Jordan Journal of Civil Engineering, Volume 11, No. 1, 2017

- 80 - © 2017 JUST. All Rights Reserved.

Investigation of Parameters Affecting Creep Behavior of

Sandy Clay Soil in Laboratory Conditions

Alireza Negahdar 1), Shima Yadegari 2) and Siyab Houshmandi 3)

1) Assistant Professor, University of Mohaghegh Ardebili, Ardebil, Iran. E-Mail: negahdar@uma.ac.ir

2) MSc Student, University of Mohaghegh Ardebili, Ardebil, Iran. E-Mail: yadegari.shima@gmail.com

3) PhD Sudent, University of Mohaghegh Ardebili, Ardebil, Iran. E-Mail: houshmandi_76@yahoo.com

ABSTRACT

Investigation of mechanisms and factors influencing the creep behavior of soils is one of the main

requirements in geotechnical engineering. In this paper, one-dimensional single-stage, stepwise and

overloaded-unloaded creep tests are carried out using oedometer apparatus on dried in air and water-saturated

sandy clay soils at different stress levels, in order to investigate parameters affecting the creep behavior, such

as: stress level, stress history and pore water. The creep mechanisms are explained with respect to contacts

and deformation of particles. Data analyses are explained based on relationships between the coefficient of

secondary compression (Cα) and change in void ratio (Δe). Test results showed that in water saturated

samples at low stress level, due to higher sliding ability and lower friction, a large amount of creep

deformation occurs and with increased stress level, the creep rate decreases. But, in dry samples, stress

increment increases creep rate. Further, the creep rate in overloaded-unloaded test is higher than that in

single-stage test, and this in turn accelerates the creep.

KEYWORDS: Creep, One-dimensional creep test, Single-stage test, Stepwise test, Overloaded-

unloaded test.

INTRODUCTION

The compressibility behavior of soils is a main

concern in geotechnical engineering, because of long

settlement of soil due to creep deformation. Therefore,

the study of soil creep behavior is necessary. The creep

deformation indicates the long responses of soil. These

responses include settlement of grounds and movement

of slopes. In other words, creep behavior reflects the

basics of soil deformation.

Creep behavior of soil was first studied at the early

nineteenth century by Terzaghi. The investigations on

the secondary compression have been started after

Terzaghi consolidation theory (1925), which states that

the compression of clay occurs after the dissipation of

pore water pressure. The mechanisms of creep

deformation in soils are extensively discussed by

several researchers from different perspectives.

Experiment results reported by Buisman (1936) and

Taylor (1942) showed the effect of time on the

compressibility of clays. Buisman (1936) presented a

Received on 7/2/2015. Accepted for Publication on 2/6/2015.

Jordan Journal of Civil Engineering, Volume 11, No. 1, 2017

- 81 -

linear behavior of settlements with the logarithm of

time under constant effective stress for clay and peat.

Taylor introduced a time-dependent model to describe

creep behavior of soils, in which primary consolidation

and secondary compression are considered as two

separate processes. Mejia (1988) and Zhang et al.

(2006) realized that in one-dimensional creep tests on

sand at low stress levels, the creep rate of sand

increases with the stress level increment. Leung (1996)

showed that at high stress levels, the creep deformation

of sand is accompanied by grain crushing and that the

amount of crushed grains increases with time. Mitchell

and Xu (2005) explained that the mechanisms of

secondary compression involve particle rearrangement

at edgeface particle contacts due to sliding and

expulsion of pore fluid from micro-pores under

constant effective stress. Meanwhile, Lambe (1958)

stated that the dissipation of pore fluids from the

micro-voids is the reason for secondary compression.

Some authors (Berry and Xu, 1972; De Jong, 1968;

Nakaoka et al., 2004) defined secondary compression

as a local mass transfer of water between macro- and

micro-pores. Budhu (2007) explained that for effective

stress to remain constant during creep stage, the

average number of clay particle contacts must be

constant.

