Stability evaluation of slope subjected to seismic effect combined with consequent rainfall

Journal Pre-proof Stability evaluation of slope subjected to seismic effect combined with consequent rainfall

You-Liang Chen, Geng-Yun Liu, Ning Li, Xi Du, Su-Ran Wang, Rafig Azzam PII:

S0013-7952(19)30595-2

DOI:

https://doi.org/10.1016/j.enggeo.2019.105461

Reference:

ENGEO 105461

To appear in:

Engineering Geology

Received date:

1 April 2019

Revised date:

15 November 2019

Accepted date:

18 December 2019

Please cite this article as: Y.-L. Chen, G.-Y. Liu, N. Li, et al., Stability evaluation of slope subjected to seismic effect combined with consequent rainfall, Engineering Geology (2019), https://doi.org/10.1016/j.enggeo.2019.105461

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© 2019 Published by Elsevier.

Journal Pre-proof Stability evaluation of slope subjected to seismic effect combined with consequent rainfall You-Liang Chen1, Geng-Yun Liu*1, Ning Li1, Xi Du1, Su-Ran Wang1, Rafig Azzam2 1

Department of Civil Engineering, University of Shanghai for Science and

Technology, 516 Jungong Rd,Shanghai 200093, P.R.China 2

Department of Engineering Geology and Hydrogeology, RWTH Aachen

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University, Lochnerstr.4-20 Haus A, D-52064 Aachen, Germany

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*Correspondent Author

The failure mechanism of a slope is analyzed taking a combination effect of

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

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Highlights

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earthquakes and post-seismic rainfall into account. A model is developed that allows to investigate the main effects on the slope with

software.

The Plant Reinforcement Technology is suggested and how biological

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

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consideration of dynamic load and unsaturated infiltration using GeoStudio

remediation would act in this case on the slope and improve strength are explored.

Abstract A large number of field investigations of earthquake damage have shown that sometimes slopes instability occurs as a result of a series of post-seismic ripple effects rather than of merely seismic load or ground motion, among which a combination of earthquakes and post-seismic rainfall is a typical contribution for slope instability. In this paper, taking a slope in Southwest China as an example, failure mechanism and stability of slope under seismic load and heavy rainfall are analyzed with

Journal Pre-proof consideration of dynamic load and unsaturated infiltration using GeoStudio software. Results indicate that, the acceleration of earthquake is sharply amplified at the top of the slope, resulting in instantaneous tension stress, which causes tension cracks and therefore reduction in tensile strength at the top of the slope. The existence of cracks contributes to the post-earthquake infiltration of rainwater slows down the dissipation of excess pore water pressure. Slope instability is ultimately caused by permanent damage due to seismic load or ground motion and secondary damage on account for

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heavy rainfall. Furthermore, the Plant Reinforcement Technology is put forward to reinforce the slope and satisfactory results are obtained. Therefore, this research

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provides a theoretical basis to analyze and control slope stability in other similar

Slope

stability;

seismic response;

infiltration;

Plant

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Reinforcement Technology 1 Introduction

unsaturated

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Keyword:

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engineering geology problems.

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Earthquake triggered landslides have been studied in the seismic regions as a major type of geological disasters (Sergio et al.,2005; Chang et al.,2012; Jibson and

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Harp 2016; Marc et al.,2017; Jorge et al.,2019). However, a large number of field investigations of earthquake damage show that slopes instability is a result of a series of ripple effects caused by earthquakes rather than of merely seismic load or ground motion (Lin et al., 2006; Chen and Hawkins, 2009; Liu et al.,2013; Yano et al., 2019). Especially, seismic activity with consequent rainfall pose a challenging threat to slope stability. The crustal stress, stress field or failure at a fault that triggers earthquakes will affect precipitation in the short term after an earthquake. A related effect of massive landslides after earthquakes will increase a large number of dust and particles in the air, which are the best condensation nodules of water droplets; Furthermore, the seismic shock wave will continuously release energy into the air, causing air vibrations (shock waves) as well as a large number of condensation nodules and

Journal Pre-proof nodules over the earthquake area. Water vapor molecules collide constantly, combine fully, which may eventually cause heavy rainfall (Huang and Fan,2013). Slope instability due to earthquake-induced rainfall have been reported in some cases. After the 1999 Taiwan Earthquake, the area of landslides induced by heavy rains can reach three times that of landslides directly induced by earthquakes (Lin et al., 2006). After the 2008 Wenchuan Earthquake, Beichuan area with high seismic high intensity suffered the largest heavy rainfall with the precipitation amount up to 250-350 mm,

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which led to further activity of landslides and a large number of new landslides (Yin et al., 2009; Huang and Fan, 2013; Tang et al., 2011; Jun et al., 2018). Rainfall also

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triggered many small-scale landslides from May to October after the Kobe earthquake

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on January 17, 1995 in Japan (Kanaori and Kawakami, 1996).

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Theoretical analysis (Bishop, 1995; Pantelidis and Griffiths, 2013a; Pantelidis and Griffiths, 2013b) have been performed to study the dynamic response and failure

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characteristics of slopes. Although the theories to assess slope stability is becoming increasingly mature, the mechanism of slope instability under complex

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external forces remains confusing. Shaking table model tests were conducted to investigate the failure modes as well as causes of landslides, and discuss the stage

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failure characteristics of slopes under shaking table conditions (Hong et al., 2005; Lin and Wang, 2006; Hu and Chen, 2011). Aided with numerical simulation, a large number of practical slope instability cases have been studied (Iwata at al., 2013; Huang and Xiong, 2017) whereas there is usually no information about the actual pore pressure development, especially the dynamic pore water pressure due to the earthquake, which largely limits the reliability of the calculation. Heavy rainfall is another main trigger of slope instability (Tang et al., 2011; Kimoto et al., 2013; Sasahara and Sakai, 2014). The infiltration of pore water under rainfall elevates the free surface of the groundwater in the slope, weakens the rock and soil at the potential slip surface, and thus reduces the stability of the slope, leading to the formation of the landslide. Actually, it is crucial to the slope stability

Journal Pre-proof that the infiltration of rainfall in rock and soil mass of slope experienced a seepage unsaturated-to-saturated process, during which the physical and mechanical properties of rock and soil in unsaturated areas change continuously. In recent years, research on slope instability caused by rainfall have been carried out (Okura et al., 2002; Moriwaki et al., 2004; Kimoto et al., 2013). Typically, Fredlund (1987) first took the influence of unsaturated soil seepage into account in slope stability analysis. Alonso et al. (1995) calculated and analyzed the stability of

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the slope with consideration of the soil type, soil-water characteristic curve, rainfall duration and infiltration intensity of the slope. Okura et al. (2002) and Moriwaki et al.

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(2004) have successively carried out full-scale tests on mobile landslides and the

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results show that the increase of pore water pressure is a result from collapse of loose

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soil structure in the upper slope while that is a result of a mix of soil compression and shearing in the lower slope. Kimoto (2013) concluded that rainfall infiltration can

numerical simulation.

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reduce matrix suction and increase permeability coefficient of unsaturated soil by

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A large number of geology engineering practices show that the slope instability is multifactorial among which seismic activity and heavy rainfall are two major

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incentives, but their failure mechanisms are completely different (Clague and Stead, 2012; Sorbino and Nicotera, 2013). Thus, the study on the mechanism of slope instability under seismic load with subsequent rainfall infiltration and the corresponding treatment or remediation measures urgently needs to be carried out from both theoretical and practical aspects. In this paper, taking a slope in Southwest China as an example, failure mechanism and stability of slope under continuous rainfall after earthquake are investigated with consideration of seismic load and unsaturated infiltration using GeoStudio software. Further treatment measures are suggested as well. 2 Project overviews and calculating conditions 2.1 Geological conditions

Journal Pre-proof The slope, located in Sichuan Province, southwest China, has a narrow and long strip-like topography that inclines from North to South gradually. The terrain dips north with a slope of about 46-55°. Due to several earthquakes in the past, annual earthquake related effects like landslides, slumps and barrier lakes can be observed. the underlying bedrock is silty shale, limestone and dolomite. The rock and soil mass of the slope is a layered structure, which mainly consists of the underlying bedrock (mostly silty shale, limestone and dolomite), weathering

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zones and colluvium and talus accumulation (mostly clayey and sandy soil, with gravel and debris). According to the division of seismic zones in Sichuan Province,

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the slope lies in the Anning River-Zemuhe seismic zone with estimated

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earthquake intensity of VIII and seismic acceleration (horizontal acceleration in this

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paper) of 0.3g.

