ORIGINAL PAPER

Effects of Soil Aging on Mechanical and HydraulicProperties of a Silty Soil

Mounir Ltifi • Tarek Abichou • Jean Paul Tisot

Received: 22 May 2011 / Accepted: 21 May 2014

� Springer International Publishing Switzerland 2014

Abstract The aging phenomenon, which produces

changes in material state over time, is associated with

significant modification of mechanical and physical

soil properties. This change should be accounted for

during geotechnical design. Although soils sometimes

improve with aging, the opposite effect is occasionally

observed. This paper describes a study performed to

investigate the effect of aging on the mechanical

behavior and the permeability of a silty soil.

Undrained unconsolidated triaxial shear tests and

triaxial permeability tests were performed on dis-

turbed and compacted samples. Upon conclusion of

these tests, the samples were sealed from air and

moisture. The results show an important increase in

both the undrained shear strength and the deformation

modulus caused by silt rigidification during the aging

process. These changes cause an over estimation of

laboratory measured shear strength. For instance, the

increase in the deformation modulus and undrained

cohesion can approach 100 % for an approximate

328 day storage period. Sample permeability was

found to decrease with aging. This reduction can be

ascribed to several causes including micro-organisms

growth, secondary sample consolidation and progres-

sive filling caused by the migration of very fine

particles. These phenomena might have negated the

expected increase in permeability with aging time

reported in the literature.

Keywords Soil aging � Permeability � Shear

strength

1 Introduction

The aging phenomenon, which reflects a material state

change as function of the time, can produce considerable

changes in the mechanical and physical soil properties.

The long term change in these properties should be

considered during geotechnical design. Previous inves-

tigators (Mitchell 1960, 1986; Leonards and Ramiah

1960; Leonards and Altschaefl 1964; Zeevaert 1949,

1983; Schmertmann 1983, 1987, 1993) developed a

variety of reliable ideal models which account for soil

aging. The aging process itself, however, remains a

complex and uncontrolled phenomenon.

Generally, aging produces an improvement in soil

properties. In select cases however (Schmertmann

1991), degradation of soil properties was observed. All

soils age with time (Mitchell 1986). ‘‘Pure’’ aging is

M. Ltifi

Civil Engineering Department, National Engineering

School of Gabes, Gabes, Tunisia

T. Abichou (&)

College of Engineering, Florida State University, 2525

Pottsdamer Street, Tallahassee, FL 32310, USA

e-mail: abichou@eng.fsu.edu

J. P. Tisot

National Geology Higher School of Nancy, Nancy, France

123

Geotech Geol Eng

DOI 10.1007/s10706-014-9784-1

often characterized as the phenomenon which

involves time-dependent changes only. Chemical

weathering, freezing–thawing, swelling–desiccating

and changes in ground-water level are the main in-situ

processes observed during aging. These processes

occur simultaneously and their individual influence is

difficult to quantify during ‘‘pure’’ aging (Schmert-

mann 1993). Also, many references have focused on

changes caused by later stage processes such as creep

and/or secondary consolidation aging.

Thixotropy is widely linked to aging mechanisms

(Mitchell 1960). Most geotechnical engineers associ-

ate thixotropy with an increase in compressive

strength remolding. Mitchell (1960) has refined his

definition of aging to include isothermal conditions

and reversible behavior at constant composition and

volume. Mesri (1993) similarly defines thixotropic

hardening as a reversible process that can occur under

conditions of constant composition and volume.

Schmertmann (1991), presents examples of

improvement in preconsolidation, modulus, strength,

and bearing capacity based on lab and field studies. He

also describes soils where little aging effect was

observed. Schmertmann (1991) suggests that in those

solid displaying a lack of observable improvement

with age, other processes might be at work. Schmert-

mann (1991) also argues that the effects of aging can

be explained by mechanics and, that strength gain is

due to an increase in frictional resistance and not from

any increase in cohesion.

Mitchell and Soga (2005) also address the effect of

time on the strength and deformation of soils. Again,

their focus was on how time changes the structural,

deformation, and strength properties of soils under

stress. They present suggestions and methods to

quantify and mathematically predict the effects of

time on soil properties. In recognizing that macro-

scopic stress on a soil results in tangential and normal

interparticle stresses which can be modeled by discrete

particle simulation. These models ‘‘show that changes

in creep rate with time can be explained by changes in

the tangential and normal force ratio at inter-particle

contacts that result from particle rearrangement during

deformation.’’ They conclude that changes in the

micro-fabric lead to a non-homogeneous strong par-

ticle network with locally weak clusters which explain

the mechanical aging process.

