Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

6

DISCUSSION

Yuefeng Zhoua, W. M. Yana, L. G. Thama, Fuchu Daib and Ling Xub

aDepartment of Civil Engineering, The University of Hong Kong

bInstitute of Geology and Geophysics, Chinese Academy of Sciences

Stability Analysis of a Loess Slope with Water Stability Analysis of a Loess Slope with Water Infiltration Affected by CracksInfiltration Affected by Cracks

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Introduction Field investigation Numerical modeling Conclusion

Outline

In China, loess covers proximately 6.6% of the total area. Loess Plateau is the greatest bulk accumulation of loess on earth, the area of which is 31700 km2 (containing 50% of loess in China).Loess plateau is one of the most severe terrain in China for geohazards. In recent years great range of loess plateau is subject to serious and accelerated soil-water erosion. Loess landslide, as a typical geological disaster in China, occurs frequently in loess plateau and has been paid special attentions in recent years.

Introduction

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Introduction

Location: Location: Heifangtai Loess Plateau60 km southwest of Lanzhou, China

Description of the Study Area: Description of the Study Area: Loess thickness is 40 to 50 m.The bedrock is mainly mudstone and siltstone.Extensive cracks developed on the edge of loess plateau so that failure can be easily initiated.Agricultural production is the major economic source for local residents.

The Site

Climate:

The climate at Heifangtai plateau is temperate semiarid.

The average annual precipitation amount is 287 mm.

Under such a dry condition, a typical character shown

at Heifangtai plateau is that extremely steep slope.

Introduction

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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historic landslide

Yellow River

Introduction

groundwater seepage

From 1968 to 2010, more than 60 major landslides occured at Heifangtai Loess Plateau (13.7km2).

The reason for large amount of landslides at Heifangtai Plateau is the rise of groundwater table due to long-term and continuous irrigation of agricultural lands.

Based on early stage investigation, a typical slope was chosen at the edge of plateau to conduct this field test to simulate flooding irrigation to study stability of slope with water infiltration.

Introduction

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Field test

Range of installation

Principal section

C5

C4

C6 C7

C10

F1

F2

F3

MC1

MC2 C3

C8

C9

T0123

IN1 IN2

A A

Field test

Details of flooding irrigation

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Date (yyyy-mm-dd)

Mo

istu

re c

on

ten

t (%

)

S1_M1 (0.5m)

S1_M2 (1m)

S1_M3 (2m)

S1_M4 (3m)

Field investigation

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Date (yyyy-mm-dd)

Su

cti

on

(k

Pa

)

S1_T1 (0.5m)

S1_T2 (1m)

S1_T3 (2m)

S1_T4 (3m)

Field investigation

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Field investigation

The slope angle was measured by compass

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Numerical modeling

Seepage (FE method) and stabilility analysis (LE method)

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Numerical modeling

Four layers of undisturbed samples were taken at a slope nearby at

the depth of 5m, 10m, 15m and 30m respectively. Then the samples

were airproofed by preservation membranes and wax.

ICU and CS tests were performed on undisturbed loess samples.

Soil properties (strength)

Kv is several times to several dozens times higher than Kh.

Permeability adopted here: Kv= 4.8 × 10-6 m/s; Kh=7 × 10-7m/s

Numerical modeling

Soil Properties (permeability)

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Numerical modeling

Suction (kPa)

0.01 0.1 1 10 100 1000 1e+004 1e+005 1e+006

Vol. W

ater Content (m

3/m3) (x 0.001)

0

50

100

150

200

250

300

350

400

450

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

0 1 10 100 1000

Suction (kPa)

Pe

rme

ab

ility

(m

/s)

Kv Kh

Fitting curves of SWCC for whole range by equation (Fredlund et al. 1994)Prediction of unsaturated permeability curves(Fredlund et al. 1994)

FOS=1.101 FOS=1. 115 (a)

(c)

(b)

(d)

Suction for (a) with cracks and (b) without cracks, and volumetric water content for (c) with cracks and (d) without cracks after irrigation on 17 Oct., 2009(3rd day)

Numerical modeling

Suction for (a) with cracks and (b) without cracks, and volumetric water content

for (c) with cracks and (d) without cracks after irrigation on 20 Oct., 2009 (6th day)

Numerical modeling

FOS=1.090 FOS=1.110 (a)

(c)

(b)

(d)

Suction for (a) with cracks and (b) without cracks, and volumetric water content

for (c) with cracks and (d) without cracks after irrigation on 24 Oct., 2009 (10th day)

Numerical modeling

FOS=1.068 FOS=1.108 (a)

(c)

(b)

(d)

Suction for (a) with cracks and (b) without cracks, and volumetric water content

for (c) with cracks and (d) without cracks after irrigation on 26 Oct., 2009 (12th day)

Numerical modeling

FOS=1.061 FOS=1.107 (a)

(c)

(b)

(d)

0.95

1.00

1.05

1.10

1.15

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Date (yyyy-mm-dd)

Fac

tor

of

Saf

ety

with cracks

without cracks

initial value (strength from CS tests)

initial value ( strength from ICU tests)

Numerical modeling

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Conclusions

With the consideration of cracks, the wetting zone is much deeper than without consideration.

Short term irrigation could only trigger local failure rather than global failure of slope.

The existence of cracks could be regarded as an effective way of discharging water for the stability of slope in short time.

Strength parameters from CS tests are more reliable than that from ICU tests

Investigation of Landslide Failure Mechanism in Liquefiable Loess Triggered by Water Infiltration

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Thank you!

Numerical modeling

0

0.5

1

1.5

2

2.5

3

3.5

-160 -140 -120 -100 -80 -60 -40 -20 0

Suction (kPa)D

ep

th (

m)

Initial value (numerical model) Initial value (field monitoring)

0.5m to ponding area(with cracks) 0m to ponding area(with cracks)

field monitoring results measuring span0.5m to ponding area(no cracks) 0m to ponding area(no cracks)

Comparison between simulated and monitored data

Numerical modeling

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

VWC

De

pth

(m

)

Initial value (numerical model) Initial value (field monitoring)0.5m to ponding area (with cracks) 0m to ponding area (with cracks)

0.5 m to ponding area (no cracks) 0 m to ponding area (no cracks)Field monitoring results

Comparison between simulated and monitored data

Due to substantial cracks, joints and cavities existing in the

site, the accurate matching between field testing and

numerical modeling results are not expected in this study.

The overall comparison between monitored and simulated

data is acceptable.

Numerical modeling