Shear strength deterioration effect and slope reliability analysis

10 Extreme ice-snow melting in winter affects the infiltration process of snow water on the slope surface 11 significantly, and plays an important role in the deformation stability of landslide. The fluctuation trend 12 of slope stability under ice-snow melting is the same as that of soil volume water content. The 13 deterioration effect of mechanical parameters will directly affect the deformation stability of bank slope. 14 Based on this, the ice-snow melting cycle model test of slope soil was designed and carried out. The 15 results are showed.(1) We were established an ice-snow melting model based on physical process. In the 16 process of ice-snow melting, the soil cohesion and internal friction Angle have obvious deterioration 17 effect .The deterioration of cohesion is obviously larger than that of internal friction Angle. In the early 18 part of the ice-snow melting cycle, the deterioration of shear strength parameters is very obvious. Among 19 them, the deterioration of shear strength parameters caused by the first four ice-snow melting cycles 20 accounted for about 70% of the total deterioration. After the G2/T2 ice-snow melting cycle, the degree of 21 phase deterioration gradually decreases. The deterioration trend of shear parameters of soil samples 22 gradually tends to be gentle. (2) In the ice-snow melting cycle, the inside of the soil samples have 23 micro-cracks, fissures repeatedly opened and closed, gradually developed and converged. The result is 24 that the soil samples change from dense state to loose state where internal cracks develop. The internal 25 damage of soil samples is the fundamental reason for the gradual deterioration of shear 26 strength.(3)We are keep to the relative independence principle of creep model and unsaturated 27 seepage equation. We are studied and improved the parameter solving method of creep model. The 28 modified model is reasonable and effective. The creep trend and main characteristics of the unsaturated 29 soil can be described well. Shear strength deterioration effect and slope reliability analysis under 30 extreme ice-snow melting conditions .It has important reference significance to the protection of extreme 31 snow and ice disaster on the bank slope. 32 KEYWOEDS: ice-snow melting; deterioration effect; physical process; matric suction; creep model; 33 copula 34 35 Introduction 36 The rapid huge landslide occurred in Yi gong River in the Bo mi, Tibet, on the April 9, 2000. The Yi gong 37 Landslide is caused by the melting of snow and ice on Xuefeng mountain, resulting in the formation of debris 38 flow, which was a huge disaster.Geological disasters caused by melting ice and snow are common. The volume 39 of the Alps is larger than that of the Swiss Alps 10m.There are dozens of extremely large landslides . Most of 40 them are caused by rock mass loosening and water saturation caused by the melting of ice and snow during the 41 retreat of Quaternary glaciers.A large number of landslides occurred in Japan from February to March 2006 42 due to the melting of ice and snow. The extreme ice snow disaster climate rarity appears in the Three Gorges 43 Reservoir Area, the probability is very small. The geological disaster caused by it has not been paid enough 44 attention it . There is a great difference with the alpine region,in the topography, geological environment, 45 climate environment. In early 2008, the rare ice-snow climate occurred and induced large geological disasters in 46 southern China.One high loess slopes, which was located in Yi li Region of Xinjiang ,reactivated and 47 caused long-distance road burying and river blocking on April 30,2019. The rapid snow ablation and 48 infiltration drive by the abnormal temperature rising in spring was the most important influencing factor on 49 deformation evolution of the loess slope,and the coupling of thawing water and rainstorm in spring was the 50 fundamental reason controlling the occurrence of loess landslides.A large number of soil slope are 51 instability occurred after the peak of ice-snow melting, indicating that there is a very close relationship between 52 soil slope instability and ice-snow melting . The ice-snow melting infiltration were greatly reduces the shear 53 strength of soil, and collapsible deformation occurs under the softening action of water. With the change of its 54 hydrologic properties, the mechanical strength will greatly decrease, which is an important factor inducing soil 55 landslide . The cohesion and internal friction angle of soil were show different variation rules under 56 different condition. The main reason of soil slope is shown instability caused as follows, there have two 57 facts. On the one hand, ice-snow melting is leads to the disappearance of matrix suction in soil .On the other 58 hand , the temporary of pore water pressure will increase in the transient saturated zone.From the perspective 59 of effective stress, under the condition of the increase of pore water pressure, the decrease of effective stress in 60 the soil body of the slope is the main reason for the decrease of shear strength of soil .From the perspective of 61 effective stress, the increase of pore water pressure and the decrease of effective stress are the main reasons for 62 the decrease of soil shear strength. 63 On the basis of laboratory tests, a large number of creep models have been established by predecessors 64 through theoretical analysis. In terms of creep characteristics of saturated soil, a multi-dimensional 65 logarithmic functional model is describing strain time relationship in uniaxial compression creep test. 66 Hyperbolic model or power function model is generally used to describe the strain time relationship in the 67 triaxial creep tests. Experimental study of creep characteristics under different water content, one-dimensional 68 consolidation and the triaxial compression tests were carried out on saturated, wet, air-dried and dried clays .It 69 is pointed out that the viscosity coefficient of saturated sample is higher than that of unsaturated sample. Many 70 scholars were carried out the uniaxial compression and the triaxial creep tests of mudstone with different water 71 content. The results have shown the uniaxial compression strength and elastic modulus of mudstone samples 72 decrease significantly with the increase of water content, and the creep strain rate increases with the increase of 73 water content . 74 Snow melt model is a tool to quantitatively describe the process of snow melt. It is closely related to the 75 development of snowmelt runoff hydrological model. Snowmelt models are included single point model 76 and spatial distributed model. We are integrated Hydrological Confluence Module. The single point melt 77 water process is converged to the section to calculate. The single point snowmelt model is coupled to the snow 78 scheme in GCM to realize the snow cover calculation. According to different melt water algorithms, snow 79 melt models can be divided into temperature index method and energy balance method. According to the 80 temperature index method, there is a certain correlation between snow melt amount and temperature. 81 The bank slopes are covered by extreme ice and snow. There are many uncertainties in the stability of bank 82 slope. Geotechnical parameters are often difficult to obtain, especially in situ parameters. The small sample 83 problem is arises. They 49 had proposed a small sample expansion method based on Bootstrap. According to this, 84 we are try to determine the optimal edge distribution function of the variable and the optimal Copula function 85 recognition method. They 50-52 also discussed the system reliability of geotechnical structures (such as retaining 86 walls) with the help of Copula function. Theyhad analyzed the shear strength parameters of rock mass and 87 their correlation with Q value of rock mass and deformation parameters with the help of Copula function. Try to 88 predict some parameters that are hard to measure, such as the elastic modulus of rock mass. Theyhad 89 established the joint distribution model of landslide scale, occurrence frequency and stable state with the help of 90 Copula function. Copula is successfully applied to regional landslide disaster assessment using conditional 91 probability theory. 92 At present, there are few experimental studies on extreme ice-snow melting in the Three Gorges Reservoir 93 area. We are discussed the deterioration effect of bank slope soil under ice-snow melting. The deterioration of 94 physical and mechanical properties of soil mass during ice-snow melting action is a gradual and long-term 95 process. The macro phenomenon in the short term is local bank collapse or small scale deformation and failure. 96 However, the cumulative damage effect cannot be ignored for the overall deformation and stability of the bank 97 slope. 98 Material and methods 99 Study on ice-snow melting model 100 We are chooses a homogeneous slope in the Three Gorges reservoir area as the research object. In accordance 101 with the “Geotechnical Test Procedure” , we have tested and analyzed the basic physical and mechanical 102 parameters of the unmodified soil samples retrieved from the site. The mechanical parameters of the material 103 are as follows: Elastic modulus E is 2.6MPa,Poisson's ratio μ is 0.31, cohesion c is 14kPa, angle of internal 104 friction φ is 19o,γ is 19.7kN/m,Saturation permeability coefficient Ks is 2.28*10 (m/s). Natural moisture 105 content is 17.92%.The parameters are shown in table 1. Grading curves of soil sample is shown in Figure 106 1.P≤0.075 mm is 85.963%,P≤0.075mm is represents the percentage of cumulative mass of particles with 107 particle size no more than 0.075mm in total mass. Where,% is the mass percentage of soil less than a particle 108 size. 109 110 Figure 1. Grading curves of soil sample 111 112 γ( 3 kN m ) Ks(m/s) C( kPa ) φ E( MPa ) μ 18 2.28*10 19.3 18.5 2.6 0.31 Table 1. The basic property of soils 113 In this experiment, two small scale (2mx1mx1.5m) glass tanks of the same size were prepared to carry out the 114 ice-snow melting model test. 115 In this experiment, we were carried out total of 6 times ice-snow melting model tests. There are two groups 116 with three times in each group. One time test lasted 24 hours, and three times lasted 96 hours. Each test tank was 117 evenly covered with snow 30cm at a time. The results of many tests are show as follow. The multi-stage model 118 test of ice-snow melting is shown in Figure 2. 119 120 (a) Matrix suction sensor (b)Data logger 121 10 1 0.1 0.01 0.001 %


