This study presents the experimental results of the effective utilization of polypropylene (PP) fibre and silica fume (SF) to stabilize the expansive soil subgrade as a possible alternative from the environment, economic, and technical perspective. During the drying cycle, clayey soil undergoes unpredictable shrinkage deformation. The nonrecurring shrinkage behaviour of expansive soil caused the adverse effect on lightweight civil engineering structures. Therefore, expansive soil is not considered suitable subgrade material for paved structure construction. Paved structures constructed on clayey soil subgrade experience severe damages occur in its life cycle. The numerous mechanical, biological, and chemical techniques have been successfully demonstrated to reduce the swelling–shrinkage nature and improve the strength behaviour of expansive soil subgrade. This study aims to utilize industrial waste SF as chemical and PP fibre as a mechanical stabilizer. The mecho-chemical stabilization of expansive soil has been carried out by reinforcing the expansive soil with PP fibre and chemically stabilizing with SF. To evaluate the performance of waste material as a stabilizer, compaction, Atterberg’s limits, free swelling index (FSI), electrical conductivity (EC), pH, swelling pressure, and California bearing ratio (CBR) tests have been conducted on treated and untreated specimens. Over the past few years, images from digital cameras have been successfully used to study crack morphology and various parameters of soils. The image processing technique has been used to evaluate the shrinkage limit by developing the MATLAB program code. The microstructural analysis has been carried out using scanning electron microscopy (SEM) analysis. The varying percentage of silica fume (2%, 4%, and 8%) and polypropylene fibre (0.25%, 0.5%, and 1%) by weight of dry soil were used to stabilize the expansive soil. The results show that the value of CBR increases and the shrinkage area decreases with an addition of SF and PP fibre. The experimental results also show a reduction in Atterberg’s limit with the addition of SF and PP fibre; as a result, the shrinkage behaviour of clayey soil reduces exponentially. The digital image processing technique showed a higher potential to understand the soil morphology for the large area.
The infrastructure development like paved structure, building, and dams involves exhaustive use of natural resources, and has a serious concern to the environmentalist . The durability of paved structures majorly affected by the performance of the subgrade material. Expansive soil causes a considerable amount of damage in the various part of the world  due to the presence of moisture-sensitive fine-grained clay mineral content, which rapidly caused volume change. The construction of paved structures on expansive soil subgrade experiences extensive large-scale damage caused by swelling accompanied by loss of strength and shrinkage cracks during monsoon and summer, respectively . Several conventional methods such as chemicals , mechanical [5, 6], biochemical [7, 8], and geosynthetic [9, 10] were demonstrated to reduces the swelling–shrinkage nature of expansive soil.
The chemical stabilization is a substantiate quick-fix method to improve the shear strength and to reduces the swelling shrinkage nature of expansive soil . The cement is the traditional material used in soil treatment to increase stability and durability , which causes global warming due to CO2 emission . India is the second largest cement manufacturer causing the threat of higher CO2 emission; therefore, researchers are looking for an alternate substitute of cement . Silica fume is one of the non-expansive industrial waste materials, which significantly reduces the swelling pressure in expansive soil subgrades and can be used as an alternative of cement . However, the chemical stabilized expansive soil subgrade shows the brittle shear failure and causes tensile cracking, which is not suitable for paved structures subjected to cyclic loading.
Natural or synthetic fibre has been used to strengthen the soil against tensile cracking, which causes due to the drying period. Innumerable lab and in-situ pilot studies have also been performed to show the possible use of discrete fibre components to soil mass stabilization [16, 17]. Various researcher studies show that the bearing strength of the soil subgrade rises, and the thickness of the base course and the surface layer of the paved structure is minimized by the addition of fibres [18, 19]. Polypropylene fibre is highly acid and salt resistance, and its higher tensile strength shows a higher potential to use the material as microfine reinforcement . The chemically inert, hydrophobic, and non-corrosive nature of PP fibre also indicates its effective utilization in the soil reinforcement due to low susceptibility of absorption or reaction with soil moisture or leachate, inert with water and soil; hence, it can be used for the soil reinforcement [21, 22].
Punthutaecha et al. presented the study to establish the effect of fibres with fly ash and bottom ash on the swelling and shrinking properties of expansive soils. The results show that fly ash treatment decreased soil swelling and shrinking characteristics. The addition of fibres raises swelling resistance capacity by more than 0.2% . With the reinforcement of PP fibre and carpet fibres along with cement tensile and, shrinkage strengths are increased [24, 25]. Fibre effect on volume changes characteristics of expansive soil combined with lime  and observational analysis to assess soil compaction and CBR combined with fly ash and discrete fibre  have been conclusively present.