The first sliding of the slope occurred in the late 1980s, about 78 km away from

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the epicenter (Magnitude 7. 2) and 54 km away from hanging or foot wall in the fault zone. After that, the slope slided again under the influence of M8.0 Wenchuan

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earthquakes and aftershocks in May 2008. The slope failed partially and ground cracks appeared on the slope surface. In January 2012, a further large-scale

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deformation occurred again. Several days before that event, there was continuous rainfall in the area, and consequently the slope was saturated, which eventually caused instability and sliding. The landslide was formed in the early Quaternary collapsing or landslide deposits, mainly composed of debris-rich clay that is loose and low permeable. The main scarp dips to the southeast and develops crown cracks with length of tens to hundreds of meters in fault fracture zones. There are often several secondary scarps in the zones, forming complex rupture structures. The geological profile of the slope and the arrangement of monitoring points are shown in Fig. 1.

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Elevation / m 520

Geological fault colluvium & talus accumulation Severe weathering strata

460

Mild weathering strata

1# (C#)

Bedrock

400

2# 3#

A#

D#

340 4# 5# (E#)

280

B# 220 160 75

150

225 300 Distance / m

450

375

525 565

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0

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6#

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Fig.1 Cross section of the slope used for modeling with location of control points

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2.2 Hydrology

Groundwater is Quaternary loose-rocks pore water, which occurs in sandstone

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and glutenite. It is seasonal temporary diving and mainly receives meteoric water

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infiltration and groundwater runoff recharge. Precipitation variation along with seasons in this area is remarkable, mostly concentrated from May to October.

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According to the data of the meteorological stations in Sichuan Province from 2002 to 2016, as shown in Table 1, the annual average rainfall is 873.79 mm; the daily maximum rainfall (24h MX) reaches 420.5 mm; the maximum rainfall is 105.4 mm in an hour; and the maximum continuous rainfall lasted 13 days.

Table 1. Average rainfall and rainstorm statistics(Unit：mm) mouth Average

1

2

3

4

5

6

7

8

9

10

11

12

34.3

36.9

126.7

284.7

361.6

406.7

384.7

262.7

144.7

54.7

33.5

32.4

52.3

71.2

111.4

145.4

154.3

358.9

403.0

422.7

341.9

323.2

157.6

62.6

24h MX

2.3 calculating conditions

Journal Pre-proof The structure, lithology, and stress state of rock mass are the main influence factors to the sliding mode of a slope. The failure mechanism analysis for rock slope follows the principle of rock mass control. Combined with the actual working conditions of a slope in Southwest China, the instability mode of the slope is analyzed. And there emerges an arc sliding surface with vertical cracks. It is of significance to study and determine and confirm the potential sliding surface of the slope by research on the stability of mixed rock / soil slope based on limit equilibrium method. The

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automatic search of procedures in light of the slope sliding mode was adapted to determine the potential sliding surface.

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For the slope in Southwest China, a model will be developed that allows to

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investigate the main effects of the earthquake and consequent rainfall on the slope

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with consideration of dynamic load and unsaturated infiltration using GeoStudio software Furthermore, the Plant Reinforcement Technology, a biological remediation,

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is suggested and how biological remediation would act in this case on the slope and improve strength are explored. Three types of working conditions were analyzed with

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consideration of seismic effect and consequent rainfall, underground water, and slope reinforcement treatment using GeoStudio software. We will be creating the model to

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simulate the working conditions in three stages as follows: (1) natural state plus earthquake conditions in the first stage; (2) post-earthquake rainfall conditions based on the first stage of stress and displacement fields and underground water level in the second stage; (3) reinforcement conditions after earthquake and rainfall in the third stage. Detailed analysis of the three conditions was carried out. 3 Stability evaluation of seismic effect combined with consequent rainfall The damage of slope caused by earthquake mainly depends on the failure ground acceleration and the large fluctuation of pore water pressure along with earthquake. Ground acceleration (dynamic load) acts as additional load or force and increases driving forces, which makes an increase in the sliding force along the sliding surface in potential sliding body. Besides, due to cyclic weakening, the potential sliding

Journal Pre-proof surface enters the plastic state, which leads to the decline in sliding resistance of slope body and the increase in the instability of slope body. Traditional idea holds the view that shear failure caused by seismic load is a main contribution to slope failure under earthquake with a similarity to that under static condition. However, based on the investigation of landslides caused by the Wenchuan earthquake, many landslides show that at the mountain ridge tensile fractures develop behind the ridge in the rear slope, and the slope below sheared off, forming a unified high-speed sliding surface. (Yin et

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al., 2009; Jun et al., 2018) Obviously, the traditional concept of slope seismic damage only considers the shear failure without a consideration of the weakening of slope due

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to the previous tensile failure. In fact, compared with its compressive and shear

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strength, the tensile strength of rock and soil is considerably lower. Under seismic

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load, the slope is more prone to tensile failure due to the cyclic loading. After the earthquake, many cracks appeared on the slope surface, and the heat

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inside the earth was released a lot. At the same time, some water vapor underground entered the air along the crack, thus forming a strong updraft on the ground together

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with dust and particles, so rainfall usually occurred after an earthquake. Research has indicated that the areas stricken by an earthquake would experience a prolonged

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influence (Huang and Li, 2014; Jun et al., 2018). Secondary hazards triggered by the earthquake have greatly changed the land use and cover in the area, in particular, the serious natural vegetation loss and degradation, which results in increased soil erosion and accumulated massive loose debris layer (Jun et al., 2018). Secondary disaster induced by heavy rainfall after seismic activity increases the susceptibility to slope instability. The heavy rainfall event on January 17-22, 2012 provided an opportunity to study the contribution of the earthquake to the occurrence of the subsequent, rainfall-induced landslides. In this section, the variation law of the stress and displacement fields of the slope under earthquake conditions was discussed using quake/w module in GeoStudio. Furthermore, SEEP/W module based on saturated-unsaturated seepage model is used to simulate slope stability under the

Journal Pre-proof condition of post-earthquake rainfall through numerical modeling. Subsequently, the failure mechanism is further explored. 3.1 Effect of additional horizontal dynamic stress

1

1

3 s

3 s

1

1

(a)

(b)

3 s

3 s 1

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3

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3

1

(c)

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Fig. 2 Stress state under natural state and seismic load: (a) Natural stress state;(b-c)

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stress state under seismic action

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In this Section, the representative elementary volume of the rock slope is taken to discuss the stress state and failure law of the element under cyclic load. The seismic

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load in slope is simplified as horizontal load with the assumption that the initial stress of rock mass is caused by gravity of rock masses and geological structure. Fig. 2

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shows the stress state of rock and soil under natural state and seismic action. Eq. (1) is satisfied at the natural state:

1   h ；  3  k0 h

(1)

where σ1 is the vertical principal stress (major principal stress); σ3 is the horizontal principal stress (the third principal stress); γ is the bulk density; and k0 is the lateral pressure coefficient. When rock and soil mass are subjected to horizontal seismic load, additional horizontal dynamic stress, divided into the same and opposite direction to horizontal principal stress, is acting compressive and dilative on the rock and soil mass due to cyclic character of the horizontal seismic load.

Journal Pre-proof (1) When dynamic load and static horizontal stress act the opposite direction (have the opposite vector),

1   h ；  3   3   s  k0 h  Csvs where: vs is shear wave velocity; Cs is the velocity of S wave, Cs 

(2) G



therein

and G is the shear modulus; and σs is the additional horizontal dynamic stress. The horizontal seismic load causes a decrease in the horizontal principal stress in

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the rock and soil mass. At the time when k0γh > ρCsvs, that is σ3´ > 0, the decrease of

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the third principal stress in the rock and soil mass causes shear failure. When third principal stress turns negative, tensile failure occurs.

