J. Cent. South Univ. (2016) 23: 2688-2694

DOI: 10.1007/s11771-016-3330-7

Plasticity, strength, permeability and compressibility characteristics of black cotton soil stabilized with precipitated silica

R. Gobinath^{1, 2}, G. P. Ganapathy³,I.I.Akinwumi⁴, S. Kovendiran², S. Hema², M. Thangaraj²

1. VIT University, Vellore, India;

2. Department of Civil Engineering, Jay Shriram Group of Institutions, Tiruppur, India;

3. Centre for Disaster Mitigation and Management, VIT University, Vellore, India;

4. Department of Civil Engineering, College of Engineering, Covenant University, Ota, Ogun State, Nigeria

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The suitability of using precipitated silica (PS) from the burning of rice husk was investigated to improve the geotechnical engineering properties of a black cotton soil. A laboratory experimental program consisting of series of specific gravity, Atterberg limits, compaction, California bearing ratio (CBR), unconfined compression and consolidation tests was conducted on the untreated and PS treated soil samples. The application of PS to the soil significantly changed its properties by reducing its plasticity and making it more workable, improving its soaked strength, and increasing its permeability and the rate at which the soil gets consolidated. An optimal PS content of 50%, which provided the highest soaked strength, is recommended for the improvement of the subgrade characteristics of the BC soil for use as a pavement layer material.

Key words: black cotton soil; expansive soil; precipitated silica; rice husk ash; soil stabilization

1 Introduction

One of the most difficult soils to use as construction or earth foundation material is an expansive soil [1-2]. They are usually characterized by high plasticity, swell potential and shrinkage potential, low permeability, and very low strength, when in wet condition, and are volumetrically unstable. These characteristics make most expansive soils a significant hazard to engineering construction and consequently, neglected for use as construction materials. Their high plasticity makes them difficult to work with during construction. The drastic reduction of their strength, when they get saturated with water, high swell pressure, and high shrinkage have led to the failure of several buildings, road pavements and earth structures.

Expansive soils are found in India [3], Nigeria [4], United Kingdom [2], United States of America [5-6], Australia [7], South Africa [8], China [9] and many other countries of the world. These soils cover a significant land area in some of these countries. Annually, property owners typically loose more of their finance to expansive soils than to a combination of earthquakes, floods, hurricanes and tornadoes [2, 10]. Excavating expansive soils at proposed project sites and borrowing of suitable soil to replace them have most times increased the overall project cost of constructing roads and buildings. Consequently, many researchers have been concerned about finding alternatives by investigating the use of stabilizers. Portland cement and lime are the commonest and most extensively researched soil stabilizers [3, 11] but they are expensive and cement production significantly contributes to the global emission of greenhouse gases [12]. This has made researchers focus on searching for locally-available and low-cost stabilizers.

Some of the low-cost stabilizers that have been recently investigated include fly ash [13], bottom ash [14], rice husk ash [15], groundnut shell ash [16], calcium carbide residue [17], to mention only a few.

This work investigates the effects of stabilizing a black cotton (BC) soil with precipitated silica (PS) on its plasticity, strength, permeability and compressibility characteristics.

When ashes are used for soil stabilization, their silica contents have been reported to be chiefly responsible for their pozzolanic reactivity [18-19]. This research work is therefore significant for using rice husk ash of high silica purity for the stabilization of a black cotton soil.

2 Materials and methods

2.1 Materials and preparation

The BC soil sample was collected from Tiruppur district in the South Indian State of Tamil Nadu (latitude 11^o10′ N and longitude 77^o35′ E). The soil was collected by method of disturbed sampling after removing the top soil of 150 mm depth. The chemical composition of the black cotton soil is presented in Fig. 1 while the micrograph of the black cotton soil, obtained using a scanning electron microscope (SEM), is shown in Fig. 2.

