Collapse Behavior of Coal Ash
A. Trivedi
1
and V. K. Sud, M.ASCE
2
Abstract: The paper describes an investigation carried out to examine the factors influencing collapse settlement of the compacted coal
ash due to wetting. The ashes produced by the coal fired thermal power plants are stored as a high mound in the disposal dump. Some of
the ash dumps and ash fill structures wetted under certain conditions of loading may exhibit collapse. Attempts have been made to
correlate the ash characteristics and the specific placement parameters such as ash type, soluble content, degree of compaction, overcon-
solidation ratio, moisture content, and stress level at wetting with collapse. This was based largely on the oedometer and partly on the
model and the field test results. 378 single oedometer tests were conducted to obtain the collapse potential. The collapse potential was
correlated with the mean size of the ash. The favorable pressure, moisture, fines, compaction, soluble substance, and prestressing decrease
the collapse potential of an ash fill. The collapsible and the noncollapsible ashes were identified by the results of oedometer test and
laboratory model test. Normally, if the collapse potential in the oedometer is more than 0.01, the collapse of soils may occur in the field.
However, the model test demonstrated this to be an unconservative criterion. A value of 0.0075 at 80% degree of compaction was found
appropriate for the ashes examined. The paper explains the technique of field test performed at the controlled densities. The field test
confirmed incidence of collapse on a rising water table for a collapsible ash.
DOI: 10.1061/ASCE1090-02412004130:4403
CE Database subject headings: Fly ash; Collapse; Oedometers; Field tests.
Introduction
Coal ash is a waste product of the coal based thermal power plant.
The power generation based on coal produces a huge amount of
coal ash. There is a serious problem of its storage and disposal.
Turgon 1988 reported use of blended ash in land reclamation. In
recent years the coal ash was studied as a structural fill material
without additives Indraratana et al. 1991; Sood et al. 1993; Walia
et al. 1995; Trivedi et al. 1996; Trivedi 1999; and Trivedi and Sud
2002. It was recognized that a loose ash fill structure may be
prone to collapse on wetting. Therefore an investigation was car-
ried out to examine the factors influencing the collapse of com-
pacted ash fill on inundation.
Morland and Hastings 1973 evaluated collapse of the dry
porous volcanic tuff which is similar to the coal ash in its char-
acteristics. The general characteristics of collapsing soils are a
sudden and a large volume decrease at a constant stress when
inundated with water. According to Lutenegger and Saber 1988
the collapse is associated with the meta-stable structure of a large
open and porous fabric of the material. Holtz 1948 suggested
that earthen structures such as embankments, road fills and struc-
tural fills may collapse when the placement moisture content is
dry of optimum. Fourie et al. 1999 found that infiltration of the
rainfall may be sufficient to reduce the matric suction within the
ash to a low value enough to trigger a shallow failure. Indraratna
et al. 1991 studied the engineering behavior of pozzolanic
ASTM class C Thailand ash and indicated its collapsible nature.
However, little information is available on the collapsibility a
typical nonpozzolanic ASTM class F coal ash. It has been ob-
served that in a wide range of placement parameters the ash re-
mains vulnerable to the collapse on submergence in working
stress range. Fourie et al. 1999 reported the susceptibility of an
ash slope to instability induced from a prolonged infiltration. A
slip failure was described by Dhillon 1995 at the ash dump of
Vijyawada thermal power plant resulting in the destruction of
several houses and the swamping of land with fly ash. Indraratana
et al. 1991 reported the sudden failure of a large fly ash disposal
dump after rainfall and associated mudflow at Panki, Kanpur.
Such failures are not quite representative of conventional failures.
Several studies have indicated that compaction control of coal ash
in the field by usual methods is often poor. It adds to the vulner-
ability of ash fill to a wetting induced collapse.
The soils that exhibit collapse have an open type of structure
with a high void ratio as expected in the case of ashes. According
to Barden et al. 1969 the collapse mechanism is controlled by
three factors; 1 a potentially unstable structure, such as floccu-
lent type associated with soils compacted dry of optimum or with
loess soils; 2 a high applied pressure which further increases the
instability; and 3 a high suction which provides the structure
with only temporary strength which dissipates on wetting. As per
an empirical study by Meckechnie 1989, the dry unit weight and
water content are generally considered as important parameters
that control the collapse of metastable structure of soils, if the dry
unit weight is less than 16 kN m
-3
. The tentative dry unit weight
of the coal ashes in Ropar ash pond was often found to be less
than 10 kN m
-3
suggesting possibility of collapse. Jennings and
Knight 1975 indicated that collapse behavior is also dependent
1
Professor, Dept. of Civil Engineering, Res-8 Type V, Delhi College
of Engineering, Bawana Rd., Delhi, India 110042. E-mail:
atrivedi14@yahoo.com
2
Formerly, Professor, Dept. of Civil Engineering, Thapar Institute of
Engineering and Technology, Patiala, India 147004.
Note. Discussion open until September 1, 2004. Separate discussions
must be submitted for individual papers. To extend the closing date by
one month, a written request must be filed with the ASCE Managing
Editor. The manuscript for this paper was submitted for review and pos-
sible publication on April 17, 2001; approved on July 31, 2003. This
paper is part of the Journal of Geotechnical and Geoenvironmental
Engineering, Vol. 130, No. 4, April 1, 2004. ©ASCE, ISSN 1090-0241/
2004/4-403– 415/$18.00.
JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / APRIL 2004 / 403