Mineral composition (i.e., mineral content of

particles), stress level, stress history, pore fluid

chemistry, drainage condition and soil structure are all

important parameters in the understanding of creep

behavior of soils. However, the effects of some of these

parameters on sandy clay soils have not been studied

yet. Therefore, this paper focuses on identifying the

effects of stress level, stress history and pore water

chemistry on creep behavior of sandy clay soils.

Moreover, in geotechnical engineering, it is a challenge

to predict what the creep behavior of soil after months

or years will be (Mitchell and Soga, 2005). In a

conventional creep test, a soil sample is usually loaded

to a specific effective stress and allowed to creep at this

specific effective stress. Therefore, it is valuable to

develop a methodology to accelerate creep test.

In the rest of the paper, creep behavior of soils

under one-dimensional consolidation test is explained

and the materials used, as well as the standard

oedometer apparatus and test set up are introduced.

Then, the test program and discussions on the one-

dimensional compression tests of sandy clay are

presented. The test results include the effects of stress

level, stress history and pore water chemistry on creep

behavior of sandy clay, as illustrated. Finally,

conclusions are drawn.

One-Dimensional Creep Behavior of Soils

When soil is subjected to a load, effective stress

increases with time. As a result of dissipation of

induced excess pore water pressure, a primary

consolidation occurs. Significant amount of settlement

occurs during primary consolidation. After the

complete dissipation of the excess pore water pressure,

if the load is continuously maintained on the soil,

further deformation can be observed over a long period

of time, which is known as secondary compression or

creep. Secondary compression is represented by an

index called the coefficient of secondary compression

(Cα). Figure 1 shows a typical void ratio-log time

relation of saturated soil in the one-dimensional

compression test at a sustained stress level. In this

study, Casagrande curve fitting method is used to

determine the time (t100) taken to completely dissipate

the excess pore water pressure at the particular stress

level and the void ratio (eEOP) at the end of primary

consolidation. In Casagrande curve fitting method, two

linear portions, the initial portion of primary

consolidation and that of secondary compression stage,

are plotted and the intersection of both lines is taken as

the end of primary consolidation point. Fig. 1 clearly

shows elastic, primary consolidation and secondary

compression regions. Taylor (1942) introduced a

logarithmic model based on the constant (Cα) concept

to represent secondary compression. This can only

represent the stages with decreasing creep rate. The

relation is expressed as follows:

Investigation of Parameters… Alireza Negahdar, Shima Yadegari and Siyab Houshmandi

- 82 -

(1)

where e is the void ratio, eEOP is the void ratio at the

end of primary consolidation, t is time and t100 is time

at the end of primary consolidation. Cα is the

coefficient of secondary compression.

Figure (1): Typical void ratio-log time relation of saturated soil in 1D compression

The coefficient of secondary compression is one of

the most useful parameters to describe the behavior and

the magnitude of secondary compression and it is less

affected by testing conditions. Coefficient of secondary

compression can be expressed in several ways, but the

following equation is commonly used:

(2)

where ∆e is the change in void ratio during the

secondary compression stage.

Sand Creep at Low Stress Level

Bowman and Soga (2003) conducted a series of

creep tests on Leighton Buzzard sand and Montpellier

sand at low stress levels (50 and 500 kPa). In their

tests, the change in micro-structure of the sands in 1D

creep was investigated using the techniques of resin

injection and optical microscopy of sections. It was

observed that the creep of sandy soils at low stress

levels is due to the rearrangement of grains over time.

Consistent with Bowman and Soga’s observation, in

the triaxial creep tests of Ham River sand, Kuwano and

Jardine (2002) found that creep deformation of sand at

low stress levels (200 and 400 kPa) is caused by the

gradual stabilization of micro-structures.

According to the theory of Mitchell and Soga

(2005), the deformation of sandy soil is induced by

contact deformation, grain sliding and rolling, as well

as grain crushing. The contributions of these factors are

various at different stress levels. At very low stress

levels, the contact deformation dominates the

deformation of the soils and so, the soils behave

elastically and the deformation is reversible.

