1. No category

Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference
Stavanger, Norway, June 17-22, 2001
Copyright © 2001 by Tile International Society of Offshore and Polar Engineers
ISBN 1-880653-51-6 (Set); ISBN 1-880653-53-2 (VoL II); ISSN 1098-6189 (Set)
Sample Quality Evaluation of Soft Clays Using Six Types of Samplers
Masanori Tanaka, Hiroyuki Tanaka and Dinesh R. Shiwakoti
Port and Harbour Research Institute
Yokosuka, Japan
ABSTRACT
based on the assumption that strong relationship exists between
residual effective stress and shear strength of soil. The validity of
such assumptions has also been investigated in this paper.
To investigate the effect of sampling method in sample quality, six
types of samplers were used and three boring methods were
employed to retrieve coastal clays of Japan (Ariake), Thailand
(Bangkok), and United Kingdom (Bothkennar). Unconfined
compression tests and residual effective stress measurements were
carried out in soil samples taken by the various types of samplers
and sampling methods. It has been found that the sample quality
from the Japanese sampler is equivalent to that of the large
diameter Laval sampler and Sherbrooke sampler. Sample quality of
Japanese sampler is significantly influenced by the method of
boring.
SAMPLE DISTURBANCE BASED ON SAMPLER TYPES
SAMPLER TYPES USED
Since there is no international standard or unified code on
sampler types and sampling methods, various types of samplers
are being used in various countries based on the characteristics of
soil, availability of regional expertise and technology, and
subjective preferences of geotechnical engineers. In this research
6 distinct types of samplers being used in practice in various
countries have been investigated. Table 1 shows main feature of
these samplers, and are briefly described below.
KEY WORDS: sampling method, residual effective stress, shear
strength, shear modulus, sample disturbance, soft clay, boring method
Table 1 Main features of samplers
INTRODUCTION
Sampler
type
Geotechnical engineers have recognized the importance of
sample quality in evaluating reliable design parameters.
However, although some countries or organizations have
developed their own sampling methods suitable for their soil
conditions and characteristics, still there is no international
standard for collecting undisturbed soils. Similarly, although
the measurement methods of residual effective stress of soil is
getting easier and more precise than before due to the
advancement of technology, there is no internationally accepted
standard method for suction measurement and for the evaluation
of the sample quality.
In this paper influences of samplers and sampling techniques
in sample quality have been investigated using six types of
samplers and the corresponding sampling techniques for several
clay deposits. Some researchers (e. g. Hight et at., 1992) have
claimed that residual effective stress of a soil reduces with
storage time. We have checked the p h e n o m e n a o f residual
effective stress, and compared our results with the data available
in literature. Some researchers (e. g. Shogaki, 1995) have been
insisting that the residual effective stress of samples can be
estimated, even if the samples are disturbed, and therefore, shear
strength of a ground can be estimated from it. Such ideas are
Inside Sampler
diameter length
(ram)
(ram)
Sample Thicklength
hess
(mm)
(ram)
Area
Inside Piston
ratio~ clearance
(%) ratio" (%)
Developer
JPN
75
1000
800
1.5
7.5
0
SHT
72
610
500
1.65
8.6
I
yes Japanese Geotechnical Society
no ASTM D1587
NGI
54
768
650
0.5
yes
ELE
101
500
330
1.7
6.4
0
LVL
208
660
600
4
7.3
0
SS
250 ~
350
13
42
Norwagean Geotechnical Institute
yes Engineering Lab.
Equipmem, UK
no Laval University, I
Canada
no Sherbrooke University, Canada
l):sample diameter
2): (Do:-D~:)/D," × 100 where, Do: outer diameter o f sampler, D,: tip diameter of sampler
3) : (D,-D.)/Do × 100 where. D,: inner diameter o f sampler
a) Japanese fixed piston sampler (JPN or Japanese sampler)
Japanese stationary piston sampler has been specified by
Japanese Geotechnical Society (JGS, 1995). This sampler is the
most common type of sampler used in Japan (Adachi, 1979).
Tube of this sampler has inner diameter of 75 mm and length of
sample that can be obtained is 800 ram. Extension inner rods are
used to fix the piston in the tube.
493
b) Shelby tube sampler (SHT)
On the other hand, Shelby tube is an open drive sampler
without a piston, and has been specified by ASTM (D1587).
