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Stability Analysis of Landslide Dam under Rainfall

Pei-Hsun Tsai1, Zheng-Yi Feng

2, Fan-Chieh Yu

3 and Jian-Han Lin

4

1 Associate Professor, Department of Construction Engineering, Chaoyang University of Technology,

168 Jifong E. Rd., Wufong District, Taichung, 41349, Taiwan; [email protected] 2 Professor, Department of Soil and Water Conservation, National Chung Hsing University, 250, Kuo-

Kuang Rd. Taichung, 40227, Taiwan; [email protected] 3 Professor, Department of Soil and Water Conservation, National Chung Hsing University, 250, Kuo-

Kuang Rd. Taichung, 40227, Taiwan; [email protected] 4 Graduate Student, Department of Construction Engineering, Chaoyang University of Technology, 168

Jifong E. Rd., Wufong District, Taichung, 41349, Taiwan; [email protected]

ABSTRACT: Failure of landslide dam may occur by river discharge and rainfall. A

rise of water level in the upstream of landslide dam and rain infiltrate into the dam

body will increase pore water pressure and weight of the dam. In this study, transient

seepage analysis of rainfall infiltration and dam stability analysis is performed. The

two-phase flow simulation using the finite difference code, FLAC, is adopted to

analyze unsaturated seepage flow in transient fluid-mechanical calculations. The

safety factor of dam stability is evaluated by the shear strength reduction technique.

The parameters discussed in the study include the rising speed of water level, rain

infiltrating and the hydraulic conductivity of soil. The results show that the slope

failure time of the dam is about 221 minutes when only the effect of rising water level

in the upstream is considered. The failure time becomes faster as 131 minutes when

both the rain infiltration and rising water level in the upstream are considered. The

results also indicate that the hydraulic conductivity of the dam influencing the failure

time of the dam.

INTRODUCTION

Numerous landslides and debris flows occur after heavy rainfall. The wasting

materials due to landslides and avalanches cound obstruct a river and create a

landslide dam. Unlike manmade dams with compacting process and filtering materials,

a landslide dam is formed by a mixture of unconsolidated soil/rock in a naturally

unstable state. Water level uasually rises rapidly in the upstream side of a landslide

dam due to continuous rainfall. The landslide dam is always unstable and dangerous

because flash flood could occur in the downstream area due to failure of the landslide

dam. Failure of landslide dam may occur in a number of processes which includes

overtopping, sliding and piping, etc. The sliding failure could occurr in the dam body

due to the increase of pore water pressure. When rainfall or rising water level in

upstream side occurs, the water infiltrating into dam causes the increase of water

content and pore water pressure in the landslide dam. This is a transient seepage flow

in unsaturated soils. The pore water pressure profile can be analyzed using a numerical

transient analysis of saturated/unsaturated seepage flow model. For stability analysis

of the landslide dams, the shear strength reduction technique is used to obtain the

safety factor of the dams and to locate the corresponding critical slip surface. A

number of studies including finite difference and finite element method performed 2D

transient seepage flow analyses using saturated-unsaturated seepage theory (Ng et al.,

1998; Xu et al., 2003; Huang and Jia, 2009; Fu and Jin, 2009). Some experimental

testing about the stability analysis of unsaturated slope has been studied by small-scale

models (Orense et al., 2004; Tohari et al., 2007; Schnellmann et al., 2010; Egeli and

Pulat, 2011).

We examine the relationship between the safety factor of dam stability by transient

analysis and the distribution of moisture inside the dam induced by influencing

parameters. In addition, parametrical study was performed by varying the rising speed

of water level and the saturated hydraulic conductivity of soil.

NUMERICAL MODEL

Landslide Dam Configuration

The FLAC code with the two-phase flow model was adopted to analyze the timing of

slope failure when water level rising in the upstream side. A typical configuration and

finite difference mesh for the landslide dam is shown in Fig. 1. The dam with height

H=4 m and length L=14 m was assumed to be situated above a riverbed. The slope

angles of the upstream and downstream faces of the dam were assumed as 34 , which

is close to the angle of repose of the dam materials.