At low stress levels, grain sliding and rolling

dominate the deformation of the soil, while at high

stress levels, grain crushing dominates the deformation

of the soils. Because the maximum capacity of our

laboratory one-dimensional consolidation test device is

1280 kPa, in this work, creep deformation is studied at

low stress levels.

EXPRIMENTAL STUDIES

The creep behavior of soil has been extensively

investigated in one-dimensional (1D) and triaxial creep

tests. In general, the phenomenon is more pronounced

in clay than in sand. Therefore, most of the existing

 

100

logEOP

te e C

t

 

loge

Ct

Jordan Journ

studies focus

are only a f

creep behavi

observations

of creep of sa

Materials U

Sandy c

dimensional

kaolinite cla

40% sand. T

g/cm3. The O

standard. In

dioxide is ar

is 2.652 and

Kaolinite

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density of

Kaolinite is

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which is sm

Details of ka

Figure 2 sho

of the sandy

One-Diment

Overloaded-

Clay Sample

Creep be

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few studies c

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on soil creep

andy clay soil

sed

clay soil wh

creep tests c

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The density o

Ottawa sand

the grain s

round 99.8%.

the grain shap

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Figu

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havior of san

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behavior of c

conducted to

In this sectio

p are presented

ls.

hich is used

consists of O

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of sandy clay

is according

solid, the con

The specific

pe is round.

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ganic clay i

ding material

y particle has

between 10

nic clay are gi

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ure (2): The i

PROGRAM

gle-Stage, S

ompression T

ndy clay soils

lume 11, No. 1,

clay, while the

investigate t

on, experimen

d in the subje

d in the on

Ottawa sand a

f 60% clay a

sample is 2.

to ASTM-C7

ntent of silic

gravity of sa

eudo- hexagon

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is 2.45 g/cm

l when water

a specific ar

0 and 20 m2

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istribution cur

nitial grain s

Stepwise a

Tests on San

is analyzed a

, 2017

- 83 -

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Table 1. C

Density

(LL) L

Plastic

(PI) Pl

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The purpos

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andy clay soi

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edometer ring

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equired to com

Characteristic

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astic Index

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oedometer

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guide resin pi

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f the samples

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e water is add

test.

sandy clay sam

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2.4

5

33.

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aratus and Te

experimental

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dimensional

ding to ASTM

apparatus co

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m. To preserve

ded into the

mple

ession index

mpression (Cα)

to determine

solidation (eE

ry consolidati

clay soil

47

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behavior of

consolidation

M D 2435-90

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using LVDT

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consolidation

(Cc) and the

). Casagrande

the void ratio

EOP), the time

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o

f

n

0

n

t

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T

d

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o

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Investigation of Parameters… Alireza Negahdar, Shima Yadegari and Siyab Houshmandi

- 84 -

the coefficient of secondary compression (Cα).

Creep tests are perforemed on samples in three

stress levels as follows: in single stage test, the soil

sample is loaded to a specific stress level and is

allowed to creep at this stress level. In stepwise test,

the soil sample is loaded at different stress levels and is

then allowed to creep. In overloaded-unloaded test, the

soil sample is loaded to σoverload. After the end of

primary consolidation, the sample is unloaded to σcreep

and is allowed to creep at the same stress level (σcreep).

In all tests, the samples with a specified mass (73 g)

are poured into the confining ring. In water-saturated

samples, loads increase from 0 to 50 kPa for a duration

of 16 minutes between two consecutive loads to

complete the dissipation of excess pore water pressure.

16 minute time scale is selected based on the one-

dimensional consolidation tests on water-saturated

sandy clay samples, but in dry samples, the load is

applied in a single increment. According to the

Casagrande curve fitting method, it can be shown that

the water-saturated sandy clay samples have a

thickness of 22 mm, requiring 16 minutes to complete

the primary consolidation under the single drainage

condition. Single stage tests at the stresses of 300, 600

and 1200 kPa are carried out on the dried in air and

water-saturated sandy clay samples. Notice that the

samples under the aforementioned stresses are loaded

for about 120 hours. Stepwise compression creep tests

are carried out on dry and water-saturated sandy clay

samples and samples are subjected to stepwise loads at

different σcreep values of 50, 300, 600 and 1200 kPa for

120 hours.