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(2) When dynamic load and static horizontal stress act the same direction (have

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the same vector),

(3)

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1   h ；  3   3   s  k0 h  Csvs

Additional horizontal dynamic stress is acting compressive and dilative on the

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soil due to cyclic character of the horizontal seismic load. When σ3´ < σ1, the increase in third principal stress makes the soil tend to stable. When σ3´ > σ1, the maximum

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principal stress and the minimum principal stress are reversed where the horizontal principal stress becomes the major principal stress. At this time, the increase of the major principal stress will lead to shear failure of rock and soil. 3.2 Strength criterion and seepage equation Fredlund (1978) proposed the shear strength formula for unsaturated soils, which is expressed as follows:  f  c  (  ua ) tan    (ua  uw ) tan  b

(4)

Where c′ is cohesion; φ is the total normal stress on the failure surface; ua and uw are net normal stress and matrix potential, respectively; φ′ is the internal friction angle related to the net normal stress state variable ( σ − ua ); and φb is the rate at which the shear strength increases with the matrix suction (ua − uw ).

Journal Pre-proof A combination of stress state variables (σ − ua) and (ua − uw) with the pore gas pressure ua as a benchmark was adopted by Eq. (4), which is essentially an extension of the Mohr-Coulomb shear strength formula for saturated soils. When the soil is saturated, uw is equal to the ua, so the matrix suction (ua − uw) is equal to zero, achieving a transition from unsaturated to saturated. Domenico and Schwartz (1997) provided a comprehensive theoretical review of groundwater flow through porous media. Saturated-unsaturated porous seepage

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continuum equation is obtained on the assumption that fluid seepage in porous media follows the law of conservation of mass and Darcy's law (Lam, et al.,1987; Gottardi

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and Venutelli, 1993).

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  h    h  h w  kx    kz   m  w x  x  z  z  t

(5)

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where x and z indicate the horizontal and vertical directions, respectively; h is the total hydraulic head, kx and kz are the permeability coefficients function of unsaturated

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soil; γw is the unit weight of water; mw is the coefficient of water volume change in respect of the change in matric suction, which can be obtained from the slope of the

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soil-water characteristic curve; and t is the time. The vectorial equation of soil unsteady seepage can be expressed as follows (John, 2008):

KH +  M H =Q

(6)

where [K] is the unit eigenmatrix; {H} is the node head vector; [M] is the unit mass matrix and {Q} is the node flow vector. 3.3 Boundary conditions and parameters 3.3.1 Dynamic boundary conditions ： The dynamic boundary treatments included that: (1) the bottom boundary (i.e. the bottom of bedrock) was fixed horizontally and vertically; (2) in order to reduce the impact of seismic wave rebound

Journal Pre-proof in the sliding body, the impedance boundary was set at the front and rear boundary of the slope and displacement consolidation boundary vertically and horizontally were set at lower part of the slope; (3) the top of the slope was set as free boundary horizontally and vertically. 3.3.2 Seepage boundary conditions: Fixed boundary is often used in numerical simulation due to the complexity of rainfall infiltration boundary. Actually, it is a constantly changing boundary with rainfall duration, so runoff should be taken into

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account (Bandara et al., 2016; Cho, 2016). (1) No runoff

where k sij

kis3kr (h) ni

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h xj

p

(7)



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kijs kr (h)

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The rainfall infiltration boundary can be treated as the flux boundary:

is the saturated permeability tensor; kr is the relative permeability

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coefficient, which can be determined by the permeability coefficient curve of unsaturated soil in unsaturated region; ni is the normal direction vector of slope

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surface; and p is the rainfall intensity on slope surface. At this time, the rainfall intensity is less than the infiltration capacity of soil, so

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rainwater will all infiltrate. Therefore, the pressure head at each point on the rainfall boundary should be satisfied: h ≤ 0. (2) Runoff

The rainfall boundary can be treated as the head boundary. Considering the inclination of the slope, surface water will flow fast. Therefore, the pressure head on the slope is 0, i.e., h = 0. At this time, the rainfall intensity is greater than the infiltration capacity of soil, and rainwater cannot infiltrate completely. Therefore, the discharge on the rainfall boundary should be satisfied:  s  h  kis3kr (h) ni  p kij kr (h) x j   

(8)

Journal Pre-proof According to the local weather station in Sichuan, the rainfall fluctuated little in five days period after the earthquake. 10e-6 m/s, with a considerable similarity to the actual rainfall intensity, is chosen as the precipitation boundary. The accumulated precipitation reached 86.4 mm in 24 hours. 3.3.3 Selection of calculation parameters: A strong motion record of the earthquake on May 23rd recorded by the seismic station was chosen to simulate the earthquake with 10 s and a peak acceleration of 0.81m/s2. The attenuation relations of

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ground motion were reported that the isoseismal lines of earthquakes above moderate intensity are mostly in the shape of ellipse in NWW direction. Thus, the

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constrained model of elliptical attenuation of earthquake intensity considering long

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and short axis was adopted in this work. On the basis of the Seismic Hazard Analysis

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Package (ESE) which considers the heterogeneity of seismic activity and is recommended by the China Earthquake Administration(CEA), the time history curve

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of ground motion with the exceedance probability of 10% was set up. Comprehensive analysis for the earthquake conditions was conducted at each interval step (A time

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step of 0.02 s for modeling, totally 500 interval steps for a 10-s vibration). The revised simulated time series of seismic ground motion with a seismic acceleration

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(horizontal acceleration in this paper) of 0.3g is shown in Fig. 3 The calculation parameters were selected for stability evaluation in accordance with actual geological conditions as well as the modes of destruction in simulation areas. The engineering geological survey was carried out and the field direct shear test and field deformation test of a rock mass were conducted. Then combined with relevant field reports, the material characterizations such as internal friction angle, cohesion force, elastic modulus, and Poisson’s ratio were determined synthetically. In this paper, Mohr-Coulomb elastic-plastic model is used and the calculation parameters are shown in Table 2.

Journal Pre-proof 0.3

Acceleration / ms-2

0.2 0.1 0.0 -0.1

-0.3 0

2

4

6

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-0.2

8

10

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Seismic duration / s

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re

-p

Fig. 3 Revised simulated time series of seismic ground motion

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Table 2. Mechanical parameters for rock and soil mass structural surfaces Elastic Poisson’s modulus Before After Internal friction (MPa) ratio (μ) reinforcement reinforcement Angle φ/(º)

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Category of rock and Unit weight soil (KN/m3)

Colluvium & talus accumulation Intense weathering Mild weathering Geological fault Bedrock

Shear strength (C/(MPa))

18.9

20

43

31

767.28

0.27

21.5 25.5

90 500

114 500

35 41

2,496 39,936

0.3 0.18

21

60

92

30

39936

0.35

26.5

1200

1200

43

39936

0.49

In addition, the soil-water characteristic curve (SWCC) representing the relationship between water content and suction, and the hydraulic conduction equation (HFC) representing the relationship between permeability coefficient and suction are two important hydraulic characteristics in the analysis of unsaturated soil

Journal Pre-proof infiltration. Soil-water characteristic curve according to VG model (Van, 1980) is as follows:

s  r  m  r  1   h n   (h )       s

h<0 (9)

h0

where θ is volumetric water content (%); θs is saturated water content (%); θr is residual water content (%); α and n are curve shape parameters, and the unit of α is

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m-1; m=1-1/n.

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The corresponding functional expression of the hydraulic conduction equation is as follows (Mualem and Yechezkel, 1976; Ippisch, et al., 2006)

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m 1   h n       n m /2 1   h     n 1

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h

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   1     k (h)  ks   ks

2

h<0

(10)

h0

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where ks is saturated permeability coefficient. Based on the field data, the saturated permeability coefficients in bedrock and

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mild weathering strata (including geological faults) are close to constant, 1.25e-6 and 1.0e-10 m/s, respectively. The saturated permeability coefficients in colluvium and talus accumulation and intense weathering strata are 0.75 and 1.02 m/s, respectively whose soil-water characteristic curve and permeability coefficient curve are estimated by Eq. (9) and Eq. (10), as shown in Fig. 4.

