地盤工学会 北海道支部 技術報告集 第 5 7 号 平成 29 年 2 月 於 北 見 市

Stiffness and strength mobilization in steel slag-mixed dredged clays with various curing

time

Hokkaido University Student Member W.M.N.R Weerakoon

Hokkaido University Student member Haruna Sato

Hokkaido University International Member Satoshi Nishimura

1. Introduction

Dredged clays are conventionally considered as slurry like fluid. Recently dredged clay is mixed

with steel slag to improve their engineering properties and is used for several engineering

applications. Steel slag is retrieved from source of waste in steel processing plants and the

presence of high lime (CaO) indicates potential hydration capacity similar to that widely

recognized in manufactured cement. Some researchers have worked on this topic to understand the

strength mobilization with curing time. From them, various factors affecting strength mobilization

after 3 days curing time were identified, such as optimum content of slag, particle size of slag and

curing time (Sun et al., 2009; Chan et al., 2012; Chan et al., 2014). However, the strength and

even stiffness mobilization are not fully understood from immediately after mixing to prolonged

curing time. The availability of such data provides a comprehensive understanding of strength and

stiffness development with curing time. From this, strength or stiffness increment characteristics

can be estimated at early stages of design and suitable materials and mixing conditions can be

selected for particular applications. Any relationship between the stiffness and strength also

serves as a useful tool for monitoring the quality of steel slag-clay mixtures in practical

applications, since stiffness form shear wave velocity can easily be investigated in the field. These

aspects were investigated in this study.

2. Materials for testing

In this study, three different types of natural clays were used; A, B and C. Natural clay samples,

which was obtained under wet conditions, were firstly sieved through 425μm to remove coarser

particles such as large sea shells. Removal of coarse materials was required to ensure the

homogeneity of the specimens. At natural states, all clays behave like a viscous fluid because

their natural water contents are greater than their liquid limits. Their index properties are

summarized in Table 1. C is a medium plasticity clay while the others can be classified as high

plasticity clays. As shown in Fig.1 (particle size distribution), clay C is coarser than the other

clays. Different amounts of soluble silica were identified for clay A, B and C; 12 mg/g, 12 mg/g

and 6 mg/g, respectively. These values indicate soluble silica supplied to pore water from dredged

soil. Two steel slags were used in this study; S1 and S2. The dry densities of S1 and S2 steel slags

are determined as 3.5 g/cm3 and 3.8 g/cm3, respectively. The slags, as received from a

manufacture, contained particles of irregular shapes and different sizes. The maximum particle

sizes were 9.5 mm for S1 and 4.75 mm for S2. From XRD tests, broadly identical mineralogical

compositions were identified for all clays, while S1 contained a higher CaO amount than S2. In

order to observe the strength and stiffness increment due to addition of the steel slag, the slag

content is defined as a volume-based ratio, as expressed by Eq.1.

297

∗ 100% (1)

where, α is the slag content, VS and VC are the volume of slag and volume of dredged clay,

respectively.

Table1: Physical properties of dredged clays

Clay Plastic limit (%) Liquid limit (%) Plastic index Particle density

(g/cm3)

A 30.7 66.2 35.5 2.71

B 29.1 44.1 15.0 2.71

C 28.4 73.4 45.0 2.78

Fig.1. Particle size distribution of the dredged clays and steel slags

To understand the influence of different volumetric mixing ratios, three values for α were used in

this study; 20%, 30% and 40%. Initially, for preparing specimens, the water content of a clay

sample was adjusted by using artificial sea water as 1.5 times of its liquid limit, i.e., 1.5wL. Then

adding the required slag amount into the dredged clay, it was thoroughly mixed for 3 minutes by

using a spatula. Immediately after mixing, sample still remained as very soft. Therefore

conventional geotechnical tests such as unconfined compression tests (UCTs) and triaxial

compression tests cannot be carried out for initial curing time because the specimen was not

strong enough to stand by itself. To avoid this operational problem, direct shear tests (DSTs) were

