Effects of surcharge loads on liquefaction parameters of pond ash improved w

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 4, July-August (2013), © IAEME
225
EFFECTS OF SURCHARGE LOADS ON LIQUEFACTION PARAMETERS
OF POND ASH IMPROVED WITH STONE-SAND COLUMNS
H.P. Singh1
1
Department of Civil Engineering, NERIST, Itanagar, Arunachal Pradesh, India,
ABSTRACT
Huge quantities of coal ash produced from thermal power plants are very fine, non-plastic
and of low unit weight and are loosely disposed into lagoons or ponds covering an area of several
square kilometres. These ashes in ponds, called pond ash are having low load carrying capacity and
poor settlement characteristics. This material in their saturated condition may be susceptible to
liquefaction during earthquakes. On the other hand the performance of such materials can
substantially be improved by applying soil reinforcement techniques and their liquefaction potential
can be reduced. Keeping this in view and in order to rehabilitate the abandoned ash ponds, it is
therefore necessary to evaluate the liquefaction resistance and settlement susceptibility of pond ash
In the present study, a number of tests were performed on a small Vibration (Shake) Table imparting
harmonic excitation of 0.3g amplitude under the frequency of 5 Hz to pond ash samples prepared at
relative densities of 20% without and with stone-sand columns at 4d c/c spacing, where d is diameter
of stone-sand columns. Tests were also conducted on improved pond with various surcharge loads.
The liquefaction parameters of pond ash such as maximum excess pore water pressure (Umax),
maximum excess pore water pressure built up time (t1), Maximum pore water pressure stay time (t2),
complete excess pore pressure dissipation time (t3) were measured with the help of glass tube
piezometer and stop watch. The liquefaction resistance of pond ash was evaluated in terms of
maximum pore water pressure ratio (rumax = Umax/σv’) for all the tests. It was observed that the
liquefaction resistance of pond ash increases with the inclusion of stone-sand columns. It was also
observed that the liquefaction resistance of pond ash further increases when various surcharge loads
are applied on the samples of pond ash improved with stone-sand columns Thus there is a significant
increase in liquefaction resistance of pond ash due to surcharge loads.
Key Words: Harmonic Excitation, Liquefaction Parameters, Pond ash, Stone-Sand Column,
Surcharge Loads,
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 4, Issue 4, July-August (2013), pp. 225-235
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2013): 5.3277 (Calculated by GISI)
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IJCIET
© IAEME

226
1. INTRODUCTION
A major source of electrical energy in India is thermal power, which is being generated by
burning of low-grade coal of high ash content. The current production of ash in India is over 100
million tons per year out of which a very small percentage (3 to 5%) is being used for various
applications such as brick making, cement manufacture, soil stabilization, etc. Disposal of the
remaining quantity of ash has created a major problem of availability of land. In many thermal power
plants ash deposit is spread over a large area usually for a height of 10 to 30 m. Toth et al.[1] studied
the use of fly ash as a structural fill and found that the physical behavior of fly ash is similar to that
of silt. Sridharan et al.[2] investigated the geotechnical characterization of various ash ponds in India
and reported that pond ashes in general possess low unit weight, good frictional properties, low
compressibility and low Permeability and they are well suited for their use as a structural fill.
Choudhary and Verma [3] used the soil reinforcement technique to improve the TISCO pond ash and
found that there is significant increase in the CBR value of pond ash due to inclusion of
reinforcement and pond ash can be used as a pavement subgrade. Singh et al. [4] also observed that
these materials are very fine, loose and non-plastic similar to fine sand and may be vulnerable to
liquefaction during earthquakes. They described the procedure of determining the liquefaction
resistance of pond ash based on shake-table tests. Zand et al. [5] gave the procedure of evaluating
liquefaction potential of the impounded fly ash using cyclic triaxial tests and found that fly ash is
susceptible to liquefaction during earthquakes. A challenging task is to improve these areas for
further construction of civil engineering structures like buildings, roads etc. Therefore, it is essential
to study the liquefaction behaviour of pond ash and an appropriate ground improvement technique to
improve its liquefaction strength. Gandhi and Dey [6] studied the improvement of fly ash by blasting
techniques and found that fly ash is densified to great extent in deeper depths. Boominathan and Hari
[7] used the soil reinforcement technique with randomly distributed fibres and mesh element and
concluded that addition of fibres/mesh elements increases significantly the liquefaction resistance of
fly ash at low relative density. Singh et al.[8] used the stone-sand columns to improve the
liquefaction resistance of pond ash using one dimensional shake table and found that there is
significant increase in liquefaction resistance of pond ash when improved with stone-sand columns at
3d c/c spacing of the columns(where d is diameter of stone-sand column). However, the studies on
liquefaction behaviour of pond ash improved with stone-sand column under various surcharge loads
are rarely available in the literature. This paper is an extension of Singh et al. [8] and it exclusively
deals with effects of surcharge loads on various liquefaction parameters of pond ash.
A number of vibration table studies for liquefaction are reported in the literature e.g. Florin and
Ivanov [9], Finn [10], DeAlba et al. [11], and Gupta [12]. However, these studies deal with sands.
The present study has significance, as the pond ash covers a large area, near thermal power plants in
India. In this study, tests have been conducted on a small shake table imparting one-dimensional
horizontal harmonic excitations to the pond ash samples improved with Stone-sand columns at
columns spacing of 4d c/c without and with different surcharge loads. The effects of surcharge loads
on liquefaction parameters of improved pond ash have been investigated in the present study.
2. PROPERTIES OF POND ASH
The pond ash used in the tests was collected from the site of Anpara-D thermal power plant,
Anpara, Uttar Pradesh, India. The index properties of the fly ash are given in Table 1. Further, the
grain size distribution curve of pond ash is shown in Fig. 1.