Precipitated silica (PS) was obtained by burning of pulverized rice husk at 850 °C in a fire tube boiler, thereby producing ash residue. The silica used was precipitated by burning the rice husk in controlled air that was supplied upward in order to generate ash of higher silica content. The supplied air introduced oxygen into the system, thereby reducing the loss on ignition or carbon content of the resulting white-coloured ash. This carbon was given off as either carbon dioxide or carbon monoxide. This process generated silica of high purity (98.6%). Rice husk ashes that have been used in literature for stabilizing soils were typically produced at much lower temperatures than that used in this work and also yielded silica of lower purity than that obtained from this work. Prior to being used in the laboratory, the silica was stored in a plastic container. The micrograph of the PS, obtained using a scanning electron microscope (SEM), is shown in Fig. 3.

Fig. 1 Chemical composition of BC soil samples

Fig. 2 SEM images of BC soil sample

Fig. 3 SEM images of PS sample

2.2 Methods

The BC soil was air-dried and pulverized in order to ensure that its particles were not clumped together. The soil fraction that passed through 0.425 mm sieve was used to determine the index properties of the soil.

The determination of the specific gravity of the soil was in accordance with IS 2720: Part 3 [20]. The particle size distribution of the soil was determined by conducting sieve and hydrometer analyses in accordance with IS 2720: Part 4 [21]. The liquid and plastic limits of the soil were determined in accordance with IS 2720: Part 5 [22], while the shrinkage limit of the soil was determined in accordance with IS 2720: Part 6 [23]. The compaction characteristics of the soil were determined using the standard Proctor test performed in accordance with IS 2720: Part 7 [24]. The permeability of the soil was determined using the variable head method in accordance with IS 2720: Part 17 [25]. The consolidation test was performed according to IS 2720: Part 15 [26]. The unsoaked and soaked California bearing ratio (CBR) tests were conducted in accordance with IS 2720: Part 16 [27], while the unconfined compressive strength (UCS) of the soil were determined in accordance with IS 2720: Part 10 [28]. The pH of the soil was determined in accordance with IS 2720: Part 26 [29].

Representative samples of the soil was divided into eight equal parts and PS was added to each of the parts in 0%, 10%, 20%, 30%, 40%, 50%, 60% and 70% proportions by dry mass of the soil sample, respectively. Series of specific gravity, liquid and plastic limits, compaction, unsoaked and soaked CBR, unconfined compression, permeability and consolidation tests were conducted on each of the proportions of PS-treated soil.

The samples for the consolidation test were prepared at a moisture content of 14% and a dry density of 15 kN/m³. The coefficient of consolidation was determined for pressure ranging between 200 and 400 kN/m².

3 Results and discussion

3.1 Untreated soil and precipitated silica

The physical and geotechnical properties of the untreated BC soil are presented in Table 1. The soil sample is classified as A-7-6, according to the American Association of State Highway and Transportation Officials (AASHTO) soil classification system [30] and inorganic clay of high plasticity (CH), according to the Unified Soil Classification System (USCS) [31].

The soil has a specific gravity of 2.61 and its bearing capacity or strength, as presented by its CBR and UCS values, is generally low. It is clayey (Fig. 4) and the clay minerals are suspected to be predominantly montmorillonite because of its expansive nature. Approximately 78% of the soil particles are finer than 75 μm and this fraction significantly influences the behaviour of the soil. The swelling and shrinkage characteristics of the soil are high.

The specific gravity of the precipitated silica is 2.01 and its pH is 11.2.

3.2 Effects of adding precipitated silica to BC soil

3.2.1 Specific gravity

The effect of mixing varying proportions of PS with the soil on its specific gravity is graphically presented in Fig. 5. The specific gravity of the treated soil decreased with increasing PS content in the soil. The specific gravity of the soil admixed with 50% of PS decreased by 8.3%, when compared with that of the natural soil. The correlation between the specific gravities of the treated soil and their PS content is strong (r=-0.991). The decrease in specific gravity of the treated soil as its PS content increased was expected knowing that the specific gravity of the PS (2.01) is lower than that of the soil (2.61).