Overloaded-unloaded compression creep tests are

carried out on water-saturated sandy clay samples to

understand the effect of stress history on creep

behavior of sandy clay. The samples are overloaded to

325, 625 and 1225 kPa, respectively. Then, they are

allowed to complete primary consolidation.

Immediately after the end of primary consolidation, the

samples are unloaded to σcreep of 300, 600 and 1200

kPa, respectively and are allowed to creep. After the

creep stage, the samples are again overloaded to 350,

650 and 1250 kPa, respectively and after the end of

primary consolidation at these stress levels, the

samples are unloaded to σcreep of 300, 600 and 1200kPa,

respectively and are allowed to creep for 120 hours at

these stress levels.

At the beginning of each test, initial void ratios of

sandy clay slurry are determined using the values of

moisture content. The initial values of moisture content

are determined by drying the samples in an oven at

100° C for 24 hours.

RESULTS AND DISCUSSION

The dependency of soil creep rate on the applied

stress level is one of the concerns in most of the

existing studies. However, on one hand, it is observed

that the creep rate of soil increases with the applied

stress level in most of the tests. On the other hand, it

was shown that there is no dependency of creep rate on

the applied stress level in some other tests (i.e., at some

stress levels, creep rate increases with stress level, but

at other stress levels, creep rate decreases with stress

level). The contradictory observations might be caused

by the different strain levels at which the samples are

allowed to creep at different increased or decreased

stress levels.

Relationships between void ratio and time in the

single-stage and stepwise tests at stress levels of 300,

600 and 1200 kPa are shown in Figure 3 and Figure 4,

respectively. Figure 5 and Figure 6 show the

relationships between secondary compression

coefficient (Cα) and stress level (σcreep) of sandy clay

samples in stepwise and single-stage tests, respectively.

There is an approximately non-linear relationship

between Cα and σcreep. In saturated samples with

increasing stress levels, the creep rate decreases, while

in dry samples it increases.

In saturated samples, with increasing stress level

the variatoins of Cα in both single-stage and stepwise

tests are approximately equal, but in dry samples with

increasing duration of loading, the variation of Cα

increases.

Jordan Journ

Figure 7

and time in

water-saturat

and 1200 kP

loading step

compression

Relations

overloaded-u

creep values

Figures (8-10

Test res

unloaded sta

nal of Civil Eng

shows the rel

overloaded-u

ted samples,

Pa. The Figur

s at primary

stages.

ships between

unloaded and

of 300, 600 a

0).

sults of sin

ages are used

Figure (3)satur

Figu

ngineering, Vol

ationships bet

unloaded cree

at stress leve

re clearly sho

consolidation

n void ratio a

d single-stage

and 1200 kPa

ngle-stage an

to study the

): Relationshiated and drie

ure (4): Relati

0

void ratio

lume 11, No. 1,

tween void ra

ep tests for t

els of 300, 6

ows incremen

n and seconda

and time for t

e creep tests

are compared

nd overloade

effect of stre

ips between ved in air samp

ionships betwstress l

0

0.5

1

1

step

Ste

, 2017

- 85 -

atio

the

600

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ary

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at

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ess

hi

ov

eq

th

sin

ac

sta

tim

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sh

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void ratio andples at stress

ween void ratlevels of 50-12

100 100

log time

pwise‐dry sam

pwise‐saturat

istory on sec

verloaded-unlo

qual, but the

he overloaded

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ccelerates the

age compress

me to reach a

owever, the s

hort period

ompression cr

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00 1000000

e,s

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ondary comp

oaded and sin

required time

d-unloaded te

est and the

creep. The fi

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certain void r

same void rat

of time

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oaded test acc

gle-stage tests, 600 and 120

n stepwise te

0

pression. Cα v

ngle stage tes

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overloaded-u

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t, it takes a lo

ratio during th

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in overloa

o, it can be

celerates the c

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sts at

values in the

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than that in

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that in single-

ong period of

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d with a very

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said that the

creep.

