Treatments Technology and Mechanisms of East Asia Black Cotton Soil

Abstract

The East Asia black cotton soil (BCS) cannot be used as embankment filling directly due to its high clay content, liquid limit, plasticity index, and low CBR strength (CBR < 3%). This study evaluates the effects of treating East Asia BCS with lime, volcanic ash, or a combination of both on its engineering properties. Experiments were conducted to analyze the basic physical properties, swelling characteristics, and mechanical properties of the treated soil. Results indicate that lime addition significantly reduces the free swelling rate, improves limit moisture content, increases optimum moisture content, decreases maximum dry density, and enhances CBR value. Although volcanic ash also improves BCS performance, its effects are less pronounced than those of lime. The combined treatment with lime and volcanic ash exhibits superior performance, greatly reducing expansion potential and significantly increasing soil strength. Specifically, a mixture of 3% lime and 15% volcanic ash optimizes the liquid limit, plasticity index, and CBR value to 49.2%, 23.8, and 24.7%, respectively, meeting the JTG D30-2015 requirements and reducing construction costs. The treatment mechanisms involve hydration exothermic reactions, volcanic ash reactions, and semipermeable membrane effects, which collectively enhance the soil’s properties by producing dense, high-strength compounds.

1. Introduction

In recent years, China’s highway and railway construction technology has made considerable progress in outward exports. China’s construction engineering group often encounters engineering problems caused by the local BCS in the road and railway construction projects undertaken by East African countries. BCS is a highly expansive soil, and a major deposit was found in India, where cotton is grown in large quantities. It can cause serious issues at the start of projects, including uneven settlement, landslides, slope collapse, etc. Therefore, when it is present, developers must adopt a replacement program, which increases the amount of earthwork and project cost. The in-depth study of the expansion characteristics and treatment technology of BCS in East Africa can increase the safety of the project, reduce the project cost, and enhance the environmental benefits.

Due to the relatively backward research conditions in East Africa, there is little research on the expansion characteristics of BCS in this area. Most of the achievements came from Chinese road and railway industry technicians, as Chinese engineering companies encountered a large number of engineering issues caused by BCS in the process of construction in Africa and Southeast Asia. Investigate the survey of black soil distribution and the characteristics of macro-geological features. Through geological mapping and geophysical prospecting combined with laboratory tests, in situ tests, etc., a set of exploration methods suited to the national conditions of the country was established. By analyzing the physical and mechanical properties of black cotton soil, it was found that black cotton soil exhibits medium-to-strong expansiveness and medium-to-high compressibility engineering characteristics [1]. X-ray diffraction and thermal analysis were used to analyze the composition and porosity of the soil [2,3]. Based on the above analysis, the composition of BCs in Africa is similar to the expansive soil in Yunnan, Tibet, and other provinces in China. However, the color of expansive soils in Africa is deeper, and the self-expansion rate is very high [4,5]. The content of clay montmorillonite distributed in southern Iran reached 83%. This high concentration suggests that montmorillonite plays a major role in the soil composition and properties of southern Iran [6]. The expansive soil roadbed should be improved in two aspects: building structure and additives. In terms of structure, the subgrade drainage system shall be reinforced; the subgrade bed for the weak medium expansive soil shall be replaced by not less than 0.5 m, and the subgrade bed for the middle and strong expansive soil shall be replaced by not less than 1 m. During construction, dynamic monitoring of the subgrade should be performed. Data such as moisture content, CBR, and deflection changes of the subgrade during dry and rainy seasons should be analyzed, and dynamic construction optimization designs should be made based on the site conditions [7,8]. The compaction of expansive soil should adopt the heavy-duty compaction standard, and the water content should be 3% more than the optimum moisture content. The moisture content of the fill material shows minimal fluctuation after construction, and the CBR value meets the regulatory requirements [9]. Observations of black cotton soil from the East African region using a scanning electron microscope reveal that the soil is relatively dense and forms flaky aggregates, with unit particles primarily in face-to-face contact and a noticeable stratification. This microstructure contributes to the soil’s strong expansiveness. When piles are used to lift the bearing capacity of the subgrade, the length of the pile should be increased, and special piles such as bamboo piles should be used [10]. In terms of additives, a certain amount of lime can improve the soil structure [11]. The use of slag composites can also increase the strength of the soil, allows construction to be carried out as quickly as possible, and prevents surface and road water infiltration from damaging the subgrade, thereby improving overall subgrade stability and extending its service life [12,13]. India uses coconut fiber, banana fiber, and sisal fiber to reinforce the expansive soil subgrade and concluded that the maximum load-carrying capacity was obtained at 0.5% of the 2.5 cm long fiber. As fiber content and fiber length increase, the maximum dry density and optimum moisture content decrease. The CBR value of the soil increases with the addition of sisal fibers, with the peak CBR value achieved at a fiber content of 0.50% [14]. Microscopically, SEM images were used to create a map of the clay microstructure and explain the engineering properties of the clay [15,16].