227
Table 1: Properties of Anpara Pond ash
S. No. Properties Value
1 Specific Gravity (G) 2.31
2 % Optimum Moisture Content (OMC) 22
3
Grain Size Distribution
(a) % of Particles Coarser than 4.75 mm 2
(b) % of Particles ( 4.75 mm ~ 0.075mm) 64
(c) % of fines (finer than 0.075mm) 34
4 Plasticity Index (PI) Non-Plastic
5 Maximum Void Ratio (emax) 1.31
6 Minimum Void Ratio (emin) 0.54
Fig. 1: Grain size distribution curve of pond ash
3. EXPERIMENTAL SET UP
The tests were performed on a simple but indigenously fabricated vibration table. The test bin
is a watertight tank 1.05 m long, 0.60 m wide and 0.60 m high in which sample of fly ash was
prepared. The details of the experimental set up are described below.
The tank is mounted on a horizontal shake table. The sides of the tank consist of a rigid mild
steel frame with 5 mm thick steel panels. The shake table consists of a rigid platform, which rests on
four wheels supported on four knife-edges. This is driven in horizontal direction by a 3 H.P. A.C.
motor through crank mechanism for changing rotary motion into translatory motion. The crank
mechanism consists of a device for changing the amplitude of motion through two eccentric shafts.
By changing the relative position of two shafts, the amplitude can be fixed as desired. The equipment
has facility to change the frequency of dynamic load using the pulleys of different diameters on the
driving shafts. The pore pressure measurements were performed with the help of glass tubes
piezometer. These were attached to the tank set up with rubber tubes. At the mouth of rubber tubes,
G.I (galvanized iron) pipes were connected, which go up to the center of tank i.e. the locations of
points for measuring pore water pressure. At the outlet of these G.I. pipes, porous stone wrapped
with filter papers were placed. A Schematic diagram of test set up is shown in Figure 2.
The shaking table can produce one-dimensional harmonic excitation of varying amplitude
(0.05 to 1g) and frequency up to 10 Hz. The measurement of the pore water pressure was conducted

228
at three different depths in the shake table tank. The locations of measuring points from base of the
tank are:
Bottom Point (B): 40 mm
Middle Point (M): 125 mm
Top Point (T): 200 mm
The total effective depth of the tank is 600 mm and the ash samples are filled for a height of 500 mm
from the bottom of tank.
Fig. 2: Liquefaction table used in the study
4. EXPERIMENTAL PROCEDURE
4.1 Preparation of Samples
All the tests were performed keeping the relative density of pond ash as 20 %. By trial the
following sequential procedure has been evolved for preparing the submerged sample of pond ash to
achieve the relative density to 20 %:
a) 175 liter of water was filled in the tank, which is sufficient to submerge the points where
piezometers are connected. It ensures the removal of air bubbles from G.I. pipes, rubber tubes and
piezometer tubes.
b) About 350 kg of pond ash was dropped through funnel keeping the tip of the funnel at a height of
25 cm above water surface.
c) The water with pond ash was left for 12 hours so that all the fines and coarse particles settle
properly in the tank. This procedure ensured the full saturation of the pond ash sample.
d) The surplus water on the top of the saturated pond ash sample was then carefully removed and its
weight was measured.
e) The height of the saturated pond ash sample in the tank was measured and the volume of the
space occupied by the sample in the tank was evaluated using plan dimensions of the tank.
f) Total weight of the pond ash plus the net amount of water divided by the volume of sample
obtained in step (e) gives the saturated unit weight of pond ash. Using this saturated unit weight
(γsat) and with the known value of specific gravity (G), the value of void ratio (e) can be obtained
using the following equation:

229
wsat
e
eG
γγ
+
+
=
1
(1)
Where γw is the unit weight of water.
g) Using the values of maximum voids ratio (emax) and minimum voids ratio (emin), Table 1, the
relative density (R.D.)
can be found from the following equation:
minmax
max
..
ee
ee
DR
−
−
= (2)
This procedure gave the relative density as 20 %
h) For the preparation of the pond ash sample with stone-sand columns, the mild steel hollow open-
ended pipes (750 mm long and 50 mm dia) were inserted at the desired locations. The bottom ends
of the pipes were plugged with detachable wooden cones while inserting in the tank. These pipes
were filled with the mixture of stone chips (10 mm down size) and fine sand, in proportion of 2:1 by
weight and mixed thoroughly in a mixer, in five layers. Each layer was given 25 blows of the 2.5 kg
rammer dropped from a height of 450 mm.
After each layer was compacted, the pipe was withdrawn by the same amount. Thus finally
whole pipe was withdrawn. The center-to-center spacing between pipes was kept as 4d. Further, it
was observed that the inclusion of stone-sand columns increases the submerged unit weight of pond
ash sample by 24 % .
Fig. 3: Location of stone-sand columns in the shake table tank (dimensions are in mm)
200
200
200 200200200
1050
600
15 Stone–Sand Columns φφφφ= 50

230
4.2 Test Procedure
As per IS: 1893[13], the site from where the pond ash was procured falls under seismic zone
III and the expected peak ground acceleration at the site is 0.16g. The predominant frequencies in
many past earthquakes in alluvial deposits have been observed to be of the order of 2 to 5 cycles per
second. Therefore, to be on the conservative side, the tests were performed at an acceleration of 0.3g
and at a frequency of 5 Hz.
Before imparting the shaking, the values of static pore water pressures in all the three piezometer
were recorded. Then shaking was imparted for 60 seconds. The pore water pressures were recorded
at the interval of 10 seconds till these get completely dissipated. This procedure was adopted in
studying the liquefaction behavior of pond ash, pond ash with stone-sand columns and Pond ash
improved with stone-sand columns under various surcharge loads.
The tests were conducted for following four cases:.
(a) Pond Ash with Stone-Sand Columns @4d c/c spacing with total 15 columns.
(b) Pond Ash with Stone-Sand Columns @4d c/c spacing with total 15 columns under the surcharge
loads of 2.77kN/m2
.
(c) Pond Ash with Stone-Sand Columns @4d c/c spacing with total 15 columns under the surcharge
loads of 7.62kN/m2
.
(d) PondAsh withStone-Sand Columns @4d c/cspacing with total 15 columns under the surcharge
loads of 12kN/m2.
Where d is the diameter of the stone columns. These spacing of stone-sand
columns were selected keeping in view the spacing of actual stone columns used in the field. In each
test, variation in excess pore water pressure with time was recorded using three piezometer tubes and
stopwatch. Since the actual earthquake duration normally does not exceed 60 seconds, the results in
this paper have been presented for shaking of 60 seconds.
5. TEST RESULTS
The test results for above four cases are shown in Figs. 4 (a-d), respectively. In these figures,
variation in excess pore water pressure with time is shown at the location of all the three points in the
tank. The general trend of results is similar in all the four cases. The pore water pressure rises
significantly even after the shaking was stopped. This is attributed to the fact that it takes some time
in pond ash to develop pore water pressure after shaking starts and rise of pore water pressure
continues quite some time under free vibration even when shaking is stopped. In second stage, the
pore water pressure remained constant for some brief duration before dissipation starts, this time lag
is attributed to free vibration phase of shaking, which ultimately converts in deceleration. Finally,
dissipation led pore-water pressure to zero, which occurred after more than an hour. Thus dissipation
of pore water pressure took very long time in pond ash as compared to time taken in sands. It was
due to the fact that the pond ash contained significant amount of fine particles (34 %).
It can be observed that in all the four cases, the pore water pressure is the maximum at bottom point
and minimum at top point. This behavior was expected, as the effective overburden pressure is the
maximum in bottom and decreased upward. However, the maximum value of pore water pressure
ratio rumax (defined later) was almost the same at all the three points. Also in all four cases the
maximum pore water pressure first reached at bottom point and then proceeds upwards.

