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Article

Prediction of Compression Index from Secant Elastic Modulus and Peak Strength of High Plastic Clay Ameliorated by Agro-Synthetic Waste Fibers for Green Subgrade

by
Ayesha Zubair
1,
Zainab Farooq
1,
Khalid Farooq
1,
Zubair Masoud
1,
Hassan Mujtaba
1,* and
Abdullah Mohamed
2
1
Department of Civil Engineering, University of Engineering and Technology, Lahore 54890, Pakistan
2
Research Center, Future University in Egypt, New Cairo 11835, Egypt
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(22), 15871; https://doi.org/10.3390/su152215871
Submission received: 28 September 2023 / Revised: 20 October 2023 / Accepted: 23 October 2023 / Published: 12 November 2023
(This article belongs to the Special Issue Sustainable Development of Geotechnical Engineering)
Browse Figures
Review Reports Versions Notes

Abstract

:
Agro-synthetic stabilization of high-plastic clay is trending due to its vital role in sustainable geotechnical construction and maintenance of clay subgrade. Remoulded samples of high plastic clay (C), ameliorated by optimal doses of 1.2% polyester (P) and 0.9% banana (B) at maximum dry density (γdmax) and optimum moisture content (OMC), were subjected to swell potential, unconsolidated undrained (CU) triaxial, consolidation, and California bearing ratio (CBR) tests. The outcome of this research presents that the use of an optimal clay-polyester-banana (CPB) mix enhanced the secant elastic modulus (E50), peak strength (Sp), and CBR by 2.5, 2.43, and 2.7 times, respectively; increased E50/Cc increased from 12.29 to 53.75 MPa; and lowered the swell potential by 48% and compression index (Cc) by 42.8%. It was also observed that the increase in moisture content (mc) of the optimal CPB mix from 20% (unsaturated phase) to 32% (wet phase) decreased Sp from 212 kPa to 56 kPa and E50 from 8.42 MPa to 2.16 MPa, whereas Cc was increased from 0.16 to 0.26, depicting the potential use of the CPB mix as a stable and sustainable geotechnical material even in wet seasons. Novel correlations are developed for the prediction of Cc from mc, E50, and Sp for an optimal CPB mix to achieve sustainable geotechnical systems and designs in sustainable geo-environmental engineering.

