Safety assessment of weathered slopes by measuring shear wave velocitySafety assessment of weathered slopes by measuring shear wave velocityMohsin U. Qureshi, Ikuo Towhata and SuguruYamadaMohsin U. Qureshi, Ikuo Towhata and SuguruYamada

Department of Civil Engineering, The University of Tokyo, Tokyo, Japan. Department of Civil Engineering, The University of Tokyo, Tokyo, Japan. 東京大学東京大学April 3-8, 2011 April 3-8, 2011 Vienna, AustriaVienna, Austria

1. Introduction1. Introduction

The reduction in shear strength of slope surfaces due to weathering is

ubiquitous phenomenon in the regions where extreme environmental conditions

prevail i.e. repeated change of temperature and moisture. Natural disasters such

as earthquakes or heavy rainfalls thwart the potential stability of weathered

slopes. Close examination of rock slope surfaces in Kashmir, Pakistan (hit by

7.6 magnitude earthquake in 2005) indicated that the rock is highly weathered

and lost its shear strength. The area is also under the cyclic temporal effects (in

winter temperature falls below 0°C and in summer average temperature is 35-

40°C) with dry and rainy seasons. The cyclic effects of the environmental

agents physically deteriorate the rock to make the slopes potentially unstable.

2. Objectives2. Objectives

3. Reproduced laboratory weathering tests3. Reproduced laboratory weathering testsField direct shear tests, Dynamic cone penetration tests and seismic refraction

tests were performed to evaluate shear strength, shear wave velocity and depth

of surface weathered layer. Tests were performed at four localities in Japan

(Yokosuka (JPYZ), Nagano (JPNGO), Izu (JPIZU) and Kobe (JPKOBE)) and

two sites in Pakistan (Muzaffarabad (PKMZD) and Taxila (PKTXL)).

4. In-situ tests for mechanical properties4. In-situ tests for mechanical properties

6. Concluding remarks6. Concluding remarks1. Freeze-thaw weathering in laboratory deteriorates the stiffness of soft

rock. However confinement thwarts this deterioration which is rational

with natural weathering process taking place at shallow depths.

2. A reasonable relationship between S-wave velocity and penetration Nd

value was established with is a useful tool

3. As an end result, safety can be assessed by knowing the slope angle and

S-wave velocity of surface weathered layer.

Reference: Geophysical Research Abstracts, EGU2011-1576-4Reference: Geophysical Research Abstracts, EGU2011-1576-4CONTACT: CONTACT: Mohsin Usman Qureshi (Ph. D. Student)Mohsin Usman Qureshi (Ph. D. Student)Geotechnical Engineering Laboratory, Department of Civil Engineering, Tokyo UniversityGeotechnical Engineering Laboratory, Department of Civil Engineering, Tokyo University7-3-1, Hongo, Bunkyo-ku, 113-8656, Tokyo, JAPAN , Email: qureshi@geot.t.u-tokyo.ac.jp7-3-1, Hongo, Bunkyo-ku, 113-8656, Tokyo, JAPAN , Email: qureshi@geot.t.u-tokyo.ac.jp

In dealing with the slope instability problems for such regions, present as well

as future mechanical properties of those slopes have to be elucidated.

Therefore, envisagement of negative ageing behavior of geo-material in

laboratory, and elucidation of in-situ mechanical properties and depth of

weathered surface layer in field takes precedence.

Muzaffarabad, Pakistan Weathered sandstone

• Reproduce the mechanical weathering

process in laboratory to evaluate the

change in mechanical properties at various

confining levels.

• Propose a method to evaluate the safety

factor of weathered slopes by measuring

the shear wave velocity in field.

Change in mechanical properties is studied by subjecting soft rocks to freeze-

thaw cycles in a triaxial system which is capable to maintain a temperature

range of -5 to 45°C under pressure in the confining cell.

A typical freeze-thaw cycle

consisted of freezing the partially

saturated rock to a maintained

temperature of -6°C. Freezing

allows the water in matrix

porosity and cracks to expand by

9% in volume, creating sufficient

pressure which is if greater than

the tensile strength of rock, results

in expansion of pores and

widening of cracks and joints.

