109

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

110

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,

111

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

112

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

113

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

114

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

115

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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