Research On Surface Water Migration And Control Methods For Expansive Soil Slopes Covered By Carrying Soil

Expansive soil is a kind of clay formed in natural geological process,rich in hydrophilic minerals.It swells with water absorption and shrinks with water loss.It is widely distributed and geological disasters related to expansive soil occur frequently.The instability of expansive soil slope is one of the most serious geological problems.Moisture is the key factor to determine the physical and mechanical properties of expansive soil.The complex interaction of soil and water upon the cycle of dry and wet is an important inducing factor to cause the shallow damage of expansive soil slope.The external soil layer with non expansive clay and sand as the main materials has the functions of moisture storage,evapotranspiration,side drainage and other advantages,such as green environmental protection,low cost and easy maintenance.It is of great significance for lightweight protection to control the surface water content by the external soil layer fixed on the expansive soil slope by geocell.In order to solve the problem of shallow instability of expansive soil slope under the action of dry wet cycle,a method of controlling the surface water content of expansive soil slope based on external soil cover is proposed.Based on the theoretical model of rainfall infiltration considering dip angle and soil layer,the effects of soil layer thickness,permeability coefficient,matric suction head,rainfall intensity and duration on rainfall infiltration of expansive soil slope covered with soil layer are analyzed.In this paper,the field test base of expansive soil slope covered with carrying soil is established.The hygrothermal characteristics of slope surface under artificial rainfall and natural climate are studied,and the anti-seepage and moisture retention performance of carrying soil layer on expansive soil slope is discussed.By means of numerical simulation,the seepage control performance of the carrying soil layer is studied,and its design is optimized.On this basis,the long-term hygrothermal behavior,deformation and stability of the expansive soil slope covered by the carrying soil layer are analyzed.The main research work and achievements are as follows.(1)Based on the analysis of water balance factors and water transfer mode of slope surface,an improved infiltration model of slope considering dip angle and carrying soil layer is established,and the influence rules of carrying soil layer thickness,permeability coefficient,matric suction head,slope dip angle,rainfall intensity and duration on rainfall infiltration of expansive soil slope are analyzed.The results show that when the slope angle is more than 60°,the rainfall duration of rainwater infiltration into expansive soil slope layer increases with the increase of slope angle;when the rainfall intensity is less than 20mm/h,the rainfall duration of rainwater infiltration into expansive soil slope layer decreases significantly with the increase of rainfall intensity.The infiltration water from the slope is easy to drain out of the slope in the inclined coarse-grained soil layer,which is conducive to the long-term effective play of the anti-seepage effect of the carrying soil layer.For the double carrying soil layer composed of fine-grained soil and coarse-grained soil,the thickness of coarse-grained soil has little effect on the rainfall duration required for rainwater infiltration into the expansive soil slope layer.(2)The results show that the influence of rainfall on the soil water content of expansive soil slope covered by sand double carrying soil layer is less than that of expansive soil slope covered by single carrying soil layer.The lateral drainage of sand layer begins to play a role when the rainwater moves to the sand expansive soil interface.The lateral drainage rate increases with the rainfall,reaches the peak at the end of the rainfall,and decreases after the rainfall stops.The greater the rainfall intensity is,the earlier the lateral drainage of sand layer occurs,and the stronger the lateral drainage effect is.The lateral drainage can continue for a long time after the rainfall stops After the rainfall stopped,the lateral guide displacement accounted for a large proportion of the total guide displacement.(3)In this paper,the behavior of expansive soil slope covered by different soil layers under natural climate was monitored for one year,and the anti-seepage and moisture retention performance of expansive soil slope covered by different soil layers was discussed.The results show that the influence of climate factors on the water content and water potential of expansive soil layer of slope covered by double carrying soil layer is obviously weakened.During the monitoring period of one year,the final permeability of single carrying soil layer is close to 200mm,accounting for17.7%of the total rainfall;the permeability of double carrying soil layer is almost not affected by the cumulative rainfall,and the growth rate is slow,the final permeability is only 40mm,accounting for only 3.7%of the total rainfall.The final cumulative water loss of expansive soil layer covered by single and double carrying soil layers in dry season is 27mm and 9mm respectively,and the water loss rate is 0.351mm/d and0.117mm/d respectively.The double carrying soil layer with sand layer has better moisture retention effect on expansive soil slope than single carrying soil layer.(4)A numerical model for seepage analysis of expansive soil slope covered with double carrying soil layers is established.Sensitivity analysis is made on the slope dip angle,thickness of carrying soil layer,permeability coefficient,inlet pressure,initial pore pressure and other parameters affecting seepage.The optimization design method for seepage control and an example are proposed and verified.The results show that the thickness of surface clay and pore pressure almost have no effect on the anti-seepage performance of double carrying soil layer on expansive soil slope,and the smaller the saturated permeability coefficient of surface clay is,the stronger the anti-seepage performance of carrying soil layer is,and the critical saturated permeability coefficient is 1.5×10^-8m/s.Laying sand layer can significantly improve the anti-seepage performance of carrying soil layer on expansive soil slope,but the thickness of sand layer should not be too large,and the best laying thickness is about20cm;the larger the saturated permeability coefficient of sand layer is,the better the anti-seepage performance of carrying soil layer is;when the water inflow value is not more than 12k Pa,the smaller the water inflow value of sand layer is,the stronger the anti-seepage performance of carrying soil layer is,and the lower the water inflow value(smaller particle size)is preferred for sand layer coarse particles.(5)The long-term hygrothermal characteristics,deformation and stability of expansive soil slopes with different overburden layers are analyzed numerically.The results show that the variation range and rate of pore pressure of expansive soil slopes with double overburden layers are significantly slower than those with bare overburden and single overburden layers.The volume water content of expansive soil of slopes with double overburden layers maintains a relative equilibrium state around0.32m³/m³,and the storage water content of soil layers is lower than that with bare overburden layers and single overburden layers Under the condition of 7379mm total accumulated rainfall in six years,the total accumulated runoff on the slope is1659.2mm,accounting for 22.5%of the total accumulated rainfall;soil evaporation and vegetation transpiration are 1023.8mm and 1600.1mm,accounting for 13.9%and21.7%of the total accumulated rainfall respectively.The storage capacity of soil layer and lateral guide displacement of sand layer are 209.8mm and 2886.0mm,accounting for 2.8%and 39.1%of the total accumulated rainfall,respectively.Lateral guide drainage of sand layer is the main way to dissipate rainfall.The final displacement of the soil at the 0.5m depth of the slope toe is about 6mm,4.5mm and 3.8mm respectively for the slope with no carrying soil layer,single carrying soil layer and double carrying soil layer,and the deformation growth rate of the slope with double carrying soil layer is the lowest.The results show that the soil moisture of expansive soil layer covered by double carrying soil is in a relative equilibrium state for a long time,and its safety factor has been kept in a high state;the soil of slope covered by single carrying soil and bare slope experienced many times of drying and wetting cycles,and the instability occurred in the early stage of rainfall.Double carrying soil layer can ensure the long-term stability of expansive soil slope.