1412
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Sampling of clay was performed at one
to three different
sampling levels at each test site. In all, twenty-two different
types of clay were investigated in the study. The different soils
are listed together with their base characteristics in Table 1.
2.2
Investigation methods
The investigations required large uniform samples with
preserved properties also in the “elastic” small strain region.
The size of the samples should allow taking out a fairly large
number of “identical” test specimens.
Sampling of the clay was performed with a new sampler
constructed at SGI in 2009 with the aim of obtaining large high
quality samples in all types of mainly soft to medium strength
fine-grained soil (Larsson et al. 2012). The sampling tube is
about 1 m long with an inner diameter of 200 mm. The samples
are cut into six parts, before being sealed and carefully
transported to the laboratory. In the laboratory, test specimens
of a diameter of 50 mm were trimmed from the part samples.
Table 1. Properties of the clays used in the investigations.
Test site
Depth
(m)
(Mg/m
3
)
w
P
(%)
w
L
1)
(%)
w
N
(%)
S
t
c
u-rem
1)
(kPa)
Org. cont.
(%)
Mellösa
5 1.45
35 94 101 10 1.02
3.4
8.5
1.54
25 87 86 10 1.58
1.3
Strängnäs
6 1.55
22 55 71 49 0.20
0.8
Norrköping
5 1.54
24 73 82 19 0.81
0.8
Linköping
5
1.6
24 71 73 16 1.25
1.0
Gläborg
4.5
1.58
25 50 79 185 0.08
1.3
6
1.6
25 46 75 180 0.10
1.0
10 1.69
23 42 62 190 0.11
0.8
Munkedal
5 1.83
21 39 42 28 0.86
1.8
10 1.69
25 45 61 253 0.12
1.0
Fultaga
6.5
1.63
28 64 81 94 0.21
1.1
10 1.66
24 53 63 95 0.37
0.9
Onsjö
3.6
1.68
22 56 59 25 0.96
0.6
7 1.59
27 57 71 219 0.14
0.8
Torpa
3.5
1.60
27 58 70 41 0.50
1.2
5.5
1.54
27 71 79 42 0.48
1.0
8 1.57
29 76 79 26 0.93
1.0
Fråstad
6.5
1.60
28 65 71 49 0.47
1.2
Äsperöd
2.7
1.69
24 54 54 15 1.60
1.9
7 1.59
26 57 74 100 0.27
1.6
Kattleberg
4.5
1.46
26 69 108 151 0.08
0.8
8 1.59
25 55 81 224 0.07
1.3
1)
Determined by the fall cone method
Control of the homogeneity showed no significant
differences across the diameter or along the length of the
samples. Evaluation of the specimen quality (or disturbance) in
accordance with the method proposed by Lunne et al. (1997),
where the change in void ratio,
e
, during reconsolidation to in-
situ stresses in the triaxial cell or in the oedometer is compared
to the initial void ratio,
e
0
, showed that the specimens generally
met the criteria of “very good to excellent quality”. For each test
site and sampling level, comparative CRS-oedometer tests and
static active triaxial tests were performed on samples taken with
the Swedish standard piston sampler (St II) for control and
comparison of the sample quality. For most part also the latter
samples met the criteria of very good to excellent quality
showing that the Swedish standard piston sampler in normal
cases is adequate for routine sampling of soft clays (Larsson et
al., 2012). The main testing programme was performed on
samples taken with the new large diameter sampler.
CPTs and static and cyclic full-flow penetration tests with a
T-bar penetrometer were carried out at each test site. The CPTs
were performed according to the European standard (ISO 2012)
with higher demands for accuracy corresponding to the
recommendations by the Swedish Geotechnical Society for soft
clays (SGF 1993). The T-bar tests were performed using
equipment with recommended dimensions and according to
recommended practice (DeJong et al. 2010). Cyclic T-bar tests
were performed at all levels where sampling had been
performed and the cycling was made over the same 1 m depth
interval. The static phase of the T-bar tests was normally
continued one or a few metres below the deepest cycling level
The laboratory testing involved classification tests of basic
geotechnical properties, CRS-oedometer tests and active static
and cyclic triaxial tests. The classification tests comprised the
normal Swedish routine tests of bulk density, natural water
content, liquid limit and undrained and remoulded shear
strength, the last three determinations being made with the
Swedish fall cone test (ISO 2004). They also comprised
additional tests of plasticity limit, organic content through
analyses of organic carbon, clay content through sedimentation
tests, pH in the soils by use of electrodes and resistivity by use
of a so-called Soil-box (Camitz 1980).
The triaxial tests were performed on specimens that were
first anisotropically consolidated for about 80% of the estimated
preconsolidation stresses in both vertical and horizontal
directions, giving an overconsolidation ratio of 1.3. An
overconsolidation ratio of about 1.3 or slightly lower is typical
for soft clays in Sweden. In tests with higher overconsolidation
ratios, the specimens were then unloaded to the estimated stress
conditions after a corresponding unloading. The consolidation
process was usually completed within 24 hours. The static
undrained active tests were then performed at a rate of
compression of 0,01 mm/min (approximately 0,6%/h), which is
the normal testing rate in undrained tests on clay used at SGI.
Cyclic triaxial tests were performed with both stress-
controlled and strain-controlled cyclic loading. The stress-
controlled cyclic tests were performed as undrained
compression triaxial tests with an initial static shear stress state
corresponding to a factor of safety of 1.3, and an additional
cyclic stress oscillating around this initial stress state. The tests
were performed with different sizes of the cyclic stress
components. The specimens for the strain-controlled cyclic tests
were consolidated in the same way as those for the stress
controlled tests and started at the same initial static shear stress
conditions. The strain-controlled cyclic tests were performed
with two levels of strain; up to the failure strain at static loading
and up to two times this strain.
The cyclic loading was for most part performed with a
frequency of 1Hz, but some tests were also performed at slower
rates (lower frequencies) to study the influence of frequency.
3 RESULTS
T-bar testing was tried out with the main purpose of testing
the rate of shear strength degradation during cyclic tests. The
results have led to new interpretation methods for Swedish clays
similar to those used for CPT-tests. New correlations for
interpretation of remoulded shear strength and sensitivity have
also been brought forward. However, the measuring accuracy of
the equipment was found to be insufficient for accurate
determinations in the soft sensitive Swedish clays where the
remoulded shear strength often is very low (Åhnberg and
Larsson 2012).
The laboratory investigations together with tests in the field
directly after the samples had been taken showed that the
parameters liquid limit, remoulded shear strength and sensitivity
change with time of storage in the laboratory. This is in
agreement with earlier experience (e.g. Larsson 2011). Since
only a small increase in the remoulded shear strength has a large
effect on the sensitivity in quick clays, it is important that these
properties be determined as soon as possible. However, further
control tests showed that a storage time of up to three months
had little influence on other properties or the behaviour during
static and cyclic strength and deformation testing.
The classification tests in the laboratory largely verified
earlier established correlations between properties found for