1. Introduction

Shrinkage and swelling in high plastic subgrade clays results in cracks in the wearing course layer of flexible pavements, which requires the mitigation of high swell and shrinkage of clay by stabilizing material [1,2]. Road infrastructure is frequently required to be constructed on problematic expansive clays where the subgrade poses large settlement problems in response to traffic loads [3]. Soil stabilization is a good option to ameliorate strength and settlement parameters by using waste fibers for project sustainability on a long-term basis [4]. Environment-friendly modification of compressibility properties of high-plastic expansive clays by use of fibers is gaining high trend in this era [5].
Cement and lime are frequently used in the construction of road infrastructure for the stabilization of subgrade soils, resulting in CO2 emissions in the environment [6]. Calcium and alumina in cement and lime react with sulfates in expansive clay, resulting in expansive minerals, i.e., ettringite [7]. Due to severe environmental pollution in the recent decade, natural fibers are an excellent option for use as green stabilizers on a large scale to reduce carbon footprints and protect the environment in the future [8,9].
The use of waste fibers in soil stabilization is acclaimed as an alternative to chemical stabilization. Now-a-days, waste fibers are replacing lime for stabilization of clays as an environmentally-friendly green alternative material to attain sustainable and eco-environmental construction goals [10,11]. The addition of fibers causes the enhancement of the elastic properties of clays, which are requirements for effective subgrade design for heavy-loaded pavement [12]. Eco-friendly use of fibers is an effective method to improve the shear strength and bearing capacity, along with reducing the expansiveness and settlement of weak clayey strata [13].
Appreciable application of randomly distributed fibers to enhance the post-peak strength, secant deformation modulus (Es), unconfined compressive strength (UCS), and California bearing ratio (CBR) of clays for use as significant parameters in construction of road infrastructures [11,14], whereas peak shear strength and secant deformation modulus can be derived from the UCS stress-strain curve [15]. Fibers are commonly used as sustainable reinforcing smart materials for reduction of compressibility as fibers make a net due to random distribution in clay to resist deformation and swelling [16,17].
Polyester fibers have been extensively used for fabric manufacturing for many decades, so large volumes of waste textiles are being faced as a significant problem for landfills [18,19]. It is worth mentioning that polyester fibers resist the piping in soil during erosion, and cracking in clay is reduced [20,21] Waste banana fibers are frequently available due to large cultivation volumes and are being used for enhancement of the mechanical properties of soil to attain high-strength, affordable fiber-clay design mixes [22]. Banana fibers, after conversion from hydrophilic to hydrophobic by chemical treatment, can be used effectively for the stabilization of high-plastic clays for use in subgrade construction [12].
Banana fibers, which are socio-economic waste material extracted from the banana pseudo-stem, can be used in soil stabilization [23,24]. High cellulose and low micro-fibril angle of banana fibers are significant for their use in soil strengthening [12]. Both polyester and banana fibers act in a combined pattern, due to which the clay shows adhesive forces between clay and fibers. In addition to the cohesiveness of clay particles, the unconfined compressive strength of clay is enhanced ([12]).
An isotropically consolidated undrained (CU) triaxial test is conducted for the evaluation of the undrained shear strength of clays [25], whereas the hydro-mechanical behavior of saturated clays in compression is influenced by saturation level. It is worth mentioning that the undrained shear strength of clays is strongly related to the initial moisture content, whereas an increase in moisture content causes a significant decrease in the undrained shear strength of high-plastic clays [26]. The behavior of the compression curve (e-log p curve) of clays is significantly affected by the initial moisture content; hence, the intrinsic compressibility of clay is strongly related to the initial soil-water interaction, especially in kaolinitic soils [27]. Elastic modulus (E) is a significant parameter used in the calculation of settlement for foundations where moisture content (mc) governs the extent of elastic modulus (E) and shear strength, which are strongly affected by the matric suction in unsaturated clays [28]. The assessment of saturation and matric suction level is significant for the prediction of the shear strength of clays.
Nano-clay and nano-silica can be used to stabilize the expansive clays to improve their strength characteristics ([29]). Silica (SiO2) and Alumina (Al2O3) from rice husk ash can effectively improve the strength of expansive clay ([30]).
Nano-materials are used in the improvement of materials and also in clay stabilization for the improvement of elastic, strength, and swell parameters ([31,32,33,34,35]). The characterization of clay samples can be precisely conducted by Fourier Transform Infrared Spectroscopy (FTIR) for the identification of different minerals and X-ray fluorescence (XRF) for the quantification of different minerals ([31]).
The compressibility of clays is a critical aspect for the design of geotechnical structures, whereas Cc is an essential part of consolidation settlement models. A lot of work has been conducted and widely referenced on the correlations of compression index with initial moisture and void ratio (e) for clays [36]. The compression index (Cc) can be evaluated by a time-consuming laboratory consolidation test [37]. The consolidation test on high-plastic clay may take 10 to 12 days, starting from the preparation of the sample to the completion of the loading and unloading stages. A large number of consolidation tests may cause considerable delay in geotechnical projects; hence, empirical correlations are frequently used by geotechnical engineers for the prediction of Cc. The existing empirical correlations of Cc in the literature oversimplify the complex phenomenon of compressibility. In the literature, correlations of Cc with index properties like moisture content (mc), initial void ratio (e), and liquid limit (LL) are common; however, the compressibility of clays is substantially enhanced by increasing the initial moisture content (natural moisture content) of clays [37].
Cc is frequently used in settlement analysis; however, the correlations of Cc with the elastic and strength properties of fiber-clays mixes in unsaturated and wet phases are rare in the literature. Hence, the need is felt for the correlation of Cc with elastic modulus (E50) and peak strength (Sp) evaluated from consolidated undrained (CU) triaxial tests for high plastic clay to develop suitable correlations in both triaxial and consolidation tests by simulation of saturation phase, dry density, and initial moisture content. These correlations may be used for speedy geotechnical estimates for projects, especially for the performance models of clay subgrades in wet climates. The results of this research will be evaluated using a well-known statistical tool like analysis of variance (ANOVA), which presents the precision of the data sets to determine their health. In the past, rare studies have been seen on agro-synthetic stabilization, especially clay-polyester-banana fiber blends to achieve green stabilization of subgrade clay. This research intends to illustrate the use of agro-synthetic PB fibers in clay stabilization to achieve the following objectives:
i.
Evaluation of optimal contents of polyester (P), banana (B), and polyester-banana (PB) combined fibers for amelioration of high plastic clay for enhancement of secant modulus (E50), peak strength (Sp), and CBR along with reduction in swell potential and compression index (Cc).
ii.
Hydro-mechanical studies on the optimal clay-polyester-banana (CPB) mix in unsaturated and wet phases and the development of correlations of Cc versus mc, E50, and Sp.
iii.
Cost-benefit analysis and environmental impact of the CPB mix to achieve eco-economic green stabilization.

