Black Cotton Soil Modification using Sea Salt Rajesh Prasad Shukla
Former undergraduate Student Government Engineering College, Rewa, (M.P.), India e-mail:
[email protected]
Niraj Singh Parihar
Former undergraduate Student Government Engineering College, Rewa, (M.P.), India e-mail:
[email protected]
Rajendra Prasad Tiwari
Principal Government Engineering College, Nowgong (M.P.), India
[email protected]
Bal Krishna Agrawal
Head of Department Department of Civil Engineering, Government Engineering College, Rewa, (M.P.), India
ABSTRACT Black Soil is a most common soil in India, which covers more than 20 percentage land area of India and is also known as ‘regur’. This soil is spread over Madhyapradesh, Gujarat, Andhrapradesh, Tamil Nadu, Maharashtra, Karnataka and other parts of India as well. Presence of montmorillonite clay minerals is mainly responsible for swelling and shrinkage characteristic of black cotton soil. The swelling and shrinkage characteristics of black cotton soil pose a serious threat to foundations and structures constructed on them. Light weight and small structures are generally more susceptible to damage due to their less amount of overburden pressure. In this study, experiments have been conducted to find out the effect of addition of sea salt on the behaviour of black cotton soil. From the laboratory test results it is observed that the addition of salt in black cotton soil significantly reduces the liquid limit, plastic index, swelling and plasticity index of soil with a minimum cost. Optimum moisture content and dry density of soil has also found to be changed with addition of sea salt.
KEYWORDS: Black cotton soil, Swelling, Sea salt, Clay, Soil modification.
INTRODUCTION The swelling soil used for study is popularly known as black cotton soil, the name black cotton has been given due to its black colour and its favourability for production of cotton. This soil covers about 73 million hectare area of India which is more than 22% of total geographical area of India [1]. It is widely found in Madhyapradesh, Gujarat, Andhrapradesh, Tamil Nadu, Maharashtra, Karnataka and other parts of India as well. These soils are not only widespread in - 8807 -
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India but also at many other parts of the world. Black Cotton Soils consist of a variety of minerals such as montmorillonite, kaolinite and illite. Its expansive characteristic mainly depends on content of montmorillonite and illite minerals. Its chemical composition consists of silica dioxide, aluminium oxide, calcium carbonate and iron oxide [2]. These soils exhibit a very high affinity towards water, which may be attributed to their small grain size and positive ions present in the particles [3]. Black cotton soil undergoes a drastic volume change when it comes in contact with excess water content. These soils swell and shrink with increase and decrease in water content respectively [4]. In the rainy season, the expansive clay minerals attract water molecules and water enters into the inter-plane space between the minerals that leads to an increase in volume. The loss of water from soil minerals results in the reduction in volume which causes the shrinkage of soil and desiccation cracks [5-7]. The location of free water table and its seasonal variation affects the swelling potential of the soil to a large extent. Small structures with shallow foundation and light weight are generally more susceptible to damages and cracks due to less overburden pressure. Uneven swelling and contraction do cause reduction in serviceability, emergence of hairline cracks and many a times differential settlements with severe cracks which can even lead to total collapse. The performances of the structures as well as foundations on expansive soils generally depend on the engineering properties, type of loading, composition of soil and climatic conditions. The most important engineering properties of expansive soils include water content, shear strength, dry density and other index properties of soils. These all properties change with change in constituents of soil and water content. Literature is full of studies and discussion about problems and damages posed by the black cotton soil. A lot of research has been done on improvement of expansive soils with some good and useful come outs, but some results are contradicting and show that the plasticity of soil and characteristics of water has a crucial role in soil behaviour [8-11]. There are lots of solutions available to reduce or avoid the losses due to hazardous behaviour of soil such as, to remove the black cotton soil up to a particular depth, increase the backfill pressure, soil mixing, and provision of proper drainage, keeping the water table constant and altering the soil properties with some additives. After certain depth, removal of soil may not be an option and water table variation may not be fully controlled. Most of the time, addition of additives proves to be a good option to modify the composition of soil to improve the performance of soil. The black cotton soil causes loss of billions of dollars in the USA due to damage and has proven to be a big source of damage [3, 12, 13]. Swelling behaviour and effects of additives have been studied by many researchers [11, 14-19]. In this experimental study, the effect of salts on index properties of black cotton soil has been studied.