231
Fig. 4 (a): Pond ash with stone-sand columns @4d c/c spacing with total 15 columns
Fig. 4(b): Pond ash with stone-sand columns under the surcharge loads of 2.77kN/m2

232
Fig. 4(c): Pond ash with stone-sand columns under the surcharge loads of 7.62kN/m2
Fig. 4(c): Pond Ash with Stone-Sand Columns under the surcharge loads of 12 kN/m2

233
Table 2: Values of Excess Pore Water Pressures of Improved Pond Ash
at Different Surcharge Loads
Surcharge
Loads
(kN/m2
)
U60
(kN/m2
)
ru60 Umax
(kN/m2
)
rumax Effective over burden
Pressure,σ'0, (kN/m2
)
0
0.70 (B)
0.25 (M)
0.70 (T)
0.19 (B)
0.08 (M)
0.089 (T)
2.01(B)
1.46(M)
1.31(T)
0.55 (B)
0.48 (M)
0.53 (T)
σ'v (B) = 3.62
σ'v (M) = 3.04
σ'v (T ) = 2.47
2.77
0.41 (B)
0.24 (M)
0.22 (T)
0.064 (B)
0.041(M)
0.042 (T)
2.20 (B)
1.72 (M)
1.49 (T)
0.34 (B)
0.30 (M)
0.28 (T)
σ'v (B) = 6.39
σ'v (M) = 5.81
σ'v (T) = 5.24
7.62
0.32 (B)
0.22 (M)
0.21 (T)
0.028 (B)
0.021(M)
0.021 (T)
1.95 (B)
1.51(M)
1.33 (T)
0.173 (B)
0.142(M)
0.132 (T)
σ'v (B )=11.24
σ'v (M) =10.66
σ'v (T) = 10.09
12.00
0.25 (B)
0.20 (M)
0.18 (T)
0.016 (B)
0.013(M)
0.012 (T)
1.79 (B)
1.49 (M)
1.26 (T)
0.115 (B)
0.099(M)
0.087 (T)
σ'v (B) = 15.62
σ'v (M) =15.04
σ’v (T) = 14.47
Table 3: Values of Time Parameters (t1, t2 and t3) for Improved Pond Ash
at Different Surcharge Loads
Surcharge Loads
(kN/m2
)
t1 (s) t2 (s) t3 (s)
B M T B M T B M T
0 340 550 620 90 120 180 4970 4730 4520
2.77 370 560 620 90 120 180 3900 3680 3560
7.62 420 580 630 110 130 190 2710 2530 2420
12.00 450 620 660 120 150 200 2490 2290 2200
6. EFFECTS OF SURCHARGE LOADS
It was observed that decrease in rumax value was not significant when the pond ash was
strengthened with stone-sand columns at 4d c/c spacing. Therefore, the effect of surcharge loads on
liquefaction parameters of pond ash improved with stone-sand columns at 4d c/c spacing has been
investigated. The values of rise in pore water at all the three points (Bottom – B, Middle –M, Top –
T) for all the four cases have been presented in Table 2. Here U represents the pore water pressure at
the end of shaking while Umax represents the maximum pore water pressure before dissipation starts.
From Table 2, it can be observed that Umax for bottom point is greater than that of middle point and
so on. The last column of Table 2 represents the effective overburden pressure at the three points in
the tank, which has been used in computation of maximum pore water pressure ratio rumax defined
as follows:
rumax= Umax / σv’ (.3)
The variations of excess pore water pressure with respect to time have been presented in Fig.
4(a-d) for improved pond ash without and with three surcharge loads i.e. 2.77 kN/m2
, 7.62 kN/m2
and 12.00 kN/m2
. Further the liquefaction parameters were computed from the plots of Fig. 4(a-d)