2. Materials and Methods

High-quality plastic clay (C), polyester (P), and banana (B) fibers were used as materials in this study. High-quality plastic clay was collected from Lahore, Pakistan. Banana fibers were prepared from waste pseudo-stems of banana plants, which are initially hydrophilic, whereas the hydrophobic characteristics of fibers are mostly required in soil stabilization, especially on exposure of subgrade to wet seasons [38,39]. Fibers were air-dried for three days at an average temperature of 18 °C. To change the nature of banana fibers from hydrophilic to hydrophobic, the banana fibers are chemically treated with NaOH. In the first step, banana fibers were dipped in a 2% detergent-water mixture, completely washed, and dried in the air for a smooth drying process. In the next step, the fibers were dipped for approximately one hour in an aqueous solution prepared by 5% NaOH, which resulted in an enhancement of the mechanical strength and thermal stability of banana fibers [40,41]. Finally, the fibers were washed by dipping in distilled water and air-dried before being used in soil stabilization [42]. The separation of fibers from each other, placement in the clay, and uniform mixing are the critical processes at road construction sites that require precision for the achievement of the required properties of the clay-fiber mixture. Clay, polyester, and banana fibers used in the experimentation are presented in Figure 1.
Clay material was air-dried before classification tests. The clay used in the experimentation was classified as CH (high plastic clay) as per [43]. The index properties of clay are summarized in Table 1.
The polyester fibers contain ester functional groups, and these fibers are frequently used in the manufacturing of cloth. Polyester fibers show good resistance to sunlight, abrasion, and wrinkles. The index properties of polyester fibers are presented in Table 2.
Presented in Table 3 are the physical properties of the banana fibers used in this research. Initially, the high moisture content and high cellulose content of banana fibers were commonly observed.
The mechanical behavior of clay can be effectively studied by using remolded samples in experimentation [47]. Polyester (P) fibers in proportions of 0.4%, 0.8%, 1.2%, and 1.6%, whereas banana (B) fibers in proportions of 0.3%, 0.6%, 0.9%, and 1.2% were mixed in high plastic clay to reconstitute the clay-fiber mixtures.
Fiber mixing was performed manually using wire brushes for individual fibers and also for combined air-dried fibers in clay in the experimentation of this research. Thorough mixing of the fibers in the clay was conducted manually as per the procedure outlined by [21] to obtain a homogeneous mixture of clay and fibers. A random mixing procedure was adopted to eliminate the chance of making weak planes in the combined clay-fiber samples. Random mixing is a fast process in the construction of road subgrade. [48,49]. The sample preparation for clay-fiber mixes is a challenging job. The uniform mixing of fibers was assured so that the stresses could be distributed uniformly in the sample. The uniform mixing of both the banana and polyester fibers was also considered a critical step in sample preparation. During the pouring of the clay into the mold, both types of fibers were uniformly distributed. This random fiber mixing procedure is also strictly observed in the field to achieve the optimum strength level of the clay-fiber mix. Twenty-four clay-fiber blends were prepared with three replicates for each blend, making a total of seventy-two data sets.
In Figure 2, the typical clay, clay-polyester, clay-banana, and clay-banana-polyester samples are presented before being subjected to a triaxial test. The matric suction was taken at 25 kPa for all the samples to be tested in triaxial for evaluation of the strength characteristics of optimal mixtures of clay-fiber mixes at the same matric suction level.

2.1. Classification Tests

Grain size analysis [50,51] tests were performed on three clay samples to evaluate the average of sand, silt, and clay proportions. Atterberg limits (liquid limit and plastic limit) tests [45] were conducted on three samples of experimental clay. The soil was classified on the basis of average grain sizes and Atterberg limits data as per the procedure outlined in [43]. Specific gravity (Gs), dry density, and moisture content of the soil were determined as per [46,52,53], respectively, to assess the effect of index properties on the strength and stiffness of soil specimens.

