Document 6532968

Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference
Stavanger, Norway, June 17-22, 2001
Copyright © 2001 by The International Society of Offshore and Polar Engineers
ISBN 1-880653-51-6 (Set); ISBN 1-880653-53-2 (Vol. 11); 1SSN 1098-6189 (Set)
Sample Quality of Pleistocene Clay and Influence of Residual Effective Stress
F. Rito and N. Ohmukai
OYO Corporation
Ohmiya, Japan
1-1. Tanaka and M. Tanaka
Port and Harbor Research Institute
Yokosuka, Japan
ABSTRACT
This study aims to estimate the sample quality of Osaka Bay
Pleistocene clay obtained from the great depth (as deep as 350m),
and examines if the variation in its consolidation yield stress (py)
profile was caused by the variation in residual effective stress of the
sample. This study found that sample quality of the Pleistocene clay
is good and uniform from practical viewpoint, regardless of the
depth, and that its py values were not influenced by variation in
residual effective stress.
PyN'l/n¢
0
1000
3;00
4OO0
3000
50OO
0
--50
-100
k~12
Llicaaxn~wc~der
/
"~-15o
KEY WORDS: Sample quality, residual effective stress, Pleistocene clay,
great depth, CRS test, consolidation yield stress
.-I
0
INTRODUCTION
Osaka Bay Pleistocene clay had experienced extreme release of
insitu stress when recovered from great depth of as deep as 350m.
Disturbance of the sample was suspected to have occurred due to extreme
release of insitu stress of Osaka Bay Pleistocene clay when recovered
from great depth, and also it was suspected that variation of residual
effective stress might have influenced to consolidation yield stress (py) of
the clay.
Therefore, to address these issues, this study aims to estimate the
sample quality of Pleistocene Osaka Bay clay obtained from the great
depth, and to investigate if its py profile was influenced by the variation
in residual effective stress of the sample.
Constant Rate of Strain (CRS) oedometer tests were carried out to
examine the quality of the samples. Two parameters, obtained from CRS
tests, were examined to estimate the sample quality: e vo (change of
volumetric strain by recompressing sample to insitu effective stress), and
A e/eo (ratio of change in void ratio changed while recompressing
sample to insitu effective stress, A e and initial void ratio, e0).
To investigate if variation ofpy was caused by variation of residual
effective stress, CRS tests with different initial setting conditions were
conducted, such as subjecting the specimen in various soaking conditions
and various swelling boundary conditions prior to subjecting the
specimen to consolidation.
M~
1 D~ .iscntype ~arpier
,¢,-~0
OCf~.3
Figure 1. Consolidation yield stress (py) of
Osaka Bay Pleistocene clay
figure, samples of this site were recovered from great depths. The
deepest elevation of sample extraction was 350m. For soil sampling,
boreholes were drilled using wire line method. Two types of wire line
samplers were used. Hydraulic piston sampler was used above 160m
depths, and Denison type sampler was used below 160m depths.
The Pleistocene clay at Osaka bay area consists mainly of marine
clays and partly of non-marine clays. In Figure 1, the legend Ma
indicates that the soil is marine clay. It may be noted that these marine
clay layers have been numbered orderly against the depth (For example,
lchihara, 1993). The present study covers the layers from Ma12 to Ma3.
As is obvious from the figure, py distribution has a considerable
scatter, which varies from 1.2 to 1.6. Until now, discussions have been
going on to find the possible causes for this uneven distribution ofpy.
Some researchers have pointed out the influence of sample disturbance as
one responsible factor. Therefore, in this section, the sample quality of
ESTIMATION OF SAMPLE QUALITY
Background (py values of Pleistocene Clay Recovered from Great
Depth)
Figure 1 shows py distribution pattern of undisturbed Pleistocene
clay at Osaka bay site obtained by CRS tests. As can be seen from the
488
Pleistocene clays, which were recovered from great depth and which
were subjected to the extreme release of insitu stress due to extraction,
has been evaluated.
Table 1. Relation between e ~o and sample quality
(Andersen and Kolstad, 1979)
E v0
Volume ¢hanp
< 1%
1 -- 2%
2--4%
4-- 10 %
> 10 %
Existing Studies for Estimation of Sample Quality
For clays, the volumetric strain when consolidating the specimen to
the insitu effective stress, e ~o, can be used as an indication of sample
quality. Table 1 and Figure 2 show relations between the change of
volumetric strain ( e v0) and quality of samples having various degree of
sample disturbance and various OCRs (Andersen and Kolstad, 1979;
Lacasse and Berre, 1988). As may be noted, Table 1 uses only e v0, but
Figure 2 considers allowable strain as a function of stress history (OCR)
and depth.