1 0.1 0.01 1E-3 Severe weathering strata 1E-4 1E-5 Colluvial deosit 1E-6 1E-7 1E-8 Fitting data 1E-9 Experimental data 1E-10 1E-11 0.01 0.1 1 10 100 Matric suction / kPa

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0.5 0.4

Severe weathering strata

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0.3

Colluvial deosit

0.1

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0.2

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Volumetric water content / m3m-3

0.6

(b)

1000

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(a)

Conductivity / ms-1

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0.0 0.01

0.1

Fitting data Experimental data

1 10 Matric suction / kPa

100

1000

Fig. 4 Parameter curve for unsaturated permeability: (a) HFC; (b) SWCC

3.4 Stability analysis of earthquake and consequent rainfall Based on the Morgenstern-Price method, we calculated the slope stability for five selected areas, including underlying bedrock, weathering zones, colluvium and talus accumulation and so on by using GeoStudio software. In order to evaluate the slope stability on the two conditions, that is, natural state plus earthquake conditions in the first stage; and post-earthquake rainfall conditions based on the first stage of stress and displacement fields and underground water level in the second stage, the cutting-in and cutting-out methods were adopted to examine the potential sliding

Journal Pre-proof surface position. Detailed analysis such as stress field and displacement field on the two conditions was carried out. 3.4.1 Analysis on stress field of slope under seismic activity Minimum Effective Stress -200 - 0 kPa 0 - 200 kPa 200 - 400 kPa 400 - 600 kPa 600 - 800 kPa 800 - 1,000 kPa 1,000 - 1,200 kPa 1,200 - 1,400 kPa 1,400 - 1,600 kPa 1,600 - 1,800 kPa 1,800 - 2,000 kPa

Minimum Effective Stress -200 - 0 kPa 0 - 200 kPa 200 - 400 kPa 400 - 600 kPa 600 - 800 kPa 800 - 1,000 kPa 1,000 - 1,200 kPa 1,200 - 1,400 kPa 1,400 - 1,600 kPa 1,600 - 1,800 kPa 1,800 - 2,000 kPa

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na

(b)

lP

re

-p

ro

of

(a)

(c)

X-Total Stress ≤ -500 - 0 kPa 0 - 500 kPa 500 - 1,000 kPa 1,000 - 1,500 kPa 1,500 - 2,000 kPa 2,000 - 2,500 kPa 2,500 - 3,000 kPa 3,000 - 3,500 kPa 3,500 - 4,000 kPa 4,000 - 4,500 kPa ≥ 4,500 kPa

Fig. 5 Stress distribution under seismic load: (a) Initial effective stress;(b) Effective Stress after Earthquake; (c) Stress field after earthquake

Journal Pre-proof Comparing the initial effective stress distribution (Fig. 5a) with the effective stress contours after Earthquake (Fig. 5b), the slope performance is reduced due to the lowered shear strength, especially in the colluvium and talus accumulation at the foot of the slope with weak rocks lithology. The effective stress decreases by more than 50%. Under the action of earthquake, the seismic acceleration is sharply amplified at the top of the slope due to the topographic amplification effect, resulting in an instantaneous tension load, which causes tension cracks subsequently and weakening

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of the tensile strength at the top of the slope. Fig. 5 (c) shows the distribution of the stress field at the end of the seismic loading. It can be seen that the maximum tensile

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stress near the geological fault at the top of the slope exceeds 0.5 MPa, causing a large

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number of fine tensile cracks.

re

450

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300 250

0s 4s 6s 10s

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350

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(a)

Elevation / m

400

200

150 370

372

374

376

378

380

Pore-water pressure / kPa

382

384

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M.P 1# M.P 2# M.P 3# M.P 4# M.P 5# M.P 6#

30 25 20 15 10 5

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(b)

Excess pore-water pressure / kPa

35

0 1

2

3

4 5 6 7 Seismic duration / s

8

9

10

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0

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Fig. 6 Pore-water pressure distribution:(a) Pore-water pressure curve at different

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elevations; (b) Excess pore-water pressure curve with seismic duration

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Figs. 6a and 6b show the pore water pressure distribution at different stress states. The pore water pressure of slope increases continuously, with both the increasing

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duration of seismic motion and the elevation of the observation point. In strata with poor permeability, such as bedrock, the excess pore-water pressure increases linearly.

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While in the poor weak strata with large permeability coefficient, which are usually located at the slope surface (monitoring point 1 #, monitoring point 2 #, etc.), the excess pore water pressure increases linearly at first, and then tends to be stable gradually. It can be seen that the excess pore water pressure around the geological faults at the foot and top of the slope is relatively high where shear slip is more likely to occur. 3.4.2 Analysis on displacement field of slope under seismic activity

Journal Pre-proof X-Displacement

X-Displacement

X-Displacement

≤ -0.1 - -0.05 m -0.05 - 0 m 0 - 0.05 m 0.05 - 0.1 m 0.1 - 0.15 m 0.15 - 0.2 m 0.2 - 0.25 m 0.25 - 0.3 m 0.3 - 0.35 m 0.35 - 0.4 m 0.4 - 0.45 m 0.45 - 0.5 m ≥ 0.5 m

≤ -0.14 - -0.12 m -0.12 - -0.1 m -0.1 - -0.08 m -0.08 - -0.06 m -0.06 - -0.04 m -0.04 - -0.02 m -0.02 - 0 m ≥0m

(a)

(b)

≤ -0.6 - -0.4 m -0.4 - -0.2 m -0.2 - 0 m 0 - 0.2 m 0.2 - 0.4 m 0.4 - 0.6 m ≥ 0.6 m

(c)

X-Displacement

X-Displacement ≤ 0 - 0.1 m 0.1 - 0.2 m 0.2 - 0.3 m 0.3 - 0.4 m 0.4 - 0.5 m 0.5 - 0.6 m 0.6 - 0.7 m 0.7 - 0.8 m ≥ 0.8 m

(e)

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(d)

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≤ -0.6 - -0.4 m -0.4 - -0.2 m -0.2 - 0 m 0 - 0.2 m 0.2 - 0.4 m 0.4 - 0.6 m ≥ 0.6 m

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Fig. 7 Development of displacement field over time under seismic load: (a)

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Displacement distribution after 1s; (b) Displacement distribution after 3s; (c) Displacement

10s.

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distribution after 5s; (d) Displacement distribution after 8s; (e) Displacement distribution after

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Fig.7 shows the horizontal displacement field distribution of the slope in the first, third, fifth, eighth and tenth seconds (corresponding a, b, c, d, e, respectively) during

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the occurrence of seismic loading. During the earthquake, the maximum horizontal displacement of the slope with seismic load exceeds 0.9 m, which is mainly located in the colluvium and talus accumulation with poor lithology. It can also be seen that in the first 2-3s of the earthquake, the higher peak acceleration leads to the obvious displacement stratification phenomenon of bedrock with better lithology. This could also cause further amplification effects. Then, with the decrease of peak acceleration, the amplitude of seismic vibration decreases, and the displacement stratification phenomenon gradually disappears (see Fig. 7e).