Introduction 36
The rapid huge landslide occurred in Yi gong River in the Bo mi, Tibet, on the April 9, 2000 1 . The Yi gong 37 Landslide is caused by the melting of snow and ice on Xuefeng mountain, resulting in the formation of debris 38 flow, which was a huge disaster 2 .Geological disasters caused by melting ice and snow are common. The volume 39 of the Alps is larger than that of the Swiss Alps 10 8 m 3 .There are dozens of extremely large landslides 3  Reservoir Area, the probability is very small. The geological disaster caused by it has not been paid enough 44 attention it 9,10 . There is a great difference with the alpine region，in the topography, geological environment, 45 climate environment. In early 2008, the rare ice-snow climate occurred and induced large geological disasters in 46 southern China 11-13 .One high loess slopes, which was located in Yi li Region of Xinjiang ，reactivated and caused long-distance road burying and river blocking on April 30，2019 14 . The rapid snow ablation and infiltration drive by the abnormal temperature rising in spring was the most important influencing factor on 49 deformation evolution of the loess slope 15 ，and the coupling of thawing water and rainstorm in spring was the 50 fundamental reason controlling the occurrence of loess landslides 16 ．A large number of soil slope are 51 instability occurred after the peak of ice-snow melting, indicating that there is a very close relationship between 52 soil slope instability and ice-snow melting 17 . The ice-snow melting infiltration were greatly reduces the shear 53 strength of soil, and collapsible deformation occurs under the softening action of water. With the change of its 54 hydrologic properties, the mechanical strength will greatly decrease, which is an important factor inducing soil 55 landslide [18][19][20][21][22][23][24] . The cohesion and internal friction angle of soil were show different variation rules under 56 different condition 26, 27 . The main reason of soil slope is shown instability caused as follows, there have two 57 facts. On the one hand, ice-snow melting is leads to the disappearance of matrix suction in soil .On the other 58 hand , the temporary of pore water pressure will increase in the transient saturated zone 28  water process is converged to the section to calculate. The single point snowmelt model is coupled to the snow 78 scheme in GCM to realize the snow cover calculation 47 . According to different melt water algorithms, snow 79 melt models can be divided into temperature index method and energy balance method 48 . According to the 80 temperature index method, there is a certain correlation between snow melt amount and temperature.