2m 6m 2m 6m 2m

2m

airy boundary

seepage boundary

Impermeable boundary

saturated boundary

saturated boundary

airy boundary

saturated boundary

4m

FIG. 1. A typical finite difference mesh for the landslide dam in this study

To analyze the influence of the rising speed of water level in the upstream side on

dam stability, three different rising speed of water level (v=20, 40 and 80 cm/min)

were studied. To estimate the impacts of hydraulic conductivity of soil on stability of a

landslide dam, three different saturated hydraulic conductivity of soil (K= 31057.8 , 41057.8 and 51057.8 m/s) were used for analyses.

Seepage Flow Modeling of Unsaturated Soil

The transient seepage flow analysis of the dam after rising water level in the

upstream side is described by Richards’ equation.

1

y

h)h(K

yx

h)h(K

xt

hC yx , (1)

where h is the water pressure head, Kx(h) and Ky(h) are the hydraulic conductivity in x

and y direction, respectively. C is the specific moisture capacity, t is the time, x is

horizontal coordinate and y is vertical coordinate.

A model proposed by van Genuchten (1980) is used as the relationship between

water pressure head, moisture content and hydraulic conductivity. The relationship

between soil moisture and water pressure head is expressed as:

0h for 1

0h for

h1

1

Sm

e (2)

where and are constant parameters related with matric potential of soil,

)1(1m , Se is the effective saturation, which is defined as:

rs

reS

, (3)

where s and r are the saturated and residual moisture content, respectively. The

relationship between effective saturation and unsaturated hydraulic conductivity is

expressed as:

0hfor K

0h for )S1(1SKK

s

2mm1e

5.0es (4)

where s is the saturated hydraulic conductivity. For the numerical analysis, the

landslide dam and riverbed sediments were assumed to satisfy the Mohr-Coulomb

failure criteria. The engineering properties of the landslide dam and riverbed sediment

for the simulation are listed in Table 1. The landslide dam is assumed as an isotropic

medium and the parametric values of unsaturated soil model are as follows:

m=0.333, s=0.412, and r=0.0185.

Table 1. The material parameters of the landslide dam and riverbed sediment

Zone Density

(kg/m3)

Bulk

Modulus

(MPa)

Shear

Modulus

(MPa)

Cohesion

(kPa)

Friction

Angle(°)

Saturated

Hydraulic

Conductivity

(m/sec)

Dam 1571 49 19 1 35 8.57×10-4

Riverbed 1669 49 19 3 38 8.57×10-5

Procedures of the Simulation

The initial values of water pressure head and moisture content in the dam were firstly

specified. In the mechanical boundary condition, the bottom and two sides of the

riverbed sediment were assumed to be fixed, i.e. the deformability is constrained and

sliding will be prevented. The bottom of the riverbed sediment was assumed as a no-

flow boundary. The boundary condition on dam surface under water level in the

upstream side and the upper, left and right side of riverbed sediment have to be

specified for their pressure head and saturation. The seepage boundary is on the

downstream side of the dam, which indicates the water leaves the soil and water

pressure head is zero. The height of seepage face at the seepage boundary is not

known initially. The upstream side over the water level and the upper side of the dam

are airy boundaries. For rainfall infiltration case, the infiltration rates at the airy and

seepage boundaries are assumed as the product of rainfall intensity and Cosine of the

angle of the boundary regarding to a horizontal line.

An initial static equilibrium in the dam body was carried out during the forming

process of the landslide dam. An initial stress state in the dam can be obtained before

the transient seepage flow analysis. The water level rises in the upstream side was

simulated for river blockage by landslide materials. To estimate the failure time of the

landslide dam, the safety factors of dam stability were calculated during the 10 water

levels for the water retaining stage. The purpose of the various water levels simulation

is to ensure a reasonable pressure head and saturation distribution in the dam. The

boundary values of pressure head and saturation at the upstream boundary were will

be re-specified during each water level raises. The various rates of the boundary

values depend on the rising speed of water level. The transient seepage analysis with a

time step of 0.5 s was performed and the analysis is decoupled with the mechanical

analysis. The stress field of static equilibrium of the dam will be computed again after

each transient seepage analysis. The shear strength reduction technique was used to

obtain the safety factor of stability of dam and locate the corresponding critical slip

surface during each water level raises. We assume safety factor of one to maintain the

stability of the dam.