As mentioned earlier, the OCR of Osaka Bay Pleistocene clay varies
from 1.2 to 1.6. Therefore, Table 1 and Figure 2 imply that if the range of
e ~0 is within 4%, the Osaka Bay Pleistocene clay can be considered as
having good quality.
Table 2 shows the relation between A e/Co and sample quality
(Lunne et al., 1997), where A e is the change in void ratio in
recompressing the sample to insitu effective stress, and e0 is the initial
void ratio. According to this figure, when the range of Ae/e0is within
0.07, we can regard that the samples are of good quality.
/3e/eo is equal to change in pore volume divided by initial pore
volume while ~ vo is equal to change in pore volume divided by initial
total volume. It is reasonable to assume that a certain change in pore
volume will be more detrimental to the particle skeleton the lower the
initial pore volume. It is therefore suggested to use A e/eo rather than e ~0
when quantifying sample disturbance.
1
Teat spemiman quality
Very good to excellent
Good
Fair
Poor
Very poor
Volumechange. E"vo
2
3
4
0|
.,o,,o
10
OCR
30
3.0-8.0
,0
Figure 2. Strain at insitu overburden stress from oedometer tests
on high quality clay samples (Lacasse and Ben'e, 1988)
Table 2. Relation between A e/eo and sample quality
(Lunne et al, 1997)
Influence of Dissolved Gas in sample quality
While sampling soil from great depth, it is possible that expansion
of specimen might have occurred due to the release of dissolved gas. The
expansion of specimen, if any, caused by the release of dissolved gas, can
be estimated by examining the degree of saturation of soil specimen. This
is because partial degree of saturation of soil specimen is an indicator of
such phenomenon.
Figure 3 shows the degree of saturation (St) of the soil samples
obtained from various depths of Osaka Bay Pleistocene clay. It may be
observed that, in general, Sr is about 100% for all the depths. In other
words, soil specimens are completely saturated throughout the depths,
implying that there were no dissolved gases in the soil samples.
Therefore, it is not necessary to take into account the influence of
dissolved gas while studying the soil quality of this site.
At&
~eroaa~dation V~g00d '
Good
Poor
V~
to
to
exedlc~t
1-2
<~.04
O.Oa,-O.O'/ 0.07-0.14I >0.14
24
<0.03
0.03-0.05 0.05-0.10t >OlO
90
95
I
fair
Sr
(%)
100
105
110
0
Results of ¢ v0 and A ere0 from CRS Test
Figure 4 shows the change of volumetric strain (~ v0) profile of
Osaka Bay Pleistocene clay samples with recompression to the
corresponding insitu effective vertical stress. With few exceptions, the
range of e v0 varies within 2% to 4%. Comparing these results with
Table 1 and Figure 2, it is apparent that majority of the samples of Osaka
Bay Pleistocene clay site are of good quality. A distinct feature that can
be noted from Figure 4 is that ~ v0 distribution is almost uniform with
depth. Besides, although the sampler type was changed from hydraulic
piston sampler to Denison sampler at a depth of 160m, e v0 does not
change due to the change in sampling method. It means that Osaka Bay
Pleistocene clay samples are of good and uniform quality from practical
viewpoint.
Figure 5 shows the A e/eo profile of Osaka Bay Pleistocene clay
samples with recompression to corresponding in situ effective vertical
stress. Values of A e/eo vary in a narrow band of 0.04 to 0.07, and are
not sensitive with depth. It may be recalled that if Ae/eo is within 0.07,
soil samples can be considered to be of good quality. These results are
consistent with the tendency shown by ~ ~o profile in Figure 4. This
confirms that all of the Osaka Bay Pleistocene clay samples have good
-50
•
-100
"E -150
._1
• A •
•
°c -200
,-n
-250
~x
~r~ ~ ' -
.