In this paper, through the free expansion rate test, limiting moisture content, compaction test, and CBR test, the treatment effect of different lime and volcanic ash contents on the BCS in East Africa was studied. It was found that the lime and volcanic ash conformation scheme is better than the single use of lime or volcanic ash. It is recommended that the 3% lime and 15% volcanic ash treatment plan meet the specification requirements and reduce the construction cost. At the same time, using computer molecular modeling to explain the modified mechanism of black soil in East Africa from the microscopic and mesoscopic levels.

2. Performance Test

2.1. Materials

The BCS used in the experiment was taken from South Link K2 + 130, Nairobi, Kenya. The BCS in East Africa was mainly composed of hydrophilic clay minerals with a high percentage of fine clay particles. The free expansion of BCS was determined to be between 40% and 140% and was categorized as expansive soil with medium strength according to TB 10077-2001 [17]. So, it is a kind of high-liquid-limit clays with strong expansion potential and low strength.

The volcanic ash used in this paper was produced during volcanic activities. Generally, it is dark gray, yellow, or white in color. The volcanic ash was composed of tiny debris, which became tuff after compaction. There was a certain amount of activated silica (SiO₂) and activated alumina (Al₂O₃) in the mixture. Thus, the volcanic ash could bond the clay together and reduce the water-absorbing capacity of the clay particles.

The milled volcanic ash and BCS were mixed together manually, and then the appropriate amount of volcanic ash was determined by relevant tests. The hydrated lime provided by a local company had adopted an effective calcium hydroxide (Ca(OH)₂) content of 96.2%. The 80 μm pass rate of the lime was 100%.

2.2. Test Plan

The BCS specimens were prepared. Then free expansion rate tests, limiting moisture content tests, compaction tests, and CBR tests were conducted to test the engineering properties of the modified soil. Finally, the test results of lime, volcanic ash and mixture of lime and volcanic ash, were analyzed, and the optimum proportion of additives was recommended to modify BCS.

2.2.1. Specimen Preparation

The dried BCS was crushed into particles between 0.5 mm and 2 mm diameters and sieved for the preparation of further experimental tests. The volcanic ash was crushed and sieved through a 75 μm sieve. All row materials were dried in an oven with a temperature range of 100 °C to 105 °C for 24 h, in accordance with JTG E40-2007 [18].

The effect of additives on BCS performance, consisting of volcanic ash, lime, and a combination of volcanic ash and lime, was analyzed. Four contents of volcanic ash (10%, 15%, 20%, and 25%) and four contents of lime (1%, 3%, 6%, and 9%) were utilized. The contents of the combined volcanic ash and lime were 1% lime + 15% volcanic ash, 1% lime + 20% volcanic ash, 3% lime + 15% volcanic ash, as well as 3% lime + 20% volcanic ash. After the additive content was determined, the raw materials were measured accurately and mixed together in an enamel pan or iron pot. Then the soils were poured and mixed in a small mixer with an appropriate amount of water for at least 10 min. Finally, the mixture was poured into a plastic bag, shaken manually, and conditioned for 24 h.

