Proceedings of the First Makassar International Conference on Civil Engineering (MICCE2010), March 9-10, 2010, ISBN 978-602-95227-0-9
MIXING OF RICE HUSK ASH (RHA) AND LIME FOR PEAT SOIL STABILIZATION Faisal Estu Yulianto1, Noor Endah Mochtar2.
ABSTRACT: Peat soil is known as a very soft soil with high organic content (≥ 75%). It has unfavorable behaviour, that is, low bearing capacity and very high compressibility. Many kind of soil improvement methods, such as: preloading with surcharge, sand column, and corduroy have been adopted to improve its behaviour. Those methods, however, are not environmentally friendly bacause they need a lot of irreversible materials. Because of that stabilization method using lime had been developed to improve the behaviour of peat soil. This paper will present the effectiveness of using rice husk ash (RHA) as a pozolon to enhance the lime for stabilizer material of peat soil. Besides, the effect of curing period to the behavior of stabilized peat soil is also presented. The mixture for stabilizer material is 30% lime and 70% RHA; the percentage of stabilizer chosen for peat soil stabilization is 10%. The stabilized peat soil shows a good improvement on its physical and engineering behavior. The values of wet unit weight and specific gravity increase with the increase of curing period; the water content and void ratio decrease with the increase of curing period. The increment of curing period is also altered its engineering behavior that is increasing the soil strength and reducing its compressibility. Keywords: Lime, peat soil, rice husk ash (RHA), stabilization.
INTRODUCTION Indonesian peat soil is a tropical peat formed as a product of decomposition process of many kinds of plants such as grasses, mangrove, screw pine, and others (Van De Meene, 1984). According to Wijaya, Adhi et.al. (1992), Indonesian peat covered about 15,96 million hectares spread in Sumatra, Kalimantan and Papua. Peat soil is known as a very soft soil with high organic content (≥ 75%). It has unfavorable behaviour, that is, low bearing capacity and very high compressibility. Many kind of soil improvement methods, such as: preloading with surcharge, sand column, and corduroy have been adopted to improve its behaviour. Those methods, however, are not environmentally friendly bacause they need a lot of irreversible materials. Due to that reason, stabilization method using lime had been developed to improve the behaviour of peat soil. Unfortunately, this method has not given satisfied result due to no silica content in peat soil. Based on that reason, Mochtar, N. et al (2009) carried out research on rice husk ash (RHA) as a pozolon mixed with lime for stabilizer material. This paper will present the results of study performed in the laboratory, those are: 1
Graduate Student in Geotechnical Engineering of Civil Engineering Department, Institut Teknologi Sepuluh Nopember, ITS, Surabaya, e mail :
[email protected], Indonesia
2
Professor of Civil Engineering Department, Institut Teknologi Sepuluh Nopember, ITS, Surabaya, e mail :
[email protected], Indonesia
1. The effect of stabilizer material, rice husk ash (RHA) and lime mixture, to the behavior of stabilized peat soil. 2. The influence of curing period to the physical and engineering properties of stabilized peat soil.
STABILIZER MATERIALS The lime was a chemical waste of fertilizer industry ZA (Ammonium sulphate), taken from PT. Petrokimia Gresik, Indonesia. The rice husk ash (RHA) used as a source of silica in this study was taken from MojosariMojokerto, Indonesia. The silica content of RHA was analysis in Chemistry laboratory at ITS Surabaya, Indonesia. Type of mineral contained in RHA was defined by performing X-Ray Diffraction (XRD) test at Research Center of ITS, Indonesia. From the test result received from PT. Petrokimia Gresik, Indonesia, it is known that the chemical constituent of lime is dominated by CaCO3 that is 71.37%. Rice husk ash (RHA) contains of silica, that is 77%. From the image of RHA given in Figure 1, it is understood that RHA contain of feldspar, mica, and kaolinite. It means that the stabilizer materials, lime and RHA, are the ones that fulfill the qualification for stabilized material of peat soil. Mochtar, N. et al (2009) figuring out that the percentage of stabilizer material which produce an
optimal mixture is 30% lime and 70% RHA. Besides, her study found that using of 10% stabilizer give satisfactory improvement of behavior of stabilized peat soil.
