ABSTRACT: Fly ash is a byproduct of the combustion of pulver .".... ized coal in thermal power plants. Energy and environmental con siderations over the coming ...
TECHNICAL NOTE N. S. Pandian,l K. C. Krishna, 1 and A. Sridharan 1
California Bearing Ratio Behavior of Soil/Fly Ash Mixtures
REFERENCE: Pandian, N. S., Krishna. K. C, and Sridharan, A., "California Bearing Ratio Behavior of SoillFly Ash Mixtures,"
soil on California Bearing Ratio (CBR) values is studied. Black cotton (BC) soil is expansive in nature due to the presence of clay with an expanding lattice structure. This undergoes volume change with seasonal variation in moisture content, resulting in swelling and shrinkage of the soil, causing differential movements leading to severe damage to the structures. Hence BC soil alone cannot be used in pavements as a sub-base material.
Journal of Testing and Evaluation. JTEVA, VoL 29, No.2, March "001, pp. 220-226.
ABSTRACT: Fly ash is a byproduct of the combustion of pulver .".... ized coal in thermal power plants. Energy and environmental con
siderations over the coming years point to greater use of coal and
hence a further increase in the quantity of fly ash. Gcotechnical and
chemical characterization of fly ash and its interaction behavior
with soil is i1kely to provide viable solutions for its large-scale dis
posal and utilization. Concerted efforts have been initiatcd in the
laboratory to understand the effect of fly ash addition on the Cali
fornia Bearing Ralio (CBR) values of black cotton soil both for un
soaked and soaked conditions. The study indicates thaI CBR values
of black cotton soWfly ash mixtures increase up to an optimum fly
ash content wherein fly ash acts as a coarser material. The CBR val
ues decrease beyond this optimum fly ash contcnt and again in
crease to an optimum value wherein black cotton soil acts as a
binder.
Materials and Methods Material
In the present investigation. fly ash (FA) from the Raichur ther mal plant of Karnataka Power Corporation and black cotton soil (BC soil) from Davanagere. Karnataka State. were used. Air-dried black cotlon soil was used in this study. Tables 1 and 2 summarize the physical properties and chemical composition of fly ash and black cOllon soil samples, respectively. The panicle size distribu tion curves of these materials are shown in Fig. I.
KEYWORDS: California Bearing Ratio, compaction, fly ash, black conon soil
Methods
For BC soil/fly ash mixes, compaction tests were conducted us ing the standard Proctor method as per IS:2720 (Part 7)--1980 [7] (ASTM Test Method for Laboratory Compaction Characteristics of Soil Using Standard Effort (D 698·91» . . The CBR tests were conducted as per IS: 2720 (Part 16)--1979 [8] (ASTM Test Method for CBR (California Bearing Ratio) of Laboratory·.Compacted Soils (D 1883-84)). A known quantity of BC soil and fly ash was mixed with water content corresponding to 95% maximum dry density. The mix was kept for moisture equilibration for 24 h in sealed polyethylene cov ers and stored in desiccators. Then the samples were statically com pacted to 95 % of the standard Proctor maximum dry density in the CBRmold. The diameter of the mold is 150 mm and the height is 175 mm. A circular metal spacer disk 148 mm in diameter and 47.7 mm in height is used, and a metal penetration plunger, SO mm in diameter and not less than 100 mm long, is used for penetrating the specimen in the mold. After placing the CBR mold in position under a sur charge pressure of 0.05 kg/cm2, load was applied to the penetration plunger so that the penetration rate is 1.25 mmlmin. After the test under unsoaked conditions, the plunger and surcharge weights were removed and average water content was detennined as per IS: 2720 (Part 2)-1973 [9] (ASTM Test Method for Laboratory De
The necessity of providing electrical power to the rapidly grow in!' industrial as well as agricultural sectors has resulted in the set ti lp of a number of coal-based thermal power stations in India. At present, about 100 miHiop metric tons of fly ash are being pro duced annUally. This large quantity of fly ash, apart from causing environmental pollution, is also creating problems associated with its disposal and utilization and is a threat to public heailh. Only a very small fraction of fly ash generated in India, about 5 to 6%, is used in gainful applications like manufacture of bricks, pozzolana cement, and other products. Fly ash or fly ash/soil mixtures can be used as embankment material, backfill material, and as sub-base materials. Fly ash has high shear strength (frictional angle up to 3r or more [ID and a low specific gravity as low as 1.64 [2], which could result in significant engineering benefits in addition to facil itating mass disposal of fly ash. It has been reponed [3-6] that fly ash can be used in embankments and fills. In this investigation the effect of different percentages of fly ash addition to black cotton
Manuscript received 01131/00; accepted for publication 11120/00. I Professor, research scholar, and professor, respectively, Department of Civil Engineering. indian Institute of Science. Bangalore 560 012.