2.2. Compaction and CBR Tests

Modified compaction and California bearing ratio (CBR) tests for triplicate samples were performed as per the methods outlined in [44,54] respectively. Soaked CBR tests were conducted in a mold of volume 2315.5 cm3 in five layers by applying compaction efforts of 10, 30, and 65 blows per layer per mold at γdmax, and OMC was evaluated from the modified compaction test. A surcharge load of 4.54 kg and a penetration rate of 1.3 mm/min were adopted in the loading system for the determination of loads at 0.254 cm and 0.508 cm penetration. Modified compaction and CBR tests were performed on the clay and clay-fiber mixes by mixing individual fibers, i.e., polyester and banana, as well as combinations of fibers, i.e., polyester-banana.

2.3. Triaxial (CU) Test

Triaxial (CU) tests were conducted as per the procedure mentioned in [25] on the samples with a length-to-diameter (L/D) ratio of 2, prepared at γdmax and OMC for the evaluation of the strength characteristics of remoulded, untreated, and stabilized expansive clay. The peak shear stress (Sp) and secant elastic modulus (E50) (i.e., ratio of stress to strain at 50% of Sp) were calculated on the basis of the trend of the stress-strain curve. E50 is a significant parameter for the evaluation of the stress-strain behavior of untreated and stabilized soils for use in geotechnical problems involving large strains, like the failure of clay embankments. E50 of soil was evaluated for different dosages of P and B fibers in clay-fiber mixes. After installation of samples in a triaxial machine, samples were saturated up to 98%, and isotropic consolidation stress was applied at 25 kPa, 50 kPa, and 75 kPa, where 50 kPa simulates the overburden pressure for commonly used subgrade thickness for heavily loaded pavements. Effective stress was calculated by subtracting pore-water pressure (uw) from total pressure (i.e., σʹ3 = σ3 − uw and σʹ1 = σ1 − uw), where σʹ3 is effective all-round stress and σʹ1 is effective vertical stress used for compressing the samples to failure by application of deviotor stress, i.e., σ1 − σ3 as per the consolidated undrained (CU) condition of the triaxial (CU) test. The peak strength (Sp) was evaluated from the apex of the elastic-plastic transition of the stress-strain curves of triaxial (CU) tests. The matric suction, which is the difference between pore air pressure and pore water pressure (i.e., s = ua − uw), was maintained at a level of 25 kPa for all triaxial tests so as to compare the undrained peak strength (Sp) for the same trend of soil-water interaction, especially for unsaturated, untreated, and treated clay samples, due to the fact that an increase in matric suction causes substantial enhancement in the elastic modulus of soils [55]. Triaxial (CU) tests at variable moisture content are also conducted on untreated and stabilized clays for wet-season feasibility.

2.4. Consolidation and Swell Potential Tests

Consolidation tests were performed as per [56] for the determination of compression index (Cc) for untreated and stabilized clay. The loading sequence of 25 kPa, 50 kPa, 100 kPa, 200 kPa, 400 kPa, 800 kPa, and 1600 kPa was adopted to achieve the void ratio (e) versus log p curve for evaluation of Cc from the slope of the relatively uniform compression curve portion, which ranges from 400 kPa to 1600 kPa. At the start of the test, the clay absorbed water and started swelling after inundation (free flooding for 24 h). The swell potential of soil was measured as an increase in volume compared to the original volume. After the swelling has ceased, the next loading is selected from the standard loading sequence, which should be larger than the swell pressure noted at the completion of the swelling.
It is commonly seen in high-intensity rainfall areas that the rainwater penetrates subgrade soil and causes swelling of high-plastic clay, ultimately resulting in uneven settlement of flexible pavement. Hence, the swell potential needs to be determined for high-plastic clays before use in subgrade construction. In this research, swell potential tests were performed as per the procedure described in [57] for non-treated clay, clay plus individual polyester (P) /banana (B) fibers, and clay plus PB fibers, i.e., CPB mix.