+.~-
Hy¢ raulic pis :on
sampler
_
De~dsontype
;ampler
-300
-350
-400
Figure 3. Degree of saturation (St) of
Osaka Bay Pleistocene clay
489
"'7
poor
ratio
,,
0
Volume change E vo (%)
1
2
3
4
5
• Dtc
6
2.5
o
1Mal-"
&Mal 1
.o_ 2.0
J
-50
-100
•
k
~
,iston
cdrau
s~ mpler
•
~
XMal£
~
{.'E_ ''10
~
-150
a
XMa9
t "
• Doe5
+ Ma8
-Me7
0.5
J.p_M:_~_3
0.0
= -200
o
-250
0
Deni ;on type
samF ler
-
I
~Ma4
1
2
3
4
5
Volume change 6 vo (%)
-300
Figure 6. Relation between OCR and s v0
for Osaka Bay Pleistocene clay
-350
-400
o
o
Figure 4. s ~oofOsaka Bay Pleistocene clay
c
o
a.a
2.5
p,Mall f
2.0
:::;o I
1.5
• Doe5
1.0
e"
O
0,00
0.02
A e/eo
0.04
0.06
0
°
t•i
kA~"
>Xx
lkl
o
= -200
•
[ ] -250
-300
=~x
4
•
0.00
0.02
0.04
0.06
0.08
0.10
'1"'t"
from its in situ state. When a soil element is in its insitu state, it has a
stress anisotropy. In the figure, the total stress anisotropy is shown by a
v and a h, and the effective stress anisotropy is shown by ,s ~' and ,r h'.
When the soil element is recovered on the ground from in situ state, the
total stress gets lost and the effective stress changes to the residual
isotropic stress a p'. The isotropic residual effective stress ( a p') of
perfect sampling can be represented as follows (for example, Noorany
and Seed, 1965).
o-p'= o-v' [Ko+Ao(I-Ko)]
Ko: Coefficient of earth pressure at rest
Ao : Pore pressure coefficient
The value of a p' changes with the quality of a sample. Therefore,
a p' has been considered as an index for judging the sample quality. It is
known that smaller the value of a p', larger the degree of sample
disturbance. For Osaka Bay Pleistocene clay, since the depth of the
sample extraction is as much as 350m, the proportional release of
effective stress upon extraction is significantly larger at great depth. This
suggests if residual effective stress is different, consolidation yield stress
could be become different.
Figure 9 shows the results of incremental loading oedometer tests on
stiff clay specimen (Sandbaekken, 1987). The figure compares the
characteristics of specimen mounted on dry and saturated filters. It shows
that, when saturated filter is used, soil specimen absorbs water, which
results in the significant underestimation of yield stress. This
phenomenon is related with loss of residual effective stress, which
occurred due to the saturation of specimen.
It is, thus, necessary to investigate if the initial soaking of specimen
causes any reduction in yield stress of Osaka Bay Pleistocene clay
samples. As mentioned earlier, investigation of this phenomenon for
Osaka Bay Pleistocene clay is one of the objectives of present study.
Therefore, An investigalion has been carried out to examine the
. Lk
"
x
i1P
•
Im Ma4
Figure 7. Relation between OCR and A e/e,
for Osaka Bay Pleistocene clay
alll~ll;~l
- 150
o
o
I-ydraulic ~iston
-1 O0
...I
r',,
0.10
" MaT
0.0
A e/co
I
-50
0.08
+ Ma8
0.5
Denis ~n type
..... r
m
-350
-400
Figure 5. A e/eo of Osaka Bay Pleistocene clay
and uniform quality.
Figure 6 shows the relation between s ~0 and OCR for Osaka Bay
Pleistocene clay samples. It is obvious that OCR is almost constant with
depth, and is independent of s v0- Similarly, Figure 7 shows the relation
between A e/eo and OCR, which reveals that OCR is not dependent on
A e/eo. This tendency is consistent with relation between OCR and e ~oAs a consequence, it is clear that the samples obtained from great
depth of Osaka Bay have good and uniform quality regardless of depth.
Similarly, since OCR is constant regardless of e vo, it may be concluded
that p~ values obtained from these samples are correct from practical view
point.
ESTIMATION OF R E S I D U A L E F F E C T I V E STRESS
Background
Figure 8 shows the stress change of a soil element when recovered
490
Total stress
x,/~,-'N/
Effective stress
0
O"e'=-Ur
\A~z
Water
Same Specimen
Samplint;
Under ground
Specimen
O"v
fly'
Figure 10. Setting method of CRS test
Figure 8. Stress conditions of a soil element
specimen inside water. However, due attention was given to preserve the
water content of the specimen during CRS test.
EFFECTIVE AXIAL STRESS, log Oa', kN/m z
0
1
2
I.-"
_.1
x
3
,
50
100
200
500
1000 2000
~ p,~= ~
Results of CRS Test
SO00
Figure 11 shows the results obtained from 3 sets of the CRS tests for
MalO marine clay specimen recovered from 160m depth. It is clear that,
before reaching the insitu effective stress Po, the three sets of curves have
slightly different origin and follow slightly different paths. However,
after the consolidation pressure reaches to Po, all the curves almost
coincide with each other. Resultingpv values from these curves vary from
1,195 to 1,285 kN/m 2, yielding the practically same values.