Dunn and Mitchell (1984) relate the time effect of

aging to the permeability (hydraulic conductivity) of

fine-grained soils. Hydraulic conductivity was found

to increase with the delay in time between sample

preparation and test initiation. This was attributed to

thixotropic changes in sample fabric. They argue that

flocculation of soil particles increases with time after

compaction. This change in fabric increases the

effective pore size, which explains the higher hydrau-

lic conductivity. Mitchell et al. (1965) also document

an ‘‘increased degree of flocculation’’ when they find a

high correlation between the increase in hydraulic

conductivity with time and the increase in strength and

modulus of soil.

Previous paragraphs document the complexity and

practical importance of the aging phenomenon. This

paper reports the results of experimental studies

performed with observational periods sufficiently long

to document the aging process. In particular, the main

focus of this study is the influence on the relationship

between storage time and constitutive soil properties

as measured in laboratory. Storage time is defined as

the time between sample compaction and the initiation

shear strength. The effect of aging (soil storage) on

shear strength was assessed by performing undrained

unconsolidated triaxial compression tests after the soil

had been compacted and stored for various periods of

time. The laboratory measured influence of the storage

time on soil permeability provided a secondary focus

for the present study.

2 Materials and Methods

The experimental study was carried out on a silt

originating from the Xeuilley area, located at Nancy

North–West (France). The silt had a Liquid Limit of

56, Plastic Limit of 31, and a Plasticity Index of 25 (as

determined in accordance with ASTM D4318). The

specific gravity (Gs) of the soil was 2.64 (in accor-

dance with ASTM D854-10). The silt is classified as

MH to the unified soil classification system (USCS).

The mineralogical composition of the silt used in this

study is shown in Table 1.

Shear strength tests were performed on samples

compacted with the Harvard Miniature apparatus devel-

oped by Wilson (1970). This apparatus is unique in that

the soil undergoes kneading and homogenization during

compaction. This differs significantly from the widely

used Proctor test which relies on solely on impact force

for compaction. The use of the Harvard Miniature for

Geotech Geol Eng

123

compaction is justified because of the need to obtain

more homogeneous samples. Laboratory testing with the

Harvard apparatus determined that optimum water

content of the soil was 22 % which was associated with

a maximum dry density of 1.6 g/cm3, as measured using

the Harvard Miniature, in accordance to the procedures

described in ‘‘Suggested Method of Test for Moisture–

Density Relations of Soils Using Harvard Compaction

Apparatus’’ published in 1970. Permeability testing was

performed on samples compacted according to the

standard Proctor method (ASTM D698) to a water

content of 23 %. Compaction curves were obtained for

both Harvard and Proctor compaction are shown in

Fig. 1. The compaction curves were very similar, but one

should recognize that this does not means that there no

difference in soil structure between the two method of

compaction. The similarity of both compaction curves

should not be taken to indicate similarity of soil structure.

2.1 Shear Strength Testing

After compaction testing, the samples were divided into

four groups and undrained unconsolidated triaxial

compression tests were performed on the samples in

each group in order to study soil shear strength

parameters, in accordance with the procedures described

in (ASTM D4767-04). Table 2 presents the triaxial

compression test program. The compacted samples had

a water content ranging from approximately 23–24%

with only a slight deviation in dry density The samples

were then aged for various periods of time. Air-tight bags

were used to isolate the sample from temperature and

humidity variations in the ambient air. These samples

were placed in a hermetic container to ensure a constant

temperature and a constant relative humidity. Redun-

dancy was provided by analyzing multiple samples for

each group and aging period.

After a pre-determined amount of time, the samples

were retrieved and the water content was again

computed. Triaxial testing was then performed for

the determination of the shear strength parameters.

The strain rate for all triaxial tests was maintained at a

constant 0.9 mm/min. The rate of shear was kept

constant during all tests to avoid introducing another

variable into the investigation.

For each period, every group of samples were

sheared under various confinement pressures (50, 100,

200 and 400 kPa), see Table 2. A single sample group

was sheared immediately after compaction in order to

be used as reference for the other samples. Height and

diameter of the samples were 7 and 3.13 cm

respectively.