RESULTS OF THE NUMERICAL ANALYSES

Transient Results of Dam under Raising Water Level Process

The rising speed of water level at the upstream was assumed as 40 cm/min. When

water level reached the top of dam (height = 4 m), i.e., 10 minutes after. The upstream

water level was held at the level of the dam crest. The calculated saturation and pore

water pressure profiles from the numerical analysis of the landslide dam when the time

was 200 minutes are shown in Fig. 2. The computed saturated zone of soil occurred in

the upstream side of dam and the range of saturation in the downstream side is

between 0.5 and 1.0. Because the soil is in unsaturated state, the negative pore water

pressure developed in the top portion of downstream side of the dam.

(a)

(b)

FIG. 2. Contours when the time is 200 minutes: (a) saturation and (b) pore water

pressure.

Influence of the Rising Speed of Water Level on the Dam Stability

To study the influence of rising speed of water level on the safety factor of dam

stability, the saturated hydraulic conductivity of soil was fixed as Ks=41057.8 m/s

and 3 rising speeds at 20, 40 and 80 cm/min were assumed. The simulated results

under these rising speeds of water level are compared in Fig. 3. As can be seen in Fig.

Unit: N/m2

3, the safety factors of dam stability decrease with increasing time when time is

greater than 50 minutes. However, the influence of rising speed of water level on dam

stability is insignificant because the variation is not obvious. The reason could be that

the infiltrating time in dam body from upstream to downstream is more than that of

water level rising, so that the pore water pressure profiles in these case are close.

0 50 100 150 200 250

Time (min)

0.8

1

1.2

1.4

1.6

1.8

2

2.2F

act

or

of

safe

ty

v=20 cm/min

v=40 cm/min

v=80 cm/min

FIG. 3. Influence of rising speed of water level on dam stability.

1 10 100 1000 10000

Time (min)

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Fa

ctor

of

safe

ty

Ks=8.5710-3

m/sec

Ks=8.5710-4

m/sec

Ks=8.5710-5

m/sec

FIG. 4. Influence of saturated hydraulic conductivity of soil on dam stability.

Influence of the Saturated Hydraulic Conductivity of Soil on Dam Stability

To study the influence of saturated hydraulic conductivity on the safety factor of dam

stability, two additional hydraulic conductivity were estimated, Ks=31057.8 and

51057.8 m/s. The simulated results under these hydraulic conductivities are

compared in Fig. 4. In Fig. 4, it shows that the variation of safety factor is less before a

threshold time, the safety factor of dam stability decreases with increasing time when

time is more than the threshold time. The results imply that the threshold time and the

dam failure time are almost proportional to the saturated hydraulic conductivity of

dam.

Influence of Adding Rain Infiltration on Dam Stability

It can be observed from Fig. 5 that the slope failure time of the dam is about 221

minutes when the only effect of rising water level in the upstream is considered;

however, it is 131 minutes in the case of rising water level adding the rain infiltration

condition. The coupling with rainfall infiltration enlarges the zone of saturated soil and

speeds up the wetting of unsaturated soil. Therefore, the timing of the dam failure of

the case with both rain infiltration and water level rising comes earlier than the case of

considering water level rising only.

0 50 100 150 200 250

Time (min)

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Fa

cto

r o

f sa

fety

without rain infiltration

with rain infiltration

FIG. 5. Influence of adding rain infiltration on dam stability.

CONCLUSIONS

This study used FLAC to analyze stability of landslide dam assuming rising water

level in upstream side. The influence parameters discussed include the rising speed of

water level and the hydraulic conductivities of soils. Based on the numerical analyses

presented in this paper, the following conclusions are made:

1. The influence of rising speed of water level on safety factors of dam stability is

less significant than varying hydraulic conductivities of soil.