This sampler is extensively used in practice in many parts of the
world. Samples of 72 mm in diameter and 500 mm in length can
be obtained from SHT.
c) NGI sampler (NGI)
NGI sampler, developed by Norwegian Geotechnical
Institute, is a stationary piston sampler with extension inner
rods (Andersen and Kolstad, 1979). Since this is a composite
type sampler, its thickness and area ratio are several times
larger, compared with other tube samplers. Sample obtained
using NGI sampler is 54 mm in diameter and 650 mm in length.
sample obtained using this sampler.
SAMPLE DISTURBANCE
Figure 1 shows typical s t r e s s - s t r a i n curves of Ariake clay
at a depth of l0 m, obtained by unconfined compression tests
using various samplers described above (Tanaka and Tanaka,
1999). The curves for the specimen obtained by JPN and LVL
sampler are almost identical, and their corresponding peak
shear strengths and secant moduli are practically equal. The
shear strength and secant modulus from Sherbrooke sampler
are the greatest of all samplers used in this investigation. On
the contrary, Shelby tube and ELE sampler highly
underestimate the peak shear strength and also yield large strain
at failure.
d) ELE sampler (ELE)
40
ELE sampler was developed by the Engineering Laboratory
Equipment in England. This sampler is also a stationary piston
sampler. Sample obtained using this sampler is 101 mm in
diameter and 330 mm in length. Although the dimensions of
ELE are different from that of JPN, the mechanism and
treatment are almost same for both the ELE and JPN samplers.
.'
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35
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•
JPN ~.
×
S.T I;
NGI I~
[]
E Eli"
t~
25 F-
a-
v
e) Laval sampler (LVL)
O-=
• e
~ ' xXXx
~"^~x~
L "~
~<~Z~~
15
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ss
o
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5
J~ Sherbrooke sampler (SS)
~
A
~
depth
o
XX
1
,,,x
.L~.x
x
10
0
Sherbrooke sampler was developed by the University of
Sherbrooke in Canada (Lefebvre and Poulin, 1979). This
sampler is not a tube sampler. It has three rotating cutting tools.
During sampling, water or bentonite mud is fed in from the
surface to help carve the sample, which flows out from the
c u t t i n g tools. Photo 1 shows a view o f the SS s a m p l e r and
a
2O
~4
Laval sampler was developed by the Geotechnical group at
Laval University in Canada (La Rochelle et al., 1981). A ball
valve keeps vacuum in the tube while the sample is lifted up.
Sample obtained from this sampler is 208 mm in diameter and
600 mm in length.
~
2
4
6
strain
8
f lOm
10
12
14
(~)
Fig. 1 Typical stress-strain curves of Ariake clay
I
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,
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~
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xx
' I'
I '
I'
i'
~
i''
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LVL
[
xx'~
s: 10
JPN
succPT)
oa
•
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x~
4
,
"o
15
x
20
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10
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20
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30
qu/2
(a)
shear
I
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~ tt
40
50
i t
60 o
rlllll,l,,,O,5,
~
(kPa)
strength
z
3
4
~,
(b)
strain
o LVL
S I
6
7
(%)
at
failure
Fig. 2 Strength profile of Ariake clay
Figure 2 shows the profiles of undrained strengths (qJ2) as
well as strains at failure (el) of Ariake clay, obtained by the six
types of samplers. Also included in the figure is the undrained
shear strength, s,(CPT), profile obtained by cone penetration
test (CPT), as defined below.
su(CPT) Photo 1 Soil sampling at Ariake using Sherbrooke sampler
494
(qt-cvo)
Nkt
(1)
where,
Table 3 Comparison of s, from unconfined compression, laboratory
vane shear and triaxial tests normalized by .% ofJPN (P)
s~ (CPT): Undrained shear strength from CPT
qt: Corrected cone resistance
O'v0: Overburden pressure, in total stress
Nkt: Cone factor
Inside
Since strength profile of a soil varies with depth, and since
sampling depths of all the sampler types might not always
correspond with each other, it is essential to establish a standard
benchmark strength profile against which the performance of
various samples could be tested. For this reason, a standard
benchmark undrained shear strength [s~(CPT)] profile has been
defined using the point resistance qt measured from CPT. Cone
factor (Nkt) of 10 has been derived by comparing the CPT
strength profile with the strength profile obtained by field vane
shear t e s t s .
