EVALUATIO OF U DRAI ED SHEAR STRE GTH O BUSA CLAY …
Transcript of EVALUATIO OF U DRAI ED SHEAR STRE GTH O BUSA CLAY …
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EVALUATIO OF UDRAIED SHEAR STREGTH O BUSA CLAY USIG
FLATDILATOMETER TEST
SIGH V. K.1, CHUG S. G.
2, HOG Y.P.
3, KWEO H. J.
4
1Researcher, National Research Laboratory of Soft Ground, Dong-A University, Busan, S. Korea
2Professor, Department of Civil Engineering, Dong-A University, Busan, S. Korea
3 Postgraduate Student, Department of Civil Engineering, Dong-A University, Busan, Korea
4Researcher, National Research Laboratory for Soft Ground, Dong-A University, Busan, Korea
ABSTRACT: Busan clay, usually varying from 20 to 70m thick, is widely deposited along the coastline in the Nakdong
River deltaic area located west of Busan City in Korea. Despite many geotechnical investigations for various reclamation
projects, the properties of the clay have not been yet elucidate due to their spatial variation and inadequate undisturbed
sampling. The undrained shear strength of the clay is interpreted using the flat dilatometer (DMT) with various existing
empirical equations. The results are compared with each other, and correlated with the corrected undrained shear strength
obtained from the field vane shear test (Su(FVT)). The results indicated that the KD–based empirical equation proposed by
Marchetti (1980) slightly underestimates the Su(FVT) values on the clay.Empirical equations based on Su-KD and Su-ED-ID
relationshipsare developed for the clay. The undrained shear strengths estimated from theSu-ED-ID relation give the closest
correlation.
Keywords: clay, in situ test, undrained shear strength, DMT, FVT
ITRODUCTIO
The undrained shear strength is usually obtained from
unconfined compression tests or unconsolidated undrained
triaxial compression tests on undisturbed samples or from
field vane shear test (FVT). Laboratory test results largely
depend on the quality of undisturbed samples. Sample
disturbance may result in the severe failure of the structure
due to incorrect prediction of undrained shear strength such
as failure of the Break-water at Busan New Port site that
occurred during construction of the New Busan Port
(Chung et al., 2007). Thus, it is a common practice to
undertake the field vane shear test (FVT) for the
clay;however, it is necessary to appropriately determine
the correction factor for the undrained shear strength of
clay. Besides, the field vane shear test results can be
affected by sand lenses, shells and seams.
Based on the investigation on failure of the Break-waterat
Busan New Port, Chung et al. (2007) estimated the
undrained shear strength of the Busan clay from back
analysis.With the several field vane shear tests at five
additional sites including the present site, they concluded
that the correction factor proposed by Aas et al. (1986) is
applicable to Busan clay. They found that the corrected
strength ratio (Su,corr./σ’vo) in the Aas et al. (1986)method
varies between 0.22 and 0.30, however, the lower bound
value 0.22 appears to be applicable for the design.Hong
et al. (2007) evaluated clay at Busan New Port sites with
flat dilatometer test, field vane shear test and CK0U
triaxial tests and found that Su(FVT)/σ’v ranges from 0.20
to 0.22 whereas, Su(CKoU)/σ’v ranges from 0.30 to 0.35.
The flat dilatometer test (DMT) is comparatively simple,
rapid, repetitive and applicable to both clay and sand.
The one of the main application is to estimate the
undrained shear strength of the clay. The flat dilatometer
test has been used in characterizing marine clays in the
region including Korea by number of researchers (Chang,
1992, Kamei and Iwasaki, 1995, Kim et al., 1997, Kim et
al. 2001, Lee and Seong, 2001, Byeon et al., 2004, Hong
et al., 2007, Lee et al. 2008 etc.). However, the
correlation varies location to location depending on the
clay type. This infers the requirement of calibration of the
flat dilatometer prior to use at the local sites.
In the present study, the flat dilatometer test(DMT) and
field vane shear test (FVT) were performed at Jangyu site.