Journal Pre-proof 0.06 M.P 2# M.P 4# M.P 6#

(a)

Shear strain

0.04 0.02 0.00 -0.02 -0.04

M.P 2# M.P 4# M.P 6#

-0.1

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0.0

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0.1

10

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(b)

X-relative displacement

0.2

4 6 8 Seismic duration / s

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0.3

2

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0

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-0.06

-0.2

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-0.3

0

2

4 6 8 Seismic duration / s

10

Fig.8 Deformation law for the slope under seismic load: (a) Shear strain curve; (b) Relative X-displacement curve

As shown in Fig. 8, there is a significant vertical amplification effect and hysteresis effect that the closer the monitoring point is to the top of the slope, the greater the fluctuation range of the displacement values is observed. Meanwhile, potential tension-shear slip surface gradually come to form due to seismic load and the increment in the shear strain of the slope increases (see Fig.8b), which results in the decrease of X-displacement of the slope with the increase of the slope height. The

Journal Pre-proof maximum permanent displacement at the bottom of the slope, shows a clear horizontal slip trend In the study of local seismic records, it is found that after the earthquake on February 3, 2012, several aftershocks occurred in the next few hours, with magnitudes below 6. Although the peak acceleration of these aftershocks was small, some of them lasted for a long while, some even for several days. This paper interprets some aftershock seismic records where the duration of seismic events varies from 10 to 30s,

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with an amplitude of 0.2g. As shown A, B, C, D, and E in Fig. 1, five monitoring points are selected, located at the foot of the slope, colluvium and talus accumulation,

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intense weathering layer and bedrock, respectively, to discuss the influence of the

0.8 M.P A# M.P B# M.P C# M.P D# M.P E#

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lP

0.6

0.4 0.3 0.2

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0.5

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(a)

X-Displacement / m

0.7

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duration of earthquake on the dynamic response of the slope.

0.1 0.0

10

15

20

25

30

Seismic duration / s

35

40

Journal Pre-proof 0.6

M.P A# M.P B# M.P C# M.P D# M.P E#

(b)

Y-Displacement / m

0.5 0.4 0.3 0.2 0.1 0.0

-0.2 8

12

16

20

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-0.1

24

28

32

36

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Seismic duration / s

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Fig.9 Development of cumulative displacement over time under seismic loading: (a)

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X-displacement curve; (b) Y-displacement curve

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From the simulation results, the effect of duration of shaking on the peak acceleration response is not obvious, but it is of significance to the displacement of

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the slope, as shown in Fig.9. With the continuous increase of the duration, the displacement of monitoring points increases significantly. When the duration

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increases to 20 s, the displacements at A, C and D tend to converge, while the displacement at B colluvium and talus accumulation in the sliding body is still increasing. This indicates that with the continuous release of the energy during the earthquake, the damage of soil in the slope is accumulating, but the slope with better lithology, equipped with self-resistance to resist earthquake damage, is not susceptible to instability. It also shows that the slope instability has a significant accumulation effect as a result of cyclic weakening for rock and soil. 3.4.3 Stability analysis of slope under seismic activity

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1.129

Safety Factor after earthquake

Critical Slip Surface Slip Surface 121

1.40 1.43

1.84

Slip Surface 96 1.86 Slip Surface 67

1.13

1.45 1.40 1.35 1.30 1.25 1.20 0

2

4

6

8

10

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Safety Factor

Slip Surface 41

Seismic duration / s

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Fig. 10 Development of FOS over time while earthquake

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As parts of the rock and soil mass in the geotechnical structure undergoes sudden strength damage, the stress will be readjusted and distributed. The stress field and

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displacement field under stress redistribution after earthquake have been analyzed.

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Earthquake triggers tension area near the geological fault on the top of the slope and causes a tensile stress field at the top of the slope, while shear strength reduction

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occurs at the foot of the slope in colluvium and talus accumulation. Furthermore, a certain permanent deformation of the slope occurs after the earthquake. Under seismic

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activity, the safety factor of the slope has been changing always within the range of 1.185-1.43(see Fig. 9). Fig. 10 also shows the safety factors of different sliding surfaces. According to the results of numerical simulation, the safety factor of most dangerous potential sliding surface after earthquake is 1.129. This indicates that the slope only suffers from damage and no complete failure or coherent slide develops, which agrees on the record for field observation. Under seismic load, due to the effect of topographic amplification, high displacement rates are usually located in the lower consolidated debris and soils while the acceleration of earthquake is sharply amplified at the top of the slope. This results in instantaneous tensile stress, which causes tension cracks and decreases in tensile strength at the top of the slope. The existence of cracks contributes to subsequent rainwater infiltration after earthquakes where the increase of pore water pressure in

Journal Pre-proof slope leads to the reduction of effective stress in soil mass and the activation of potential sliding mass. 3.5 Stability analysis of slope on rainfall infiltration

Geological Fault

Initial water level 1st Day

Severe Weathering Strata

2nd Day 3rd Day Colluvial Deosit & Talus Accumulation

4th Day

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5th Day

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Bedrock

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Fig.11 Development of slope water level during a 5-day rainfall period

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As shown in Fig. 11, owing to the difference between rainfall intensity and permeability of rock and soil, a transient saturation zone is formed after a rainfall

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period, resulting in the rise of water level and the increase of groundwater pressure, which negatively effects the slope stability. Before the earthquake, the matrix suction

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(namely, negative pore water pressure) increases linearly with slope height (see Fig.12). With the infiltration of rainwater, the matrix suction of soil decreases, and the permeability coefficient increases, which accelerates the water erosion into soil. On the first day of rainfall, the matrix suction increases first, and then an inflection point appears near the junction of the colluvium and talus accumulation and intense weathering layers (H = 425m).Moreover, it can be seen that the matrix suction in the unsaturated area of intense weathering layers begins to increase with the continuous rainfall, and the matrix suction tends to decrease at H = 415 m when the rainfall lasts to the fifth day.

Journal Pre-proof 440

420 410 initial state 1st Day 2nd Day 3rd Day 5th Day

400 390

-400

-300 -200 -100 0 Pore-water pressure / kPa

100

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380 -500

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Elevation / m

430

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Fig.12 Influences of rainfall on matrix suction

The process of generating and dissipating the excess pore water pressure

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adversely affects the slope. The excess pore water pressure caused by the earthquake is the main factor causing the deformation and failure of the slope. The excess pore

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water pressure tends to dissipate as time lapses when the rise of the groundwater level in the slope during the rainfall will affect the dissipation and its execution. Here,

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monitoring points A, D and E (see Fig.1) are taken in the colluvium and talus accumulation, intense weathering strata and bedrock, respectively, and the effect of different rainfall duration on the variation of excess pore water pressure is analyzed, as shown in Fig. 13. For the colluvium and talus accumulation and intense weathering strata, due to the high permeability, the excess pore water pressure dissipates rapidly. However, the water level increases, the matrix suction of the soil decreases continuously with the rainfall infiltration, leading to an increase in permeability. And rainfall infiltration makes a slower dissipation rate of excess pore water pressure, which has the greatest effect on the colluvium and talus accumulation on the slope surface. For the bedrock, due to the small permeability coefficient, the excess pore water pressure dissipates so slowly, which rainfall infiltration has little effect on.

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1st Day 3rd Day 5th Day

6

4

2

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Excess pore-water pressure / kPa

8

0 4

6 8 Time / Day

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2

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10

lP

8

2

na

6 4

12

1st Day 3rd Day 5th Day

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12

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(b)

Excess pore-water pressure / kPa

(a)

10

0

0

2

4

6 8 Time /Day

10

12

14

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(c)

Excess Pore-water Pressure / KPa

30 1st Day 3rd Day 5th Day

25

20

15

4

6 8 Time /Day

10

12

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2

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10

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Fig.13 Effect of rainfall on dissipation of excess pore water pressure in different strata:

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(a)for the colluvium and talus accumulation; (b)for the intense weathering strata; (c)for the bedrock

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2.0 1.8

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1.4

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Factor of safety

1.6

Critical Slip Surface Slip Surface 121 Slip Surface 96 Slip Surface 67 Slip Surface 41

1.2 1.0 0.8 0.6

0

1

2 3 Rainfall duration / Day

4

5

Fig. 14 Effect of rainfall duration on the factor of safety

Fig. 14 shows the change of safety factor with rainfall. The safety factor decreases with the continuous increase of rainfall, which may lead to a failure. Among them, the critical slip surface will lose stability on the fourth day of rainfall, and the range of landslide will expand with the continuous rainfall.