2.5. Effect of Moisture on Clay-Fiber Mix

A comprehensive study of the interaction of water with untreated and stabilized clay was conducted to assess the feasibility of PB-stabilized clay in wet seasons and take potential preventive measures for the protection of subgrade from severe settlement. The initial moisture content (mc) of the clay samples affects the undrained shear strength of clays significantly. The effect of mc on strength parameters, i.e., E50 and Sp, and compressibility parameters, i.e., Cc, were evaluated for untreated and stabilized clay for use in the assessment of the service life performance of clay subgrade.

3. Results and Discussion

3.1. Classification Tests

Sand, silt, and clay were determined to be 2%, 34%, and 64%, respectively, from the results of grain size analysis as shown in Figure 3.
The plasticity index (PI) was evaluated at 34%, and soil was classified as “high plastic clay i.e., CH” as per [50], which describes the engineering classification of soils. The specific gravity [46] of the soil was evaluated as 2.70. The field dry density and natural moisture content of the clay were observed to be 1.76 g/cm3 and 13.7%, respectively.

3.2. Compaction and California Bearing Ratio Tests

A decrease in maximum dry density (γdmax) and an increase in optimum moisture content (OMC) are characteristic of fibers, as the fibers are light in weight and act as water absorbents when used in clay-fiber mixes. The maximum dry density (γdmax) and optimum moisture content (OMC) for high plastic clay were observed to be 1.83 g/cm3 and 16.3%, respectively. The dry density and moisture content trend for the modified compaction test is presented in Figure 4.
The decrease in γdmax was observed to be 5.1%, 6.7%, and 9.4% in cases of optimized content of polyester (P), banana (B), and polyester-banana (PB) in clay-fiber mixes, respectively. It was observed that individual fibers, i.e., P, B, and combined PB fibers, enhanced the California bearing ratio of high-plastic swelling clay when used in pavement subgrade. It was observed that optimal proportions of individual P and B fibers, i.e., 1.2% and 0.9% of the dry weight of clay, respectively, increased the CBR of non-stabilized clay by 1.64 and 1.51 times, respectively, whereas the optimal blend of “C + 1.5% P + 0.9% B” enhanced the CBR by 2.7 times the non-stabilized clay as presented in Table 4 and Table 5.

3.3. Triaxial CU Test

Presented in Figure 5 are the samples after testing in triaxial (CU) conditions, depicting that clay-polyester (CP) and clay-banana (CB) samples showed fewer cracks as compared with untreated clay samples and also showed that minor cracks were developed in samples containing combined fibers, i.e., CPB samples, at the failure stage. The scattering of fibers in clay samples is shown in Figure 6 and Figure 7. It is also observed that larger cracks in untreated clay samples resulted in a lowering of peak stress, which is validated by the stress-strain curve in Figure 8. Shown in Figure 8 are typical triaxial (CU) curves where the clay-banana (B) mix showed a less significant increase in stress as compared with clay-polyester (P); however, the enhancement in peak stress is higher as compared with untreated clay. The stress-strain curve of the sample containing clay plus 1.2% polyester plus 0.9% banana fibers (optimal CPB mix) showed optimum peak strength as compared with clay (C), clay-polyester (CP), and clay-banana (CB) blends. Triaxial (CU) test results were analyzed for evaluation of the secant elastic modulus, i.e., E50. Figure 9 presents the evaluation of E50 for untreated and stabilized clay samples at an effective stress (σʹ) of 50 kPa as it simulates the overburden pressure of subgrades of heavy traffic road infrastructure. The ratio of secant elastic modulus to peak strength (E50/Sp) was evaluated for the untreated and stabilized clay samples for all proportions of fiber (CP, CB, and CPB). It was observed that E50/Sp is significant in the way that, by evaluating the E50 from triaxial data, the values of Sp can be predicted at different moisture contents for the clay-fiber blends. Triaxial tests were conducted in unsaturated and wet phases of clay. In the first phase of testing, the samples were prepared at OMC and γdmax. The unsaturated clay-fiber stabilized mixes with matric suction of 25 kPa showed an increasing trend (Figure 9 where end points of straight line are indicated for stress-strain determination for E50) of Sp and enhancement of E50, which is supported by the research [58] conducted for enhancement of E50 by use of synthetic fibers in expansive clay. The increase in peak stress (Sp) for CP, CB, and CPB was observed to be 1.93, 1.62, and 2.43 times higher, whereas E50 was enhanced to 1.72, 1.27, and 2.5 times higher than untreated clay. The transition from brittle to ductile failure was observed in the samples with higher fiber content. It is also observed that ductility increases with an increase in fiber content. Stress-strain curves of the CPB mix at various initial preparation moisture contents ranging from 20% (unsaturated phase) to 32% (wet phase) are presented in Figure 10 to evaluate the effect of higher moisture content (at the optimum level) on the sustainability of CPB mix in wet seasons. The expanded stress-strain curves are shown in Figure 11 (where straight lines are shown for evaluation of E50) for the evaluation of E50 at 50% of the peak stress for CPB mixes at different moisture contents.