2.2.2. Test Program

Four tests were carried out according to JTG E40-2007 and ASTM D4318 [19], including a free expansion rate test, a compaction test, a limiting moisture content test, and a California bearing ratio test (CBR test). Prior to the experiment, BCS samples with different additives were prepared and kept under wet conditions for 24 h.

For the free expansion test, the soil particles should be smaller than 0.5 mm in diameter. Then, a 10 mL soil sample was taken through the sample device, placing the 50 mL measuring cylinder on the test bench. Finally, 30 mL of distilled water and 5 mL of a 5% analytical pure sodium chloride solution were injected into the samples, and the soil sample was poured into the measuring cylinder. When the soil sample is stable, the free expansion rate should be calculated.

Heavy-duty compaction was adopted for the compaction test, where the hammer was 4.5 kg, with a 45 cm drop height. In order to obtain optimum moisture content, the soil sample was compacted using the same method as the heavy-duty compaction method. The test specimens were prepared and submerged into water for a continuous 96 h, and then the CBR expansion amount and CBR value of black cotton soil specimens were measured using the CBR expansion measuring device and penetration apparatus, respectively.

For CBR test, the penetration was applied at a rate of 1.25 mm/min, and the load was recorded at penetrations of 5.0 mm.

For limiting moisture content test, the liquid limit (w_L), plastic limit (w_P), and plasticity index (I_P) are used for quantitative characterization. The limit water content test is conducted according to the method specified in ASTM D4318.

2.2.3. Testing Results

The BCS in East Africa was modified with different proportions of lime, volcanic ash, and a combination of lime and volcanic ash. The results of the free swelling rate are shown in Figure 1 and Figure 2. Figure 1a shows that the maximum free expansion rate of BCS was 129% without modification (according to research by Zhao [5] and Wang [8], the free expansion rates of unmodified BCS are all above 100%), and such a value could drop to 55% when 6% of lime was used. The free swelling rate kept increasing slightly as a higher percentage of lime was added. Figure 1b shows that the maximum drop in the free expansion rate of BCS was 29% when mixed with 15% volcanic ash, whereas it did not drop further with additional volcanic ash.

The free swelling rate of BCS with 3% lime or 15% volcanic ash alone is 105% or 100%, respectively. While adding 3% lime and 15% volcanic ash at the same time, free swelling rate fell to 71%. Thus, the mixture of lime and volcanic ash can obtain a preferred modification effect.

The greater liquid limit and plasticity index of the expansive soil, the stronger water storage capacity and expansibility. On the contrary, reducing the boundary moisture content would increase the water stability and soil strength of the expansive soil. Therefore, liquid-plastic limit tests were conducted on plain and modified BCS and results are shown in Figure 3. If the lime content is about 9%, the plastic index significantly reduced and the plasticity index is approaching non-expansive soil. This is consistent with the conclusions drawn by Shao [3]. It is worth noting that the liquid limit increases at beginning. Because the lime particle size was very small and did not completely react with the BCS which leaded to absorption of a large amount of water and increased the liquid limit index. Liquid limit dropped slightly with lime content increasing. When the amount of added volcanic ash increases, the liquid limit decreases significantly, and the plasticity index decreases slightly. The modification effect of the mixture of lime and volcanic ash was much better than that of lime or volcanic ash. The liquid limit and plasticity index of black cotton were 68.1 and 26.2, when 3% lime was added. The liquid limit and plasticity index of black cotton were 56.5% and 25 using 15% volcanic ash. However, the liquid limit and plasticity index decreased to 49.2% and 23.8 with the mixture of 3% lime and 15% volcanic ash. According to JTG D30-2015 [20], liquid limit and plastic index of fine-grained subgrade filling should be less than 50% and 26. The additive of 3% lime and 15% volcanic ash could not only meet the requirements of the standard, but also minimize project cost.

