2927
Technical Committee 214 /
Comité technique 214
An AH sample was vigorously remoulded on a glass plate
using a spatula to form a soil slurry at a water content equal to
the liquid limit (Burland 1990).
The results of AH oedometer tests are presented in Figure 3
in a semi-logarithmic plot of void ratio, e, against the logarithm
of the vertical effective stress, log(
’
v
). The virgin AH soil
manifests initially a negligible amount of compression before
reaching the pre-consolidation stress,
’
p
. When the stress
values exceed
’
p
, the consolidation curve displays a non-linear
response with a gradient significantly higher compared to the
remoulded sample. At high pressures, the compression curve of
virgin soil is seen to approach the remould consolidation line in
an asymptotic trend.
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
10
100
1,000
BH22A: RL=‐0.9m
BH31C: RL=‐3.2m
Newland(1955): RL‐ 3.9m
DH423: RL=‐6.3m
BH31B: RL=‐7.7m
Remould Sample
Vertical Stress ‐
'
v
(kPa)
Void Ratio ‐ e
Figure 3. One-dimensional consolidation test results.
9 TRIAXIAL TESTS
The triaxial tests were carried out on virgin soil samples of
diameters varying between 50 and 70mm. For all the tests the
sample saturation was achieved by raising the back pressure to a
maximum of 300kPa. Over the duration of saturation, the stress
state was maintained at an effective stress of 20kPa. Full
saturation was achieved when Skempton’s pore pressure
parameter B has achieved a value equal or greater than 95%.
On completion of soil saturation, the samples were
isotropically consolidated before the start of undrained shearing.
The undrained shearing was conducted in a deformation
controlled mode to large axial strains between 15 to 20%.
Figure 4 presents the results of undrained shearing as plots
of effective stress paths: deviatoric stress q =
’
1
-
’
3
vs. mean
effective stress p’ = (
’
1
+2
’
3
)/3.
0
50
100
150
200
0
100
200
300
400
Deviatoric Stress: q (kPa)
Mean Effective Stress: p' (kPa)
DH522: RL=-4.3m
DH503: RL=-4.4m
DH525: RL=-5m
Figure 4. Effective stress paths of AH soil samples in undrained
shearing for a range of confining pressures p’
0
= 75 to 300 kPa.
The behaviour manifested by the virgin AH soil is of a
contractive type. The stress paths climb to a maximum
deviatoric stress, q
max
, before plummeting towards the origin of
the stress path. The strain softening past q
max
is a distinctive
feature of this suite of tests.
10 CRITICAL STATE
The critical state is a fundamental concept in soil mechanics as
it represents a reference state to assess the state and behaviour
of soil under loading. It was firstly introduced by (Roscoe et al.
1958) to describe the behaviour of remoulded clays, and
nowadays the concept was extended to represent a more general
framework of soil behaviour for: (a) Sands (Been et al 1991);
and(b) Combinations of sand with various percentages of plastic
and non-plastic fines (Bobei et al. 2009).
The results of undrained triaxial tests conducted on AH soil
are interpreted in the framework of critical state as illustrated in
Figure 5. The soil samples were considered to reach the critical
state when the following conditions are satisfied: dq=0, dp’=0,
du =0 while d
q
≠0.
The CS is found to plot along a linear relationship. On
consideration of initial soil state being located above the CS
line, and the contractive behaviour manifested during undrained
loading (refer to Figure 4), the virginAH soil appears to fully
conform to the framework laid out by CS.
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1
10
100
1,000
Critical State
Undisturbed AH Soil
Remoulded AH Soil
Natural Soil Consolidation Line
Remoulded Soil Consolidation Line
Void Ratio: e
Mean Effective Stress: p'
Figure 5. Critical state and soil state of AH soil before undrained
loading.
11 SHANSEP PROCEDURE
SHANSEP (Stress History and Normalized Soil Engineering
Properties) was proposed by (Ladd and Foot 1974) as an
empirical method to adopt in engineering practice to estimate
the undrained shear strength on consideration of stress history
effects arising from geological unloading. The over-
consolidation ratio (OCR =
’
p
/
’
v
) is chosen as a parameter to
encapsulate the stress history effects, with the OCR values
determined by high quality oedometer data. In mathematical
terms, SHANSEP correlates the undrained shear strength with
the soil OCR as follows:
m
OCR v
u
m
v
u
OCR
s
OCR S s
1
'
'
(2)
where: S = intercept with vertical axis at OCR = 1; and m =
gradient of linear relationship.
A plot of undrained shear strength ratio (s
u
/
’
v
) with OCR
based on CPTu data is shown inFigure 6. The CPTu data was
found to manifest a linear response, with slope gradients within
the bounds of linear relationships shown with dashed lines. In
the normally consolidation range, all linear relationships
intersect the vertical axis at a constant (s
u
/
’
v
)
OCR=1
= 0.35.