© 2001 by the American Society for Testing and Materials 220
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11
PANDIAN ET AL. ON FLY ASH MIXTURES
TABLE I-Physical properties offly ash and black corton soiL Propeny
Fly Ash
Black Cotton Soil
Specific Gravity Particle Size Distribution Fine sand size fraction, % Silt size fraction, % Clay size fraction. % Atterberg Limits Liquid limit. % Plastic limit, % Shrinkage limit, % Plasticity index, % Compaction Characteristics Optimum moisture content, % Maximum dry density, gmlcm3
2.03
2.74
81.33 17.67
24.05 30.07 45.88
LO 60 Non Plastic
56 23.0 10.30 33
46.00
27.76 1.507
0.895
TABLE 2--Chemical composition offly ash and black cotton soil used. Constituents
Fly Ash, % by weight
Black Cotton Soil, % by weight
Si0 2 Ah 0 3 Ti0 2 Fe203 MnO MgO CaO K 20 Na20 L.O.I."·
61.1 28.0 1.3 4.2 bd* 0.8 1.7 0.73 0.18 1.4
48.4 20.0 0.9 12.7 0.4 0.5 0.5 0.15 0.13
• Below detection.
*'" Loss on ignition.
Silt Size
Clay
Size
termination of Water (Moisture) Content of Soil and Rock (D 2216-92»). For other samples, standard procedure of soaking using
a surcharge weight was followed. After four days of soaking, each sample was allowed to drain under the surcharge weight for 15 min prior to test. The test was conducted as explained earlier. Experimental Program on Be SoiVFly Ash Mixtures The standard Proctor compaction curves of fly ash and BC soil are shown in Fig. 2. From Fig. 2 it is observed that fly ash shows lower maximum dry density and higher OMC. A relatively lower value of maximum dry density may be due to low specific gravity and relatively uniform grain size distribution [2]. Different percentages of fly ash varying between 0 to 100% in steps of 10%, based on the dry weight of BC soii., were mixed with BC soil. The standard Proctor maximum dry density and OMC val ues of these BC soil/fly ash mixes were detennined from the com paction curves of these mixes. Water content values corresponding to 95% of maximum dry density on the dry of OMC side were ob tained. Samples statically compacted at 95% of nlaximum dry den sity and their corresponding water content were tested for CBR. For each BC soiVtly a"s/} combination, six samples were prepared in as-compacted state, three samples were tested immediately un der unsoaked condition, and the other three samples were soaked' for four days and tested. Results and Discussions Figures 3 and 4 show the stress Vs penetration curves of black cotton soillt1y ash mixes for the unsoaked condition. Figures 6 and 7 plot the same for the soaked condition. (In every figure the plots for fly ash ancl BC soil alone are also given to serve as a compari son.) Figures 5 and 8 show the variation of CBR values with fly ash content for unsoaked and soaked conditions, respectively. For each
Sand Size 1------r--------r---iGravel Coarse Size
100
.....
0-0
flyash
0-0
Be soil
80
OJ
C
;;::
C
60
OJ
~
OJ
0..
40
20
o 0.001
0.01
221
0.1
Particle size (mm) FIG. I-Particle size distribuTiun cun'es offly ash and black cotton soil.
10
1.6 '(oMe, :
1.507 gm/CC o-c BC soil o-oOyash
OMC:27.76%
1.4
u
()
E
0)
..........
1
'( OMax
=
0.895 gmJcc
OMC=46%
0.8
30
20
40
50
Water content (%) FlG. 2-Compacrion characrerisrics offlv ash and black c(Juon soil.
20 .-. Be soil
fiyash
1>-1> lO%FA+90%BC soil a-A 20%FA+80%BC soil
unsoaked condition
0-0
15 .-...
(\.l
E
()
F
.........
10
(/) (/)