Comparing the undrained strength profiles obtained by
various samplers with that of CPT, it is evident that values from
JPN, LVL and SS samplers are very close to the benchmark
strength profile. Strength profile obtained by NGI samplers is in
slightly lower side, however, the result is still not so low
compared to the benchmark strength profile. Thus, JPN, LVL,
SS and NGI samplers provide reliable strength results. But, the
strength values from SHT and ELE samplers have large scatter
and consistently underestimate the strength values throughout
the soil profile. This suggests that strengths obtained by SHT
and ELE are not reliable. It is interesting to note that ELE
provides significantly underestimates the strength of a soil,
despite its significantly large tube diameter (101 mm).
Strain at failure (el) is also an indicator of level of sample
disturbance. Generally speaking, larger the ey, higher the degree
of sample disturbance. Figure 2(b) shows corresponding
strength profile data of Figure 2(a). As can be seen, er for SS,
JPN and LVL samples do not exceed 3.5 at any point in the
profile. In general, ELE yields largest er followed closely by
SHT. Position of NGl samples falls in between these two groups.
Mean values and standard deviations of erfor various samplers
from Figure 2(b) have been calculated and tabulated in Table 2.
For SS, JPN and LVL samples, efc,~) is less than 3%, while it is
significantly larger than 3% for the NGI, SHT, ELE, in
increasing order. It is also interesting to note that standard
deviation from the mean strain at failure of JPN sample is even
lower than that of SS, and that of ELE is the largest.
SS
JPN
LVL
NGI
SHT
ELE
(RB) 2'
(RB)
(RB)
(RB)
(RB)
(RB)
2.70
2.81
2.95
3.56
3.67
3.76
SD ~
0.50
0.44
0.70
0.38
0.59
1.16
ratio
J P N ( O ) ~,.4,
SHT 4
O
O
I
nI
17
20
m ~
1.00
0.74
16
q J2
0.53
n
11
10
5
(Lab. vane)
m
1.00
0.98
0.68
~'~
~'~
n
27
24
12
(CIUC)
m
1.00
0.91
0.88
of samples
1)
n: n u m b e r
2)
m: normalized
3)
JPN (P) :Japanese f~xed piston sampler, JPN (O):
4)
JPN (O)
figure by strength obtained from JPN (P)
Japanese open sampler
a n d S H T are i d e n t i c a l a c c e p t the d e f e r e n c e in c l e a r a n c e ratio
1%. Many textbooks including ASTM D1587 state that the
inside clearance ratio of a sampling tube should be increased
with an increase in plasticity index of a soil. It has, however,
been established that if the inside clearance ratio is eliminated,
sample quality of soft soils gets greatly improved. This point
has largely been overlooked by many geotechnical engineers,
and m a n y others may not even bother to think of this issue.
Table 3 shows average normalized strengths of Kinkai site
obtained by open tube samplers having clearance ratios of 0 and
1% (Tanaka et al., 1996). The soil strengths of samples
obtained by these two samplers were measured by unconfined
compression tests, laboratory vane tests and triaxial
compression tests. Resulting strengths have been normalized by
corresponding strengths of fixed piston sampler having inside
clearance ratio of 0%. As can be seen from Table 3, depending
upon the test type, strength increase of as high as 47% can be
achieved by reducing the inside clearance ratio from 1% to 0%.
Second major reason for strength reduction by SHT is due to its
not having fixed piston. As shown in Table 2, strength
underestimation due to not providing a fixed piston could be as
much as 26%.
ELE yielded the lowest strengths of all, which is quite
surprising because, it has been believed that larger the sample
diameter, better the sample quality. It may be noted that inner
diameter ratios of ELE and JPN are both 1.35. Area ratio of ELE
is even lower than that of JPN, as shown in Table 1. All the
sampling mechanisms of ELE are identical to JPN. Therefore,
ELE was expected to yield better quality sample than that of
JPN sample. If we look at the ratio of retrieved sample length
(Ls) to sample diameter (Di), JPN sample has the Ls/Di of 10.7,
while the ELE has Ls/Di of only 3.2. Even if we consider that a
sample gets disturbed as far as 1.5De on both sides due to end
effects, it can be visualized that, along the ELE sampler tube,
almost all the samples gets disturbed due to the end effects.
Similarly, large area ratio and existence of 0.5% inside
clearance ratio of NGI are believed to be the principal causes of
strength underestimation by this sampler (Table 1).
Table 2 Mean of strain at failure
•t- .... (%)
clearance
(%5
JPN(P) ~
Piston
No
Yes
No
Yes
No
Yes
BORING METHODS AND SAMPLE DISTURBANCE
1) SD: standard deviation of strain
2) RB: samples obtained by rotary boring
In this research, three types of drilling methods were
investigated to examine sample disturbance associated with
boring. Two types of pre-boring (PB) methods namely, rotary
boring (RB) and wash boring (WB), and a displacement boring
(DB) method examined in this research, have been described
below.