adopted for specimens at a wide range of curing times, from 0.5 hours to 28 days. For the tests

after 0.5 hours to 1 day curing time, the specimen was prepared in the shear box having a diameter

of a 60 mm and a height of 32 mm. For the rest of curing time conditions, the slag-treated dredged

clay was poured into a separate cylindrical mold having similar dimensions as the shear box and

cured at a room temperature (25oC). ASTM D 3080-98 recommends that the ratio of the maximum

particle size to the shear box diameter should not be greater than 0.1, but for the S1 mixed clay, it

is 0.16. However, Fakhimi and Hosseinpour (2011) found that a maximum particle size to shear

box diameter or width of 0.2 could be used in direct shear testing with only minor loss in accuracy

0

20

40

60

80

100

0.0001 0.001 0.01 0.1 1 10

Per

cen

tage

pas

sin

g (%

)

Grain size (mm)

A

B

C

S1

S2

298

of the measured shear strength when the Questa rock pile material was tested under low normal

stresses. In addition to the DSTs, UCTs were carried out for comparison of strength obtained in

the two different test methods after 3-day curing time to check the data consistency. Since height

to diameter ratio of 2 to 3 is recommended to avoid end effects and bucking of specimen,

specimens having a diameter of 50 mm and a height of 100 mm were prepared for UCTs. Similar

sized specimens were produced for obtaining stiffness by bender element tests at the same curing

time range as the DSTs.

3. Experimental method

3.1 Direct Shear Test

The adopted direct shear apparatus, illustrated in Fig.2, is applicable to perform tests in both

constant volume and constant pressure conditions by monitoring and controlling with computer.

The apparatus comprises a bellofram cylinder on the top for vertical loading and the direct-drive

motor on the side for horizontal shearing.

Fig.2. Direct shear apparatus employed in this study

The horizontal shearing load was applied to the outside of the tank, which is on the linear roller

slider (Fig.2) and then the tank transmits the load to the lower halve. After setting up the specimen

inside the shear box, a very low vertical pressure was applied to top of the specimen. The purpose

of the low vertical pressure (2-3 kPa) was to ensure a contact between the top surface of the

specimen and the loading platen. After this, to maintain the constant volume condition, the

loading ram of the bellofram cylinder was mechanically clamped just before shearing so that it

maintained a constant height of the sample by preventing a ram movement, as shown in Fig.2.

Under this condition, the dilation of specimen was suppressed by a variation of the vertical stress

during shearing at constant-volume conditions, which are considered equivalent to undrained

conditions. In this study, constant volume direct shear tests were carried out by shearing at a

constant horizontal displacement rate of 2 mm/min up to a displacement of 7 mm.

3.2 Bender Element Test

Bender element testing has been extensively adopted in the geotechnical research filed including

cement-treated clay as a nondestructive testing method (Ahnberg and Holmen, 2008; Seng and

Tanaka, 2011). It allows measuring shear wave velocity (Vs) or shear modulus (G) of a specimen

at any specific curing time. With a function generator, high frequency waves are created for

testing stiff soils (curing time greater than 3 days) and lower frequency waves are required for the

Specimen

Vertical load sensor

Horizontal load sensor

Driving motor

Vertical displacement sensor

Upper halve Shear box

Clamping screws

Bellofram cylinder

Linear roller springLower halve Shear box

Tank

299

soft soils (curing time form 0.5 hours to 3 days). Both transmitting and receiving signals are

displayed in a digital oscilloscope. Considering previous studies, the “start-to-start” method (Lee

and Santamarina, 2005; Yamashita et al., 2009) for determining the arrival time (Δt) and the

“tip-to-tip” method for determining the travel distances (Δd) of the shear wave were adopted in

this study to obtain Vs. Small strain shear modulus can be computed from shear wave velocity and

mass density as G = ρVs2, where ρ is the mass density. In addition to bender element tests, the

unconfined compression tests (UCT) were carried out on the specimens having strength to stand

its own, i.e., curing time greater than 3 days.

