Infiltration Law Of Irrigation Water And Its Effect On Slope Stability In Heifangtai,Gansu

In this paper,loess landslides induced by irrigation were selected as research object,and some necessary work,including investigation,mapping and sampling of landslides,has been done in Heifangtai,Yongjing County,Gansu Province.The soil-water characteristics and unsaturated strength of loess in Heifangtai were obtained according to physical and mechanical tests indoors.Monitoring data of the infiltration of rainfall and irrigation in loess,the change of groundwater level and temperature changing during freezing-thawing period was obtained by in situ monitoring.Infiltration law of rainfall and irrigation in loess,the relationship between irrigation and the change of groundwater level and the frozen stagnant water effect at the bottom of slope were analyzed.On this basis,the influence of the rise of groundwater level caused by irrigation and irrigating at the edge of slope and freeze-thaw action on its seepage field and stability was discussed.The following conclusions have been obtained:(1)The soil water characteristic curves(SWCC)of loess were measured by using axis translation method,sensor method,filter paper method and TRIM.The SWCC measured by sensor method and axis translation method is very close.The characteristic segment of curve has same slope,but the former is slightly less than the latter.The fitting effect of VG model is better than that of Gardner model and Fredlund&Xing model.The slope of permeability curve obtained by TRIM is greater than that of sensor method,and permeability coefficient of the same volume moisture content in hygroscopic process is slightly larger than that of the process of dehumidification.(2)Unsaturated shear strength and tensile strength of loess were measured by direct shear test and tensile test of unsaturated soil.The shear strength is increased with the increase of matrix suction and vertical load.The cohesion linearly is increased with the increasing matrix suction,but the internal friction angle changes little with the increase of matrix suction.The relation curve between matrix suction and ?b is approximately a straight line,and the ?b value is approximately constant.The tensile strength is decreased rapidly with the increase of moisture content and both of tensile strength and moisture content is power function.Compared with shear strength,the tensile strength is much smaller.(3)The infiltration of irrigation and rainfall was monitored to analyze laws of infiltration of irrigation and rainfall,irrigation recharging groundwater and frozen stagnant water.The rain is only circulating on the surface and is evaporated in shallow.So it has little effect on moisture content below the surface(>20cm).Compared with rainfall,the effect of irrigation on moisture content is much stronger in depth and time.It's a long process that irrigation can recharge groundwater.The monitoring data of groundwater level and temperature near the outlet are compared.We can find that the curve of water level is inversely related to temperature curve.It indicates that the frozen stagnant water effect can actually raise groundwater level.(4)The influence of the rise of groundwater level caused by irrigation,irrigating at the edge of slope and freeze-thaw action on seepage field and stability of slope was analyzed.The results show that with the rising of groundwater level,the pore water pressure at unsaturated zone is increased continuously and the strength of soil is decreased continuously,causing slope instability eventually.When initial water level is low,the effect of single flooding on slope stability is relatively small.With increase of initial water level,the effect of single flooding on slope stability is increased.As freezing time goes by,there is little change in pore water pressure inside the slope,but the rise of water level is obvious in affected area by freeze(the discharge opening at junction of loess and silty clay).