Reasons for strength underestimation by various samplers:
The reasons for underestimation of shear strength values by
ELE sample and SHT sample are as follows. SHT has two
distinct problems: firstly, its tube has inside clearance ratio of
495
PRE-BORING METHODS AND SAMPLE DISTURBANCE
a) Pre-boring methods:
(i) Rotary boi'ing method (RB)
In Japan, rotary boring method, which is a pre-boring method,
is commonly used for sampling. Before sampling, borehole o f
uniform diameter is made using drilling bit by rotating and
pushing the bit into the soil. Cuttings are generally transported to
the ground surface by drilling fluid (IMSSCS, 1981). After that,
the borehole is protected by casing or drilling mud.
To retrieve soil sample using Japanese sampler and sampling
method, first the JPN sampler is lowered to the bottom o f the
borehole. Position of the sampler piston is then fixed by locking
the upper end of the extension rod to a suitable point of the
drilling tower. The sampler tube is then penetrated into the
ground by drilling rod to retrieve the sample.
(ii) Wash boring method (WB)
This method is commonly used in some Asian countries. In
this method, borehole is advanced by chopping and twisting
actions of a cutting bit with jetting of water from the tip o f the bit.
In wash boring sampling method using SHT, the sampler is
set at the bottom of the borehole. Then, the sampler is penetrated
into the ground by drilling rod and the sample is retrieved.
b) Sample disturbance in pre-boring method:
Figure 3 compares th~ strengths and ef profiles of Bangkok
soil samples obtained by rotary boring and wash boring methods.
The q,/2 values obtained by Japanese sampler using rotary boring
method (JPN (RB)) are considerably larger than that obtained by
SHT sampler using wash boring method (SHT (WB)). It can be
seen that strength reduction o f samples obtained by (SHT (WB))
is more than 50% compared to that by (JPN (RB)).
0
I , I , I ' I , i ' J
2
)<
4
SHT(wJPN(RB)B)I I •
xj
•
JPN(RB)
X
SHT(W B)
DISPLACEMENT
BORING
METHOD
AND
SAMPLE
DISTURBANCE
a) Displacement boring (DB) method
In practice, displacement boring method is generally used to
retrieve samples using NGI and ELE samplers. Therefore, in
this research, to investigate the effect o f displacement boring on
sample quality, soil samples were retrieved using displacement
boring method by Japanese samplers.
Figure 4 shows a diagram o f displacement boring procedure.
In this method, until the desired sampling depth is reached,
sampler is pushed into the ground without making a borehole.
While pushing the sampler down to the desired depth, position
o f the piston relative to the sampler is fixed by a locking
mechanism such that tip o f the piston flushes out o f the sampler.
When the sampler tip reaches the sampling depth, the piston is
unlocked from the sampler, and then the sampler is penetrated to
the desired final sampling depth, to retrieve a sample.
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Ariake clay is almost equal to that of Bangkok soil. It then, follows
that ef of SHT (RB) is not likely to exceed 5% for Bangkok soil.
Therefore, it implies that wash boring for Bangkok soil caused
additional 5-10% increase in ef.
Wash boring invariably induces vibration to surrounding soil
during the boring operation, the intensity and extent o f which
depends on machine type, soil type and the skill of the operator. In
worst case, hydro- fracturing is also likely to occur in WB.
Therefore, a sample is likely to get more disturbed in WB. On the
contrary, RB cuts soils without exerting vibration and disturbance.
Therefore, the resulting sample disturbance by RB is minimum.
×
•
-s 8 - X •
gl0
•
12
•
x
14
16
•
•
[ n i , l i [ , I ,
0
20 40 60 80 100120
qu/'Z (kPa)
•
Bangkok
~x
IIF
+r
, , , + i + , , , I , + , !