e

y

n

n

t

-

f

.

y

d

e

Investigation

n of Paramete

Figure

Figure

Figure (7): w

ers…

e (5): Relation

stress lev

e (6): Relation

stress level

Relationshipwater-saturat

Cα

nship betwee

vel (σcreep) of s

nship betwee

l (σcreep) of san

ps between voted samples a

0

0.005

0.01

0

saturate

Alirez

- 86 -

n secondary

sandy clay sa

n secondary

ndy clay sam

oid ratio and at stress level

500

σcreep

d sample

za Negahdar, S

compression

mples in step

compression

mples in single

time in overlls of 300, 600

1000

p(kPa)

dry samp

Shima Yadega

coefficient (C

pwise tests

coefficient (C

e-stage tests

loaded-unloaand 1200 kP

1500

ple

ari and Siyab

Cα) and

Cα) and

aded tests on Pa

Houshmandi

Jordan Journ

Figure 11

effective axi

samples. Th

index (Cc) o

nal of Civil Eng

1 shows the r

al stress for d

e slope of th

f the dry sam

ngineering, Vol

Figure (8)wa

Figure (9)wa

Figure (10wat

relationship o

dry and satur

he relation; i.

mple and the s

lume 11, No. 1,

): Single-stagater-saturated

): Single-stagater-saturated

0): Single-stagter-saturated

of void ratio-l

rated sandy cl

e., compressi

saturated samp

, 2017

- 87 -

ge and overlod samples at

ge and overlod samples at

ge and overlod samples at σ

log

lay

ion

ple

is

0.

dr

sa

aded-unloadeσcreep of 300 k

aded-unloadeσcreep of 600 k

oaded-unloadσcreep of 1200

calculated a

24168, respec

ry sample (0.9

ample (0.7593

ed tests on kPa

ed tests on kPa

ded tests on

kPa

and found to

ctively. Also,

989) is higher

31). It is furth

o be about 0

the initial voi

r than that of

her noticed th

0.423472 and

id ratio of the

f the saturated

hat pore water

d

e

d

r

Investigation of Parameters… Alireza Negahdar, Shima Yadegari and Siyab Houshmandi

- 88 -

significantly influences the fabric structure of soil,

ratios of micro- and macro-pores, compressibility and

secondary compression coefficient. It is possible to

state that if e0 is high, then Cc is also high. Loading

steps, stress levels, test durations and initial void ratios

of samples are illustrated in Table 2.

Table 2. Results of conducted compression creep test on the samples

Type of test Type of

pore fluid

Sample

Stress (kPa)

Incremental loading steps (kPa) e0 e EOP Cα

Single-stage

Water

SS-WS-300 300 50-100-200-300 0.759 0.4396 0.00755

SS-WS-600 600 50-100-200-400-600 0.759 0.3677 0.004745

SS-WS-1200 1200 50-100-20-40-800-1200 0.759 0.2221 0.002213

Dry-air

SS-AD-300 300 300 0.989 0.9054 0.001576

SS-AD-600 600 600 0.989 0.8048 0.002030

SS-AD-1200 1200 1200 0.989 0.7492 0.002445

Stepwise

Water

SW-WS-50/1200

50 50-creep

0.759

0.5602 0.009893 300 50-100-200-300-creep 0.3379 0.006665

600 50-creep-100-200-300-creep-400-600-creep

0.2657 0.004842

1200 500-creep-100-200-300-creep-400-600-creep-

800-1200creep

0.1924 0.003816

Dry- air

50 50

0.989

0.9373 0.00005008

300 50-creep-300- 0.8904 0.006732

600 50-creep-300-creep-600 0.7053 0.03053

1200 50-creep-300-creep-600-creep-1200

0.6355 0.04903

Overloaded-Unloaded

Water

O-U-WS-300

300 50-100-200-325-300

0.759

0.3327 0.006181 300 50-100-200-350-300 0.3312

O-U-WS-600

600 50-100-200-400-625-600

0.759

0.2543 0.002886

600 50-100-200-400-650-600

0.2533

O-U-WS-

1200

1200 50-100-200-400-800-1225-1200

0.759

0.149 0.0023

1200 50-100-200-400-800-1250-1200

0.149

SS: Single stage DA: Dried in air e0: Initial void ratio SW: Water-saturated WS: Water-saturated Cα: Secondry compression Cofficient O-U: Overloaded-Unloaded eEOP: Void ratio at the end of primary consolidation