The UCT was conducted according to the procedures prescribed by the Japanese standards. The

loading rate was 1 % per minute, where the load-displacement data was automatically logged for

subsequent analysis and processing. The basic parameters derived from the test were the

unconfined compressive strength (qu) and the failure strain at peak stress or qu.

4. Strength and stiffness behavior through laboratory tests

4.1 Identification of undrained shear strength (Su)

Since strength at immediately after mixing can be very low, correction was made for the friction

along the slider (Fig.2) by performing several DSTs without specimen at a shearing rate of 2

mm/min. In these tests, three different vertical loads of 0, 5 and 10 N were used to investigate the

influence of vertical loads applied to the tank. Fig.3 shows relationship between the frictional

stress and horizontal displacement for three vertical loads. For all vertical loads, the frictional

stress increased linearly until displacement of 1 mm and then it reached a constant value with

some scatters. Eq.2 was proposed to model the relationship between the frictional stress and

horizontal displacement and the determined frictional stress was subtracted from the measured

shear stress in a specimen.

(2)

where, F and Fmax are the frictional stress and the maximum fictional stress respectively, d is

horizontal displacement and D is the gradient of the initial linear part of this relationship.

For typical example, six different experimental results obtained from the direct shear tests on clay

A mixed with S1 and S2 are illustrated in Fig.4 for two different curing periods. As can be seen in

Fig.4a), there are two distinct strength mobilization patterns between the different types of slag.

The finer-grained S2 (the slag which has a maximum grain size less than 4.75mm) shows the shear

stress increases with shear displacement until a failure shear stress τf is reached. After that, the

shear resistance remains approximately constant for any further increase with the shear

displacement. The mixture with S1, the coarser-grained slag shows a similar tendency as this

when the slag content is low (20%). For these cases, the undrained shear strength (Su) can be

defined at τf. In contrast to this, the mixture with S1 at slag contents of 30% and 40%showed

continuous development of shear resistance with an increase in shear displacement. This is

considered to be due to the role of the coarser particles in increasing the dilatancy in the mixture.

This behavior led to the absence of the peak shear strength. The grain size of slag and its

volumetric content are likely to provide dominant effect on strength mobilization patterns.

However, due to this strain-hardening characteristic, it is difficult to define a peak strength for

high contents of S1. To follow consistent criteria, normalized shear stress with shear displacement

300

plots are produced. Interestingly, as presented in Fig.4c), normalized shear stress curves show

quite a similar tendency for all the combinations. Although the stress ratio increases until

displacement of 3 mm, after that the shear stress only increases in proportion to the vertical stress

by keeping a constant normalized stress. This stress ratio corresponds to the undrained failure line

in the effective stress space, and defining a strength at first mobilization of this ratio is an

objective approach for such dilatant materials, as well as serving as a safe criterion not counting

on dilation-induced shear resistance encountered only at extremely large deformation.

Fig.3. Frictional stress in direct shear test a). τ with displacement at 2 hour curing

b). τ with displacement at 7 day curing c). τ/σv with displacement at 2 hour curing

Fig.4. Direct shear test results for clay A mixed with different slag contents of S1 and S2, 20, 30 and 40%

From Fig.4b), significant strength development is clearly identified at 7-day curing time when

compared with 2 hour. The strength increment is attributed to the formation of cementitious

hydrates, which occupy the voids within the soil spaces and binds the soil particles together (Chan

et al., 2014). For many of the clay-slag combinations, the peak strength is well defined. After the

0

0.4

0.8

1.2

1.6

2

0 1 2 3 4 5 6 7

Fri

ctio

nal

str

ess

(kP

a)

Horizontal displacement (mm)

0 N

5 N

10 N

D1

F= dDFmax /(Fmax+dD)

0

5

10

15

20

25

0 1 2 3 4 5 6 7

τ(k

Pa)

Shear displacement (mm)

AS1-20% AS2-20%

AS1-30% AS2-30%

AS1-40% AS2-40%

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7

τ(k

Pa)