2.2 Permeability Tests

The first sets of samples was used to carry out two

permeability tests immediately after compaction. The

Fig. 1 Compaction curves obtained using standard Proctor and

Harvard Miniature

Table 2 Summary of triaxial shear tests program

w (%) cd (kN/m3) r3

kPa

Aging period

(days)

Test number

23 to

24

16 50

100

200

400

Immediately

after

compaction

and up to

328 days

later

Min. two

samples for

each period

and for each

confining

pressure

Table 1 Mineralogical composition of soil used in this study

Mineral (%)

Quartz 60

Montmorillonite 20

Feldspaths 11

Kaolinite 4 to 5

Mica 4 to 5

Geotech Geol Eng

123

remaining samples were isolated from ambient at and

held for testing after various storage periods. For each

test, water content was determined and the stored

sample was divided into two separate specimens.

Table 2 shows the water content, dry unit weight, and

degree of saturation before and during aging

(Table 3).

Permeability tests were performed in accordance

with the procedures described in ASTM D5084-90.

Once the samples were prepared, they were placed in a

triaxial cell to launch the saturation phase. Variation in

soil volume during the test was monitored.

Sample consolidation, a major concern during

permeability testing, was accounted for by reducing

the effective pressure during testing. The lower

effective pressure insured against additional consoli-

dation upon application of the hydraulic gradient. The

bottom and top backpressures of 305 and 295 kPa

were applied yielding a hydraulic gradient of 10.

Testing continued until the last four hydraulic con-

ductivity values were within 25 % of the mean, inflow

equaled outflow (within 5 %), and stabilization of the

hydraulic conductivity and outflow/inflow.

3 Results and Analysis

3.1 Shear Strength Results

Figure 2 shows typical deviator stress variation versus

strain for three different times of aging (immediately

after compaction, 90 days after compaction, and

328 days after compaction for various confinement

pressures (50, 100, 200, and 400 kPa). Either two or

three replicates were used for each aging time and at

each confining pressure. (Also note that only curves

three aging times are presented in Fig. 2 to keep the

graph clear. The same patterns were observed for all

other aging times in a previous investigation (Ltifi

1998). Figure 2 shows that the soil samples acquired

more strength during the storage time, which corre-

sponds to higher deviator stress as the soil aging

increased. Moreover, a very distinct increase of the in

deformation modulus as function of sample age is

apparent in Fig. 2.

Figure 3a shows the maximum deviator stress

versus aging time for all tested specimens. For all

confining pressures, the maximum deviator stress

increases with aging time. These results are in

agreement with the finding of Yashura and Ue

(1983) Regression analysis of the data used to generate

Fig. 3a provides R2 values of 0.85 or higher.

Figure 3b shows the soil deformation modulus versus

aging time for the same soil specimens. For all confining

pressure, the soil modulus increases as the aging time

increases. Again, regression analysis on the data used to

generate Fig. 3b yields R2 values of 0.72 or greater.

Figure 4 examines the relationship between

undrained shear strength of the soil and sample age.

An important increase in the undrained shear strength

as function of sample ages is evident in Fig. 4.

Regression of undrained shear strength on soil aging

times shows a very high correlation between the two

variables (R2 = 0.92). The obtained results indicate

that aging time explains much of the increase in

undrained shear strength, as well as the increase in

deformation modulus of the soil. The results show

that, for increasing aging time, the undrained shear

strength as well as the deformation modulus of the soil

will increase in value.

Table 3 Initial and final samples characteristics for permeability testing

Sample w (%) preparation gdi (kN/m3) Age (days) w (%) after aging Sr (%) initial w (%) final gdf (kN/m3)

1 23.37 15.90 Immediate 23.37 92.8 25.28 15.54

2 23.24 15.70 Immediate 23.24 90.6 25.76 15.44

3 22.78 15.80 50 23.05 90.8 26.36 15.57

4 22.78 15.80 50 23.17 91.2 25.9 15.75

5 23.36 16.00 142 23.42 95.6 25.35 16.53

6 23.36 16.40 142 23.42 94.9 24.55 16.58

7 23.60 16.40 232 21.28 92.4 25.15 16.04

8 23.60 16.40 232 21.28 92.6 25.7 15.96

Geotech Geol Eng

123

0

100

200

300

400

500

600

0 3 6 9 12

Dev

iato

r St

ress

(kP

a)

Strain (%)

σ3=50 kPa

(a)

0

100

200

300

400

500

600

700

0 3 6 9 12

Dev

iato

r St

ress

(kP

a)

Strain (%)

σ3=100 kPa

(b)

0

100

200

300

400

500

600

700

0 3 6 9 12

Dev

iato

r St

ress

(kP

a)

Starin (%)

σ3=200 kPa

(c)

0

100

200

300

400

500

600

700

0 3 6 9 12

Dev

iato

r St

ress

(kP

a)

Strain (%)

σ3=400 kPa

(d)

Fig. 2 Deviator stress

versus strain for three aging

times (Black immediately

after compaction, Blank

90 days after compaction,

and Gray 328 days after

compaction) for four various

confinement pressures. Note

the water content of these

samples were all around

22 %

Geotech Geol Eng

123

These results lead one to question the physical

processes responsible for this distinct improvement.