The focus of this study is to presents the experimental results of the effective utilization of PP fibre and SF to stabilize the soil subgrade as a possible alternative from the environment, economic, and technical perspective. Many researchers studied the effect on consolidation and swelling of polypropylene fibre and silica fumes as individual material . However, the combined effect of PP fibre and SF on swelling–shrinkage and bearing strength were not investigated. Consequently, the study has been carried out to tackle the efficacy of silica fumes and polypropylene fibre in enhancing the geo-mechanical properties of expansive clays. This paper is a contribution to exploring the impact of mecho-chemical stabilization using polypropylene fibres and silica fume. A series of experimental tests were conducted, such as compaction, Atterberg’s limits, free swelling index (FSI), electrical conductivity (EC), pH, swelling pressure, and California bearing ratio (CBR).
The shrinkage limit of the soil is determined using the mercury displacement method. However, due to the hazardousness of the mercury, it requires proper care and cannot be employed safely. Alternative two methods were suggested, one being the manual measurement of the crack on the surface and the other being the Wax method, which was the time-consuming process and gave less accurate results. Hence, to overcome the above limitations and make the process safe, fast, and efficient, image analysis technique is employed. The shrinkage limit analysis MATLAB program code has been developed to quantify the effect of PP fibre and SF on the shrinkage limit of expansive soil.
Materials Used and Experimental Methods
The present research used expansive clayey soil (black cotton soil) from Central India. The soil specimens have been collected from 1.5 to 2.0 m depth from ground level. Various index property tests such as specific gravity, compaction, Atterberg's limit, grain size distribution, and free swell index were conducted for the initial investigation of soil samples. Table 1 summarizes the results of the index properties of expansive soil used in the research. In accordance with the unified soil classification system (USCS), the soil has been classified as inorganic clay with high plasticity (CH). A 120% Free Swell Index (FSI) has been observed at a period of 7 days; this indicates the higher potential of swelling occurs in soil.
The polypropylene (PP) fibres of an average length of 12 mm have been considered in this research work. This fibre is produced from a polymeric material and waste plastics. The various mechanical and chemical properties of PP fibre are shown in Table 2. The PP fibre is highly acid/salt resistance, and its higher tensile strength shows a higher potential to use the material as microfine reinforcement. The chemically inert, hydrophobic, and non-corrosive nature of PP fibre also indicates its effective utilization in the soil reinforcement due to low susceptibility of absorption or reaction with soil moisture or leachate.
The industrial waste derived from silicon metal or ferrosilicon alloy, silica fume (SF), was used to stabilize the expansive soil. The silica fume majorly consists of amorphous silica (SiO2). The higher specific surface area, cementing effect, and silica fume pozzolanic activity exhibit a higher potential to use as a cement and lime alternative. All the chemical compositions have been quantified using X-ray fluorescence microscopy. SF's physical and chemical properties are listed in Table 3.
Several specimens were prepared in the study with the addition of PP fibre and SF of various percentages up to 10% and 1.5%, respectively. The inclusion of more than 8.0% SF and 1.0% PP fibre content causes non-uniformity in the sample preparation. The higher concentration of PP fibre and SF content mixing would not be feasible for field applications. Also, obtaining the sound samples while extraction from the mold will not be possible. Accordingly, silica fume with 2.0%, 4.0%, 8.0%, and polypropylene fibre 0.25%, 0.50%, 1.00% (by dry weight of soil) finalized for the experimental analysis .
At first, the expansive soil samples were air-dried until a stable state was reached. Several mixtures were prepared by mixing the various percentages of water to achieve a uniform mixture of all ingredients. The behaviours of expansive soil affected by initial optimum moisture content and maximum dry unit weight . The specimens were, therefore, prepared at the soil maximum dry density and optimum moisture content. The 4.75 mm passed dry mixture of clayey soil was oven-dried at 105–110 °C, and SF (2.0%, 4.0%, 8.0%), and polypropylene fibre (0.25%, 0.50%, 1.00%) mix were prepared. The dry mix is maintained at 27 ± 2 °C and 65 ± 5% humidity in the environmental chamber to preserve the soil mass constant temperature and uniform moisture content.
In this present study, a series of experimental tests were conducted, such as compaction and Atterberg’s limit, free swelling index (FSI), electrical conductivity (EC), pH, swelling pressure, and California bearing ratio (CBR). The proportion of PP fibre (i.e., 0%, 0.25%, 0.50%, and 1.00%) and SF (i.e., 2.00%, 4.00%, and 8.00%) have been varied to provide the optimal amount of PP fibre and SF required to maintain the expansive soil subgrade. All specimens were carried out three times to understand the repeatability of experimental results. The findings of repeatability indicate 9.8% as the highest standard deviation; based on this, all tests are recorded on average for each experiment.