2. The safety factors of dam stability decrease with increasing saturated hydraulic

conductivity.

3. The timing of slope failure for the case of both with rainfall infiltration and rising

water level is earlier than the case of considering rising water level alone.

ACKNOWLEDGMENTS

This study was supported by research funding from the National Science Council of

Taiwan (NSC99-2625-M-005-009-MY3; NSC99-2625-M-005-004-MY3). Their

support is gratefully appreciated.

REFERENCES

Egeli, I. and Pulat, H.F. (2011). "Mechanism and modelling of shallow soil slope

stability during high intensity and short duration rainfall." Scientia Iranica, Vol.

18(6): 1179–1187.

Fu, J.F. and Jin, S. (2009). "A study on unsteady seepage flow through dam." Journal

of Hydrodynamics, Vol. 21(4): 499–504.

Huang, M. and Jia, C.Q. (2009). "Strength reduction FEM in stability analysis of soil

slopes subjected to transient unsaturated seepage." Computes and Geotechnics, Vol.

36(1): 93–101.

Ng, C.W.W. and Shi, Q. (1998). "A numerical investigation of the stability of

unsaturated soil slopes subjected to transient seepage." Computes and Geotechnics,

Vol. 22(1): 1–28.

Orense, R.P., Shimoma, S., Maeda, K. and Towhata, I. (2004). "Instrumented model

slope failure due to water seepage." J. Nat. Disaster Sci., Vol. 26 (1): 15-26.

Schnellmann, R., Bussllinger, M., Schneider, H.R. and Rahardjo, H. (2010). "Effect of

rising water table in an unsaturated slope." Engineering Geology, Vol. 114: 71–83.

Tohari, A., Nishigaki, M. and Komatsu, M. (2007). "Laboratory rainfall-induced slope

failure with moisture content measurement." Journal of Geotechnical and

Geoenvironmental Engineering, ASCE, Vol. 133 (5), 575–587.

van Genuchten, M.Th. (1980). "A close-form equation for predicting the hydraulic

conductivity of unsaturated soil." Soil Sci. Soc. Am. J., Vol. 44: 892–898.

Xu, Y.Q., Unami, K. and Kawachi, T. (2003). "Optimal hydraulic design of earth dam

cross section using saturated–unsaturated seepage flow model." Advances in Water

Resources, Vol. 26(1): 1–7.

Stability Analysis of Landslide Dam under Rainfall

Pei-Hsun Tsai1, Zheng-Yi Feng

2, Fan-Chieh Yu

3 and Jian-Han Lin

4

1 Associate Professor, Department of Construction Engineering, Chaoyang University of Technology,

168 Jifong E. Rd., Wufong District, Taichung, 41349, Taiwan; [email protected] 2 Professor, Department of Soil and Water Conservation, National Chung Hsing University, 250, Kuo-

Kuang Rd. Taichung, 40227, Taiwan; [email protected] 3 Professor, Department of Soil and Water Conservation, National Chung Hsing University, 250, Kuo-

Kuang Rd. Taichung, 40227, Taiwan; [email protected] 4 Graduate Student, Department of Construction Engineering, Chaoyang University of Technology, 168

Jifong E. Rd., Wufong District, Taichung, 41349, Taiwan; [email protected]

ABSTRACT: Failure of a landslide dam might occur by river discharge or rainfall. A

rise of the upstream water level of a landslide dam and rain infiltration into the dam

body increase pore water pressure and the weight of the dam. In this study, transient

seepage analysis of rainfall infiltration and dam stability analysis are performed. A

two-phase flow simulation using the FLAC finite difference code is adopted to

analyze unsaturated seepage flow in transient fluid-mechanical calculations. The

safety factor of dam stability is evaluated using the shear strength reduction technique.

The parameters discussed in this study include the rising speed of the water level, rain

infiltration, and the hydraulic conductivity of soil. The results show that the time to

slope failure of the dam is approximately 247 min when only the effect of the rising

upstream water level of the dam is considered. The failure time decreases to 189 min

when the rain infiltration and rising upstream water level of the dam are considered.