Shear displacement (mm)

AS1-20% AS2-20%

AS1-30% AS2-30%

AS1-40% AS2-40%

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7

τ/σ v

Shear displacement (mm)

AS1-20% AS2-20%AS1-30% AS2-30%AS1-40% AS2-40%

301

peak stress is attained, the shear stress shows a quick decrease and then an increase again in some

cases. For these cases, Su can be defined by initial peak strength. There is no clear strain-softening

only in the mixture with S2 at 20% and 30%, while the others show a clear drop of shear stress,

often followed by an increase again. In these cases, the stresses at the eventual constant failure

phase or the first peak before strain-softening are considered as Su. Similar patterns of strength

mobilization were observed in the other slag mixed-dredged clays.

4.2 Characteristics of strength mobilization with curing time and slag content

Fig.5 shows strength mobilization at different slag contents, i.e., 20, 30 and 40%, with curing time,

ranging from 0.5 hours to 28 days in a log-log scale. From these relationships, the strength

mobilization can be obviously divided into three stages except for clay C.

a). Clay A mixed with S1 and S2 b). Clay B mixed with S1 and S2

c). Clay C mixed with S1 and S2

1

10

100

1000

0.01 0.1 1 10

Su（

kP

a)

Curing time (Days)

AS1-20% AS2-20%

AS1-30% AS2-30%

AS1-40% AS2-40%

1

10

100

1000

0.01 0.1 1 10

S u（

kPa)

Curing time (Days)

BS1-20% BS2-20%

BS1-30% BS2-30%

BS1-40% BS2-40%

1

10

100

1000

0.01 0.1 1 10

Su（

kP

a)

Curing time (Days)

CS1-20% CS2-20%

CS1-30% CS2-30%

CS1-40% CS2-40%

0

0.5

1

1.5

2

0 10 20 30 40 50

Sti

ffn

ess

incr

emen

t co

effi

cien

t,b

Steel slag content (%)

AS1, 4h-3d BS1, 4h-3d CS1, 4h-14d

AS1, 3d-28d BS1, 3d-28d CS2, 4h-14d

AS2, 4h-3d BS2, 4h-3d CS2

AS2, 3d-28d BS2, 3d-28d CS2

Fig.5. Relationship between undrained strength and

curing time, with fitted lines

Fig.6. Variation of strength increment coefficient

with different slag content

302

Clay C indicates only the initial stage followed by no strength development during the curing time.

This initial stage can be identified as no strength improvement stage and it ranges approximately

from 0.5 hours to 4 hours for the other two clays. At this initial stage, there is no obvious

influence of slag types on the strength, only affected by slag content. The constant values of Su are

observed as the specimens remain unbonded. For cement treated soils, a similar tendency was

observed by Seng and Tanaka (2011). For further understanding, these stages were represented by

trend lines obtained from log-log scale. After the initial stage of curing time, a secondary phase of

strong strength development can be seen in all the slag contents in clay A and clay B. In this stage,

it seems that S1 shows a tendency of greater strength improvement than S2. To discuss the

characteristics in these curing stages quantitatively, the strength increment coefficient b was

defined and correlated with the slag content. The coefficient b was obtained as the rate of the

undrained shear strength against time in log-log scales.

Fig.6 illustrates the relationships between the strength increment coefficient b and the slag content

during the second and third curing stages, i.e., form 4 hours to 3 days and from 3 days to 28 days.

As there is no significant strength development for clay C after the initial stage, a single b value

represents the later stages (4 hour onwards). For clay B, the b values in the second and third

stages show sudden increases and drops at 30% of slag content because higher consumption of

CaO in the second stage can be resulted in lower b for next stage. The values for clays A and C are

nearly constant with the slag content. For most of the conditions except 40% of S2 in clay C, the b

values in S1 is higher than that those in S2 for corresponding curing time ranges. These

differences of the b values between S1 and S2 are likely to be due to the higher CaO amount in S1

providing a greater hydration capacity than S2 and the effect of grain size difference in S1 and S2.