S
10
S
Ef (~)
(a) shear strength
(b) s t r a i n at f a i l u r e
Fig. 3 Comparison between boring methods
soil s a m p l e
Similarly, ey values o f JPN (RB) are 2-3%, while that of SHT
(WB) vary between 5 to 15%. Although no direct evidences are
available to estimate the extent o f sample disturbance due to WB
relative to that o f R B , a comparison of ef of JPN (RB) and SHT
(RB) from Figure 2(b) (and Table 2) with e f o f JPN (RB) and SHT
(WB) from Figure 3(b)'can be done. Core boring was done for JPN
and SHT for the Ariake soil shown in Figure 2(b), which shows
that the resulting strain difference between JPN and SHT is not due
to the difference in boring method, but due to the difference in
sampling method. For JPN (RB) samples, eRmea,Ois 2.81, and that is
3.67 for SHT samples. Furthermore, Figure 2(b) suggests that, for
RB, ef has not exceeded 4.5% at any depth. Comparing the tf o f
JPN (RB) for Bangkok soil and Ariake soil, it can be seen that efof
tube
Fig. 4 Diagram of displacement procedure
b) Sample disturbance in displacement boring method
Figure 5 compares the profiles of normalized undrained
shear strength and ef for Ariake clay obtained by RB and DB
methods for JPN, NGI, and ELE samplers. It is clear that JPN
sampler with RB method invariably yields the highest strength.
Similarly, regardless o f boring methods, ELE samplers yield the
496
•
JPN(RB)
O
JPN(DB) I
•
NGI(RB)
=
NG I(OB) I
@ ELE(RB)
o
ELE(DB)I
•.
,' Regulator
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.-/Mr ~'fCS$IIUO
::::::::::::::::::::::::::::::::::::::::::: 4"-- u ,
:::::::::::::::::::::::::::::::::::::::
Ariake
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o o
I I=., I
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m
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o
~o
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0
s
o
@
0.2 0.4 0.6 0.8
(q~/2)/s
CeramicDisc
[
DataLogger j
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*
o •
•
0
•
"c
,o
ol
l0
I
•
•
D
o oo
I
im
~ 1.2
(CPT)
Ca) s t r a r g l t h r:ormalized by su(CFf)
0
1
2
3
4
5
6
wa%~"
--.t.
7
e, (%)
(b) strain at failure
I
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"98~4911ff~a
l
Fig. 5 Comparison of rotary with displacement methods
Fig. 6 Suction measurement apparatus
lowest strength.
It can be seen that, in general, displacement method yields
lower undrained strength for JPN as well as NGI samplers.
Figure 5 shows, that JPN (DB) underestimates the undrained
shear strength o f Ariake soil by as much as 20-30%, compared
to that of JPN (RB). Similarly, NGI (DB) underestimates %,12
typically by 5-10%, compared to that by NGI (RB). However,
the trend is not clear for ELE sampler.
Looking at the etprofile for JPN sampler (Figure 5b), it can
be seen that, in general, ef of JPN (RB) is smaller than that o f
JPN (DB). For example, in average, e f o f JPN (DB) increased by
about 13% when compared to that of JPN (RB). However, the
trends for ELE and NGI are not clear, and it seems that
regardless o f boring methods, e/ values do not change
significantly for these samplers.
The results indicate that DB induces sample disturbance to a
variable degree, depending upon the dimension o f sampler as
well as the overall quality of the sample. For JPN sampler,
which has the best performance among the three types o f
samplers, reductions in strength as well as increases in efare the
largest, when RB is replaced by DB. NGI sampler, which
retrieves medium quality sample, shows somewhat decrease in
strength but no clear trend in change o f failure strain can be seen
due to the use o f DB method. This may be explained by two
points, firstly because the sample retrieved by NGI sampler is
already moderately disturbed, and therefore, the effect o f
boring method is not as pronounced as in the case o f JPN
sampler. The other reason is that since the tube diameter o f NGI
sampler is quite small compared to ELE or JPN sampler, the
extent o f disturbance caused by DB method is not significant.
On the other hand, in case o f ELE, since there has been already
very large disturbance caused by the sampling method, the
results are not sensitive to the type o f boring.
soil. In this research, characteristics o f a', have been investigated
and the results have been compared with the data available in
literature. Furthermore, to examine the claim that G 'r of a soil
reduces with storage time, response of o"r with time has also been
investigated.
MEASUREMENT OF RESIDUAL EFFECTIVE STRESS OF
SOIL
Figure 6 shows schematic diagram of suction measurement
apparatus used. It consists of a ceramic disc, a pore pressure
transducer, inner and outer cells, a data logger and a personal
computer (PC). The pore pressure transducer can measure pore
water pressure from -98 to 491 kPa. Air entry value of the
ceramic disc is 200 kPa. The inner cell protects specimen from
drying.