Journal Pre-proof On the one hand, the infiltration of rainwater leads to an increase in the water content in the unsaturated zone of the slope, which reduces the matrix suction, resulting in a decrease in the shear resistance of the soil. Furthermore, it slows down the dissipation of excess pore water pressure. On the other hand, the contact with the loose sliding soil causes the soil to soften and the shear surface to penetrate, which further accelerates the deformation and eventually destabilizes the slope. 4 Plant reinforcement technology and its application

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Plant Reinforcement Technology (PRT) is used to stabilize slope by planting

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hierarchically fast-growing shrub branches or rhizomes in slope to form reinforced

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soil. The mechanism of biotechnical slope protection was studied from three aspect of mechanical function of roots system, hydrologic effect of stem and leaf (includes dead

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branches and fallen leaves), and transpiration effect of plants. Furthermore, we

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summarized the contribution of Plant Reinforcement Technology (PRT) to slope stability in three ways: (1) anchorage of deep roots and reinforcement of shallow roots;

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(2) rainfall interception and reduction in surface erosion; (3) change for the pore water pressure. (Gray and Sotir , 1996; Chen et al., 2007; Burylo et al., 2011). The cutting

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rhizome branches can play a reinforcing role immediately. Plant roots system can be divided into three types: herb-root system, horizontal and vertical woody plant root system. The reinforcement mechanism of roots system is different, including slope reinforcement of shallow roots and slope anchoring of deep roots. Generally, slope reinforcement of shallow roots and hydrological effect of stem and leaf are for slope erosion controlling; while slope anchoring of deep roots and transpiration effect of plants are for slope reinforcement. With the growth of the internal rhizome, the reinforcing role continues along with the slope depth and the scope of reinforcing enlarges, which is especially suitable for treating surface erosion of slope and shallow landslide. 4.1 Mechanical function of root system

Journal Pre-proof The tensile strength of grass roots will restrict the deformation of soil, and the reinforcement effect is related to the density, strength and properties of grass roots. The root-soil interaction can be regarded as a pattern of fiber-reinforced soil without strength and size differences among roots being considered. The effect of fiber reinforcement on the strength of composite media can be illustrated by the equivalent confining pressure theory and apparent cohesion (Burylo et al., 2011). Root fiber improves the shear strength of soil mainly by transforming the shear

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stress in soil into the tensile stress of roots through the friction of the root-soil

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interface (Waldron, 1977; Shewbridge and Sitar, 1990), as shown in Fig.15. Deformed Roots

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Undeformed Root



Tr

Shear Zone



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

Fig.15 Mechanical effect of root anchorage

The root-soil composite medium system is similar to reinforced concrete with relatively large tensile strength where tensile stress is transferred to the surrounding soil through root-soil bond between roots and soil. Wu et al. (1979) proposed a vertical root model for shear strength of root-soil composite medium, assuming that the root surface is sufficiently frictional and restrained to prevent the root from being pulled out. When shear stress is produced in soil, the dislocation displacement of the root causes the root to elongate and the tensile force Tr to generate in the roots. The component of Tr along the tangent direction of the shear plane can directly resist the shear deformation while that along the normal direction can increase the normal stress

Journal Pre-proof on the shear plane. Therefore, the increased shear strength of root-soil composites Δc is as follows:

c  Tr ( Ar / A)(sin   cos tan  )

(11)

where Tr is the average tensile force of the roots (N); Ar/A is the root coverage rate on the shear plane; θ is the root inclination angle (°); and φ is the internal friction angle (°). For the herb-root system and woody plant root system, the value of Tr is different, which is discussed as follow.

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Firstly, for herb-root system, the roots of shallow rooted plants twine in the

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surface of soil layer, forming a reinforced composite protection layer with a thickness of about 30 cm. Mechanical reinforcement effect of the root system is reflected in two

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aspects: (1) the increased cohesion Δc of root-soil composites;(2) the wrapping effect

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of “string bag” on soil particles - the increase of cohesion prevents the shear failure of water flow and the formation of tension cracks in slope surface. The wrapping effect

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of “string bag” weakens the dissociation of water flow on soil particles. The tension

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crack at the top of the slope is formed by the joint action of the horizontal displacement from the top of the slope to the free surface and the contraction of the

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surface soil due to water loss. And the increased tensile strength Tr by the reinforcement of plant roots is:

Tr 

1 m Ti sin i B i 1

(12)

where there are m roots in the soil with area B (cm2); T is the tensile strength of single plant root; and Θi is the acute angle between the plant root and the crack face (i = 1, 2, …, m). Besides, the strong vertical taproot of woody plants is deeply rooted into the soil layer. Vertical deep roots passing through the weak layer or sliding surface strengthen the by means of anchoring and shaft friction, which acts a bit like the bolt or anti-slide pile. The root system is simplified as a full-length adhesive bolt with the taproot as the axial and the lateral roots as the branch to calculate the anchorage force

Journal Pre-proof of vertical taproot, which focus on segments of roots with a diameter greater than 1 mm at the depth arranging from h1 to h2 under the slope surface. The maximum anchorage force provided by roots in the depth range of h1-h2 can be deduced as follow: h2

Tr  2  P( z )Q( z ) zdz h1

(13)

where μ is the static friction coefficient between roots and soil. P (z) and Q (z) are determined by field measurement and then fitting data. More detailedly, the

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vertical roots are divided into n segments horizontally within the extension range of

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roots. The number of roots Ni and the average radius Ri of roots are obtained for any segment [i, i + 1] (1 ≤ i ≤ n-1) by statistical measurement. And then, P (z) and Q (z)

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4.2 Transpiration effect of plants

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are obtained by fitting Ri and Ni, respectively.

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Woody plants with strong taproot, deeply embedded in the soil, can absorb the pore water in deep of the slope. Plants exert tension of 1-2 MPa on pore water through

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photosynthesis and transpiration before wilting (Fredlund and Rahardjo,1993). Plant roots continuously absorb water from the soil, making water in the soil and pore water

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pressure decrease. In-situ monitoring shows pore water pressure in soil is generally negative within the effected area of woody plants with strong taproot. The decrease of pore water pressure enhances the shear strength of soil, which is calculated quantitatively by the expression of unsaturated shear strength with consideration of root reinforcement as follows:  f  c  c  (  ua ) tan    (ua  uw ) tan  b

(14)

4.3 Hydrologic effect of stem and leaf The erosion effect of water flow on the slope surface is weakened, or the slope surface is protected from erosion and water-proof erosion through weakening the splash erosion of raindrops and inhibiting the erosion of runoff on the slope surface. Runoff is reduced by 68% and 98% while the sediment produced by erosion is

Journal Pre-proof reduced by 95% and 98%, respectively for slopes with vegetation cover of 60% and 100% with rainfall intensity of 0.81 mm / min and duration of 30 min (Wang et al., 2004; Li, et al., 2004). In the model, the hydrologic effect of stem and leaf was quantitatively described as the decrease of runoff on the slope surface. In the simulation, the surface layer of colluvium and talus accumulation and part of intense weathering layer are used as vegetation growth layer. And deep-rooted shrub Quercus humilis is planted near the geological faults in the stress-stretching

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area at the top of the slope and in the colluvium and talus accumulation. Herbal shallow-rooted herb vetiver was used on the downslope surface. The parameters of

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the simulation are selected according to the Morris and Stormont (1997), Zornberg et

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al (2003) and Burylo (2011). The root diameter is set 0.19-1.48 mm, and the density is

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0.5 kg/m3. LAI=1 is selected for shrubs and root depth is 0.8 m; LAI=2 is selected for herbs, and root depth is 0.2m.

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4.4 Analysis of strengthening effect of plant reinforcement

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Water level after roots-reinforce Water level without reinforce

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Initial water level

1.6

Safety of factor

1.5 1.4 1.3

1.25

1.2

1.20

1.1

Dynamic safety factor 1.15 1.10 Safety factor in rainfall 1.05

0.0

2.5

5.0

Time / s

7.5

1 10.0

2

3

4

5

Time / Day

Fig.16 With subsequent earthquake triggered rainfall for a plant reinforced slope.