The undrained shear strength obtained from the filed vane
shear testwas corrected according tosuggestionby Chung et
al. (2007). The DMT results were interpreted using various
existing empirical equations and the predicted undrained
shear strengths were compared and correlated with the
corrected undrained shear strength obtained from the field
vane shear test. Two new equations compatible to local site
were presented, one using the Su(FVT)-KD relationship and
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another using the Su(FVT)-ED-ID relationship. In the second
equation, the material index (ID) is incorporated so that the
equation can give better estimation of undrained shear
strength for different type of clays.
DILATOMETER TEST ITERPRETATIO
The flat dilatometer test (DMT) consists of pushing a flat
blade located at the end of a series of rods. Once at the
testing depth, a circular steel membrane located on one side
of the blade (Fig. 1) is expanded 1 mm horizontally into the
soil. The pressure is recorded at specific moments during
the test. The blade is then advanced to the next test
depth.The general layout of the flat dilatometer test is
shown in figure 2. The principal and test procedure of DMT
in detail can be found in Marchetti et al. (2001).
Figure 1 The flat dilatometer – Front and side view
The interpretation of DMT is primarily based on two field
readings, P0 (corrected contact stress) and P1 (corrected
1mm expansion stress), from which three intermediate
parameters, viz. material index [ID=(P1-P0)/(P0-u0)],
horizontal stress index [KD=(P0-u0)/σ’v], and dilatometer
modulus [ED=37.4(P1-P0)] are defined where u0 is the in-
situ pore water pressure prior to dilatometer insertion and
σ’v is the effective overburden stress(Marchetti, 1980).
The other soil parameters are interpreted using these
three intermediate parameters.
Marchetti (1980) suggested the first empirical equation
(Eqn. 1) to estimate the undrained shear strength as a
function of horizontal stress index KD. Several authors
have shown that the undrained shear strength predicted
from equation 1 in soft, uncemented saturated clays
compares fairly well with uncorrected field vane results,
however, the equation is not recommended for OC
cemented and/or fissured clays (Riaund and Miran, 1992).
Su = 0.22 σ’v (0.5KD)1.25
(1)
Lacasse and Lunne (1988) showed that the measured
value of Su varies depending on the type of test used.
They suggested equation 2 to estimate Su for soft
uncemented clays using field vane shear test.
Su = 0.19 σ’v (0.5KD)1.25
(2)
Figure 2 General layout of the dilatometer test(Marchetti, 1980)
Roque et al. (1988) considered the dilatometer insertion
as a footing loaded horizontally to failure and proposed
to use the classical bearing capacity formulas to estimate
the undrained shear strength (Eqn. 3).
Su = (P1 - σho)/NC (3)
Where, σho is total horizontal in-situ stress (σho=
K0.σ’v+u0), Nc = 7 for medium clay. To use in this
equation, K0 = 0.5 was chosen for Busan clay based on
the laboratory test on undisturbed samples.
The measured undrained shear strength also depends on
the clay type and thus varies from location to location.
Various authors have developed different equations in
order to make it compatible with local sites; for example,
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based on field vane shear test and laboratory test at
number of sites in Malaysia and Singapore, Chang (1992)
suggested equation 4 to use with young marine clay.
Su = 0.074 σ’v KD1.25
(4)
Similarly, based on the laboratory tests, Kamei and Iwasaki
(1995) suggested equation 5 to use with Japanese clay.
Iwasaki and Kamei (1994) also suggested equation 6 to
estimate undrained shear strength for Japanese clay which
is based on the dilatometer modulus (ED).