3.4. Consolidation and Swell Potential

The decrease in Cc causes a reduction in the settlement of the clay subgrade, resulting in sustainable and durable pavements. Figure 12 shows the consolidation test curves of clay mixed with polyester (P), banana (B), and polyester-banana (PB) fibers (Cc for clay has been evaluated by horizontal and vertical lines and mentioned in Figure 12). The swell pressure of the experimental clay was observed to be less than 100 kPa. Hence, the consolidation of the soil started after a loading of 100 kPa, i.e., a third loading step in the standard loading sequence presented in the methodology. Hence, the soil showed consolidation after attaining the swell potential of each clay-fiber mix.
It is observed that the optimal contents of CP (C + 1.2% P), CB (C + 0.9% B), and CPB (C + 1.2% P + 0.9% B) reduced the compression index (Cc) by 14.2%, 10.7%, and 42.8%, respectively, as compared to the untreated clay. Moreover, the CPB blend reduced the swell potential of clay by 48%. The porosity of the clay material was determined by the void ratio. The porosity of untreated clay and optimal CPM mix was evaluated at 42.8% and 40.3%, respectively.
A summary of the test results of the triaxial (CU), one-dimensional consolidation, and CBR tests is presented in Table 4 and Table 5. The ratios of E50/Sp representing elastic-strength characteristics and E50/Cc presenting the elastic-compressive behavior of the fiber-stabilized clay are also presented for different clay-fiber mixes.
Individual properties of C, CP, and CB show that increases in fiber content cause enhancements in secant elastic modulus (E50), peak strength (Sp), and CBR (Table 4).
Table 4 and Table 5 show variations in Cc and E50, which show that the increase in elastic modulus causes a considerable lowering in Cc. A significant parameter, E50/Cc, has been developed for untreated and stabilized clay. This parameter has great significance in the determination of compression index (Cc) by evaluating E50 (from the stress-strain curve of the triaxial (CU) test). It has been observed that the E50/Sp and E50/Cc of untreated clay are 31.8 and 12.2, respectively. The inclusion of optimum fiber contents, i.e., C + 1.2% P, C + 0.9% B, and C + 1.2% P + 0.9% B, showed E50/Sp as 28.3, 24.7, and 32.7 MPa, whereas E50/Cc came out to be 24.7, 17.6, and 53.7 MPa, respectively.

3.5. Composition of Clay

The clay sample was analyzed by X-ray fluorescence (XRF) testing for the different oxides. The quantities of different oxides are presented in Table 6.
The Fourier Transform Infrared Spectroscopy (FT-IR) test was conducted on the soil (in powdered form). The spectra of wave numbers ranging from 4000 to 400 cm−1 are used in this method. The FTIR result shows the main constituent as silica (SiO2) (corresponding to the highest peak), validating the X-ray fluorescence results of experimental clay. FTIR shows the main peak of transmittance (T) as 62% at a wave number of 997.98 cm−1 showing SiO2. Other peaks at wave numbers 779.1 cm−1, 697.14 cm−1, and 636.39 cm−1 with T values of 94.35%, 95.68%, and 96.23% show other clay minerals. FTIR validated the composition of the clay minerals mentioned in Table 6.
Figure 13 shows that compression index (Cc) decreases with an increase in secant elastic modulus (E50) as compared with untreated clay in all the blends of CP, CB, and CPB (trend lines for the correlations are shown in Figure 13). It was observed that Cc values decrease more rapidly with an increase in E50, as observed in the case of CPB blends, inferring stronger blends in comparison with CP and CB.