The results also indicate that the hydraulic conductivity of the dam affects dam failure

time.

INTRODUCTION

Landslides and debris flows commonly occur after heavy rainfall. Waste materials

resulting from landslides and avalanches can obstruct a river and create a landslide

dam. Unlike manmade dams with a compacting process and filtering materials, a

landslide dam is formed by a mixture of unconsolidated soil and rock in a naturally

unstable state. The water level typically rises rapidly upstream of a landslide dam

because of continuous rainfall. Landslide dams are often unstable and dangerous

because flash floods might occur downstream if the landslide dam fails. Sliding failure

can occur in the dam body because of an increase in pore water pressure. When

rainfall occurs or the water level rises upstream, the water infiltrating into the dam

515

causes an increase in water content and pore water pressure, which is caused by

transient seepage flow in unsaturated soils. The pore water pressure profile can be

analyzed using a numerical transient analysis of a saturated-unsaturated seepage flow

model. For stability analysis of landslide dams, the shear strength reduction technique

is used to obtain the safety factor of the dams and to locate the corresponding critical

slip surface. In numerous studies, the finite difference and finite element methods

were used to perform 2D transient seepage flow analyses using the saturated-

unsaturated seepage theory (Ng et al., 1998; Xu et al., 2003; Huang and Jia, 2009; Fu

and Jin, 2009). Experimental testing regarding the stability analysis of an unsaturated

slope has been studied using small-scale models (Orense et al., 2004; Tohari et al.,

2007; Schnellmann et al., 2010; Egeli and Pulat, 2011).

The influence of the hydraulic conductivity of dam and rising speed of the water

level on the dam failure time has not been fully examined. In this study, the

relationship between the dam stability and the distribution of moisture inside the dam

induced by influencing parameters was examined using a transient analysis. In

addition, a parametric study was performed by varying the rising speed of the water

level and the saturated hydraulic conductivity of soil.

NUMERICAL MODEL

Landslide Dam Configuration

A FLAC code with a two-phase flow model was adopted to analyze the timing of the

slope failure when the water level rose upstream of the dam. A typical configuration

and finite difference mesh for the landslide dam is shown in Fig. 1. The dam, with a

height of 4 m and a length of 14 m, was assumed to be situated above a riverbed. The

slope angles of the upstream and downstream faces of the dam were set as 34 , which

is close to the angle of repose of the dam materials. To analyze the influence of the rising speed of the upstream water level on dam

stability, three water level rising speeds (v = 31033.3 , 31066.6 and 31033.13 m/s) were studied. To estimate the effects of the hydraulic conductivity of soil on the stability of the landslide dam, three saturated hydraulic conductivities of soil (Ks

= 31057.8 , 41057.8 and 51057.8 m/s) were used for analyses.

14m 6m 2m 6m 14m

5m

4mairy boundary

saturation boundary

saturation

boundary

saturation boundary

saturation

boundary

Impermeable boundary

seepage boundary

airy boundary

FIG. 1. Typical finite difference mesh for landslide dam.

Seepage Flow Modeling of Unsaturated Soil

The transient seepage flow analysis of the dam after the upstream water level rises is

516 IACGE 2013

described by Richards’ equation (1931).

1

y

h)h(K

yx

h)h(K

xt

hC yx , (1)

where h is the water pressure head, and Kx(h) and Ky(h) are the hydraulic conductivity

in the x and y directions, respectively. C is the specific moisture capacity, t is time, x

is the horizontal coordinate, and y is the vertical coordinate.

A model proposed by van Genuchten (1980) is used to determine the relationship

between water pressure head, moisture content, and hydraulic conductivity. The

relationship between soil moisture and water pressure head is expressed as:

0h for 1

0h for

h1

1

Sm

e (2)

where and are constant parameters that express the matric potential of soil,

)1(1m , and Se is the effective saturation, which is defined as:

rs

reS

, (3)

where s and r are the saturated and residual moisture content, respectively. The

relationship between effective saturation and unsaturated hydraulic conductivity is

expressed as:

0hfor K

0h for )S1(1SKK

s

2mm1e

5.0es (4)

where s is the saturated hydraulic conductivity. For the numerical analysis, the

landslide dam and riverbed sediments were assumed to satisfy the Mohr-Coulomb

failure criteria. The engineering properties of the landslide dam and riverbed sediment

for the simulation are listed in Table 1. The landslide dam is assumed to be an

isotropic medium, and the parametric values of the unsaturated soil model are set as

follows: m = 0.333, s = 0.412 and r = 0.0185.