Su = 0.35 σ’v (0.47KD)1.14
(5)
Su = 0.018 ED (6)
SOIL PROPERTIES AT THE TEST SITE
The study site issituated atJangyu site,central-west ofthe
Nakdong deltaic plain (Fig. 3). The clay is extended from
ground level to about 32 m depth however, at the middle
part (17.5 m to 24 m), clay is frequently interlayered with
broken shell, clayey silt and sandy silt layers. Based
onthe CPT based soil classification chart (Robertson,
1990), 4 m to 17 m depth is classified as massive clay or
silty claylayer, 17 to 24 m depth is markedby
frequentlyinterlayering clay or silty clay, sandy silt and
clayey siltlayers; and below 24 m depth is again clay or
silty clay layer.The soil profile with various geotechnical
properties of the clay from the test site are shown in
figure 4. It is reported that the geotechnical properties of
Busan clay vary appreciable with its depositional
environment (Chung et al., 2003, 2005). The detailed
geotechnical properties and depositional environments of
the Busan clay can be found in Chung et al. (2003, 2005,
2007).
Figure 3 Location map
0
5
10
15
20
25
30
Depth (m)
Soil profile
Clay/silty clay
Clay/silty clay
Sandy/clayey silt
Shale/silt/sandy
Clay
Clay/silty clay
0 0.5 1 1.5 2
qt (MPa)
0 0.2 0.4 0.6 0.8
u0 and u2 (MPa)
u2
u0
0 0.01 0.02
fs (MPa)
0 20 40 60
Su(FV) corr. (kPa)
0 20 40 60 80 100
Wn (%)
14 16 18
γt (kN/m3)
0 20 40 60
Ip (%)
Figure 4 Soil properties at the test site
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0
5
10
15
20
25
300.01 0.1 1 10
Depth (m)
ID
SILT
SAND CLAY
Material index
0 1 2 3
KD
Horizontal Stress Index
0 1 2 3 4
ED (MPa)
Dilatometer Modulus
TEST RESULTS AD AALYSIS
Figure 5 shows the DMT test results in terms of three
intermediate parameters. The soil classification based
onmaterial index (ID) indicates all clay until 27 m depth;
however, clay between 5 m to 17 m appears
homogeneous. The dilatometer modulus (ED)
appearsmore sensitive to soil type compare to the
horizontal stress index (KD).
The DMT results were interpreted using six empirical
equations (Eqn.1 to 6) and compared with the corrected
undrained shear strength obtained from the field vane
shear test as shown in figures 6a and 6b. As shown in the
figure 6a, the undrained shear strength estimated from
Marchetti (1980) equationappears very close to corrected
undrained shear strength at upper massive clay layer
whereas it underestimates at lower clay layer. The Roque
et al. (1988) equation in other hand shows opposite trend,
i.e., the estimated undrained shear strength appears close
to the corrected undrained shear strength at lower clay
layer and it overestimates at the upper massive clay
layer.The Lacasse and Lunne (1988) method
underestimates the undrained shear strength for the entire
depths. Among the other two ID-based methods, Chang
(1992) method largely underestimates and Kamei and
Iwasaki (1995) method overestimates the undrained shear
strength (Fig. 6b). The ED-based method proposed by
Iwasaki and Kamei (1994) estimates the undrained shear
strength quite close to that of corrected undrained shear
strength fromfield vane shear testhowever, the values
fluctuate widely throughout the depths.
Figure 7 shows the variation of σ’v, KD and ED with
undrained shear strength (Su(FVT)).The relationship
between σ’v and Su(FVT) is linear and the regression line
can be given by Su(FVT) =0.28σ’v. The relationship of
Su(FVT) with KD and ED are quite poor, however, Su(FVT)-
KD shows better correlation than that of Su(FVT)-ED.
Figure 8 shows the linear relationship between Su(FVT)/σ’v
and KD1.25
with 0-intercept. The regression line can be
given by equation 7.
Su= 0.1025 KD1.25σ’v (7)
The relationship between Su(FVT) and ED until 17 m depth
(marked by σ’v = 30 kPa in Fig. 7) is linear whereas,
below 17 m depth, the relation is very poor.This indicates
the sensitivity of dilatometer modulus (ED) with the soil
type. Since the soil type is defined by the material index
(ID), it is plotted against thecorrected undrained shear
strength (Su(FVT))normalized by dilatometer modulus (ED)
as shown in the figure 9. The regression line for this
relationship can be given by equation 8 with reasonable
accuracy.