Table 1. Physical Properties of The Initial Condition of Peat Soil Studied Peat Studied Parameters
Peat Studied
by Other Researchers
Specific Gravity Void Ratio Wet Unit Weight
Image of X-Ray Diffraction (XRD) test of rice husk ash (RHA)
PEAT SOIL STUDIED Physical Properties of Peat Soil Studied Peat soil studied herein was taken from Bareng Bengkel, Palangkaraya, Central of Kalimantan, Indonesia. Soil samples were obtained in disturbed and undisturbed conditions. The disturbed sample was utilized for soil stabilization test; the undisturbed one was used to determine the initial properties of the peat soil studied. Beside laboratory tests, field tests (sand cone, pH meter, and vane shear) were also performed in order to find out the field unit weight, pH, and shear strength of the peat soil. All peat soil parameters determined in the field and laboratory are presented in Table 1. From those results, it is known that the values of all peat soil parameters are about the same or still in the range of the ones found by previous researchers (Hanrahan 1954, Lea 1959, MacFarlane and Radforth 1965, MacFarlane 1969, Mochtar, NE. et al. 1991, 1998, 1999, 2000, and Pasmar 2000). The permeability coefficient of peat soil studied is 6.38 x 10-3 cm/min; it means that it is still in the range of 10-3 – 10-6 cm/min (Colley, 1950 dan Miyakawa, 1960). Based on the data given in Table 1 above, that is: • Organic content = 97 % > 75% • Fiber content = 52.1% > 33% and < 67% • Ash content = 3% < 5% • pH = 3.1 < 4.5 peat soil studied can be classified as “Peat soil (Hemic) with Low Ash Content and High Acidity” (Standard Classification of Peat Samples by laboratory Testing ASTM D4427-84 Reapproved 1985).
t/m3
1.49
1.4-1.7
9.7
6.89-11.09
1.044
0.9-1.25
pH
-
3.1
3-7
Water Content
%
649.78
450-1500
Organic Content
%
97.0
62.5 - 98
Ash Content
%
3.0
2 – 37.5
Fiber Content
%
52.1
39.5-61.3
cm/min
6.38 x 10-3
10-3 – 10-6.
Permeability Coef.
Figure 1.
-
Engineering Properties of Peat Soil Studied In order to determine the shear strength of peat soil, field and laboratory tests were performed. In the field, vane shear test was performed in three different places. The results varies, those are 6 kPa, 10 kPa and 13 kPa; the average value of field shear strength is 9.67 kPa. This variation is caused by inhomogeneous condition of peat soil in the field in which the size and density of fibers are not the same from one place to the others. Direct shear test carried out in the laboratory was pointed out to check the change of shear strength after the soil loaded of 50 kPa (load equal to embankment in the field). From the test result, it is found that the shear strength value of peat soil becomes 26,85 kPa. It shows that load of 50 kPa causes the increase of the shear strength value of the soil. This condition appropriates with the one found by Landva (1982) and Hanrahan (1954). As mention by Lea & Brawner (1959) that consolidation method introduced by Terzaghi (1925) cannot be applied for peat soil. Because of that consolidation test adopted in the laboratory is the one developed by Gibson & Lo (1961). Test was done with one step loading of 50 kPa and it was applied for 10 days (14400 minutes). The results plotted in Figure 2 shows that compression behavior of soil studied is resemble with the one studied by Dhowian and Edil (1979 and 1980). The compression curve has four compression components, immediate compression (εi), primary compression (εp), secondary compression (εs), and tertiary compression (εt). From the curve (Figure 2), it is seen that the primary compression takes place in very short time, about 4 minutes; the secondary compression, however, takes place in very long time (about 3000 minutes).
The influence of the curing period to the specific gravity and unit weight of the stabilized peat soil can be seen in Figures 3 and 4, respectively. Those two parameters increase with the increase of the curing period. The curing period also affects the water content; it is decreasing with the increase of curing period (Figure5). Void ratio, however, is not like water content;
εi
εp
εs
2.200
Specific Gravity
2.000
εt
1.800
1.600
Figure 2. Compression curve of peat soil studied 1.400 1
Un it Weigh t (g r/cm 3)
1.125
20
30
Specific Gravity
-
1.44
1.68
2.12
2.16
Void Ratio
-
5.15
5.13
6.31
6.141
1.107
1.112
1.116
1.120
Unit Weight
t/m3
pH
-
5.9
6.0
6.27
Water Content
%
372.51
305.88
284.87
270.26
Organic Content
%
67.02
64.24
54.59
54.97
Ash Content
%
32.98
35.76
45.41
45.43
Source : primary data
1.110
1.105
10 20 Curing Period (days)
30
Figure 4. The effect of curing period to unit weight of stabilized peat soil
400.000
Water Content
10
1.115
1
Curing Period (days) 1
1.120
1.100
Table 2. Physical Properties of Peat Soil Stabilized with 10% Stabilizer and Subjected to Curing Parameters
30
Figure 3. The effect of curing period to specific gravity of stabilized peat soil
STABILIZED PEAT SOIL The Effect of Stabilizer and Curing Period to The Physical Properties of Stabilized Peat Soil In this study, peat soil stabilized with 10% stabilizer was prepared and subjected to curing for 1, 10, 20, and 30 days. Those prepared samples, then tested to determine the physical parameters of the stabilized peat soil. The results are tabulated in Table 2 and plotted in Figures 3 until 7. Compared to physical parameters of the un-stabilized peat soil in Table 1, specific gravity and wet unit weight of the stabilized peat soil are higher; the water content and void ratio are lower. The acidity is reduced (becomes neutral) and the organic content is also decreased. Improvements of those physical parameters of the stabilized peat soil are caused by the presence of CaCO3 in lime, and silica and other minerals in rice husk ash (RHA).