Similar soil strengthening was reported by Zeevaert

(1949), Shmertmann (1991), and Mesri (1975). These

authors attribute the strength gained during aging to

the increased rigidity which results from thixotropic

hardening during the sample storage.

Several investigators attribute the increased resis-

tance of soil during aging to chemical pore water

alteration and/or changes in solid matter material

constituents (Mitchell 1986; La Rochelleet al. 1986;

Graham et al. 1990). In this study, according to the

nature of the materials mineralogy (namely quartz), this

increase is not expected to have a chemical origin, i.e. it

must have been initiated by the transformations that the

material structure could have undergone. Mesri (1993)

defined thixotropic hardening as the purest form of soil

aging. It does not require volume change or chemical

alteration. Only the influence of aging is expected on the

samples prepared for this study.

These conclusions agree with the findings of

Zeevaert (1949) who worked with Mexico City clay.

The conditions under which the test in this study were

carried out, and the verifications made by

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300 350 400

Max

imum

dev

iato

r st

ress

(kP

a)Aging time (days)

w=22%

50 kPa100 kPa200 kPa400 kPa

(a)

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400

Mod

ulus

(M

Pa)

Aging time (days)

50 kPa100 kPa200 kPa400 kPa

w=22%(b)

Fig. 3 Maximum deviator

stress versus aging time

Fig. 4 Undrained shear strength versus aging time

Geotech Geol Eng

123

measurements, in particular water content and dry

density, show that the temperature and moisture have

an insignificant influence on material. In addition,

quality control was used to eliminate sample groups

whose preparation water content variations were

different. Thus, the results obtained during this study

seem to be caused by a pure aging mechanism defined

by Mesri (1993).

3.2 Permeability Results

After application of the hydraulic gradient, inflow and

outflow quantities through the test apparatus were

carefully monitored. Figure 5 shows the permeability

versus sample aging time. Due to experimental

constraints, permeability testing was performed on a

maximum aging time of 232 days. Sample permeabil-

ity decreases with increasing aging time. This reduc-

tion is very important during the first 5 months.

Beyond the 5 month aging duration, the permeability

becomes stable. This is probably due to the structural

changes that occur during the storage period. These

results are inconsistent with the findings of Mitchell

et al. (1965), where he reported an increase in

permeability due to the change in soil structure with.

He also argued that the increase in permeability is due

to same change in structure responsible for the

increase in strength. It is difficult to relate the

permeability results obtained during this study to the

strength results because each set of samples were

compacted using two distinct compaction methods, as

described in the previous section.

Dunn and Mitchell (1984) ascribed the reduction in

permeability during the permeability testing period

(while the sample is saturating and permeating as

being due to several causes such as the micro-

organisms growth), secondary sample consolidation

and progressive filling by the migration of very fine

particles. The effects of these phenomena could negate

the expected increase in permeability with aging time.

Sample permeability converged as aging time

increased. The difference in permeability between

the two initial specimens (aging time is zero) is around

one half order of magnitude. The difference decreased

as the aging time reached 50 days and becomes

insignificant for aging time higher than 150 days. This

might be further evidence that that soil structure

changes with aging until a steady state is reached.

4 Summary and Conclusions

This paper describes an experimental study on the

effect of storage time on the aging of a silty soil on

undrained shear strength and soil permeability

(hydraulic conductivity). Several unconsolidated

undrained triaxial shear tests and triaxial permeability

tests were performed. These tests were performed on

compacted samples which were stored for differing

time periods. Compaction was carried out using the

miniature Harvard test apparatus (shear strength

testing) and the standard Proctor method (permeability

testing).

The following conclusions can be drawn:

• The strain modulus and material rigidity increase

with increasing aging time.

• The strain at failure is not as affected or influenced

by the aging time.

• Undrained cohesion increased as time of storage

increases.