To measure the suction and also to determine unconfined
compressive strength from the same specimen, soil specimen was
trimmed with a wire saw to the dimension of 35 mm diameter and
80 mm height. Specimen thus prepared was first subjected to (7',
measuring test, and then to unconfined compression test. To
measure the tYr, t h e specimen was placed on a saturated ceramic
il
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RESIDUAL EFFECTIVE STRESS AND SAMPLE QUALITY
EVALUATION
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Some researchers have been insisting that the residual
effective stress (G'~) o f samples can be estimated, even if the
samples are disturbed, and therefore, shear strength o f the ground
can be estimated from it. Such ideas are based on the assumption
that strong relationship exists between o"r and shear strength o f
15
. . . . . . . . .
200
400
elapsed
i
. . . . . . . . .
600
time
800
(see)
Fig. 7 Typical (7'r monitoring data
497
o
1000
1200
0.3
0.2
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O
3 W~ek~7~) I
•
on-site(9m)
[]
3Woeks(8m)
•
on-slto(lOm) J
0.15
.
Ariake
. ~ 0.1
~
I
0.2
•
0.15
•
o
40
50
o
[2
0.1
O.05
~JPN(6.4m
I
I
On s i t e
3 weeks
O,05
JPN(IO.4m)I
ELE(6,2m)
ELE(IO.2•)
I
I
I
2 months
0
10
20
Distance
2 years
30
fro•
the sa•pler
60
70
80
edge(c•)
Fig. 8 Variation in effective stress with storage time
Fig. 10 Effective stress and distance from the sample edge
disc and its negative pressure was monitored until it attained a
constant value, which was taken as o"r of the specimen.
Figure 7 shows a typical o"r monitoring data of Holocene clay. As
can be seen, negative pore pressure decreases with time, and it takes
about 20-30 minutes to achieve crr.
To investigate the time effect on or'r, suction measurements
were done immediately after the sampling, as well as three weeks,
two months and two years after the sampling. Figure 8 shows a
typical relationship between o"r and sample storage time for JPN
(RB) samples and ELE samples of Ariake clay. It may be noted
that values of o"r have been normalized by the corresponding
effective overburden pressures (o"~0).
Figure 8 reveals that o'~ from JPN sampler, which yielded
good quality samples, doesn't change with storage time. On the
other hand, o", from ELE, which yielded had quality samples, has
low value immediately after the sampling, but the values of three
weeks, two months and two years of sampling are almost constant.
Thus, for ELE samples, suction measurement immediately after
the sampling showed low values of o"~ due to poor sample quality.
However, its o", did not change with storage time.
SOME CHARACTERISTICS OF RESIDUAL EFFECTIVE
STRESS
To investigate the influence of sample extraction time on soil
properties, soil samples were extruded at various time intervals
after sampling. Sample extrusion time is defined as the time
duration from the time of sampling to the time of sample
extrusion from the sampling tube. Figure 9 shows a schematic
view of G'r distribution along the sample length obtained by
Japanese sampler. Distribution characteristics of cr ~ along the
tube length of samples extruded from the tube immediately after
sampling and after three weeks of sampling have been shown. As
can be seen, if a sample is extruded immediately after sampling,
resulting o'r is distributed non-uniformly; there is lager stress
concentration n e a r front edge of sampler, which gradually
becomes smaller towards the rear edge. As shown in lower part of
the figure, if a sample is not extracted from the sampling tube
immediately after the sampling, o"r gets re-distributed along the
tube, with a reduction of (7', near the front edge, and an increase
towards the rear end, thereby setting more uniform stress
distribution.
These results suggest that, samples should be extruded from
sampler immediately after the sampling, and should be cut into
suitable pieces and preserved, in order to retain initial G'r
condition of each piece.
Figure 10 shows the relationship between o"r and distance
from the sampler edge, for samples extruded immediately after
sampling and after three weeks of sampling. The cr'r in each
sampler has larger value near its front edge, which keeps on
reduc!ng away from sampler front edge. It may also be noted that
the or, obtained from in-situ measurement are lager than that
from samples extruded three weeks after sampling. The effect is
more pronounced, especially near the front edge. This suggests
that if a sample is kept in the sampling tube without extruding it
out from the sampler, re-distribution of O"r takes place with time,
in which reduction of stress takes place near the front edge of the
sample and increase in stress takes place towards the rear edge.
