Spatial And Temporal Characteristics Of Water-movement In Soil And Shallow Stability Analysis Of Soil Slope Under Rainfall Infiltration

Natural slope sliding and engineering slope instability caused by rainwater infiltration leads to frequent occurrence.The engineering slope is closely related to the life and property of the people,and the subgrade slope in the engineering slope occupies a large proportion in the railway,highway and other transportation fields.The sudden diseases caused by heavy rainfall often affect the normal operation of the train,and even endanger the safety of driving.Many diseases,the shallow collapse of subgrade soil slope is the most typical.Therefore,it is very important to establish the mode of the shallow slope failure and the stability analysis method based on the characteristics of rain infiltration.However,at home and abroad,the study on the shallow failure mode of soil slope caused by rainfall is mostly based on the assumption of sliding mode,such as the slope plane,the arc,the broken line and so on.There is little literature to start with the stress state of the slope,and to construct a reasonable model of the shallow slope instability of the soil slope under rainfall infiltration,and the transformation relationship with the surface and deep instability mode.At the same time,in order to prevent the occurrence of shallow slide slope diseases,a variety of protective measures represented by skeleton protection were designed,but most of them are based on structural design experience.In this paper,the basic law of water movement in unsaturated soil under horizontal surface and one dimensional vertical infiltration were analyzed,and the spatial and temporal distribution characteristics of rainfall infiltration and slope runoff were probed.started from the stress state of the infinite slope soil,the failure modes and influencing factors of the soil slope under the two conditions of uniform rainfall infiltration softening and downslope seepage were analyzed.The relationship between the failure mode of slope instability and the surface,shallow and deep layers and their mutual transformation threshold were discussed,and compared with the traditional method,such as the method of circular slip surface,and the method of combined slip surface with circular arc and plane.The relationship between the shallow soil stability and the rainfall factors was also established.Finally,in order to effectively prevent the occurrence of the shallow slope instability of the soil slope,the optimal design of the frame structure under the protection of the rectangular frame based on the principle of mechanics was analyzed.The main work and achievements are as follows:1.Analysis of rain infiltration characteristics based on Green-Ampt modelBased on the Green-Ampt model,three kinds of typical time criteria for ponding in ground surface under the infiltration of rainwater are summarized:Criterion 1:when the infiltration rate of ponding infiltration condition is equal to infall intensity,the corresponding time,tpi,of ponding infiltration condition is the discrimination threshold,which is a necessary but not sufficient condition for ground surface ponding uner rainfall infiltration.Criterion 2:cumulative infiltration amount under ponding condition at time tp1 is equal to the amount of rainfall at time tP2,which is logically correct.Criterion 3:The time,tP3,required for the cumulative infiltration and the amount of rainfall.However,the saturated infiltration of the former is faster than that of the latter,and the infiltration depth is different at the same time,Therefore,criterion 3 has inherent physical defects.Under constant boundary of soil moisture at slope surface,because in the early infiltration dominated by suction pressure gradient control,slope angle have little effect on the infiltration rate q and infiltration amout Q,but with the extension of the infiltration time,the transition to gravity head gradient control,the impact of slope angle is more and more significant;Under the condition of same rainfall intensity,because of decreasing of the bear rainfall intensity with the increase of the slope angle,so the ground surface ponding time is shorter than slope runoff time,but at the same bear rainfall intensity of level ground and slope,because the infiltration capacity of slope condition is lower than the infiltration of level ground,the the opposite law is shown.2.Spatial and temporal distribution characteristics of soil water movement under rainfall infiltrationBased on van Genuchten soil-water characteristic curve and permeability coefficient K(?)equation of unsaturated soil,the water capacity C(?)equation reflecting unit matric suction changes caused by moisture change,and hydraulic pulse diffusion rate D(0)equation with impulsive property were derived.Mathematical expression of Parlange model for one-dimensional vertical infiltration of horizontal surface was improved.The results show that the C(?)curves of sand and silt are usually more significant peak feature than clay;D(?)is related to the normalized volumetric moisture content Se.At 0<Se<1,the D(?)of sand and