particles from upper part.During a single event of rainfall,the cumulative mass of soil particles carried by collected runoff basically increases linearly with time,but the law of runoff sediment concentration is not significant,besides,the electric conductivity and turbidity of the upper suspension of collected runoff gradually decrease to a stable value with the progress of rainfall.Because runoff carries away a large number of fine particles of the slope soil,the primary coarse particles gather on the slope during rainfall stop periods,whereas the number and coverage area of coarse particles decrease with the increase of rainfall events.The monotonic increase of rainfall intensity can increase the single erosion amount of the slope,but after experiencing high-intensity rainfall and then undergoing low-intensity rainfall,the increased rainfall duration time is not able to significantly increase the loss of slope soil,reflecting that the rainfall history can have an impact on the erosion degree.The middle,lower,and toe parts of slope respond quickly to rainfall,whereas the saturation state of soil at depth of about 20 cm under the upper and top platform parts has not changed significantly after a total of 8 rainfall events,indicating the poor soil permeability and the prominent surface sealing during rainfall,which is inseparable from the soil dispersity. |
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