The 100 mm-diameter soil specimen was prepared by compacting the three-layer soil sample to perform the standard compaction test. Optimum moisture content (OMC) and maximum dry unit weight (MDU) of each sample were determined from this experiment. The values of the OMC and MDU obtained were used to prepare the soil sample in all experiments. To evaluate the effect of silica fume and polypropylene fibre on expansive soil, the Atterberg’s limits, i.e., liquid limit (Lw), plastic limit (Lp), plastic index (Ip), and volumetric shrinkage limit (ws), were determined. The soil sample passing from 425 microns has been used to calculated various Atterber’s limits.
Normally, free swelling experiments are used to classify expansive soils and forecast their propensity for swelling (IS: 2720 part-40). The sediment volume is assessed against the cylinder level after 24 h. The modified free swelling index was considered in this study .
The shrinkage limit places a vital role in the performance of expansive soil characteristics. The digital camera image processing technique has been proposed to quantify the shrinkage behaviours of expansive soil [6, 30]. The MATLAB program code has been developed to quantify the volumetric shrinkage limit of expansive soil. The flow of the MATLAB program is shown in Fig. 1a and the experimental setup used for capturing the digital image is shown in Fig. 1b. The quality of the image is highly significant and influences the result, and to maintain the same quality of the image, the height of the camera has been fixed at 300 mm after trial-and-error method calculation. The rectangular cropped image was taken as an input for the MATLAB program. In MATLAB image, an oval shape was established along the edge of container taking length and width as diameter and then separated along with outside was cleared. Then obtained separated image is smoothed, cleaned, contrasted, and stretched. The obtained refined image was then converted to grayscale from RGB (Red, Green, Blue). Grayscale image was segmented by thresholding. It divides the entire image into multiple regions. The cracked and shrinkage areas have darker pixels, because light does not enter into them, while the surface of the intact soil has brighter pixels, because there is enough light on its surface. By setting a threshold, all pixels in the image are changed to 0 (black pixels) or 255 (white pixels). Pixels with values less than the threshold will be turned into black, and pixels with values greater than the threshold will be turned into white. This selection is made using incremental values until the desired threshold level is reached, which is found to be between 0.225 and 0.35 after several trials.
The swelling pressure can be determined by the swell consolidation method, different pressure method, and constant volume method. The experimental study carried out by Soundara and Robinson shows that in the constant volume method, the soil fabric remains the same and gives moderate swelling pressure . Constant volume swelling pressure test has been adopted to calculate the upward swelling pressure and expansion percentage. To evaluate the strength performance of reinforced and unreinforced lightweight compacted expansive clayey soil, soaked CBR test has been conducted. The specimens were tested at a 1.25 mm/min strain rate, and the values of vertical displacement and load-carrying capacity have been recorded with 25 mm capacity LVDT and 50 kN capacity load cell.
The electrical conductivity and pH values have been calculated with the inclusion of SF with expansive soil, the effect of PP fibre not included due to inert behaviour with soil in a chemical reaction. To verify the microstructural effect of silica fume and polypropylene fibre; SEM analysis has been carried out. The freeze-cut-dying method is used for preparing the soil specimen for the microstructural analysis. This method can preserve the original soil microstructure and minimize flaws such as erroneous orientation and false void, which occur in the SEM micrographs .
Results and Discussion
The results of the standard compaction test on expansive soil subgrade with the inclusion of different percentages of SF and PP fibre content are shown in Fig. 2a, b. Figure 2a depicts that the OMC of expansive soil decrease with the inclusion of silica fume (i.e., 2.00%, 4.00%, and 8.00%); however, with the inclusion of polypropylene fibre content (0.25%, 0.5%, and 1.00%), the value slightly increases. Due to the higher specific surface area, SF fills the air voids in the soil mass and less water needed to compact the soil; as a result, the OMC value decreased. Polypropylene fibre does not react with water, but raises the air void in the soil specimen, and more water is needed to fill the air void; hence, the OMC increased with the addition of PP fibre. It can also be seen that with the overdosages of PP fibre content reduction in the OMC observed. Figure 2b shows that MDU of the reinforced specimens increases with the addition in the polypropylene fibre and silica fume content. It may be due to soil particles is replaced by SF, which is having a higher specific surface area. However, at the 1.00% PP fibre and 8.00% SF content, MDU decreased, since a higher content of silica fume creates cementation bond between soil particles and this bond required for energy to compaction. As a result, improper compaction takes place and subsequently reduces the dry density of the expansive soil. The OMC of expansive reduces with the inclusion of silica fume, the SF grains’ coats, and binds the clay mineral, which absorbs water content and, as a result, reduction in the OMC was observed.