Fig. 16 shows the variation of safety factor of the slope and the water level change caused by post-earthquake rainfall through plant reinforcement treatment. It

Journal Pre-proof can be seen that the safety factor of the landslide increases due to the increase of cohesion of the slope soil layer, which is not only reflected in the rainfall condition, but also in the seismic condition where the stability of the slope increases significantly. On the one hand, the root system directly enhances the stability of the slope due to the improved cohesion, and greatly restricts the formation and development of the crack at the top of the slope as a result of enhanced tensile strength of roots system; On the other hand, the root system reduces the rainfall infiltration in the weak stratum of the

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slope, and strengthens the surface runoff, causing significant reduction of pore pressure values, and acceleration of the excess pore-water pressure dissipation, which

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substantially improves the stability of the slope. Thus, it comes to a conclusion that

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the plant reinforcement technology is effective in dealing with slope stability under

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seismic load due to earthquakes and rainfall.

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5 Conclusions

In this paper, taking a slope in Southwest China as an example, slope stability

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and failure mechanism under continuous rainfall after an earthquake are analyzed with consideration of seismic load, unsaturated infiltration and improved plant

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reinforcement technology using GeoStudio software. Summarizing the results from the different investigations, the following conclusions are drawn: 1. Under seismic load, high displacement rates are usually located in the lower consolidated debris and soils. Topographic amplification causes tension stress at the peak, which triggers tension cracks and decreases in tensile strength at the top of the slope. 2. The existence of cracks contributes to post-earthquake rainwater infiltration which slows down the dissipation of excess pore water pressure. Slope instability is ultimately caused by soil weakening due to seismic load and reduction in dissipation rate of pore water pressure. 3. Plant Reinforcement Technology can significantly improve the stability of the slope and is proved effective by selectively planting in weak or damaged areas for

Journal Pre-proof dealing with the damage and instability of slopes subjected to seismic activity and subsequent heavy rainfall. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51608323) and Natural Science Foundation of Shanghai (Grant No. 16ZR1423300). The authors would like to thank Dr. Rafig Azzam, Dr. Ning Li and Dr. Wang Suran for helpful discussions, research team under Dr. Chen Youliang and Li

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Ning for financing this study. Data Availability

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Some or all data, models, or code generated or used during the study are

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available from the corresponding author by request.

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Data in this paper are presented in the form of Figs. 3-15 and Fig. 18, which are

Reference

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available from the corresponding author by request if the source data needed.

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Alonso, E., Gens, A., Lloret, A., and Delahaye, C. (1995). Effect of rain infiltration on the stability of slopes. In proceedings of the first international conference on unsaturated soils/unsat'95/paris/france/6-8 september 1995. volume 1. Bandara, S., Ferrari, A., and Laloui, L. (2016). Modelling landslides in unsaturated slopes subjected to rainfall infiltration using material point method. International Journal for Numerical and Analytical Methods in Geomechanics, 40(9), 1358-1380. DOI: 10.1002/nag.2499 Bishop, A.W. (1955) The Use of the Slip Circle in the Stability Analysis of Earth Slopes. Geotechnique, 5(1), 7-17. DOI: 10.1680/geot.1955.5.1.7 Burylo, M., Hudek, C., and Rey, F. (2011). Soil reinforcement by the roots of six dominant species on eroded mountainous marly slopes (southern alps, france). Catena, 84(1-2), 0-78. DOI: 10.1016/j.catena.2010.09.007. Chang, K. T., Lin, M. L., Dong, J. J., and Chien, C. H. (2012). The hungtsaiping landslides: from ancient to recent. Landslides,9(2), 205-214. DOI 10.1007/s10346-011-0293-5 Chen, H., and Hawkins, A. B. (2009). Relationship between earthquake disturbance, tropical rainstorms and debris movement: an overview from

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Taiwan. Bulletin of Engineering Geology and the Environment, 68(2), 161-186. DOI: 10.1007/s10064-009-0209-y Chen, J. B.,Kong, L. W.,Zhao Y.L.,Lu H.B.(2007). On influence factors of suction and deformation of unsaturated soil under evaporation and transpiration effect. Rock and Soil Mechanics, 28(9), 1767-1772(in Chinese). Cho, S. E. (2016). Stability analysis of unsaturated soil slopes considering water-air flow caused by rainfall infiltration. Engineering geology, 211, 184-197. DOI: 10.1016/j.enggeo.2016.07.008 Clague, J. J., & Stead, D. (Eds.). (2012). Landslides: types, mechanisms and modeling. Cambridge University Press. Domenico, P., and Schwartz, F. W. (1997). Physical and chemical hydrogeology. Physical & Chemical Hydrogeology,28(6). DOI: 10.2134/jeq1999. 00472425002800060047x Fredlund, D. G., Morgenstern, N. R., & Widger, R. A. (1978). The shear strength of unsaturated soils. Canadian Geotechnical Journal, 15(3), 313-321. DOI: 10.1139/ t78-029 Gray, D. H., & Sotir, R. B. (1996). Biotechnical and soil bioengineering slope stabilization: a practical guide for erosion control. John Wiley & Sons. Gottardi, G., & Venutelli, M. (1993). Richards: computer program for the numerical simulation of one-dimensional infiltration into unsaturated soil. Computers and Geosciences,19(9), 1239-1266. DOI: 10.1016/0098-3004(93)90028-4 Hong, Y. S., Chen, R. H., Wu, C. S., & Chen, J. R. (2005). Shaking table tests and stability analysis of steep nailed slopes. Canadian Geotechnical Journal, 42(5), 1264-1279. DOI: 10.1139/T05-055 Hu, X., and Chen, Z. L. (2011). Dynamic response of horizontal bedded rocky slopes under earthquakes and influence of ground motion parameters. Applied Mechanics and Materials,90-93, 570-573. DOI ： 10.4028/www.scientific.net/AMM.90-93.570 Huang, R., and Fan, X. (2013). The landslide story. Nature Geoscience, 6(5), 325-326. Huang, R., & Li, W. (2014). Post-earthquake landsliding and long-term impacts in the Wenchuan earthquake area, China. Engineering Geology, 182, 111-120. DOI： 10.1016/j.enggeo.2014.07.008 Huang, Y., & Xiong, M. (2017). Dynamic reliability analysis of slopes based on the probability density evolution method. Soil Dynamics and Earthquake Engineering, 94, 1-6. DOI: 10.1016/j.soildyn.2016.11.011 Iwata, N., Yoshinaka, R., and Sasaki, T. (2013). Applicability of the seismic response analysis using multiple yield model for discontinuous rock. International Journal of Rock Mechanics and Mining Sciences, 60(8), 196-207.DOI: 10.1016/j.ijrmms.2012.12.039 Ippisch, O., Vogel, H. J., & Bastian, P. (2006). Validity limits for the van Genuchten-Mualem model and implications for parameter estimation and numerical