Figures 12 and 13 show the results for Ma7 recovered from 260m
depth and Ma3 recovered from 330m depth, respectively. The curves for
both the sets have tendency similar to that of MalO. For Ma7 and Ma3,
however, the e ~ logp curves converge more closely with each other.
It is clear from the above results that, after the consolidation
pressure exceeds Po consolidation characteristics of Osaka Bay
Pleistocene clays are not influenced by initial setting conditions. In other
words, yield stresses of Osaka Bay Pleistocene clays are not influenced
by the change in residual effective stress caused by variation in degree of
soaking.
Dry filters
I~r~- z=15.Sm
L
NV,=J7 w.
Saturated filtersl~%
z=lS.Om i
u~=14.8"N
!
I
6
Figure 9. Results of incremental loading oedometer tests
on stiff clay specimens mounted with dry and saturated filters
influence of residual effective stress on py of Osaka Bay Pleistocene clay.
Setting Methods of CRS Tests
To investigate the influence of initial setting condition on yield
stress of Osaka Bay Pleistocene clay samples of great depths, three sets
of CRS tests were conducted. Rate of strain of all tests is 0.02%/minute.
In each case, three identical soil specimens recovered from a single
piece of sample obtained from the same depth were used for three
different sets of tests. Figure 10 shows the three different setting methods
adopted in CRS tests. These setting methods are as follows.
-5
0
5
a) CASE 1 (Wet 1 method)
In this method, the head of piston was fixed, water was poured in
oedometer, and then the backpressure was applied to the specimen. A
backpressure of 200kN/m 2 was applied to the sample. After that, CRS
test was started. This procedure is similar to the Japanese Geotechnical
Standard Method of conducting CRS test.
tu
15
2O
b) CASE 2 (Wet 2 method)
In this method, head of pedestal was kept free and the specimen was
allowed to soak inside water for 24 hours. During this period, the
specimen was allowed to swell due to absorption of water. At the end of
24 hours, the head of pedestal was fixed, and then CRS test was started.
In this method, backpressure was not applied because the soil specimen
was regarded as completely saturated by soaking inside water for 24
hours.
-4-
25
3O
10
.d.
10o
10oo
Consolidation pressure p (kN/m z)
Figure 11. e ~ l o g p curve of M a l 0 clay
c) CASE 3 (Dry method)
In dry method, CRS test was started without soaking the soil
491
1o0oo
Progressing o f The International Symposium o f Soil Sampling, Singapore,
pp.13-21.
-5
For example; Ichihara, M (1993). THE OSAKA GROUP, Sougensha,
pp.68-86.
0
A
5
Lacasse, S, and Berre, T, (1988). "Triaxial Testing Methods for Soils."
Advanced Triaxial Testing o f Soil and Rock, ASTM STP 977, pp.
264-289.
to
10
Lunne, T, Berre, T, and Strandvik, S, (1997). "Sample disturbance effect
in soft low plastic Norwegian clay" Symposium on Recent Developments
in Soil and Pavement Mechanics, Rio de Janeiro, pp.81-102.
15
20
10
100
1000
10000
For example; Noorany, I, Seed, H.B, (1965).
"ln-situ Strength
Characteristics of Soft Clays" Proc. ASCE, Voll. 91, No. SM2, ppA9-80.
Figure 12. e ~ l o g p curve of Ma7 clay
Sandbaekken, G, Berre, T, and Lacasse, S, (1987). "Oedometer Testing
at Norwegian Geotechnical Institute" Norwegian Geotechnical Institute
Report. 168, pp. 1-25.
Consolidation pressure p (kN/m 2)
-5
5
v
to
lO
15
20
10
100
1000
10000
Consolidation pressure p (kN/m 2)
Figure 13. ~ ~ l o g p curve of Ma3 clay
CONCLUSIONS
From the present study, it may be concluded that:
o The samples of Osaka Bay Pleistocene clay recovered from
great depth have good quality from practical viewpoint. In
addition, the sample quality is constant regardless of the
depth.
• Conducting at three different setting conditions with and
without soaking the specimen in water, CRS tests show that py
values of Osaka Bay Pleistocene clay do not change due to
variation in degree of release of effective stress.
• py values obtained for the Osaka Bay Pleistocene clay are
correct values, and any scatter in their values are due to the
inherent properties of soil, not due to the soil disturbance
during sampling.
REFERENCES
Andresen, A, and Kolstad, P (1979). "The NGI 54-mm Samplers for
Undisturbed Sampling of Clays and Representative Sampling of Coarser
Materials," State of the Art on Current Practice of Soil Sampling,
492