Jordan Journ

Single-sta

creep tests a

dried in air a

order to illu

history and p

soils. Finally

At low cr

saturated

in air sam

higher sl

slide ver

makes th

soil stru

decreases

levels, t

incremen

In satura

level, the

stepwise

duration

secondry

Berry, P.L., a

peat.” Geo

nal of Civil Eng

Figure

CONC

age, stepwis

are carried ou

and water-satu

ustrate the ef

pore water on

y, the followin

reep stress lev

sandy clay sa

mples, because

liding ability

ry easily. But

he samples de

ucture becom

s; whereas, in

the creep r

nt.

ated samples,

e variatoins of

tests are ap

of loading d

compression

REFE

and Poskitt, T.J

otechnique, 22

ngineering, Vol

e (11): Relatio

LUSIONS

e and overl

ut at different

urated sandy c

ffects of stre

creep behavio

ng conclusions

vels, the value

amples is high

e in saturated

and lower fr

t, increasing

enser and sma

mes stable a

n dry sample

rate increase

, with increa

f Cα in both s

pproximately

does not affe

n coefficient

ERENCES

J. (1972). “Th

2 (1), 27-52.

lume 11, No. 1,

onship betwedry an

loaded-unload

stress levels

clay samples,

ess level, stre

or of sandy cl

s are drawn.

e of Cα in wat

her than in dri

samples, due

riction, partic

the stress lev

aller, and so t

and creep ra

es at low stre

es with stre

asing the stre

single-stage a

equal and t

ct the value

t. But, in d

e consolidation

, 2017

- 89 -

een void rationd saturated s

ded

on

in

ess

lay

ter-

ied

e to

les

vel

the

ate

ess

ess

ess

and

the

of

dry

th

str

sa

sa

sa

sig

ra

se

th

n of

Bo

o and log effecsamples

samples, w

the variation

The values

single-stage

required ti

overloaded-

single-stage

accelerates

tests, by ap

rearrangeme

particles oc

creep rate.

There is an

he void ratio

ress. It can be

andy clay sam

andy clay sam

amples is high

gnificantly in

atios of micro

econdary com

hat at higher e0

wman, E.T., a

micro-structu

Soils and Fou

ctive axial str

ith increasing

n of Cα increa

of Cα in the

e tests are app

ime for a

-unloaded tes

e tests, and the

the creep. In o

plying σoverload

ent, further s

ccur, resulting

n approximate

and the log

e said that the

mples is higher

mples, as the

her than that o

nfluences the f

o-and macro-p

mpression coe

0 values, Cc is

and Soga, K.

ural change in

undations, 43 (

ress for

g the duration

ases.

e overloaded-u

proximately e

specific poro

st is lower

e overloaded-

overloaded-un

ded and σcreep t

liding and de

g in an accele

ely linear rela

garithm of ef

e compression

r than that of

e initial void

of dry sample

fabric structur

pores, compr

fficient. It is

also higher.

(2003). “Cree

n dense granul

(4), 107-117.

n of loading,

unloaded and

equal, but the

osity in the

than that in

-unloaded test

nloaded creep

to the sample

eformation of

eration of the

ation between

ffective axial

n index of dry

f the saturated

ratio of dry

es. Pore water

re of the soil,

essibility and

also noticed

ep, ageing and

lar materials.”

,

d

e

e

n

t

p

e

f

e

n

l

y

d

y

r

,

d

d

d

”

Investigation of Parameters… Alireza Negahdar, Shima Yadegari and Siyab Houshmandi

- 90 -

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