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

simulation. Advances in Water Resources, 29(12), 1780-1789. DOI: 10.1016/j. advwatres.2005.12.011 Jibson, R. W., Harp, E. L. (2016). Ground Motions at the Outermost Limits of Seismically Triggered Landslides. Bulletin of the Seismological Society of America, 106(2), 708-719. DOI: 10.1785/0120150141 John K. Seep modeling with SEEP/W2007 version[R]. Calgary, Canada: Geoslope International Ltd., 2008: 266-268. Jorge A. Salinas-Jasso., Luis G. Ramos-Zuñiga., & Juan C. Montalvo-Arrieta. (2019). Regional landslide hazard assessment from seismically induced displacements in Monterrey Metropolitan area, Northeastern Mexico. Bulletin of Engineering Geology and the Environment, 78(2), 1127-1141. DOI: 10.1007/s10064-017-1087-3 Jun, X., Ming, W., Kai, L., & Coulthard, T. J. (2018). Modeling sediment movement and channel response to rainfall variability after a major earthquake. Geomorphology, 320,18-32. DOI：10.1016/j.geomorph.2018.07.022 Kanaori, Y., & Kawakami, S. I. (1996). The 1995 7.2 magnitude Kobe earthquake and the Arima-Takatsuki tectonic line: implications of the seismic risk for central Japan. Engineering Geology, 43(2-3), 61-82. DOI ： 10.1016/S0165-1250(97)80006-5 Kimoto, S., Oka, F., and García, E. (2013). Numerical simulation of the rainfall infiltration on unsaturated soil slope considering a seepage flow. Geotechnical Engineering, 44(3), 1-13. Lam, L., Fredlund, D. G., & Barbour, S. L. (1987). Transient seepage model for saturated-unsaturated soil systems: a geotechnical engineering approach. Canadian Geotechnical Journal, 24(4), 565-580. DOI: 10.1139/t87-071 Lin, C. W., Liu, S. H., Lee, S. Y., and Liu, C. C. (2006). Impacts of the Chi-Chi earthquake on subsequent rainfall-induced landslides in central Taiwan. Engineering Geology, 86(2-3), 87-101. DOI：10.1016/j.enggeo.2006.02.010 Lin, M. L., & Wang, K. L. (2006). Seismic slope behavior in a large-scale shaking table model test. Engineering Geology, 86(2-3), 118-133. DOI: 10.1016/j.enggeo. 2006.02.011 Liu, S. H., Lin, C. W., & Tseng, C. M. (2013). A statistical model for the impact of the 1999 Chi-Chi earthquake on the subsequent rainfall-induced landslides. Engineering Geology, 156, 11-19. DOI: 10.1016/j.enggeo.2013.01.005 Marc, O., Meunier, P., & Hovius, N. (2017). Prediction of the area affected by earthquake-induced landsliding based on seismological parameters. Natural Hazards and Earth System Sciences, 17(7), 1159-1175. DOI: 10.5194/nhess-17-1159-2017 Moriwaki, H., Inokuchi, T., Hattanji, T., Sassa, K., Ochiai, H., and Wang, G. (2004). Failure processes in a full-scale landslide experiment using a rainfall simulator. Landslides, 1(4), 277-288. DOI: 10.1007/s10346-004-0034-0 Morris, C. E., and Stormont, J. C. (1997). Capillary barriers and subtitled covers: estimating equivalency. Journal of Environmental Engineering, 123(1), 3-10. DOI: 10.1061/(asce)0733-9372(1997)123:1(3)

Journal Pre-proof

Jo ur

na

lP

re

-p

ro

of

Mualem, & Yechezkel. (1976). A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resources Research, 12(3), 513-522. DOI: 10.1029/wr012i003p00513 Okura, Y., Kitahara, H., Ochiai, H., Sammori, T., and Kawanami, A. (2002). Landslide fluidization process by flume experiments. Engineering Geology, 66(1), 65-78. DOI: 10.1016/S0013-7952(02)00032-7 Pantelidis, L., & Griffiths, D. V. (2013a). Stability of earth slopes. Part I: two dimensional analysis in closed-form. International Journal for Numerical and Analytical Methods in Geomechanics, 37(13), 1969-1986. DOI: 10.1002/nag.2118 Pantelidis, L., & Griffiths, D. V. (2013b). Stability of earth slopes. Part II: three dimensional analysis in closed‐ form. International Journal for Numerical and Analytical Methods in Geomechanics, 37(13), 1987-2004. DOI: 10.1002/nag.2116 Sasahara, K., and Sakai, N. (2014). Development of shear deformation due to the increase of pore pressure in a sandy model slope during rainfall. Engineering Geology, 170(4), 43-51. DOI: 10.1016/j.enggeo.2013.12.005 Sergio A. Sepúlveda, Murphy, W., Jibson, R. W., & Petley, D. N. (2005). Seismically induced rock slope failures resulting from topographic amplification of strong ground motions: The case of Pacoima Canyon, California. Engineering Geology, 80(3-4), 336-348. DOI：10.1016/j.enggeo.2005.07.004 Shewbridge, S. E., and Sitar, N. (1990). Deformation‐ based model for reinforced sand. Journal of Geotechnical Engineering, 116(7), 1153-1170. DOI： 10.1061/(asce) 0733-9410(1990)116:7(1153) Sorbino, G., and Nicotera, M. V. (2013). Unsaturated soil mechanics in rainfall-induced flow landslides. Engineering Geology, 165, 105-132. DOI ： 10.1016/j.enggeo. 2012.10.008 Tang, C., Zhu, J., Li, W. L., and Liang, J. T. (2009). Rainfall-triggered debris flows following the wenchuan earthquake. Bulletin of Engineering Geology and the Environment, 68(2), 187-194. Van Genuchten, M. T. (1980). A closed-form equation for predicting the hydraulic conductivity of unsaturated soils 1. Soil science society of America journal, 44(5), 892-898. DOI: 10.2136/sssaj1980.03615995004400050002x Waldron, L. J. (1977). The shear resistance of root-permeated homogeneous and stratified soil. Soil Science Society of America Journal, 41(5), 843-849. DOI: 10.2136/ sssaj1977.03615995004100050005x Wu, T. H., McKinnell III, W. P., & Swanston, D. N. (1979). Strength of tree roots and landslides on Prince of Wales Island, Alaska. Canadian Geotechnical Journal, 16(1), 19-33. Yano, A., Shinohara, Y., Tsunetaka, H., Mizuno, H., and Kubota, T. (2019). Distribution of landslides caused by heavy rainfall events and an earthquake in northern Aso Volcano, Japan from 1955 to 2016. Geomorphology, 327, 533-541. DOI： 10.1016/j.geomorph.2018.11.024

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Figure captions Fig. 1 Typical profile of the slope and monitoring points Fig. 2 Stress state under natural state and seismic action: (a) Natural stress state ;(b-c)

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stress state under seismic action Fig. 3 Revised simulated time series of seismic ground motion

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Fig. 4 Parameter curve for unsaturated permeability: (a) HFC; (b) SWCC

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Fig. 5 Stress distribution under seismic load: (a) Initial effective stress distribution;(b)

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Effective stress distribution after earthquake; (c) distribution of stress field after earthquake

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Fig. 6 Pore-water pressure distribution:(a) Pore-water pressure curve at different elevations; (b) Excess pore-water pressure curve with seismic duration; (c)

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Distribution of excess pore-water pressure

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Fig. 7 Development of displacement field over time under seismic load: (a) Displacement distribution after 1s; (b) Displacement distribution after 3s; (c) Displacement distribution after 5s; (d) Displacement distribution after 8s; (e) Displacement distribution after 10s. Fig. 8 Deformation law with seismic duration: (a) Shear strain curve; (b) Relative X-displacement curve Fig. 9 Development of cumulative displacement over time under seismic loading: (a) X-displacement curve; (b) Y-displacement curve Fig. 9 Safety factor during earthquake Fig. 10 Development of FOS over time while earthquake Fig.11 Development of slope water level during a 5-day rainfall period Fig.12 Influences of rainfall on matrix suction

Journal Pre-proof Fig.13 Effect of rainfall on dissipation of excess Pore water pressure in different strata: (a)for the colluvium and talus accumulation; (b)for the intense weathering strata; (c)for the bedrock Fig. 14 Effect of rainfall duration on the factor of safety Fig. 15 Mechanical effect of root anchorage Fig. 16 With subsequent earthquake triggered rainfall for a plant reinforced slope. Table 1 Average rainfall and rainstorm statistics (Unit：mm)

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Table 2 Mechanical parameters for rock and soil mass structural surfaces

Journal Pre-proof CRediT author statement You-Liang Chen: Conceptualization, Methodology, Project administration, Funding acquisition. Geng-Yun Liu: Conceptualization, Methodology, Software, Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing, Visualization, Investigation. Ning Li: Software, Formal analysis , Visualization, Funding acquisition.

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Xi Du: Supervision, Writing- Reviewing and Editing.

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Su-Ran Wang: Validation

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Rafig Azzam: Validation.

Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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may be considered as potential competing interests:

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☐The authors declare the following financial interests/personal relationships which

Journal Pre-proof Highlights 

The failure mechanism of a slope is analyzed taking a combination effect of earthquakes and post-seismic rainfall into account.

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A model is developed that allows to investigate the main effects on the slope with consideration of dynamic load and unsaturated infiltration using GeoStudio software. The Plant Reinforcement Technology is suggested and how biological

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remediation would act in this case on the slope and improve strength are

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explored.

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