Figure 2c, d shows the effect of SF and PP fibre inclusion on the liquid limit and plastic limit properties of expansive soil, respectively. It can be observed that the addition of SF reduces a liquid limit and plastic limit. The value of liquid limit and a plastic limit for expansive soil was observed as 89% and 51.15%, which reduces up to 54.36% and 34.44%, respectively. The maximum reduction in the liquid limit and plastic was observed with the inclusion of 1.00% PP and 4.00% SF content, which can be considered as optimum value. The values of liquid limit and plastic limit start increasing with the addition of 8% silica fume since the presence of higher content of SF absorbing more water to the pozzolanic reaction, which, in result, increases the plastic limit and liquid limit. Figure 3 demonstrates the effect of PP fibre and SF material on expansive soil plasticity index properties. With the inclusion of 2.00% SF content, the exponential reduction in plasticity index, i.e., 34.18%, 20.47%, 23.82%, and 21.43% has been observed at 0%, 0.25%, 0.5%, and 1.0% PP fibre content, respectively. The value of the plasticity index has been decreased with the addition of PP fibre content in the expansive soil. The presence of SF content processes the cementation effect in the expansive soil. The reduction in the plasticity index observed with the inclusion of SF content; however, higher SF content, i.e., 4% and 8% SF having more cementation effect and slight increment in the plasticity index, was observed.
The value of shrinkage limit majorly increased with the addition of polypropylene fibre, as shown in Fig. 4. The maximum shrinkage limit 35% was observed for the section BC + 8.0%SF + 1.0% PP, which is improved from shrinkage limit 9.21% of the unreinforced section. The alteration in silica fume content leads to aggregation of particles and, thus, increases the shrinkage limit. The soil was initially in the dispersed state and had high plasticity index. The reduction in diffuse double layer thickness and the repulsion between the clay particles takes place on account of rising in electrolyte concentration following the change in the content of silica fume. This results in the formation of aggregated clusters due to the movement of the soil particles towards each other. The increase in the resistance provided by these aggregate clusters against capillary suction induced volumetric shrinkage, causing an increase in the void ratio, and consequently, the water content also increases [33, 34]. The fibre content makes soil mass more resistant to the tensile cracking, and hence, a higher value of shrinkage limit has been observed at higher content of the PP fibre and SF. The processed results of the MATLAB program code for computing the shrinkage limit are shown in Fig. 5. The crack pattern can be clearly seen for all the specimens. The result of the mercury displacement shrinkage limit experimental test and results obtained from the image processing technique have been compared and are shown in Fig. 6. The results are very close to the experimental value, which shows the great potential to compute the shrinkage behaviours of the expansive soil using the digital image process technique. Thee free swell index (FSI) has been decreased with the inclusion of PP fibres and SF, as shown in Fig. 7. The maximum reduction has been observed at 8% SF content 40%, 35%, 30%, and 25% with the addition of 0%, 0.25%, 0.5%, and 1.0% PP, respectively. Both SF and PP fibres are non-expansive material. With the inclusion of non-expansive material in the expansive soil, the specific surface area of the expansive product reduces, and the free swell index decreases.
Figure 8 shows the effect of silica fume and polypropylene fibre on the CBR penetration value of 2.5 mm, and 5.0 mm, and SF and PP fibre show great potential to increase the CBR value of expansive soil. The CBR value has been increased by 107.89%, 86.84%, 78.94%, and 78.94%, for the section BC + 8.00% SF + 0.25% PP, BC + 8.00% SF + 0.50% PP, BC + 0.50% PP, and BC + 4.00% SF + 1.00% PP, receptively. The maximum improvement has been observed for the BC + 8.00% SF + 0.25% PP section. The higher percentage of SF content has a cementing effect in the expansive soil, and as a result, the expansive soil becomes steeper, and more CBR value has been observed. The polypropylene fibre reinforces the soil grain and offers higher resistance against a load. The improvement in the CBR value increases the load-carrying capacity of the pavement surface, and this will increase the performance of the paved structure.