234
and are given in Tables 2 and Table 3. The level of acceleration and exciting frequency considered
were the same as in case of improved pond ash without surcharge loads (i.e. 0.3 g and 5 Hz). The
interpretation of tests results are discussed in the following sections.
6.1 Variation of rumax with Surcharge
It can be observed from Table 2 that the value of rumax decreases when the surcharge loads
are applied on the pond ash. At higher value of surcharge load, the magnitude of developed
maximum pore water pressure decreases (Gupta 1977 also observed the same trend for sand) and
there is substantial reduction in rumax corresponding to largest surcharge load. e.g. the value of rumax at
bottom point for zero surcharge loads is 0.55. When the surcharge load is increased from 0 to 2.77,
7.62 and 12 kN/m2
the values of rumax decreases from 0.55 to 0.34, 0.17 and 0.11 respectively.
Similarly for middle and top points also the same decreasing trend of rumax is observed with respect to
surcharge loads. Since the decrease in maximum pore water pressure ratio indicates an increase in
liquefaction resistance, thus the liquefaction resistance of pond ash improved with stone-sand
columns (at 4d c/c spacing), is improved due to placement of surcharge loads. The similar
observation was made by Gupta (1977), though with respect to sand. This may be because of the fact
that the status of vibration of pond ash changes as it is no free field motion and the effective
overburden pressure within the pond ash sample increases due to application of surcharge loads. This
is why the value of rumax decreases significantly due to surcharge loads.
6.2 Variation of Time Parameters with Surcharge
It is observed from Table 3 that there is a change in the values of time parameters (t1, t2 and
t3) due to surcharge loads. The maximum pore water pressure built up time (t1) and the maximum
pore water pressure stay time (t2) increase whereas pore water pressure dissipation time (t3)
decreases. The similar trend was found by Gupta (1977) with sand. This is attributed to the fact that
surcharge load increases the effective overburden pressure at all the three points within the pond ash
and test sample becomes more stiff which inhibits the development of excess pore water pressure
during shaking and exhibits the dissipation of pore water pressure when shaking is stopped. That is
why the built up time (t1) increases and dissipation time (t3) decreases when surcharge loads are
applied over the test sample. Although there is a marginal increase in maximum pore pressure stay
time (t2) but its effect will not be significant when the maximum pore water pressure ratio (rumax) is
less than 0.5
7. CONCLUSIONS
There is a significant effect of surcharge loads on the liquefaction behaviour of pond ash
improved with stone-sand columns installed at 4d c/c spacing. The value of maximum pore water
pressure ratio decreases when the surcharge loads are applied on the pond ash. At higher value of
surcharge load, the value of maximum pore water pressure ratio decreases and there is substantial
reduction in rumax corresponding to largest surcharge load. e.g. the value of rumax at bottom point for
zero surcharge loads is 0.55. When the surcharge load is increased from 0 to 2.77, 7.62 and 12 kN/m2
the values of rumax decreases from 0.55 to 0.34, 0.17 and 0.11 respectively.There is a change in the
values of time parameters (t1, t2 and t3) due to surcharge loads. The maximum pore water pressure
built up time (t1) and the maximum pore water pressure stay time (t2) increase whereas pore water
pressure dissipation time (t3) decreases. The effect of increase in maximum pore water pressure stay
time (t2) will be negligible for the value of rumax less than 0.5 and hence liquefaction resistance of
improved pond ash increases due to application of surcharge loads.

235
8. ACKNOWLEDGEMENT
Author is thankful to Prof. D.K. Pal, Prof. Swami Saran and Dr. B.K. Maheshwari of
Earthquake Engineering Department, IIT Roorkee, for their valuable supports, encouragement and
guidance for conducting the tests in the Soil Dynamics Laboratory. The help and support extended
by Sri Subodh Jain, Laboratory Assistant in preparing the test set up is gratefully acknowledged.
REFERENCES
[1] P.S. Toth, H.T Chan and Cragg, Coal ash as structural fill with special reference to Ontario
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[4] H.P. Singh, B.K.Maheshwari, S. Saran and D.K Paul, Evaluation of liquefaction potential of
pond ash, Proc. of 14th World Conference on Earthquake Engineering, Beijing, China,
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[9] V.A. Florin and P.A. Ivanov, Liquefaction of saturated sandy soils, Proc. of Fifth Int. Conf.
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[12] M.K. Gupta, Liquefaction of Sands during Earthquakes, doctoral dissertation, University of
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[14] Machhindra S.Purkar and Sunil Y. Kute, “Numerical Modeling of Reinforced Soil Segmental
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