1010
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
(2003) proposed the general shear strength relationship for
critical state as follows
tan tan
a
b
v
s
(4)
where,
a
,
b
,
are the critical state friction angles in respect to
the vertical stress,
v
and suction,
s
. The critical state friction
angles can be found using the following functions:
2
1
2
'
kb
b
r
r
r
r
S S
S S
(5)
2
1
2
max
max
1
'
'
'
ka
a
a
a
r
r
r
r
S S
S S
(6)
where S
r1
and S
r2
are two reference degrees of saturation and
’
is the saturated friction angle. For
S
r
exceeding
S
r1,
a
/
’
and
b
/
’
ratios are equal to
’
whereas for
S
r
smaller than
S
r2,
a
/
’
=(
a
/
’)
max
and
b
/
’
=0
. In this study the reference degree of
saturation was taken as S
r1
=1 (full saturation conditions) and
S
r2
=0.75,
kb
=2
and
ka
=1
(Figure 7 b).
Figure 7. Prediction of (a) the ultimate shear strength using the average
skeleton approach and (b) the critical friction angles with S
r
measured at
critical state.
The predictions of the ultimate shear strength obtained with
the two approaches are shown in Figure 8. Although both
approaches require approximately the same number of
parameters, the critical friction angles approach seems superior
in predicting the shear strength of soil prepared at a wide range
of moisture contents and energy levels. The predictions may be
considered satisfactory given that in this study a single set of
parameters is used for modelling compaction states that differ in
both water content and energy level and thus soil structure.
0 20 40 60 80 100 120 140 160 180 200
0
20
40
60
80
100
120
140
160
180
200
10kPa
1:1
Measured shear stress,
(kPa)
Predicted shear stress,
(kPa)
byAverage skeleton approach
byCritical friction angles approach
Figure 8. Prediction of the ultimate shear strength using the average
skeleton approach and using the critical friction angles approach
4 CONCLUSION
This study presented the results on the shear strength of
compacted silty sand using constant water content direct shear
tests. The as-compacted water retention data showed that,
regardless of the energy level applied during compaction, there
is a unique relationship between water content and suction.
The peak and ultimate shear strength envelopes results show
that there is a relatively well defined non-linear relationship
between shear strength and matric suction. Also, the envelopes
suggest the existence of critical suction, after which the shear
strength increase is less significant. A decrease of peak shear
strength was observed for increasing compaction energy that
was interpreted to be associated with a difference in soil
structure and the compaction history of the specimens.
Constant water content direct shear tests on saturated
specimens show that the ultimate shear strength is relatively
independent of the initial compaction state. The ultimate shear
strength is modelled using two different approaches, that is, the
average skeleton stress and independent critical stress ratios.
The first is usually associated with the consideration of a
Bishop type of effective stress whereas the latter considers the
net stress and suction effect on the shear strength to be
independent. Although both approaches provide reasonable
estimations of shear strength, for this particular study the
independent stress variables approach is superior. Worth noting
that the prediction exercise catered for different compacted
states and represents a step forward in understanding the
implications of the inherent variability of compaction conditions
on the soil shear strength.
5 ACKNOWLEDGEMENTS
The authors acknowledge the financial assistance provided by
the Australia Research Council, Penrith Lakes Development
Corporation, Ltd and Coffey Geotechnics and assistance from
Mr. Robert Golaszewski and Mr. Alan Grant.
6 REFERENCES
Cokca, E., Erol, O. and Armangil, F. (2004). Effects of compaction
moisture content on the shear strength of an unsaturated clay.
Geotechnical and Geological Engineering
22(2), 285-297.
Jotisankasa, A. and Mairaing, W. (2010). Suction-Monitored Direct
Shear Testing of Residual Soils from Landslide-Prone Areas.
Journal of Geotechnical and Geoenvironmental Engineering
136(3), 533.
Kodikara, J. (2012) New framework for volumetric constitutive
behaviour of compacted unsaturated soils,
Can. Geotech. J.
49,
1227-1243.
Oloo, S. Y. and Fredlund, D. G. (1996). A method for determination of
fb for statically compacted soils.
Can. Geotech. J.
33(2), 272-280.
Proctor, R. R. (1933). Fundamental Principles of Soil Compaction.
Engineering News Record
111(9), 245-248.
Seed, B. and Chan, C. K. (1959). Compacted clays: Structure and
strength characteristics.
Journal of soil mechanics and Foundations
division Transactions
126(I), 1344.
Shibuya, S., Mitachi, T. and Tamate, S. (1997). Interpretation of direct
shear box testing of sands as quasi-simple shear.
Géotechnique
47(4), 769-790.
Tarantino, A. and Tombolato, S. (2005). Coupling of hydraulic and
mechanical behaviour in unsaturated compacted clay.
Géotechnique
55(4), 307-317.
Toll, D. G. and Ong, B. H. (2003). Critical-state parameters for an
unsaturated residual sandy clay.
Géotechnique
53(1), 93-103.
Vanapalli, S. K., Fredlund, D. G. and Pufahl, D. E. (1996). The
Relationship Between the Soil-Water Characteristic Curve and the
Unsaturated Shear Strength of a Compacted Glacial Till.
Geotechnical Testing Journal
19 (3), 259-268.
Vilar, O. M. (2006). A simplified procedure to estimate the shear
strength envelope of unsaturated soils.
Can. Geotech. J.
43(10),
1088-1088.
Wheeler, S. J. and Sivakumar, V. (2000). Influence of compaction
procedure on the mechanical behaviour of an unsaturated
compacted clay. Part 2: Shearing and constitutive modelling.
Géotechnique
50(4), 369-376.
Zhan, T. L. and Ng, C. W. W. (2006). Shear strength characteristics of
an unsaturated expansive clay.
Can. Geotech. J.
43(7), 751-751.
0
0
20 40 60 80 100 120 140 160 180 200
20
40
60
80
100
120
140
160
180
200
(a)
Measuredshearstress,
m
(kPa)
Predicted shear stress,
p
(kPa)
v
(S
rm
)
v
(S
r
)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
10
20
30
40
50
(b)
a
b
Critical friction angles,
(
o
)
Averagedegreeofsaturation,S
r
S
r2
a
')
max
'