MATERIAL USED IN LABORATORY TEST A typical black cotton soil sample was collected from eastern part of Madhya Pradesh at a depth of 1.5m below the ground surface. This area comes under hot and sub humid region and consist of red soil mixed with clay and black cotton soil as well. Colours of soil found in this area vary mainly from dark black to reddish grey. In this region, cracks are common in houses as soils have medium to high shrinkage and swelling potential. Figure 1 shows the cracks on the field, on typical mud house and cracks in PCC roads in north-eastern part of Madhya Pradesh.
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(a)
(b)
(c)
Figure 1: (a) Cracks in the field; (b) Typical cracks in houses; (c) Cracks in PCC roads in north-eastern part of Madhya Pradesh Samples were collected from ten different places and the variation has been observed in their properties and their composition as well. The average shear strength of soil samples compacted to OMC was 315.00 kPa. The material properties determined in laboratory are shown in table 1.0. A typical grain size distribution of a sample of soil is shown in figure2.0. Figure 3 shows the stress– strain behaviour of black cotton soil.
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Percentage finer (%)
100 90 80 70 60 50 40 30 20 10 0 0.0001
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0.001
0.01 0.1 Particle size (mm)
1
10
Figure 2: Grain size distribution of a sample of soil 350 300
Stress (kPa)
250 200 150 100 50 0
0
2
4
6
Strains (%)
8
10
12
Figure 3: Stress–strain behavior of Black cotton soil Table 1: Properties of Expansive Soil used in study
Soil Properties Clay % Silt % Sand % Liquid Limit Plastic Limit Specific Gravity Plasticity Index
Values 41-52 28-36 13-20 60-81 21-33 2.66 38-48
Soil Properties Free Swell Index, FSI (%) Cohesion (kN/m2) Angle of internal friction OMC Maximum Dry Density (kN/m3) Unsoaked CBR (%) Soil Classification
Values 82-102 120-138 3.89-4.18 17-22.45 15.78-16.45 3.56-4.05 CH
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EXPERIMENTAL SET-UP AND PROCEDURE There are two methods namely direct and indirect methods available for determination of swelling properties of soil. Indirect methods are based on the empirical correlation of different physical properties, chemical properties and mineralogical composition of the soils whereas direct methods mainly consist of laboratory tests. Here both methods have been used to determine the swelling properties of the soil. First of all, the soil samples were collected from field and soil were preserved in desiccator. First, the index properties; liquid limit, plastic limit, shrinkage limit, in-situ moisture content, grain size distribution curve and maximum dry density of all soil samples were determined in laboratory as per the Indian standard codes. Then the compaction properties were also determined in the laboratory. All the tests were again repeated with the addition of sea salt. From literature review it is found that the optimum amount of salt used varies from 15mg/l to 20mg/l and in the present study 17mg/l of salt concentration was used. The specific gravity, water content, liquid limit, plastic limit, shrinkage limit and grain size distribution curve of the black cotton soils samples were determined as per IS: 2720( Part III :(1980)19, IS : 2720 (Part II) – 1973 [20], IS : 2720 (Part V ) – 1985 [21], IS : 2720 (Part VI) – 1972 [22], and IS : 2720 ( Part IV ) – 1985 [23] respectively. The procedure for determination of OMC was determined by same method as described in IS: 2720, Part VII: 1980 [24]. For the determination of free swell index of soils, the method described in IS: 2700(Part XL): 1997 [25] have been used in this study. Similarly, for determination of swelling pressure of soil, a method described in IS: 2700, Part XLI: 1997, second section [26] has been used. Indirect classification of potential swell of a soil can be easily assessed based on empirical equation given by Casarande’s plasticity chart [27] and physical properties of soils. Chleborad et al., 2005 [28] has provided a chart to assess the presence of type of clay mineral in soil. Holtz and Gibbs, 1956 [29] presented a method to assess the potential swell based on the Atterberg’s limit of soil. Potential swell of soil can be also classified based on the physical properties of soils. Skempton, 1953 [30], Seed, et al., 1960 [31], Van Der Merw, 1964 [32], Carter and Benley, 1991 [33] have established some very easy and useful empirical relationships between the physical properties of soils such as colloids contents (clay contents) and soil suction and expansion potential.