silt had no significant change with the increase of Se,but the former was higher than that of the latter,and the increased rapidly to infinity when the Se was close to 1.Based on the Parlange model of the horizontal surface vertical infiltration,the inclined surface,the soil water movement partial differential equation of water content and slope slope constant flux boundary conditions were derived,and the slope rainfall infiltration analytical expression was eastabilished.By comparing with the Philip exact solution,it can be seen that the relative error is less than 7%,and has a good engineering accuracy for the common slope of 1:13.Stability analysis of shallow soil slope under the effect of wetting and softening by rainfall"Parallel slope-curves" failure mode of shallow slope has been made,including upper tension zone,main sliding zone in the middle and lower squeeze zone.The mechanical effects of the tension zone and squeeze zone were considered,and determined the body in the upper and lower may use logarithmic spiral mathematical expressions,and offered soil slope shallow stability analysis method,which meets the moment balance of the upper and the lower,the main zone of middle section meets static force balance.For the different rain softening depth,three types of modes of soil subgrade slope failure were summarized respectively for "unit width soil slice type of failure mode 1","composite type of failure mode 2",and "integral type of failure mode 3".The slip body length and softened depth ratio ? was characterization parameter of deep and shallow slope failure category,and the thresholds ?cr and ?f of three modes were presented.When ? is larger than ?cr,it is failure mode 1.When ? is between ?cr and ?f,,it is failure mode 2.When ? is less than ?f,it is failure mode 3.According to the rigid body limit equilibrium condition,unified formula for safety factor which based on composite type failure mode and included these three kinds of modes was constructed.The results show that if ? was less than ?cr applying parallel slope-plane mode would produce conservative results due to ignore the mechanical effect of the upper and lower edge.If ? was more than ?f applying integral curve represented by circular slip surface would be dangerous.But if ? was less than ?f applying logarithmic spiral or circular arc sliding mode would no significant difference.4.Shallow slope stability and critical rainfall duration evaluation with the condition of seepage parallel to the slope surfaceThe logarithmic spiral equation expressing the rationality of the slip surface in the upper and lower edge is justified.The method of "parallel slope-curve" composite sliding,taking into account factors such as penetration of soil and adopting effective shear strength was proposed.Compared to combination sliding search method of the single arc and center plane,the downslope-curve surface composite slip method is composed of the middle plane,and the bipolar logarithmic spiral of he lower and upper edges,with no need to search and directly determine the shallow dangerous sliding surface,and overcome the search smooth method at the intersection of surface flaws in plane curved mutation.Based on the Green-Ampt infiltration model,to meet saturated soil conditions of the rainwater infiltration depth zw,functional equation beween shallow stability coefficient of slope downslope-curve surface instability mode and rainfall duration was constructed,the slope instability critical rainfall intensity duration relation was obtained.Comparison of"slope plane instability evaluation method,the rainfall intensity is constant,because the former is considered on the lower edge of anti slide effect,the corresponding he rain infiltration depth when slope reaches ultimate state,was deeper than the latter,so the required duration should be longer.5.Shallow stability analysis of subgrade slope protected by rectangular frame structure and structural optimizationBased on the shallow translational slip of infinite slope,the mechanical effect produced by horizontal and vertical frame structure on shallow slope sliding was analyzed.Two models were proposed,respectively shear sliding model along the bottom of frame structure and passive earth pressure failure model due to retaining effect of horizontal frame structure,and the threshold lsp formula between the two models was determined,It indicated that if the vertical distance lv is less than lsp,the failure model is shear sliding,E increases with increasing lv,to maximum Ep.If lv is more than lsp,E is equal to Ep.Under rainfall softening and downslope seepage conditions,the factor of safety equation of slope shallow stability protected by rectangular frame structure was established.An optimization mode was established,which included aim function of masonry with unit protection area,limited conditions of slope stability and frame structure strength.In the condition of satisfying the factor of safety for shallow stability of soil slopes and the frames structure strength,the type of horizontal window with depth cross comparing to vertical window with applanate cross can save more masonry amount.