10 20 Curing Period (days)
350.000
300.000
250.000 1
10 20 Curing Period (days)
30
Figure 5. The effect of curing period to water content of stabilized peat soil
it increases when the curing period is longer than 10 days (Figure 6). It may be due to the difficulties that occurs during mixing process between stabilizer and peat soil; consequently, it produces inhomogeneous stabilized peat soil. Therefore, the samples taken from different places show inconsistency tendency of void ratio. The acidity of stabilized peat soil is also influenced by the curing period in which it decreases with time (Table 2); it means that the increment of curing period is able to make the stabilized peat soil becomes neutral. The organic content of the stabilized peat soil has similar behavior like its acidity, it decreases with the increase of curing period, as seen in Figure 7.
6.400 6.200
V o id ra tio
6.000 5.800 5.600 5.400 5.200 5.000 1
10 20 Curing Period (days)
30
The Influence of Stabilizer and Curing Period to The Engineering Properties of Stabilized Peat Soil Shear Strength Engineering properties determined in this study are shear strength and compression behavior of the stabilized soil. Test carried out to determine field shear strength was vane shear test. In the laboratory, the shear strength was defined by performing the direct shear test. The average value of field shear strength is 8.7 kPa; the one determined in the laboratory is 26.85 kPa if the soil is compressed by the vertical stress of 50 kPa (appropriates with field embankment load). It shows that soil compression is able to increases shear strength value. Shear strength of the stabilized soil can be seen in Figure 8; it increases with the increase of the curing period whenever curing period is more than 10 days. For curing period of 1 day, the opposite condition happened that is the shear strength is the highest. It may be caused by the presence of fibers which are less decomposed. When curing period increases become 10 days, degree of decomposition is higher and produce more gasses in the stabilized soil; as a result, the soil strength becomes lower. When the curing period higher than 10 days, gas produced during decomposition process disappears and the soil becomes denser. Consequently, the soil shear strength increases with the increase of curing period as shown in Figure 8.
Figure 6. The effect of curing period to void ratio of stabilized peat soil 41.00 39.00 Shear Strength (kPa)
70.000
O rg an ic C o n ten t
65.000
60.000
37.00 35.00 33.00 31.00 29.00 27.00
σ = 50 kPa
25.00
55.000
1
10
20
30
Curing period (days)
50.000 1
10 20 Curing Period (days)
30
Figure 7. The effect of curing period to organic content of stabilized peat soil
Figure 8. The effect of curing period to shear strength of stabilized peat soil As mention above that shear strength of the initial condition of undisturbed sample is 26.85 kPa. After the soil stabilized with 10% stabilizer and curing in 30 days the shear strength of the soil becomes 34.6 kPa. It means that due to 10% stabilizer and 30 days of curing period, the shear strength increases about 25% from the initial conditon (undisturbed peat soil).
Compression Properties Compression behavior of the soil stabilized with 10% stabilizer materials is given in Figure 9; it also shows the influence of curing period to its compression properties. These curves were obtained from consolidation test performed in laboratory with one step loading of 50 kPa and it was applied for 10 days ( 14400 minutes). All of the curves in Figure 9 show the same trend with the un-
3.
4.
5.
The acidity and organic content of the stabilized peat soil decrease so that it becomes neutral peat soil with low organic content. Curing period very much improve the stabilized peat soil properties; the increment of curing period causes the increase of specific gravity and wet unit weight; and it decreases water content, organic content, and acidity level. 10% stabilizer and 30 days of curing period is able to increase the shear strength about 25% from the initial conditon (undisturbed peat soil) and significantly decrease the total compression.
ACKNOWLEDGEMENTS This research is supported by DGHE (Directorate General of Higher Education). It is allocated as a Research for National Priority of Batch II. This research has to include minimum of 2 (two) junior lecturers and 1 (one) graduate student. I am the graduate student of Prof. Noor Endah Mochtar who is in this research team. As a Graduate student in Department of Civil Engineering of ITS, I am really grateful to DGHE because this program can speed up my research and my study completion. Figure 9. Compression curves of peat soil stabilized with 10% stabilizer at different curing period stabilized peat soil in which they have 4 (four) compression. It means that curing period of 30 days does not enough time to decompose all fibers in the stabilized peat soil; as a result, macro pores and micro pores are still exist in the soil, that means, the compression behavior of fiber peat is still occurred. The curing period also affects the compression of the stabilized peat soil as seen in Figure 9. The curves show similar tendency that is the longer the curing period the smaller the compression.
CONCLUSION From the data and their analysis given above, it can be concluded that: 1. Peat soil studied can be classified as “ Peat soil Hemic with Low Ash content and High Acidity. 2. The use of 10% stabilizer material (30% lime + 70% rice husk ash) is able to improve physical and engineering characteristics of peat soil, those are: specific gravity and wet unit weight increase; water content and void ratio decrease; soil shear strength increases and its compression decreases.
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