81
The bank slopes are covered by extreme ice and snow. There are many uncertainties in the stability of bank 82 slope. Geotechnical parameters are often difficult to obtain, especially in situ parameters. The small sample 83 problem is arises. They 49 had proposed a small sample expansion method based on Bootstrap. According to this, 84 we are try to determine the optimal edge distribution function of the variable and the optimal Copula function 85 recognition method. They 50-52 also discussed the system reliability of geotechnical structures (such as retaining 86 walls) with the help of Copula function. They 54

100
We are chooses a homogeneous slope in the Three Gorges reservoir area as the research object. In accordance 101 with the "Geotechnical Test Procedure" 21 , we have tested and analyzed the basic physical and mechanical 102 parameters of the unmodified soil samples retrieved from the site. The mechanical parameters of the material 103 are as follows: Elastic modulus E is 2.6MPa,Poisson's ratio μ is 0.31, cohesion c is 14kPa, angle of internal 104 friction φ is 19o,γ is 19.7kN/m 3 ,Saturation permeability coefficient Ks is 2.28*10 -6 (m/s). Natural moisture 105 content is 17.92%.The parameters are shown in table 1. Grading curves of soil sample is shown in Figure   106 1.P≤0.075 mm is 85.963%,P≤0.075mm is represents the percentage of cumulative mass of particles with 107 particle size no more than 0.075mm in total mass. Where，% is the mass percentage of soil less than a particle 108 size.

130
We have routinely two kind observation snowfall. There are from meteorological and hydrological services 131 department 13. One is the depth of snow in centimeters; the other is snow equivalent, which is the depth of snow 132 into a layer of water-the amount of rainfall, measured in millimeters. In this test, the amount of snow 30cm thick 133 was laid, and the ambient temperature and snowmelt rate were controlled during the test. The experimental 134 scheme and part of the measured data are shown in Table 2   176 177  Where, n is the number of times that the bank slope encounters ice-snow melting. The deterioration equations 214 are shown in the equation (3) and (4)

314
Where ，θ is liquid water content; θ i is Capillary water content; n is porosity; S e is saturated moisture content; 315 K s is saturation conductivity, kg/s; d is snow particle size, m; a is parameter ，the recommended values is 2. 8.

316
Along with the melting transfer process, the possible conditions are described below.

317
(1) The snow layer are becomes thinner and the density decreases;

319
(3) When the density is lower than a critical value ρ min , the snow layer collapses and melts completely.   (1

409
The slope model is shown in Figure 14. We are be seen from

Conclusions 478
(1)We were established an ice-snow melting model based on physical process. In the process of ice-snow 479 melting, the soil cohesion and internal friction Angle have obvious deterioration effect .The deterioration of 480 cohesion is obviously larger than that of internal friction Angle. In the early part of the ice-snow melting cycle, 481 the deterioration of shear strength parameters is very obvious. Among them, the deterioration of shear strength 482 parameters caused by the first four ice-snow melting cycles accounted for about 70% of the total deterioration.

483
After the G2/T2 ice-snow melting cycle, the degree of phase deterioration gradually decreases. The 484 deterioration trend of shear parameters of soil samples gradually tends to be gentle.

496
The soil deformation of landslide is the result of the combined action of seepage and creep. The seepage force 497 generated by seepage will affect the creep characteristics of soil. The next step will focus on the coupling of 498 creep and seepage.

Availability of data and material 501
The authors confirm that the data supporting the findings of this study are available within the article.