The maximum dry density and optimum moisture content of BCS was obtained in order to control the subgrade filling quality. The compaction curves of BCS with different additive was tested and presented in Figure 4. As seen in the figure, with the increase in lime, the optimum moisture content increase, varies the maximum dry density. With the volcanic ash content increasing the optimum moisture content of the mixture decreased, while the maximum dry density increased. Figure 4c shows that if lime and volcanic ash were used jointly together, lime and volcanic ash exert the opposite effect on the compaction characteristics.

CBR test and CBR expansion test were conducted to evaluate the effect of additives on strength properties, as well as to recommend an optimum content of additive. Testing results are presented in Figure 5. As noted, the CBR value increases steadily with the increase in lime content or volcanic ash, with an increase in growth rate, which is consistent with the conclusions drawn by Li [12]. In contrast, the CBR expansion capacity decreases gradually at the beginning and tends to plateau. It is also observed that the CBR value is more sensitive if lime is utilized compared to the volcanic ash. It is also noted that if the mixture of lime and volcanic ash are both added, the CBR value is significantly increased and CBR expansion rate is greatly reduced, compared to the sole addition of lime or volcanic ash. This provides an option to utilize more volcanic ash since the price of lime in local East Africa is high.

3. Mechanism of Performance Improvement

This section contains the research and analysis on the treatment mechanism of lime and volcanic ash.

3.1. Hydration Exothermic Reaction

Ca(OH)₂ was generated when quick lime or slaked lime was added to a high-liquid-limit BCS. On the one hand, the reaction could reduce the moisture content through hydration and evaporation processes. On the other hand, the reaction generated Ca²⁺, OH⁻, and Ca(OH)₂, which provided the necessary conditions for the carbonation reaction and the Ca(OH)₂ crystallization. The additive was carbonated by absorbing CO₂ and H₂O from the air, resulting in dense calcium carbonate. CaCO₃ and Ca(OH)₂ can combine with each other, enabling soil particles to form an integral structure. Meanwhile, the Ca²⁺ adsorbed around clay minerals produced flocculate and bonded mineral particles and changed the pore structure of the entire soil particles. With an increase in lime dosage, the chemical bond forces rapidly decreased the expansion volume and force, which resulted in a continuous decrease in hydrophilicity and an increase in water stability and strength stability. This is one of the most important reasons for lime-modified soil strength improvement.

3.2. Volcanic Ash Reaction

The OH ionized by Ca(OH)₂ makes the soil alkaline. Under alkaline conditions, Ca(OH)₂ reacts with active SiO₂ and Al₂O₃ in volcanic ash and BCS, producing hydrated calcium silicate (CSH), hydrated calcium aluminate (CAH), or hydrated calcium sulfoaluminate. Partial chemical reaction formulas are shown in the following Formulas (1) and (2):

$$5\mathrm{Ca}{(\mathrm{O}\mathrm{H})}_2+6\mathrm{S}\mathrm{i}{\mathrm{O}}_2+\mathrm{n}{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{alkaline}}{\to }\mathrm{C}{\mathrm{a}}_2\mathrm{S}{\mathrm{i}}_6{\mathrm{O}}_{16}{(\mathrm{O}\mathrm{H})}_2\cdot (4+\mathrm{n}){\mathrm{H}}_2\mathrm{O}$$

(1)

$$\mathrm{Ca}{(\mathrm{OH})}_2+\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+\mathrm{n}{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{alkaline}}{\to }\mathrm{CaO}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\cdot (1+\mathrm{n}){\mathrm{H}}_2\mathrm{O}$$

(2)

This reaction was similar to the hydration of volcanic ash cement to obtain strength, known as the volcanic ash reaction. From

Table 1. Chemical composition of volcanic ash (unit: %).