Simulation Procedures

First, the initial values of the water pressure head and moisture content in the dam

were specified in transient seepage flow analysis. The bottom of the riverbed sediment

was assumed to be a no-flow boundary. The pressure head and saturation of the dam

surface under the water level upstream and the upper, left, and right side of the

riverbed sediment must be specified. The seepage boundary was on the downstream

IACGE 2013 517

side of the dam, which indicated that water left the soil and that the water pressure

head was zero. The height of the seepage face at the seepage boundary was not

initially known. The upstream side above the water level and the upper side of the dam

were airy boundaries. For the rainfall infiltration case, the infiltration rates at the airy

and seepage boundaries were assumed to be the product of rainfall intensity and the

cosine of the angle of the boundary face to the horizontal. This is because the direction

of the discharge normal to the surface of the slope was specified on the boundaries,

but the rainfall direction was assumed to be vertical. In mechanical analysis, the

bottom and the 2 sides of the riverbed sediment were assumed to be fixed (i.e., the

deformability was constrained and sliding was prevented).

Table 1. The material parameters of the landslide dam and riverbed sediment

Zone Density

(kg/m3)

Bulk

Modulus

(MPa)

Shear

Modulus

(MPa)

Cohesion

(kPa)

Friction

Angle

(°)

Saturated

Hydraulic

Conductivity

(m/s)

Dam 1900 49 19 1 35 8.57×10-4

Riverbed 2000 49 19 3 38 8.57×10-5

The initial static equilibrium in the dam body was established during the forming

process of the landslide dam. An initial stress state in the dam can be obtained before

the transient seepage flow analysis is conducted. A rising water level upstream was

simulated for the case in which the river was blocked by landslide materials. To

estimate the failure time of the landslide dam, the safety factors of dam stability were

calculated for 10 different water levels during the river blocked stage. The simulation

of various water levels was conducted to ensure a reasonable pressure head and

saturation distribution in the dam. The boundary values of the pressure head and

saturation at the upstream boundary were re-specified for each water level. Transient

seepage analysis with a time step of 0.5 s was performed separately from the

mechanical analysis. The static equilibrium stress field of the dam was recomputed

after each transient seepage analysis. The shear strength reduction technique was used

to obtain the safety factor of dam stability and to locate the corresponding critical slip

surface of each water level rise. To maintain the stability of the dam, a safety factor of

one was assumed.

NUMERICAL ANALYSES RESULTS

Transient Seepage Results of a Dam Undergoing Rising Water Levels

The rising speed of the water level upstream was assumed to be 31066.6 m/s. The

water level reached the top of dam (height H = 4 m) after 10 min. The upstream water

level was held constant at the level of the dam crest. The calculated saturation and

pore water pressure profiles from the numerical analysis of the landslide dam at 200

min are shown in Fig. 2. The computed saturated zone of the soil occurring upstream

of the dam and the range of saturation downstream is between 0.5 and 1.0. Fig. 2(a)

518 IACGE 2013

shows the wetting speed of the unsaturated soil is high in the bottom of the dam,

possibly because of the effect of gravity. The positive pore water pressure is induced

by rising water level upstream of dam, as shown in Fig. 2(b). Because the soil was in

an unsaturated state, negative pore water pressure developed in the upper portion

downstream of the dam.