Figure 9 shows the effect of SF and PP fibre on the upward swelling pressure of expansive soil. With the addition of SF and PP fibre content, the swelling pressure was reduced. When mixed 8.0% SF with expansive soil, a 49.38% decrease in swelling pressure was measured. Reduction in swelling pressure due to non-expansive behaviour and higher specific surface area of silica fume was observed. Calcium silicate hydrate (C-S-H) gel is produced when SF content associated with the water while undergoing the pozzolanic reaction, as shown in Eq. 1. The chemical reaction of silica fume and calcium allows the specimens prepared with SF more brittle and steeper because of decreased swelling pressure and expansion. The swelling pressure for the non-reinforced expansive soil samples decreased to 13.63%, 18.88%, and 30.25% with the inclusion of 0.25%, 0.50%, and 1.00% PP, respectively. The polypropylene fibre shows no expansion and control of the swelling pressure due to its inert behaviour with water by reducing the specific surface area of expansive material. The discredited distribution of the PP fibre intact the specimen and has shown effectiveness to control the upward swelling pressure. However, at 1% PP fibre content, the swelling pressure starts increasing, because due to the presence of higher content of fibre in the soil specimen required more compaction energy to achieve the maximum dry density. The fibre content creates air voids in the specimen, and upon saturation, the voids filled with water and exerted more pressure.
The electrical conductivity and pH value test conducted to understand the chemical reaction of the silica fume with expansive soil, and test results are presented in Fig. 10. The value of EC is increased continuously by increasing the percentages of SF, which confirms the chemical reaction occurs when silica fume mix with expansive soil and water. It is well established that a rise in the temperature of the atmosphere will improve the hydration of cement [35, 36]. The EC values increased after SF has been added. On the other hand, decreasing the swelling pressure caused due to the development of clay-SF reactions and the production of C-S-H gel binds the clay lumps together and reduces the hydration of clay mineral. The pH values have also been increased with increasing of SF content, which signifies changes in the nature of soil-silica fume solution and confirms the chemical alteration in the specimen. The increment in the pH value increases the shear strength parameter of the soil specimen . The higher specific surface area of the soil matrix makes it an effective filler. This reduced the volume of soil voids and induced the homogeneous production of C-S-H gel, i.e., sound microstructure. This behaviour can reduce the adsorption capacity of the clay surface and reduce the expansion in the treated samples [38, 39].
Figures 11a shows the results of expansive soil (BC), and it can be observed that the various cavities in the specimen are present. Figure 11b, c shows the effect of 2.0% and 8.0% silica fume content, respectively. It confirms the presence of C-S-H gel (calcite content) in the soil specimen and filling and coating the expansive soil particles, which significantly reduces the swelling nature of the expansive soil. Figure 11d depicts the effect of polypropylene fibre on expansive soil, and it can be seen that the PP fibre tightly packed with the expansive soil grain and give higher resistance during tensile stress.
The potential use of PP fibre and SF to improve the engineering characteristics of expansive soils has been presented. SF and PP fibres are industrial waste products, which have an adverse impact on the environment if not disposed of safely. Its utilization in the construction industry can reduce the disposal problem of waste and the use of conventional natural materials. The PP fibre and SF have a higher potential to enhance the engineering properties of expansive soil subgrade and can be used for the field trial for future investigation. Based on the results and discussion presented, the following conclusions have been made:
The digital image processing technique showed a higher potential to understand the soil shrinkage limit. It shows similar results as obtained by the mercury displacement method, and this method can be employed to understand the soil morphology in the large area.
The presence of SF and PP decreased the liquid limit and plasticity index has been increased in all the samples. For this reason, silica fume treatment to the expansive soil classification has been changed from high-plastic clays (CH) to low-plastic clays (OH) under UCSC classification.
The maximum dry density of the treated samples has also been observed and improved with the addition of both the additive and optimum values which has been observed at 0.5% PP with 2.0% 4.0%, and 8.0% SF content.
The maximum reduction in free swell index has been observed at 8% SF content 40%, 35%, 30%, and 25% with the addition of 0%, 0.25%, 0.5%, and 1.0% PP, respectively. The upward swelling pressure has been reduced exponentially due to the cementation effect of SF content.
The CBR value has been increased by 107.89% for the section BC + 8.00% SF + 0.25% PP. The improvement in the CBR value increases the load-carrying capacity of the pavement surface.
The value of EC and pH has also been increasing, which shows the chemical reaction takes place with the addition of silica fume with expansive soil. The presence of the calcite content with the inclusion of SF has been observed in the SEM micrographs.
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Tiwari, N., Satyam, N. & Patva, J. Engineering Characteristics and Performance of Polypropylene Fibre and Silica Fume Treated Expansive Soil Subgrade. Int. J. of Geosynth. and Ground Eng. 6, 18 (2020). https://doi.org/10.1007/s40891-020-00199-x
- Expansive soil
- Shearing strength
- Paved structure
- Image processing