Geoscience Reference
In-Depth Information
When the value of wetness index (m) becomes 0, the safety factor is determined
by:
¼
c
m
c
w
ð
Þ
D cos
2
€
tan
u
FS
ð
6
:
8
Þ
c
tan
€
6.2.2 Cohesion (c) and Friction Angle (
φ
)
The shear strength of the soil was described as the function of normal stress on the
slip surface, cohesion, and angle of internal friction. The angle of internal friction
(
) and cohesion are the two important physical properties of the soil which
determines angle of rupture, shearing strength, safety factor as well as stability
condition of the slope materials by developing
u
. The relationship
within all these properties to other characteristics of the soil was introduced by
Terzaghi (
1950
) and Wu and Siddle (
1995
). The geo-technical factors like angle of
repose of the debris were measured after Bloom (
1991
). The tangent of angle of
repose of dry granular materials is slightly greater than, but approximately equals to
the co-ef
'
stress circle
'
) (Van
Burkalow
1945
; Bloom
1991
). The range of cohesion and friction angle of different
soil was also adopted from Foundation of Engineering Geology (Tony Waltham
2002
) for the analysis of slope stability. All the tests were carried out under drained
condition using 100 mm diameter and 25 mm thick specimen in Geotechnical
Laboratory, GSI, Kolkata.
The major stress (
cient of sliding friction of the material or its mass friction (
u
˃
3
) and cohesion (c) were estimated through
tri-axial soil testing mechanism (Fig.
5.1
) from Geo-technical Laboratory of GSI,
Kolkata (22/com/soil/GTL/ER/O6-07). On the basis of these three major attributes a
Mohr stress circle was developed to obtain angle of internal friction and angle of
rupture. At
˃
1
), minor stress (
first, a circle was drawn through
˃
3
and
˃
1
with the centre on the
˃
1
+
˃
3
)/2 and the radius was
horizontal axis; the centre of the circle was obviously (
The values of con
(
˃
3
, and compressive stress,
˃
1
were plotted on
ning pressure,
horizontal axis where stress difference was
˃
1
− ˃
3
. On a plane parallel to the
greatest principal stress axis (2
ʱ
= 0) the normal stress across the plane was
˃
3
and
the shearing stress was 0. If the plane makes an angle of 45
°
with the greatest
principal stress axis (2
ʱ
= 90), the shearing stress is at a maximum and the normal
stress is (
˃
1
+
˃
3
)/2. If the plane makes an angle of 90
°
with the greatest principal
stress axis (2
˃
= 180
°
), the shearing stress is 0 and the normal stress is
˃
1
(Billings
1987
).
Þ
¼
r
1
þ r
3
2
r
1
r
3
2
r
n
normal stress
ð
cos
2
a
ð
6
:
9
Þ
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