1325
On the Permanent Deformation Behavior of Rail Road Pond Ash Subgrade
Sur le comportement en déformation permanente d’une assise ferroviaire en cendres volantes
de bassin
Mohanty B.
Dept. of Civil Engg., IISc Bangalore, India
Chandra S.
IIT Kanpur, Indian
ABSTRACT: In this study repeated load triaxial tests were conducted on reconstituted pond ash specimens and permanent
deformation calculations have been made taking the stress history and number of passes of vehicular traffic loading into
consideration. Tests were performed at different moisture content levels with varying dry unit weights, and at different stress levels
simulating the environmental and traffic conditions. Specimens were prepared using moist tamping technique so as to obtain density
closer to the field density. Test results were analyzed to study the effects of confining pressure, deviatoric stresses, and degree of
saturation on the permanent deformation response of pond ash. Results show that both traffic and environmental condition play an
important role in the permanent axial strain behavior of the material. Furthermore, the shakedown limit describing a critical stress
level that exists between stable and unstable condition is also examined for the design purpose.
RÉSUMÉ : Dans cette étude des essais triaxiaux à charges répétées ont été effectués sur des échantillons reconstitués de cendre
volante de bassin et des calculs de déformation permanente ont été effectués, prenant en compte l’historique de contraintes et le
nombre de passages du chargement de circulation des véhicules. Des essais ont été réalisés à différents niveaux de teneur en eau et
avec des densités sèches variables, et à différents niveaux de contrainte simulant les conditions environnementales et de trafic. Des
échantillons ont été préparés utilisant la technique du compactage humide afin d'obtenir la densité la plus proche de la densité en
place. Les résultats d'essai ont été analysés afin d’étudier les effets de la pression de confinement, des contraintes déviatoriques et du
degré de saturation, sur la réponse en déformation permanente de la cendre. Les résultats prouvent que le trafic et l'état
environnemental jouent tous deux un rôle important dans le comportement axial en déformation permanente du matériau. De plus, la
limite de shakedown caractérisant un niveau de contrainte critique séparant l'état stable de l’état instable est également examinée vis-
à-vis du dimensionnement.
KEYWORDS: Pond ash; Train loading; Triaxial tests; Permanent deformation.
1 INTRODUCTION
Pond ash is a by-product of coal-fired electric power plants
found abundantly in India. In order to avoid environmental
problem, it can be used in the construction of transportation
facilities in bulk quanties. The use of pond ash containing a
large fraction of bottom ash in rail road pavements or subgrade
will experience repeated rail traffic loading while in-service.
This material should be assessed for its suitable use by
considering resistance to permanent deformation measured from
repeated load triaxial tests. It has not been adequately
researched in the past and is investigated in this study
simulating the environmental and traffic conditions.
In laboratory, one-way cyclic triaxial tests are generally
conducted to obtain the deformation characteristics of subsoils
under repeated traffic loading simulating the in-service loading
conditions induced by passing vehicles. Hence, one-way cyclic
triaxial tests on remolded pond ash specimens were performed
in this study under undrained conditions with a constant
confining pressure and different cyclic applied compressive
(non-reversal) deviatoric stresses for each test. In this case the
axial deviatoric stress remains the major principal stress and
shear reversal does not occur during the test. The permanent
axial strain accumulated with respect to number of applied
loading cycles were recorded for each test and analyzed to study
the influence of different controlling parameters on the one-way
cyclic behavior of pond ash.
2 TEST MATERIALS AND SAMPLE PREAPRATION
The pond ash used for preparation of remolded samples were
sampled near the discharge point, near the margins of the wet
disposal ash pond of a thermal power plant producing fly ash
and bottom ash with a typical production ratio of approximately
80:20 by weight. The disturbed, completely saturated pond ash
samples were oven-dried, and then thoroughly mixed to obtain
the representative homogeneous samples. The specific gravity
(
G
s
), optimum moisture content (
w
opt
), and maximum dry unit
weight (
d max
) of the pond ash were found to be 2.36, 33.6%,
and 11.2 kN/m
3
, respectively. From grain size distribution, it is
observed that the dominant particle size is in the sand size
range. It contains 77.81% sand, and 20.56% silt size particles.
The coefficient of uniformity,
C
u
is obtained as 7.39, while the
coefficient of curvature,
C
c
is 2.07. The ash was classified based
on the classification system proposed by Prakash and Sridharan
(2006). It is found to be
Non-plastic sand-silt size fractions
and
is designated as
SMN
.
The samples were reconstituted at different initial dry unit
weights [Relative Compaction: RC = 90%, 95%, 97%, and at
standard proctor maximum dry unit weight (at MDD)] and at
different water contents giving different degrees of saturation
(
S
r
). The relative compaction, RC is defined as the percentage
of desired dry unit weight (
d
) to the maximum dry unit weight
(
d max
) that obtained from the standard proctor compaction
curve. The specimen at 97% RC was reconstituted using water
content value on the wet side of the standard proctor curve. The
samples were prepared in accordance with the conventional
moist-tamping technique, as it is a simple and easy method to
provide good control over obtaining the wide range in target
density (Ladd 1978). First, an appropriate quantity of oven-
dried representative pond ash sample was calculated with
respect to the desired dry unit weight. Then, de-aired water
corresponding to desired moisture content was measured and
mixed to form a mixture. Cylindrical split mold of 50 mm in
diameter and 100 mm in height was selected for sample
preparation. The prepared mix material (wet or moist ash
mixture) was carefully placed and compacted inside the
specimen mold in five identical layers, subdividing the total
mass into five equal parts approximately. The specimen was
prepared on a trial for at least three times to check the desired
