Water Percolation In The Unsaturated Loess Layer And Its Influence On The Slope Stability And Collapse Deformation

Loess deposits are widely distributed in arid and semi-arid regions of Northwestern China.These soils are formed in nature with a loose meta-stable structure that is susceptible to a significant loss of stiffness and shear strength during wetting.The sensitivity of the hydro-mechanical properties of loess deposits contributes to various problems including the differential settlement,ground cracks,and landslides.Therefore,investigation of the water infiltration process in the thick loess area is a prerequisite for studying the mechanism of these disasters.For this reason,this thesis aims to investigate the water migration in the unsaturated soils and the mechanism of infiltration-induced disasters.To be specific,the objectives include(i)understanding the water infiltration process in the unsaturated soil subjected to respectively natural rainfall conditions and multiple infiltration events;(ii)investigating the water infiltration characteristics as well as variation of ground water table(GWT)associated with years of irrigation activities and its influence on the slope stability in Heifangtai loess highland;(iii)analyzing the wetting-induced collapse deformation characteristics in the irrigation and non-irrigation areas respectively in the Heifangtai loess highland during the years of irrigation;(iv)investigating the influence of soil structure on the soil retention characteristics which is a key parameter to analyze the infiltration process.The water migration characteristic in unsaturated soils was investigated from field monitoring,column tests and numerical analysis.For the field test,various climate factors,soil temperature and water content at several depths within the top 7.0 m were monitored for a period of one year.The climate factors include precipitation,air temperature,air pressure,air relative humidity,and actual evaporation.Results show that in the surface 0-1.0 m,the soil temperature and water content are sensitive to climate factors.There is a variation of water content associated with the cumulative influence of infiltration and evaporation in the precipitation and non-precipitation periods respectively.Vapor flow and liquid flow are significant.In the 1.0-7.0 m,the water content is relatively constant;however,temperature varies in different seasons.Water percolation in this zone is both in liquid and vapor phases.However,the liquid flow is dominant as the vapor flow is significant less.At the depth of 7.0 m,the soil temperature and water content are both constant.Water migrates in liquid.A laboratory column test was conducted for analyzing the water flow in unsaturated soils under multiple infiltration events.The test lasted for a period of 62 days with adding water at daily intervals and collecting moisture and suction data at different heights of the column every day.The evaporation was not considered.Results highlight the formation of two wetting fronts;namely,wetting front I and wetting front II that induced by the first and subsequent infiltration events respectively.There is a stable zone where the water content is approximately constant that forms between the two fronts,where the soil water content,suction,flow velocity,and unsaturated permeability are constant.Based on results of the field and laboratory tests,a novel conceptual model of suction profile is proposed for interpreting in-situ water flow by dividing the unsaturated zone into four distinct zones;namely,active,steady,transition and capillary fringe zones.This division is helpful for providing a rational explanation of water flow in different zones.This model is useful to understand and interpret the relationship between water infiltration and suction or water content profile in the unsaturated zone as well as variation of GWT level.In addition,it helps to understand the mechanism of rainfall-induced deep-seated landslides,which has confused the engineers for a long time.As for the case study of Heifangtai loess highland,variation of hydrological process due to years of irrigation activities was investigated using the numerical simulations.The corresponding response of slope stability was obtained.Results show that the initial hydraulic state with low water content and no GWT in Heifangtai transforms to a new hydraulic equilibrium state with a constant water content in the unsaturated zone and a stable GWT.The variation of hydrological process can be divided into four stages:(?)the wetting front moves downwards and leads to the formation of GWT below the irrigation area;(?)the groundwater moves towards the slope toe;(?)the GWT rises gradually and discharges at the slope toe;(?)a new hydraulic equilibrium state form.The variation of slope stability state can be divided into four stages,namely gradual decrease,sharp decrease,little decrease and no decrease.The four stages correspond well to the four stages of the water infiltration process above.For a certain slope in Heifangtai,its stability state must be in one of the four stages.Slopes failing at different stages have different contributing factors for triggering the failure.The most feasible way to control the occurrence of landslides is to control or reduce the total amount of irrigation water per year.A simple method is proposed from easy to obtain parameters to predict the wetting-induced collapse deformation associated with infiltration.Constant water content oedometer tests and the wetting soil-water characteristic curve(SWCC)were conducted to obtain the corresponding parameters.The collapse deformation in the irrigation and non-irrigation areas of the Heifangtai loess highland was investigated.Results suggest that there is a significant difference in total subsidence between the irrigation and non-irrigation areas.The collapse in the irrigation area occurs during the period when the wetting front advances and the GWT forms and rises until the formation of a new hydraulic equilibrium state.However,the collapse in the non-irrigation area is mainly related to the GWT variation.The difference in the total subsidence between the irrigation and non-irrigation areas is predominantly associated with the propagation of the wetting front.The soil water characteristic curve(SWCC)is one of the key parameters in analyzing the water infiltration process.To investigate the influence of the soil structure on the SWCC,loess from Heifangtai compacted at different initial water contents representing dry of optimum,optimum,and wet of optimum conditions were prepared.The SWCC in full suction range was obtained using the filter paper method.And the mercury intrusion pore size distribution method(MIP)along with scanning electron microscope(SEM)tests were conducted to investigate the microstructure both qualitatively and quantitatively.Results highlight the influence of compaction water content on the microstructure and the dependence of the SWCC on the pore size distribution.