Figure 11 shows the relationship between undrained shear
strength and distance from the sampler edge, for the
corresponding samples of Figure 10. Data for two types of
samples have been included, one series of samples were extruded
immediately after the sampling, while the other series of samptes
were extruded three weeks after sampling. It can be seen that in
immediatelyaftersampling
,
O"~r /
0
10
~
3 weeksaftersampling
I
I
I
I
I
I
I
20
30
40
50
60
70
80
(cm)
front edge
1) samplenumber
JPN sampler
rear edge
Fig. 9 Schematic view of residual effective stress distribution
498
0.3 l
]
0
3 Woeks(Tm)
•
on-site(9~)
the vicinity o f the front edge, q,,/2 of in-situ sampler is larger
than that o f the sample extruded three weeks after sampling.
Thus, this result also suggests that, after the sampling, we need
to extrude samples as soon as possible.
3Weoks(Sm)
0.25 I
I
on-site(lO=)
i
- ~
EVALUATION OF SOIL PROPERTIES FROM Or'
O.2~-
i
t~
0,15
o
D
o
•
•
o
40
50
To examine the relationship between (r'~ and engineerin~
properties of soils, case studies of Bothkennar and Ariake site
have been done. Ariake site is located 1000 km south-west o~
Tokyo in Japan, while Bothkennar site is located 50 km west o~
Edinburgh in United Kingdom.
Figures 12 and 13 show typical soil profile of Ariake and
Bothkennar clay deposits. Ariake site has clay deposit frorr
ground surface to G.L. -18m, while the Bothkennar site has cla2,
deposit from surface to G.L. - 1 9 m. Particle density of Ariak~
clay (typical value: 2.60-2.66) is lower than that o f Bothkennal
(typical value: 2.70). Distinct feature o f Ariake site is that it.,
water content (typically: 75-I 50%) is larger than its liquid l i m i t
yielding liquidity index of more than unity throughout the depth
Plasticity index o f Ariake clay (60-110) is significantly higher
El
0'I I
r
0.05 ~I
t
0 I
~0
20
30
60
70
80
Distance from the sampler edge(cm)
Fig 11 Shear stress distribution with distance from the sampler
edge
I
I
I
'
I
'
]
'
I
'
'
I
~
I
I
I
'
A•
A
El
'
•oO
I
I
I
o•
•
•;
o
o
silt
13
A•
Z3.•
a¥
°11
10
g
m
D
,Se
A•
e
15
Z~
~
Am
20
2.5
,
I
,
2.6
density
I
of
part icles
2.
20
40
60
80
gradation
(g/cm ~)
WL
Wp
El
o
El•Z3.
-4
t I T I ~
, I ~I
50 100 150 200 250
,
2.7
soil
A
m
100
•ater
(~)
content
e
I
.2
~
I
1.3
~
1,4
wet d e n s i t y
(~)
I
,
,
1,5
I
,
50
.6
I
,
100
I
r
150
200
c o n s o l i d a t ion
y i e l d s t r e s s (kPa)
(g/cm s)
Fig. 12 Soil profile of Ariake clay
I
I
1
'
I
'
I
'
I
I ' I ' I '
'
I
,'
'
'
'
I
'
'
'
I
'
'
'
El
sand
e
v
a=
$
7
:°,
A
WL
•
oA
W t
W
D
eA
silt
o
o
~
o
•
z~
D
g
•O
o.
I
".azx
OA
-5~_L~\
e,
r
2.5
I
2.6
density
particles
,
I
,
2.7
of soil
(g/cm 3)
2.8
20
40
60
gradation
80
(~)
100
-
A
~ I ~ I ~ I ~ I ,
20
40 60 80 1 0 0 1 . 5
water content
(~)
I ~ 1 ~ 1 ~
1.61.71.81.90
wet d e n s i t y
Fig. 13 Soil profile of Bothkennar clay
499
.
(g/c~)
I00
200
c o n s o l i d a t ion
y i e l d s t r e s s (kPa)
300
site, there isn't any correlation between these parameters. Thus,
Figures 14 and 15 reveal that depending upon soil type, o'r may
be or may not be correlated with soil properties. Therefore, one
should be careful in correlating soil properties with o'r.
JPN
NGI
t • 5 --r--r"o.
x
ELE
SHT
LVL
SS
>
u_
CONCLUSIONS
.<>~°I,,<>
x~
× ¢
'ax
=
o" O, 5
I
o
I
I
0.1
I
I
0.2
I
The following conclusions have been drawn from the present
study.
1. It has been found that the sample quality from the
Japanese sampler depends on the method of sampling,
and its sample quality is equivalent to that of the large
diameter Laval sampler and Sherbrooke sampler.
2. Rotary boring method provides superior quality samples
compared to that o f wash boring or displacement boring
method.
3. To avoid the change in properties o f soil after sampling,
samples should be extruded from sampling tubes as soon
as possible.