Clay C had a mineralogical composition similar to the other clays, but its b mobilized by both slag

types shows considerably smaller values, i.e., from 0.05 to 0.2. It means that the strength

increment depends not only on the CaO amount in slags and mineral composition of clays. The

reason for this smaller b may be the lower amount of soluble silica in clay C than clays A and B;

as described earlier, the amount of soluble silica in clay C is half of that in clays A and B. Except

for clay C and 20% of slag content in clay B, b in the second stage has greater values than in the

third stage. In the second stage, the ranges of the b values seen for S1 and S2 are, 0.67 to 1.2 and

0.6 to 0.78, respectively. In the third stage, except for 20% of slag content in clay B, the range of

the b values becomes smaller for both slags, to 0.45 to 0.78 and 0.36 to 0.54, respectively. The

decreasing values of b observed in the third stage can be related to consumption of CaO and

soluble silica as hydration progresses. The variations of b are thus likely to be affected

simultaneously by CaO amount and grain size in slag, soluble silica amount in clay and curing

time.

4.3 Characteristics of stiffness mobilization with curing time and slag content

Fig.7 shows the shear stiffness (G) mobilization for the slag content of 20, 30 and 40% with

curing time, ranging from 0.5 hours to 28 days. Form this relationship, the stiffness mobilization

can be obviously categorized into three stages except clay C, in a similar way to the undrained

shear strength mobilization. Clay C indicates only two stages with no significant stiffness

development during curing time. For most of the conditions at the initial stage, there is no obvious

influence of slag types on the stiffness, only affected by the slag content in a similar way to the

strength. At this initial stage, an increase in the stiffness was detected in contrast to no strength

development at the same stage, as shown in Fig.5. For further understanding, these stages were

303

represented by trend lines obtained in log-log scales. After the initial stage of curing time, the

second stage with strong stiffness development can be seen for all the slag contents in clay A and

clay B.

a). Clay A mixed with S1 and S2 b). Clay B mixed with S1 and S2

c). Clay C mixed with S1 and S2

To discuss the stiffness development characteristics in these curing stages, the stiffness increment

coefficient, d, was defined and correlated with the slag content. The d value was obtained from the

rate of the stiffness increase against time in log-log scales in a similar way to b. Fig.8 illustrates

the relationship between the stiffness increment coefficient d and the slag content in the three

stages. The three stages represent, for clay A, 0.5 hour to 1 day, 1 day to 3 days and 3 days to 28

0.1

1

10

100

1000

10000

0.01 0.1 1 10

G（

MP

a)

Curing time (Days)

AS1-20% AS2-20%

AS1-30% AS2-30%

AS1-40% AS2-40%

0.01

0.1

1

10

100

1000

10000

0.01 0.1 1 10

G（

MP

a)

Curing time (Days)

BS1-20% BS2-20%

BS1-30% BS2-30%

BS1-40% BS2-40%

0.01

0.1

1

10

100

1000

10000

0.01 0.1 1 10

G（

MP

a)

Curing time (Days)

CS1-20% CS2-20%CS1-30% CS2-30%CS1-40% CS2-40%

0

1

2

3

4

5

0 10 20 30 40 50

Sti

ffn

ess

incr

emen

t co

effi

cien

t,d

Steel slag content (%)

AS1, 0.5h-1d BS1, 0.5h-8h CS1, 0.5h-1dAS1, 1d-3d BS1, 8h-7d CS1, 1d-28dAS1, 3d-28d BS1, 7d-28d CS2, 0.5-1dAS2, 0.5h-1d BS2, 0.5h-8h CS2, 1d-28dAS2, 1d-3d BS2, 8h-7d CS2AS2, 3d-28d BS2, 7d-28d CS2

Fig.7. Relationship between stiffness and curing

time with fitted lines

Fig.8. Variation of strength increment coefficient

with different slag content

304

days and for clay B, 0.5 hour to 8 hours, 8 hours to 7 days and 7 days to 28 days, respectively.

