Safety assessment of weathered slopes by measuring shear wave velocity
Mohsin U. Qureshi, Ikuo Towhata and SuguruYamada
April 3-8, 2011
Vienna, Austria
東京大学
Department of Civil Engineering, The University of Tokyo, Tokyo, Japan.
Change in mechanical properties is studied by subjecting soft rocks to freeze-
Field direct shear tests, Dynamic cone penetration tests and seismic refraction
ubiquitous phenomenon in the regions where extreme environmental conditions
thaw cycles in a triaxial system which is capable to maintain a temperature
prevail i.e. repeated change of temperature and moisture. Natural disasters such
as earthquakes or heavy rainfalls thwart the potential stability of weathered
range of -5 to 45°C under pressure in the confining cell.
A typical freeze-thaw cycle
slopes. Close examination of rock slope surfaces in Kashmir, Pakistan (hit by
consisted of freezing the partially
7.6 magnitude earthquake in 2005) indicated that the rock is highly weathered
saturated rock to a maintained
and lost its shear strength. The area is also under the cyclic temporal effects (in
temperature of -6°C. Freezing
winter temperature falls below 0°C and in summer average temperature is 35-
allows
the
water
in
40°C) with dry and rainy seasons. The cyclic effects of the environmental
agents physically deteriorate the rock to make the slopes potentially unstable.
9% in volume, creating sufficient
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
Seismic refraction analysis was
done by using Intercept time
method
No. of drops for 10 cm of
penetration is recorded as
Nd value.
and
Seismic refraction
widening of cracks and joints.
at
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,
levels of 30, 60 and 100kPa to
envisagement of negative ageing behavior of geo-material in laboratory, and
elucidation of in-situ mechanical properties and depth of weathered surface
on mechanical deterioration of
layer in field takes precedence.
soft
2. Objectives
• Reproduce the mechanical weathering
tan 
FOS 
tan 
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.
Weathered
surface
g, 
C=0
Vs
stiffness
Plot
of
against
n

H
Depth (m)
2.0
0
200
400
600
0.0
0.5
Weathered
granite
Weathered
mudstone
1.0
0.5
0.5
1.0
1.5
1.5
1.5
2.0
2.0
2.5
2.5
3.0
3.0
3.5
3.5
4.0
4.0
4.5
2.0
0 10 20 30 40 50
JPKOBE
and
safety
is
shown
in
100
a
200
300
400
500
S-wave velocity, Vs (m/s)
dry
60
50
Wet FOS=1
Dry FOS=1
UNSTABLE
ZONE
POTENTIALLY
UNSTABLE
ZONE
40
STABLE
ZONE
30
Nagano W. sandstone
Izu volcanic ash
Chengdu
Tochigi W. pumice
Kobe W. granite
20
10
200
4.5
YZ W. sandy mudstone
MZD dolomite scree
MZD W. mudstone
MZD W. dolomite
NGO E sandstone
4
TXL W. limestone
KOBE W. granite
IZU volcanic ash
400
600
800
Moreover the relationship between slope and S-wave velocity is
3
developed for the FOS=1 from wet and dry cases and stable, potentially
2
stable and unstable zones are marked. For some recent problems, slope
1
0
0
1
2
3
4
angels and S-wave velocity of some jeopardizing slopes are verified .
Depth elucidated by PDCP (m)
1.0
1.0
500
400
1.5
1.5
JPYZ
PKTXL
JPIZU
PKMZD
JPKOBE
JPNGO
Vs=110Nd
0.27
300
2.0
PKMZD
2.0
0 10 20 30 40 50
Nd value
Nd value
6. Concluding remarks
1. Freeze-thaw weathering in laboratory deteriorates the stiffness of soft
200
0 10 20 30 40 50
Nd value
10
100
Average Nd value in weathered layer
rock. However confinement thwarts this deterioration which is rational
with natural weathering process taking place at shallow depths.
Typical test results from seismic refraction and dynamic cone penetration tests
Weathered mudstone
freeze-thaw
Typical freeze-thaw test results
After a freezethaw cycle
20
f=18.2kPa
D50=3.05mm, Gs=2.67
f=13.2kPa
15
10
32cm
0
f=9.4kPa
0
5
10
n=9.5kPa

8cm
32cm
15
20
30
MZDS1
MZDS2
MZDS3
UET
KPR
UET
MZDS1
n=6.0kPa
n
5
n=1.8kPa
25
PKMZDS3
30
35
Shear displacement (mm)
40
MZDS3
MZDS2
KPR
20
10
0
3
6
Weathered mudstone
Dolomite Scree
Limestone scree
9
12
Normal stress, n (kPa)
Typical test results from field direct shear tests
θ
Laser scanning of surface deterioration
wet
2
0
measured in dry conditions..
Field direct shear
Vs (m/s)
200 400 600 800
0.0
0.0
JPNGO
due to freeze-thaw process.
Fresh surface
both
wave velocity in the field which is
5
0.0
0.5
normalized
pressure resists the deterioration
0.0
0
1.0
1.5
freeze-thaw
cycles indicated that confining
600
1.0
Shear stress , (kPa)
process.
during
400
0.5
confining
study the effects of confinement
rock
200
Weathered
sandstone
vacuum (-100kPa). These tests
performed
0
specimen
were

Vs (m/s)
0.5
17kN/m3 was saturated under
Weathered sandstone
for
4
Wet
0
conditions. The calculated factor
32cm
Depth elucidated by SR (m)
0.0
having a dry unit weight of 16Muzaffarabad, Pakistan
by assuming
8cm
32cm
Cone penetration
Shear stress at failure, f (kPa)
rock
stability analysis is performed for
60o,
Shear wave velocity of
weathered layer, Vs, (m/s)
Vs (m/s)
up to a maintained temperature of
Soft
infinite
6
Slope Angle Dry
o
15
o
30
o
45
o
60
S-wave velocity Vs (m/s)
Freezing is followed by thawing
45°C.
 and H
Vs and H
Depth (m)
pores
investigations,
relationship with the measured S-
Depth (m)
of
field
of
the tensile strength of rock, results
expansion
By using the available data from
slope angle of 15o, 30o, 45o and
pressure which is if greater than
in
8
weathered slope
two sites in Pakistan (Muzaffarabad (PKMZD) and Taxila (PKTXL)).
matrix
porosity and cracks to expand by
5. Factor of safety and shear wave velocity
Safety factor, FOS
4. In-situ tests for mechanical properties
o
The reduction in shear strength of slope surfaces due to weathering is
3. Reproduced laboratory weathering tests
Slope angle 
1. Introduction
Negative ageing
Reference: Geophysical Research Abstracts, EGU2011-1576-4
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.
7. Acknowledgement
The 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
CONTACT: Mohsin Usman Qureshi (Ph. D. Student)
Geotechnical Engineering Laboratory, Department of Civil Engineering, Tokyo University
7-3-1, Hongo, Bunkyo-ku, 113-8656, Tokyo, JAPAN , Email: qureshi@geot.t.u-tokyo.ac.jp