Study On The Hydrological Effect And Stability Of Loess Slope On Seasonal Freezing-Thawing Process

The Losses plateau situates in the secondary topography ladder in China,one sources of the birthplace of the Chinese nation,brings up the long history Chinese civilization and farming culture.Meanwhile,it is the typical and unique geomorphic unit in the world.The tattered landform,ravines and gullies criss-cross and huge slop development are the major hidden dangers of loess disaster due to long term water loss and soil erosion.The main triggeringfactors for geo-hazards are precipitation,human activities and freeze thawing in loess Plateau.Many researchers have done lots of investigations in precipitation and human activities triggered geo-hazards,therefore fruitful results were achieved.However,more works are needed to reveal the mechanism and main influence factors of loess landslides triggered by freeze-thaweffect.Statistical data shows Heifangtai landslides,the loess landslides triggered by freeze-thaw effect,occurred densely between January and April in Gansu province,which caused significant person and pecuniary loss.Moreover,the freeze thawing loess landslide is more hidden,emergent and predicted to other typesof geo-hazards.So,research on the hydrological effect and stabilization of loess slope caused by freeze-thaw process has theoretical and impractical meanings.On the basis of the current situation and development trend,this paper choosing the loess slope in Heifangtai of Gansu province,as the research area,and takes the hydrological effect and stability of loess slope during freeze-thaw process as the main line.Moreover,guided by saturated-unsaturated thermal water movement and slope stability assessment,combining in-door hydraulic physical test and numerical simulation and in-situ observation with physical stimulation and survey,it is successful for analyzing the change of groundwater-temperature and hydrological effect during the freeze thawing process of the loess slope deeply.Meanwhile,it also assesses the stability variations of loess slope during different freeze-thaw stages.All those works put forward the scientific evidence and approach for the predictionof landslides triggered byfreeze-thaweffect.loess.The main research results are as follows:1.Designed an instrument for saturated-unsaturated hydraulic parameters in different temperatures.It can be used to measure flux,positive and negative pressure,soil moisture and temperature at the same time.The saturated hydraulic conductivity,specific yield,soil moisture characteristic curve,capacity and diffusivity were obtained via Darcy and Van Genuchten equation.This instrument has the advantage of multi-parameters and high automatically measured with large size samples minimized the random.2.Established the relationship of saturated hydraulic conductivity,diffusivity,capacity and temperature,soil water content base on the indoor test and theoretical analysis.It shows the high nonlinear relationship between loess hydraulic parameters and temperature.The loess saturated conductivity is the function of temperature,which means saturated conductivity increased with the increase of temperature as well as viscous coefficient as the key influence factor.Structure of loess reconstitutes along with the increase of freeze-thawingcycle.And saturated conductivity increases then levelled off.3.The frozen depth of loess slope is about 52 cm in the monitor site indicated by one seasonal temperature,soil water content and groundwater level monitor data.Furthermore,three temperature zones were identified according to the change of soil temperature from ground surface deep to groundwater level,i.e.,temperature remarkable alternately zone from 0 to 0.3 m,temperature oscillation zone from 0.3 to 1.5 m and stabilized transmitting zone from 1.5 to 2.0 m.After thawing,soil water moves downward and reaches saturated around 1.2 m with 0.32 cm3/cm3 soil water content which is the main factor for shallow loess erosion in Heifangtai.The process of freeze and thawing has the role of controls on water movement in loess slope.It can be divided to three stages for change of soil water content to temperature: stage 1: soil water content nonlinear increases with temperature below 2?;stage 2: soil water content declines gradually with temperature beyond 2 and 6?;Stage 3: soil water content increase slightly then leveled off with temperature large than 6? means slight impact of temperature.4.Numerical model of thermal soil-water movement for unsaturated loess was established base on the saturated and unsaturated thermal conduct and water movement.Then the change of soil temperature and soil water is simulated for freeze thawing process.It is the base for further investigation seasonal distribution of loess temperature and soil water content.5.The numerical model for the groundwater response to freeze-thaw process of loess slope is established and then water movement and overflow change was simulated.Peer-freeze,the overflow amount of groundwater diminished and groundwater level increased at the foot of slope about 1.5 m.During freeze,the depth of frozen enlarged and overflow zone block partly which caused groundwater level increased to 8.18 m,and the saturated zone enlarged continually.During thawing period,discharge recovered in the overflow zone with temperature large than zero in the whole day.And the groundwater decreased beyond 2 m variation resulted in saturated zone shrink and recovered.The area and amount of overflow zone controlled by freeze and thawing process.That means the large area of overflow zone and low overflow with low temperature.6.The loess stability analyzed by traditional slices and finite element method during freeze-thaw process.The change of porosity pressure of slop along with groundwater level fluctuation was obtained.Results show that shear stress zone band distribution and shearing strain increase zone perforated in slope with groundwater level increased.Meanwhile,the status of plastic zone changed from foot of slope and postmedian to the whole zone resulted easily instability.During freeze-thaw process,it showed the negative relationship between slope stability and groundwater level.And the slope stability can ascribe to three stages: stage 1: low stability coefficient from Oct.2,2012 to Jan.9,2013;stage 2: risk between Jan.9,2013 to Feb.10,2013 and stage 3: stability recovery after Jan.10 in 2013.The research results afforded the theoretical evidences and technological supports for assessment and prevention of loess landslidesduring freeze-thaw process.Moreover,it also has the reference value for landslide integrated control in the Loess Plateau.