Clay C has two stages, and the d values represent for 0.5 hours to 1 day and 1 day to 28 days. For

clays A and B, two ranges of the d values can be identified, each for the second stage and for the

rest of the stages; they are 1.36 to 3.5 and 0.25 to 0.85, respectively. The d values for clay B and

clay C are nearly constant with slag content. At the second stage, for most of the conditions except

clay A with 40% slag content, the d values in S1 are higher than those in S2. This difference of the

d values between S1 and S2 is likely to be explained by the same factors affecting the undrained

shear strength development represented by b. If a same slag type is used, different clays give

various d values. Clay C has lower d values than the other two clays possibly due to the presence

of a smaller amount of soluble silica. From these characteristics, such variations of d reveal that it

depends on the similar factors as b.

4.4 Comparison of undrained shear strength obtained by different methods, and its correlation

to stiffness

By using clays A and B, a comparison of the undrained shear strength obtained by the direct shear

tests (DSTs) and the unconfined compression tests (UCTs) was made for curing time of 3 to 28

days. The comparison is shown in Fig.9. For the most data points, Su measured by the UCT is

higher than that from DST. However, the difference is within 45 % if the data for AS2-40%,

BS1-20% and BS2-20% are excluded. The reason for these two conditions’ discrepancy is unclear

at the moment. The systematically larger UCT strength may be attributed to the specimens’

anisotropic properties and the difference in the loading rates between the UCT and the DST.

However, the results obtained by these two methods are comparable within the accuracy required

for this paper’s discussion.

Fig .10 shows the correlation between G and Su derived from the present study. Since Su obtained

by DST is constant until the end of the first curing stage, i.e., 4 hours, this figure represents the

values for Su from 4 hours. The regression in Fig.10 was therefore applied for the data after 4-hour

curing time. A linear relationship between log G and log Su exists, meaning that the relationship is

fitted well by a power function with a regression coefficient of about 90%, as expressed by

1 : 1

1

10

100

1000

1 10 100 1000

S ufr

om U

CT

(kP

a)

Su from DST (kPa)

AS1-20% BS1-20%AS1-30% BS1-30%AS1-40% BS1-40%AS2-20% BS2-20%AS2-30% BS2-30%AS2-40% BS2-40%

G = 0.12Su1.43

R² = 0.90

0.01

0.1

1

10

100

1000

10000

1 10 100 1000

G (

MP

a)

Su (kPa)

AS1-20% BS1-20%AS1-30% BS1-30%AS1-40% BS1-40%AS2-20% BS2-20%AS2-30% BS2-30%AS2-40% BS2-40%

Fig.9. Correlation between undrained shear

strength obtained by DST and UCT

Fig.10. Correlation between undrained strength

obtained by DST and stiffness

305

G=0.12Su1.43 in (Su in kPa and G in MPa). For this relationship, the measured G ranges from 0.125

MPa to 1100 MPa. There are outlying points at medium strength levels for clays A and B because

of clear difference between strength and stiffness increment coefficients. Since the stiffness from

shear wave velocity can be easily obtained in the field, this relation can serve as useful tool to

monitor the strength of materials in practical applications.

5. Conclusions

Characteristics of strength and stiffness mobilization in steel slag-mixed dredged clay were

discussed in this paper based on experiment results by direct shear tests (DSTs), unconfined

compression tests (UCTs) and bender element tests. Several conclusions are as follows. For clay A

and B, strength and stiffness mobilization patterns with curing time can be clearly categorized

into three stages, while clay C with less soluble silica lacked the later stages with strong strength

and stiffness development. Both strength and stiffness coefficients depend not only on curing time

but also on amount of CaO in steel slag and soluble silica in dredged soil. The material which

contains smaller amount of these resulted in less significant values of b and d. The DST was

introduced to measure lower Su of the stabilized soils, since conventional UCTs cannot be adopted

due to difficulty in preparing self-supporting specimens. A relationship between the stiffness and

strength was observed for the second stage of curing to be used for assessing the strength with

non-destructive probes.

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