4, Residual effective stress o f a soil sample does not
change with storage time.
5. Depending upon soil type, effective residual stress may
be or may not be correlated with soil properties.
Therefore, one should be careful in correlating soil
properties with effective residual stress.
o
%
i
0.3
0.1
.4
0.2
o,/ov0
o
Ar i a k e
(a)
o
(b)
",
0.3
1.4
io',0
Bothkennar
Fig. 14 Relationship between q~/2 and o-'~
140
I
I
I
120
100
•.
JPN
NGI
ELE
SHT
LVL
SS
o
x
,~,
m
i
t
-
•
¢s
.~ 8 0 -
o
LU
~
JPN
ELE
LVL
REFERENCES
6O
e.m
Adachi, K. (1979). "Current practice of the sampling of soft clay in
Japan," Proc. of the International Symposium of soil sampling,
Singapore, pp. 1-12.
Andersen, A. and Kolstad, P. (1979). "The NG154-mm samplers for
undisturbed sampling of clays and representative sampling of coarser
materials," Proc. of the International Symposium of soil sampling,
Singapore, pp. 13-21.
Hight, D. W., BSese, R., Butcher, A. P., Clayton, C. R. I. and Smith P. R.
(1992)." Disturbance of the Bothkennar clay prior to laboratory testing,"
G~otechnique, Vol. 42, No. 2, pp. 199-217.
La Rochelle, P., Sarrailh, J., Tavenas, F., Roy, M. and Leroueil, S.
(1981). "Causes of sampling disturbance and design of a new sampler
for sensitive soils," Canadian Geotech. Journal, Vol. 18, No. 1, pp.
52-66.
Lefebvre, G , and Poulin, C. (1979). "A new method of sampling in
sensitive clay," Canadian Geotech. Journal, Vol. 16, No. 1, pp.
226-233.
Shogaki, T. (t995). "Effective stress behavior of clays in unconfined
compression tests," Soils and Foundations, Japan Geotechnical Society
Vol. 35, No. 1, pp. 169-171.
Tanaka, H., Sharma P., Tsuchida T. and Tanaka M. (1996). "
Comparative study on sample quality using several types of samplers,"
Soils and Foundations, Japan Geotechnical Society, Vol. 36, No. 2, pp.
57-68.
Tanaka, H, and Tanaka M. (1999). "Key factors governing sample
quality," Proc. of the International Symposium on Characterization oJ
~
40
o
ox
0
0
o
0.1
0.2
,
0.3
M
L
0.2
0.3
~'/(~'0
o,/a'~o
(a)
I
0.1
(b) Bothkennar
Ar l a k e
Fig. 15 Relationship between E50 and
O" r
than that of Bothkennar clay (30-45). Similarly, over
consolidation ratio o f Ariake clay varies from 1.1 to 1.6 while
that o f Bothkennar clay is about 2.
Figure 14 shows the relationship between q,/2 a n d o " r for
Ariake and Bothkennar clays. The q J2 values have been
normalized by s,(FV) and o'r is normalized by o",0, where s~ (FV)
is shear strength of corresponding depth obtained by field vane
test. It can be seen that for Ariake clay, (qu/2)/s,(FV) values are
strongly correlated with (r'Jo"v0 values, the relation being almost
linear. On the other hand, for Bothkennar clay, there is no obvious
relation between (q J2)~ su(FV) and (r'flo" ~0.
Figure 15 shows relationship between modulus of
deformation (Es0) and (Y',, both o f these parameters having been
normalized by the corresponding o'v0. Here, Es0 is defined by
equation (2).
E50 = ~
x 100
Soft Marine Clays -Bothkennar, Drammen, Quebec and Ariake clays-.
Yokosuka, Japan, Balkema, pp,57-81.
The Sub-committee on Soil Sampling International Society for Soil
Mechanics and Foundation Engineering (1981 ). "International Manual
for the Sampling of Soft Cohesive Soils," Tokai University Press,
pp.23-42.
JGS ! 22 l-1995 (1998). "Method for obtaining undisturbed soil samples
using thin-walled tube sampler with fixed piston," Standards oJ
(2)
where, q,: unconfined compressive strength and. es0: strain at qu/2
Japanese Geotechnical Society for soil sampling-standards
explanation, Japan Geotechnical Society, pp. 1-7.
Figure 15 reveals that for the Ariake site, Es0/o-~0 values are
almost linearly correlated to (r'/o"v0, whereas for the Bothkennar
500
and