RESULTS AND DISCUSSIONS Index Properties Addition of salt in black cotton soil reduces the average liquid limit of all samples to 46.82% from 68%. It may be concluded that the sea salt has reduced the thickness of the diffused double layer by flocculation of clay particles that leads to the reduction of liquid limit of black cotton soil clay. [33-37] have made similar observations. After addition of salt in black cotton soil, average value of plastic limit reduced to 14.84% from 27%. [33], [34] and [35] have concluded the same outcome. Plasticity index is decreased to 28.78% from 44%. This reduction is huge compared to plastic limit. [33] and [34] observed a reduction in plasticity index of expansive soil after addition of salt. High value of plasticity index and high activity of a soil indicates the high swelling potential. So addition of sea salt do cause reductions in plasticity.
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Optimum Moisture Contents and Maximum Dry Density Optimum moisture content and max dry density has been found to be decreasing and increasing respectively with mixing of salt. An increase of 4.39% in dry density of soil and 12.77% decrease in optimum moisture content was observed and shown in figure 4.0. [34-38] have made similar observations.
Maximum dry density (gm/cc)
1.85 1.8
with sea salt without salt
1.75 1.7 1.65 1.6 1.55 5
10
15
20
25
30
Water content(%)
Figure 4: Optimum Moisture content
Swell Potential The soil analysed in present study has liquid limit more than 70% and the plasticity index more than 35% and according to Holtz and Gibbs, 1956 [29], it can be concluded that soil has very high potential swell. Swell potential has also been determined from Seed, et al., 1960 [31] that confirms that soil has high potential swell. The swelling potential as determined from the chart for swelling potential proposed by Cartel et al., 1991 [33] comes out to be 24%.
Percentage Swell Total ten samples were tested in laboratory and it was found that the free swelling has been reduced by a considerable amount as shown in figure 5. Percentage average swell reduced to 71.316% from 93.877%. Observations are well matching with [37, 42, 43].
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Figure 5: Influence of sea salt on swell percentage of soil
Soil Minerals Soil minerals composition have been predicted through a chart presented by Chleborad et al., 2005 [28] that confirms that soil samples used in present analysis contains a large amount of montmorillonite and illite clay minerals.
Activity Activity of soil samples have been determined using relationship given by Seed et al., 196031 and Van Der Merwe D. H., 1964 [32]. Both method confirm that the soil is active. All theoretical formulations give different values of soil activity that are not discussed here.
Swell Pressure The swell pressure of black cotton soil was determined by conducting constant volume method. Without salt addition, an average swell pressure was 75.74054 kN/m2. With the addition of salt, the average swell pressure reduced to 71.6820 kN/m2. So, due to addition of salt, swell pressure was reduced by approximately 5%. Effect of sea salt in the swelling potential of soil is shown in figure 6.0. Some researchers such as [39-41] have made similar observations.
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Figure 6: Influence of sea salt on swell pressure of soil
CONCLUSION All the laboratory tests were conducted as per Indian standards. Index properties of black cotton soil were determined in laboratory. A reduction in value of index properties such as liquid limit, plastic limit and plasticity index of black cotton soil have been observed with addition of sea salt in black cotton soil. The most interesting change observed in swelling potential of black cotton soil with the addition of salt is that the swelling has been reduced with a significant amount. The reaction between sea salt and fine grained black cotton soil is mainly responsible for the flocculation of soil particles and decrease in thickness of double diffused layer. This may have resulted in increase in density and shear strength of soil. Optimum moisture content was decreased with addition of salt in soil.
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