Su= ED/(418 ID1.2343
) (8)
Figure 5 DMT test results
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0
5
10
15
20
25
30
0 20 40 60
Depth (m)
Su (kPa)
FVT (corr.)
Marchetti (1980)
Lac. & Lum. (1988)
Roque et al. (1988)
0
5
10
15
20
25
30
0 20 40 60
Depth (m)
Su (kPa)
FVT (corr.)
Chang (1991) MC
Iwa. & Kam. (1994)
Kam. & Iwa. (1995)
(a) (b)
Figures 6a & b Comparison between undrained shear strengths estimated from DMT with FVT
0 2 4 6
ED (MPa)
Su = 0.28σ'v
0
10
20
30
40
50
60
0 200 400
Su(FVT) (kPa)
σ'v (kPa)
0 1 2 3
KD
y = 0.1025x
0
0.1
0.2
0.3
0.4
0 1 2 3 4
Su(FVT)/σ' v
KD1.25
Figures 7 Variation of σ’v, KD and ED with SuFig. 8. Su(FVT)/σ’v - KD1.25 relationship
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y = 418x1.2343
R² = 0.9334
0
10
20
30
40
50
60
70
0 0.1 0.2 0.3
ED/S
u(FVT)
ID
0
10
20
30
40
50
60
0 10 20 30 40 50 60
Su(FVT) (kPa)
Estimated Su (kPa)
Marchetti (1980)Eqn. 7Eqn. 8
Figure 10 shows the comparison between estimated
undrained shear strength from equation 7, 8 and
Marchetti (1980) with measured undrained shear strength
from field vane shear test (Su(FVT)). The undrained shear
strength estimated from the equation 8 appears closerto
the measured undrained shear strength (Su(FVT))than other
0
5
10
15
20
25
30
0 20 40 60
Depth (m)
Su (kPa)
FVT (corr.)
Marchetti (1980)
Eqn. 7
Eqn. 8
Figure 11 Comparison of undrained shear strength profiles
methods. The undrained shear strength profiles estimated
from equations 7, 8 and Marchetti (1980) are shown in
figure 11 along with the corrected undrained shear
strength (Su(FVT)). Compare to Marchetti (1980), the
undrained shear strength estimated from the equation 7
shows slightly improved profile whereas, the equation 8
gives better estimation throughout the depths. Since the
equation 8 is newly developed equation incorporating
material index ID, it needs to be tried out with additional
flat dilatometer tests (DMT) on different clays.
COCLUSIO
The field vane shear test(FVT) and dilatometer tests
(DMT) were performed on Busan clay at Jangyu site. The
corrected undrained shear strength from FVT was
compared with DMT results interpreted using various
empirical equations. It is found that the KD-based
empirical equation proposed by Marchetti (1980) shows
a close correlation with the corrected undrained shear strength obtained from the field vane shear test; however,
it still underestimates the undrained shear strength. The
ED-based method proposed by Iwasaki and Kamei (1994)
method is also able to estimate the undrained shear
strength close to that of corrected undrained shear
strength obtained from the field vane test; however, the
values fluctuate widely.
The strength ratio,Su(FVT)/σ’v0,was found 0.28.The
correlations of Su(FVT) with KDand EDwere quite poorat
the lower clay layer, in which the KD and EDvalues varied
sensitively with soil type. Thus, two empirical equations
were developed based on the Su(FVT)-KD and Su(FVT)-ED-ID
relationships, whichcan be used in the local site. It
appeared that Eq. (8) based on the Su(FVT)-ED-IDresulted
in a better correlation throughout the depths. However,
Figure 10 Comparison of estimated Su from DMT using
equations 7, 8 and Marchetti (1980) with Su(FVT)
Figure 9 ED /Su(FVT) - ID relationship
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further study is needed to prove whether the developed
formula is applicable to different types of clays.
ACKOWLEDGEMET
This work was supported by the Korea Science and
Engineering Foundation (KOSEF) NRL Program grant
funded by the Korea government (MEST) (No. R0A-
2008-000-20076-0), and by Dong-A University, Busan
Korea.
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