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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: [email protected], Hongo, Bunkyo-ku, 113-8656, Tokyo, JAPAN , Email: [email protected]
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 .