Saturation contours

6.00E-016.50E-017.00E-017.50E-018.00E-018.50E-019.00E-019.50E-011.00E+00

Flow vectorsMax vector = 6.966E-04

(a)

Pore pressure contours

0.00E+00

2.00E+044.00E+04

6.00E+048.00E+04

Flow vectorsMax vector = 6.966E-04

(b)

0.00E+002.50E-085.00E-087.50E-081.00E-071.25E-071.50E-071.75E-07

Max. shear strain-rate

Exaggerated Grid DistortionMagnification = 2.864E+03Max Disp = 1.164E-03m

FS=0.99

(c)

FIG. 2. Contours when the time is 200 minutes: (a) saturation (b) pore water

pressure and (c) maximum shear strain rate.

Unit: N/m2

IACGE 2013 519

Influence of the Rising Speed of the Water Level on Dam Stability

To study the influence of the rising speed of the water level on the safety factor of

dam stability, the saturated hydraulic conductivity of soil was fixed at Ks = 41057.8 m/s, and the three rising speeds were assumed to be 31033.3 , 31066.6 , and 31033.13 m/s. The comparison of the simulated results under these

water level rising speeds are shown in Fig. 3, which indicates that the safety factors of

dam stability decrease with increasing time after 50 min. However, the influence of

the water level rising speed on dam stability is insignificant because the variation is

not obvious, possibly because the rain infiltration in the dam body from upstream to

downstream is more rapid than the rise of the water level; therefore, the pore water

pressure profiles under these water level rising speeds are similar.

0 50 100 150 200 250 300Time (min)

0.8

1.2

1.6

2

2.4

2.8

Fa

ctor

of

Safe

ty

v=3.3310-3m/s

v=6.6610-3m/s

v=13.3310-3m/s

FIG. 3. Influence of rising speed of water level on dam stability.

Influence of the Saturated Hydraulic Conductivity of Soil on Dam Stability

To study the influence of the saturated hydraulic conductivity on the safety factor of

dam stability, two additional hydraulic conductivities were estimated, Ks = 31057.8

and Ks = 51057.8 m/s; Fig. 4 shows a comparison of the simulated results. The

variation of the safety factor is small before a threshold time is reached, and, after the

threshold time, the safety factor of dam stability decreases when the time increases.

The threshold time indicates that the time is when seepage starts to affect the slope

stability of the dam. The results also imply that the threshold time and the dam failure

time increase when the saturated hydraulic conductivity of the dam is reduced.

Influence of Rain Infiltration on Dam Stability

Fig. 5 shows that the slope failure time of the dam is approximately 247 min when

only the effect of rising water level upstream is considered; however, it is 189 min

520 IACGE 2013

when the rain infiltration condition is added to the rising water level. The coupling of

the rising water level with rainfall infiltration enlarges the saturated soil zone and

increases the wetting speed of the unsaturated soil. Therefore, the dam fails more

quickly when both the rain infiltration and the rising water level are considered than

when only the rising water level is considered.

1 10 100 1000 10000Time (min)

0.5

1

1.5

2

2.5

3

3.5

Fa

ctor

of

Safe

ty

Ks=8.5710-3 m/s

Ks=8.5710-4 m/s

Ks=8.5710-5 m/s

FIG. 4. Influence of saturated hydraulic conductivity of soil on dam stability.

0 50 100 150 200 250 300Time (min)

0.5

1

1.5

2

2.5

3

Fa

ctor

of

Safe

ty

without rain infiltration

with rain infiltration

FIG. 5. Influence of adding rain infiltration on dam stability.

IACGE 2013 521

CONCLUSIONS

This study used FLAC to analyze the stability of a landslide dam assuming a rising

water level upstream. The influencing parameters discussed include the rising speed of

the water level and the hydraulic conductivity of the soil. Based on the numerical

analyses presented in this study, the following conclusions are made:

1. The influence of the rising speed of the water level on the safety factors of dam

stability is less significant than that of varying the hydraulic conductivity of the

soil.

2. The dam stability is reduced when saturated hydraulic conductivity is increased.

3. The dam failure is more rapid when both rainfall infiltration and rising water

level are considered than when rising water level alone is considered.

ACKNOWLEDGMENTS

This study was supported by research funding from the National Science Council of

Taiwan (NSC99-2625-M-005-009-MY3; NSC99-2625-M-005-004-MY3). Their

support is gratefully appreciated.

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522 IACGE 2013

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