1055
Experimental study of resilient modulus of unsaturated soil at different temperatures
Etude expérimentale du module de résilience d’un sol non saturé à différentes températures
Zhou C., Ng C.W.W.
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water
Bay, Kowloon, HKSAR
ABSTRACT: Fatigue cracking and failure in asphalt and concrete layer of a road pavement are of great concerns to pavement
designers and users. The deformation of this layer is related to resilient modulus (
M
R
) of subgrade soil under cyclic traffic loads. In
the field, subgrade soil is subjected to daily and seasonal variations of soil suction and temperature. Although thermo-hydro-
mechanical behaviour of soil has attracted intense attention, suction and thermal effects on
M
R
under cyclic loading-unloading have
rarely been reported. In this study, three series of cyclic triaxial tests have been carried out to investigate MR of an unsaturated silt at
different temperatures in a newly developed suction and temperature controlled cyclic triaxial apparatus. The new apparatus is
employed to investigate MR of the unsaturated silt at six different suctions (0, 30, 60, 100, 150 and 250 kPa) and two different
temperatures (20 and 40 �). To enhance the accuracy of strain measurements, Hall-effect transducers are adopted to measure the local
axial and radial deformations of each specimen. The influence of suction and temperature on
M
R
are presented and discussed.
RÉSUMÉ : Fissuration et rupture par fatigue dans la couche d'asphalte et de béton de la chaussée sont des grandes préoccupations
pour les concepteurs et les utilisateurs des chaussées. La déformation de cette couche est liée au module de résilience (
M
R
) du sol de
fondation sous charges cycliques du trafic. Le sol de fondation est soumis à des variations quotidiennes et saisonnières de la succion
et de la température sur le terrain. Bien que le comportement thermo-hydro-mécanique du sol ait intensément attiré l'attention, les
effets de la succion et de la température sur
M
R
sous cycles de charge-décharge ont été peu étudiés. Dans cette étude, trois séries
d'essais triaxiaux cycliques sont réalisés pour étudier le MR d’un limon non saturé à différentes températures dans un nouvel appareil
de test triaxial cyclique à succion et température contrôlées. Le nouvel appareil est utilisé pour étudier le MR du limon non saturé à
six succions différentes (0, 30, 60, 100, 150 et 250 kPa) et à ux températures différentes (20 et 40 °C). Afin d'améliorer la précision
des mesures de contrainte, des transducteurs à effet Hall sont adoptées pour mesurer les déformations axiales et radiales locales de
chaque échantillon. L'influence de la succion et de la température sur les
M
R
est présentée et interprétée.
KEYWORDS: Unsaturated subgrade soil, resilient modulus, cyclic, suction, thermo-hydro-mechanical
1 INTRODUCTION
Fatigue deformation, cracking and failure in asphalt and
concrete layer of any road pavement are of great concerns to
pavement designers and users. Their incidence may be caused
by many reasons such as increase in traffic volume,
deterioration of asphalt and concrete, rutting of unbound
granular materials and differential settlement of subgrade soils
(Brown, 1997). According to Seed (1962), resilient modulus
(
M
R
), is defined as the ratio of repeated deviator stress to axial
recoverable strain in cyclic triaxial test. This ratio is widely
used as a stiffness parameter to determine soil deformation
under cyclic traffic loads in pavement engineering (Brown,
1997).
In the field, unsaturated subgrade soil is subjected to daily
and seasonal variations of pore water pressure (or soil suction)
and temperature (Jin et al., 1994). It is generally recognized that
the behaviour of unsaturated soil is governed by two stress state
variables, namely net normal stress (
-
u
a
) and matric suction
(
u
a
-
u
w
), where
,
u
a
and
u
w
are total normal stress, pore air
pressure and pore water pressure, respectively (Coleman, 1962).
By controlling these two stress state variables in cyclic triaxial
test, Yang et al. (2008) and Ng et al. (2012) observed that
M
R
increases with an increase in suction but at a reducing rate.
Although matric suction is very important for understanding
M
R
of subgrade soil, it is rarely controlled or measured in resilient
modulus tests. This is possibly due to some complexities and
difficulties in suction control and measurement. Moreover,
suction-controlled tests on unsaturated soil are generally time-
consuming and so they are not very welcome by many
engineers and researchers.
On the other hand, thermo-hydro-mechanical behaviour of
soil has attracted intense attention because of its importance in
the field of geo-environmental engineering and energy
foundation. Previous experimental studies have illustrated that
temperature significantly affects unsaturated soil behaviour such
as swelling pressure, collapse potential, shrinkage property,
compressibility, water retention behaviour and shear strength
(Romero et al., 2003; Tang et al., 2008; Uchaipichat and
Khalili, 2009). One important examples of thermal effect on soil
behaviour is that the yielding stress of soil specimen at elevated
temperature is lower than that observed at room temperature. As
far as the authors are aware, thermal effects on
M
R
of
unsaturated soil under cyclic loading-unloading have rarely
been reported.
In this study, the influence of two stress-state variables
(matric suction and net stress) and temperature on
M
R
of
unsaturated soil under cyclic loading-unloading was
investigated in a newly developed suction and temperature
controlled cyclic triaxial apparatus. Effects of the number of
load applications were also studied.
2 TESTING APPARATUS AND MEASURING DEVICE
Figure 1 shows the newly developed suction and temperature
controlled cyclic triaxial system. It consists of two main parts: a
suction controlled triaxial apparatus and a heating system. The
suction controlled cyclic triaxial apparatus for testing saturated
and unsaturated soils was originally developed by Ng and Yung
(2008) using the axis-translation technique. In addition to a
conventional pore water pressure transducer installed at the
bottom of soil specimen, a mid-plane suction probe can be
mounted to negative pore water pressure ; at the mid-height of a
specimen. More details are given by Ng and Menzies (2007);
Ng and Yung (2008); Ng and Xu (2012) and Ng et al. (2012).
The heating system installed inside the triaxial cell consists
of a thermostat, a cylindrical heater with air serving as
circulating fluid, two small fans for circulating air and two
thermocouples. One thermocouple provides feedback to
thermostat for temperature control, while the other one is used