Table 1 Physical and geotechnical properties of untreated BC soil

Fig. 4 Particle size distribution of soil

Fig. 5 Specific gravity of treated soil with varying proportion of precipitated silica

3.2.2 Atterberg limits

The results of liquid and plastic limits and plasticity index of the untreated soil indicate that the soil has high potential for volume change. The predominance of montmorillonite, a clay mineral that is known to be expansive, in soils with similar properties as this particular soil has been reported to be responsible for such a high potential for volume change [32]. The variation of the liquid and plastic limits and plasticity index of the treated soil with their PS contents is graphically presented in Fig. 6.

Fig. 6 Atterberg limits of treated soil with varying proportion of precipitated silica

Each of the liquid and plastic limits and plasticity index of the treated soil decreased with increasing PS content in the soil. It can be noted that when 50% PS was mixed with the BC soil, the plasticity index of the treated soil became reduced by 46.7% of that of the untreated soil. There exist a strong correlation (r=-0.996) between the plasticity indices of the treated soil and their PS content.

The Atterberg limits results give an indication that addition of the PS to the untreated soil provides relatively beneficial effects on the plasticity and volume stability of the soil. The effect on the plasticity of the treated soil is better appreciated using the graphical illustrations in Figs. 7 and 8.

Figures 7 and 8 show the plasticity charts for USCS and AASHTO systems, respectively. The plot for the untreated soil, which was within the CH (inorganic clay of high plasticity) and A-7-6 part of the charts, became improved (in terms of its plasticity) by increasing PS content in the BC soil such that the plot for the soil treatment with 40% PS falls within the CL (inorganic clay of low plasticity) and A-6 part of the USCS and AASHTO charts, respectively.

Fig. 7 USCS plasticity chart showing effect of adding PS to soil

Fig. 8 AASHTO plasticity chart showing effect of adding PS to soil

The decrease in the plasticity index of the treated soil with increasing PS content is attributed, firstly, to the fact that the clay content in the soil decreases as the soil is being partially replaced with PS [33]. Secondly, some of the clay particles of the treated BC soil get coated with the PS, thereby minimizing the direct interaction between the clay minerals of the soil and water [34]. Consequently, these clay minerals are not allowed to maximize their moisture-holding potential; this makes the treated soil more workable.

3.2.3 Compaction characteristics

The results of compaction tests conducted on the untreated and the PS treated soil samples are graphically illustrated in Fig. 9. The maximum dry unit weights of the treated soil samples decreased with increasing PS contents and this correlation is strong (r=-0.989). On the other hand, the optimum moisture content (OMC) of the treated soil samples increased as the PS content increased. This correlation is also strong (r=0.995).

Fig. 9 Variation of compaction characteristics with precipitated silica

The decrease in the maximum dry unit weight of the treated soil as its PS content increased is attributed to the lower specific gravity of the added PS. Similar effect on the maximum dry unit weight were observed for the stabilization of soils with stabilizing agents that have lower specific gravities than those of the soil samples being stabilized [35-37]. The increase in the OMC may be attributed to the coating of the clay particles by the precipitated silica such that it prevents the agglomeration of most of the clay particles in the PS treated soil and thereby increased the water content it requires to reach optimum. Usually, the clumping together of clay particles will, in the short term, prevent water from reaching the clay particles within the interiors of the clods. Consequently, decreasing the water content it requires to reach optimum [38].

3.2.4 Strength characteristics

Unsoaked and soaked CBR tests and UCS were conducted on the untreated and treated soil samples and their results are presented in Fig. 10. When 10% PS was added to the BC soil, its unsoaked CBR value and UCS increased by 15.07% and 3.60%, respectively, but subsequent increase in PS content of the soil led to a decrease in the unsoaked CBR value and UCS of the treated soil. The correlation between the PS content and each of the unsoaked CBR value and UCS of the treated soil is strong (r=-0.741 and r=-0.940, respectively). However, the soaked CBR value of the treated soil samples increased as the PS content increased after the addition of 50% PS. This relationship is also strongly correlated (r=0.842). When 50% PS was added to the natural soil, its soaked CBR value increased by 414.02%.