Ash Sample Number	CaO	MgO	Fe2O3	Al2O3	SiO2	K2O	Na2O	SO3	P2O5	MnO	TiO2	LOI
Sample 1	10.69	11.69	12.43	13.35	43.26	1.29	2.76	0.065	0.54	0.17	2.85	0.32
Sample 2	10.88	12.18	12.41	13.07	43.08	1.26	2.75	0.061	0.54	0.17	2.81	0.22

, it can be seen that the oxide composition of all volcanic ash is very similar in East Africa. Among the oxides, the content of SiO₂ was over 40%, and the content of Al₂O₃ reached 13%, which provided raw materials for volcanic ash reaction. The hydrated calcium silicate produced by the reaction between SiO₂ and Ca(OH)₂ is the most important hydration product of Portland cement hydration and the most important strength source of cement-based composite materials. Therefore, the main reason for the high strength of the BCS mixed with lime and volcanic ash is that there were a large number of volcanic ash reactions in the soil. In addition, hydrated calcium silicate (C-S-H) and hydrated calcium aluminate (C-A-H) were gelling materials with a cementation effect. The gelling materials accumulate on the surface of the soil particles, forming a solidification layer through hardening and crystallization, which prevents the diffusion of particles’ moisture and reduces the expansion potential and liquid limit of the soil.

3.3. Semipermeable Membrane Effect

Due to the high moisture content of the BCS, the montmorillonite crystals are surrounded by the pore solution, as shown in Figure 6. In order to study the interaction between the solution in montmorillonite layer and the pore solution, the P point in Figure 6 was enlarged, as shown in Figure 7.

As shown in Figure 5, Figure 6 and Figure 7, there was an imaginary interface between the montmorillonite layer and the pore solution, which was a dotted line MN. Zheng (2013) believes that under certain conditions, MN could be equal to a semipermeable membrane, which means only water could pass freely [21]. There were various electrolyte molecules and ions in pore solutions, and the solution between montmorillonite layers also contained a certain concentration of cationics. It could be used to explain the treatment technology of East Africa BCS from the microscopic level:

Montmorillonite minerals with a certain amount of water added are shown in Figure 7a. According to the diffusion theory and the semipermeable membrane principle, the osmotic pressure makes the water molecules continuously enter the crystal layer from the pore solution (Figure 7a, arrow direction) until the osmotic pressures achieve a balance. In the continuous flow of water molecules, montmorillonite in BCS would appear to have significant expansion.

After treatment with a mixture of lime and volcanic ash, a large number of cations flowed into the pore solution. The concentration of ions in the pore solution increased significantly, and the solutions in the pores and between the montmorillonite layers are shown in Figure 7b. The ion concentration between the layers was smaller than the pore solution; thus, the osmotic pressure difference caused the water molecules to enter the pore solution from the intercrystalline layer (as shown in Figure 7b). The spacing of montmorillonite decreases and contracts, which can reduce the expansion potential of BCS.

4. Conclusions

In this paper, the engineering properties of East Asia BCS treated with lime, volcanic ash, or a mixture of both were evaluated through four experiments. The results showed that the addition of lime significantly reduced the free swelling rate, improved the moisture content limit, increased the optimum moisture content, decreased the maximum dry density, and increased the CBR value.

While volcanic ash also improved the performance of BCS, its effect was not as pronounced as that of lime. The compound treatment of lime and volcanic ash demonstrated superior results compared to using lime or volcanic ash alone, significantly reducing the expansion potential and greatly increasing the soil strength.

The optimal mixture of 3% lime and 15% volcanic ash improved the liquid limit, plasticity index, and CBR value to 49.2%, 23.8, and 24.7%, respectively. These improvements meet the requirements of JTG D30-2015 and help reduce construction costs.

Considering molecular simulations, phase composition, and engineering properties, the treatment mechanisms of BCS were identified as the hydration exothermic reaction, volcanic ash reaction, and the semipermeable membrane effect. The hydration exothermic reaction provides the basic conditions for other reactions. The semipermeable membrane effect and ion exchange can reduce the layer spacing of montmorillonite cells, weaken water absorption ability, and decrease the water film thickness on soil particle surfaces, resulting in reduced expansion potential. The carbonation reaction, Ca(OH)₂ crystallization, and volcanic ash reaction produce dense, high-strength compounds such as calcium carbonate, hydrated calcium silicate (C-S-H), hydrated calcium aluminate (C-A-H), and hydrated calcium sulfoaluminate, which significantly enhance the strength of the treated soil.