Numerical Limit Analysis Of Unsaturated Slopes Considering Hydraulic Hysteresis

Slope is a common geological form and both natural slope and artificial slope are mostly unsaturated.Water is an important factor to cause slope instability.The hydraulic characteristics of unsaturated soil are complicated and with hysteresis.In the process of moisture absorption and dehumidification,the soil water characteristic curve and the permeability function are not consistent.In different hydraulic paths,the difference of matric suction corresponding to the same moisture content may be as high as twice or more,which makes the stability analysis of unsaturated slope very difficult.In order to solve this problem,a unified numerical limit analysis method considering the influence of hydraulic hysteresis is established in this paper,which combines unsaturated seepage analysis,limit analysis theory and second-order cone programming.On this basis,the influence of hydraulic hysteresis on the seepage field and stability of unsaturated slope under the condition of water level rise and fall is studied.The main work and conclusions are as follows:(1)The effective stress of unsaturated soil based on suction stress is introduced into the limit analysis theory,and the influence of hydraulic hysteresis on the seepage and stability of unsaturated slope is considered.Combined with the finite element method and the second-order cone programming,an upper limit analysis method considering the influence of hydraulic path is established.The loading process is not needed to be tracked accurately,and therefore high efficiency can be attained.(2)Consider the effect of hysteresis during the dehumidification phase: Under a given changing rate of water level,the pore water pressure and saturation in the slope body increase during the water level rise,and the hydraulic hysteresis has little effect on the calculation results of the seepage field.In the stage of water level decreasing,the pore water pressure and saturation in the slope body decrease.In the process of water level falling,neglecting the effect of hydraulic hysteresis will lead to the calculation result of pore water pressure being larger and the saturation being smaller.(3)Consider the effect of hysteresis during the dehumidification phase: At a low slope position,as the rate of water level decline increases,the influence of hydraulic hysteresiseffect on pore water pressure gradually weakens;while at a high slope position,the influence of hydraulic hysteresis effect on pore water pressure is gradually strengthened with the increase of water level decline rate.Similarly,at a low slope position,as the velocity increases,the effect of hydraulic lag effect on saturation gradually weakens.At a high slope position and when the descent rate is less than 1.5m/d,as the rate increases,the hydraulic lag effect gradually increases the saturation;while the descent rate is greater than 1.5m/d,as the rate increases,the effect of hydraulic lag effect on saturation gradually decreases.(4)Consider the effect of hysteresis during the dehumidification phase: In the process of water level rising,ignoring the influence of hydraulic hysteresis will lead to slightly larger slope stability factor.With the increase of water level fluctuation rate,the deviation of stability factor will gradually decrease.In the process of water level decline,the slope stability factor is obviously larger when considering the hysteresis,and the difference increases with the increase of water level fluctuation rate.(5)Consider the effect of hysteresis during the moisture absorption phase: During the rising stage of the water level,the pore water pressure and saturation in the slope gradually increase,and the effect of hydraulic hysteresis effect in this stage is more obvious.If the effect of hydraulic hysteresis is ignored,the resulting pore water pressure and saturation will be too large.In the stage of water level decline,the pore water pressure and saturation in the slope gradually decrease,and the hydraulic hysteresis effect has little effect.(6)Consider the effect of hysteresis during the moisture absorption phase: In the rising stage of water level,with the increase of time,the stability factor gradually increases,the effect of hydraulic hysteresis effect is more obvious,and with the increase of time,the influence gradually increases.At the stage,the stability factor obtained when considering the hydraulic hysteresis is obviously small,and the result is conservative.In the stage of water level decline,the stability factor gradually decreases,and the hydraulic hysteresis effect has little effect.