Freezing is followed by thawing

up to a maintained temperature of

45°C. Soft rock specimen having

a dry unit weight of 16-17kN/m3

was saturated under vacuum (-

100kPa). These tests were

performed at confining levels of

30, 60 and 100kPa to study the

effects of confinement on

mechanical deterioration of soft

rock during freeze-thaw process.

Plot of normalized stiffness

against freeze-thaw cycles

indicated that confining pressure

resists the deterioration due to

freeze-thaw process.

32cm

PKMZDS3

Typical test results from field direct shear tests

3 6 9 120

10

20

30

KPR

UET

MZDS3MZDS2

She

ar s

tres

s at

fai

lure

, f (kP

a)

Normal stress, n (kPa)

MZDS1 UET MZDS2 KPR MZDS3

MZDS1

0 5 10 15 20 25 30 35 400

5

10

15

20

32cm

32cm

f=18.2kPa

f=13.2kPa

She

ar s

tres

s , (

kPa)

Shear displacement (mm)

n=1.8kPa

n=6.0kPa

n=9.5kPa

Weathered mudstone

f=9.4kPa 8cm

n

D50

=3.05mm, Gs=2.67

Silty clay

Limestone scree

Weathered mudstoneDolomite ScreeLimestone scree

5. Factor of safety and shear wave velocity5. Factor of safety and shear wave velocity

Negative ageing

By using the available data from

field investigations, infinite

stability analysis is performed for

weathered slope by assuming

slope angle of 15o, 30o, 45o and

60o, for both wet and dry

conditions. The calculated factor

of safety is shown in a

relationship with the measured S-

wave velocity in the field which is

measured in dry conditions..

7. Acknowledgement7. AcknowledgementThe University of Tokyo and the Ministry of Education, Culture, Sports,

Science and Technology (MEXT: Government of Japan) are gratefully

acknowledged for the research facilities and financial support

Weathered surface

n HH

θ

tan

tanFOS

C=0Vs

Laser scanning of surface deterioration

Fresh surface After a freeze-thaw cycle

Typical freeze-thaw test results

Seismic refraction Field direct shear

32cm 8cm

No. of drops for 10 cm of penetration is recorded as

Nd value.

Vs and H and H Cone penetration

Seismic refraction analysis was done by using Intercept time method

200

300

400

500

10 100

Average Nd value in weathered layer

She

ar w

ave

velo

city

of

wea

ther

ed la

yer,

Vs, (

m/s

) JPYZ PKMZD PKTXL JPKOBE JPIZU JPNGO

Vs=110N

d

0.27

Typical test results from seismic refraction and dynamic cone penetration tests

2.0

1.5

1.0

0.5

0.0

0 10 20 30 40 50

Vs (m/s)

Nd value

Dep

th (

m)

Weathered sandstone

2.0

1.5

1.0

0.5

0.00 200 400 600

JPNGO4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0 10 20 30 40 50

Vs (m/s)

Nd value

Dep

th (

m)

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.00 200 400 600 800

Weathered granite

JPKOBE 2.0

1.5

1.0

0.5

0.0

0 10 20 30 40 50

Vs (m/s)

Nd value

Dep

th (

m)

2.0

1.5

1.0

0.5

0.00 200 400 600

Weathered mudstone

PKMZD

0

1

2

3

4

5

0 1 2 3 4

YZ W. sandy mudstone TXL W. limestone MZD dolomite scree KOBE W. granite MZD W. mudstone IZU volcanic ash MZD W. dolomite NGO E sandstone

Depth elucidated by PDCP (m)

D

epth

elu

cida

ted

by S

R (

m)

10

20

30

40

50

60

200 400 600 800

Wet FOS=1

UNSTABLE ZONE

POTENTIALLY UNSTABLE ZONE

Nagano W. sandstone Izu volcanic ash Chengdu Tochigi W. pumice Kobe W. granite

S-wave velocity Vs (m/s)

Slop

e an

gle o

STABLE ZONE

Dry FOS=1

0 100 200 300 400 5000

2

4

6

8Slope Angle Dry Wet

15o

30o

45o

60o

Safe

ty f

acto

r, FOS

S-wave velocity, Vs (m/s)

Moreover the relationship between slope and S-wave velocity is

developed for the FOS=1 from wet and dry cases and stable, potentially

stable and unstable zones are marked. For some recent problems, slope

angels and S-wave velocity of some jeopardizing slopes are verified .