Fig. 10 Variation of strength characteristics with precipitated silica

The increase in the unsoaked strength (unsoaked CBR and UCS values) of the treated soil is limited to the application of 10% PS to the soil while the increase in the soaked CBR value is limited to 50% PS content in the soil. The microstructure of the BC soil (Fig. 2) shows that the soil consists predominantly of an assemblage of plates. Application and mixing of PS with the soil tends to introduce the PS in between these plates, thereby increasing the frictional resistance of the plates against sliding and the formation of some pozzolanic products. This is thought to be responsible for the limited increase in the CBR and UCS values of the treated soil.

3.2.5 Permeability characteristics

Figure 11 shows the results obtained from carrying out the falling head permeability tests on the untreated and PS treated soil samples. The coefficient of permeability of the treated soil samples increased with increasing PS contents. After the addition of 50% PS to the soil, the coefficient of permeability increased by 186.7%. The increase in the permeability of the treated soil as its PS content increases is strongly correlated (r= 0.982).

Fig. 11 Variation of coefficient of permeability with precipitated silica

The application of the PS to the BC soil distorted the platy structure of the soil such that it increased the interconnected pore spaces within the soil, thereby permitting water to flow through the soil more easily as the PS content in the treated soil increases.

3.2.6 Compressibility characteristic

In order to ensure the stability of a structure placed on expansive soils, it is essential that the soil gets consolidated prior to placing the structure on it. Consequently, it is necessary to know the rate at which the compression of the soil layer takes place, so that the design engineer can decide on the appropriate preloading technique for ground improvement. The value of the coefficient of consolidation is a measure of the rate at which the compression of a soil layer takes place.

The variation of the coefficient of consolidation with the PS content in the treated soil is presented in Fig. 12.

Fig. 12 Variation of coefficient of consolidation with precipitated silica

The coefficient of consolidation of the treated soil samples increased with increasing PS contents until 60% PS was added to the soil. Further addition of PS led to a decrease in the coefficient of consolidation. The increase in the coefficient of consolidation of the treated soil as its PS content increases is strongly correlated (r= 0.833).

The increase in the coefficient of consolidation of the BC soil as the PS content of the treated soil increases is consistent with the general knowledge that the coefficient of consolidation of a soil is directly proportional to its coefficient of permeability and inversely proportional to its liquid limit. The increase in the rate at which the treated BC soil gets compressed as its PS content increases is attributed to the increase in the interconnected pore space in the treated soil with increasing PS content. This permitted the drainage of water from the treated soil at an increasing rate as its PS content increased. This implies that the rate of settlement caused by the drainage of water from the pore space will also increase with increasing PS content.

4 Conclusions

This work presents the geotechnical properties of the untreated and PS treated BC soil. The application of PS to the soil significantly changed its properties by reducing its plasticity and making it more workable, improving its soaked strength, and increasing its permeability and the rate at which the soil gets consolidated.

BC soils are problematic to use for construction, especially in areas that experience regular high groundwater table; they have been responsible for the failure of many pavements and other earth structures. Thus, these findings are significant.

Though the treated soil did not meet the standard requirements for use as subbase or base material for flexible pavement construction, it became better suited for use as a subgrade material. This has the potential to reduce the pavement construction project costs associated with the excavation of unsuitable BC soil and the transportation of suitable materials to a project site.

An optimal PS content of 50%, which provided the highest soaked strength, is recommended for the improvement of the subgrade characteristics of the BC soil for use as a pavement layer material.

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(Edited by YANG Bing)

Received date: 2015-07-08; Accepted date: 2015-10-31

Corresponding author: R. Gobinath, Professor; Tel: +91-9003394964; E-mail: gobinathdpi@gmail.com