Mechanical behavior change is consistent with

Zeevaert (1949), Shmertmann (1991), and Mesri

(1975). This increase in shear strength was attributed

to aging by thixotropic hardening during the sample

storage. The effect of aging should be accounted for

when determining the properties of compacted and/or

natural materials stored at the laboratory for extended

periods of time before testing. Shear strength is

commonly overestimated in aged soils. The increase

in the deformation modulus and undrained cohesion

Fig. 5 Permeability versus aging time

Geotech Geol Eng

123

can reach up to 100 % for a storage period of about

328 days. Even though in-situ soil modification takes

place under drained conditions, (which differ from the

laboratory conditions), this study provides a helpful

reference point in describing and quantifying the aging

time effect on undrained silty soils.

The soil permeability decreases as storage time

increases. This is not consistent with Mitchell et al.

(1965), who reported an increase in permeability due

to the change in soil structure with time. Mitchell et al.

(1965) also argued that the increase in permeability is

due to same change in structure responsible for the

increase in strength. Dunn and Mitchell (1984) suggest

that the reduction in permeability during the testing

period (while the sample is saturating and permeating)

is caused by a variety of phenomena including micro-

organisms growth, secondary sample consolidation

and progressive filling by very fine particle migration.

These phenomena might have negated the expected

increase in permeability with aging time.

References

Dunn RL, Mitchell JK (1984) Fluid conductivity testing of fine-

grained soils. J Geotech Eng ASCE 110(11):1648–1665

Graham RC, Daniels RB, Buol SW (1990) Soil–geomorphic

relations on the Blue Ridge front: I. Regolith types and

slope processes. Soil Sci Soc Am J 54:1362–1367

La Rochelle P, Leroueil S, Tavenas F (1986) Discussion of the

causes and effects of aging in quick clays by Lessard G. and

Mitchell J. K. (1985). Can Geotech J 23:407–408

Leonards GA, Altschaefl AG (1964) Compressibility of clays.

J Am Soc Civ Eng 90:133–156

Leonards GA, Ramiah BK (1960) Time effects in the consoli-

dation of clays. Spec Tech Publ Am Soc Test Mater

254:116–130

Ltifi M (1998) Experimental study of aging of soils. Disserta-

tion, National Polytechnic Institutes of Lorraine, France

Mesri G (1975) Discussion of new design procedure for stability

of soft clays. Proc Am Soc Civ Eng 101(4):409–412

Mesri G (1993) Discussion on Initial investigation of the soft

clay test site at Bothkennar, by Nash DFT, Powell JJM,

Lloyd IM. Geotechnique 43:503–504

Mitchell JK (1960) Fundamental aspects of thixotropy in soils.

J Soil Mech Found Div ASCE 86(3):19–52

Mitchell JK (1986) Practical problems from surprising soil

behaviour. J Geotech Eng ASCE 112(3):259–289

Mitchell JK, Soga K (2005) Fundamentals of soil behavior,

Chapter 12—time effects on strength and deformation.

John Wiley & Sons, Inc., pp 465–520

Mitchell JK, Hooper DR, Campanella RG (1965) Permeability

of compacted clay. J Soils Mech Found Div Proc Am Soc

Civ Eng 91(4):41–65

Schmertmann JH (1983) A simple question about consolidation.

J Geotech Eng ASCE 109(1):119–122

Schmertmann JH (1987) Discussion of time-dependent strength

in freshly deposited or densified sand. J Geotech Eng

ASCE 113(2):173–175

Schmertmann JH (1991) The mechanical aging of soils. J Geo-

tech Eng ASCE 117(9):1288–1330

Schmertmann JH (1993) Update on the mechanical aging of

soils. The symposium ‘‘Sobre Envejecimiento de Suelos,’’

The Mexican Society of Soil Mechanics, Mexico City

Wilson SD (1970) Suggested method of test for moisture–den-

sity relations of soils using Harvard compaction apparatus.

Special procedure for testing soil and rock for engineering

purposes, ASTM, STP 479

Yashura K, Ue S (1983) Increases in undrained shear strength

due to the secondary compression. Soils Found

23(3):50–64

Zeevaert L (1949) An investigation of the engineering charac-

teristics of the volcanic lacustrine clay deposit beneath

Mexico City. Dissertation, University of Illinois, Urbana

Zeevaert L (1983) Foundation engineering for difficult subsoil

conditions. Van Nostrand Reinhold, New York

Geotech Geol Eng

123