3.6. Prediction of Cc for CPB Mix

A comprehensive study was conducted for the evaluation of the effect of mc on Cc, E50, and Sp for an optimal clay-polyester-banana (CPB) mix. Developed correlations show the feasibility of using CPB mixes in subgrades subjected to wet climates where clay absorbs high moisture content. E50 and Sp of the CPB mix are higher as compared with CP and CB mixes (Table 4 and Table 5), which infers that polyester fibers contribute a high elastic modulus as compared with banana fibers. Moreover, the most critical parameters like E50 and Cc for the CPB mix are observed to be reduced significantly with an increase in initial mc. Elastic properties of clays play a vital role in estimating the compressibility of clay strata; hence, the relationship between E50 and Cc has vital importance. Evaluation of Cc from the consolidation test is a time-consuming process [36].
A large number of correlations of Cc with natural (initial) moisture content (mc), liquid limit, and void ratio (e) have already been developed in the past for clays of low to high plasticity [36], which lack the effect of significant parameters like soil fabric, stress history, and cementation. However, these factors can be represented by incorporating the elastic and strength parameters of experimental clay for the evaluation of the compression index (Cc). Hence, in present research, novel correlations of Cc versus mc, E50, and Sp for CPB mix (Figure 12, Figure 13 and Figure 14) are developed for use in the remediation of high plastic clay subgrade. In the past, untreated high plastic clays were studied by [36,37] for the prediction of Cc, showing a relatively linear correlation of Cc with natural moisture content; however, clay-fiber mix is rarely explored for the correlation of Cc versus moisture content, strength, and elastic parameters of clays. Cc is a significant and essential parameter for the prediction of settlement by using E50 from the stress-strain curves of triaxial (CU). Presented in Figure 14, the data on natural moisture content versus Cc described by [37] for high plastic clays are compared with the Cc determined in this research, which shows that fibers plus clay mix show relatively low Cc values due to the additional strength added by fibers.
The novel proposed correlation of Cc versus mc and E50 is given as Equation (1) (for the data sets presented in Figure 14 and Figure 15) as per Figure 16, whereas the correlation of Cc versus mc and Sp is shown as Equation (2) (for the data sets presented in Figure 14 and Figure 17) as per Figure 18 for optimal CPB (clay + 1.2% polyester + 0.9% banana) for the mc range of 20% (unsaturated condition) to 32% (wet condition), by using MATLAB.
Cc = −0.0280 + 0.0086 × mc + 0.0024 × E50   (mc in% and E50 in MPa)
(where R2 = 0.9672, Adjusted R2 = 0.9636, Root mean square error (RMSE) = 0.0059, and Sum of Squares due to Error (SSE) = 0.0006. The very low value of RMSE presents the high precision of the proposed model)
Cc = 0.0339 + 0.0069 × mc − 0.0428 × Sp   (mc in % and Sp in MPa)
(where R2 = 0.9662, Adjusted R2 = 0.9625, Root Mean Square Error (RMSE) = 0.006, and Sum of Squares due to Error (SSE) = 0.0007)
The very low value of RMSE shows the high precision of the proposed models presented in Equations (1) and (2) as per MATLAB. The proposed models support the concept of higher settlement in weakly saturated clays, as clay showed lower values of Sp and E50 at 32% moisture content (wet condition) as compared to 20% moisture content. In the past, most of the correlations of Cc were based on a single parameter, i.e., moisture content (mc), plasticity index (PI), or void ratio (e) of untreated clay, which did not cover all aspects in the same model. The newly developed correlations in this research are valid for CPB mixes and include elastic and strength parameters along with the basic parameter of moisture content (mc) in unsaturated and wet phases, especially for subgrades exposed to a wet climate.
The Cc (compressibility parameter) values for the optimal CPB mix at a mc value of 32% are near the Cc values of the original clay samples at OMC (16.3%) and γdmax, which shows the feasibility of using an optimal blend of 1.2% polyester plus 0.9% banana fibers in clay with confidence even in wet conditions. It was observed that the optimal dosage of polyester and banana mixed with clay ensures 2.41 MPa of E50 even at 32% moisture content (a wet season moisture), hence providing confidence to geotechnical designers for the construction of subgrades using stabilized expansive clays in wet climates. The outcome of this study may be utilized to enhance the “Wet-Climate Performance” of stabilized clay.
The correlations presented in Equations (1) and (2) were validated (Figure 19) by using additional large data from twenty-one triaxial (CU) and consolidation tests for evaluation of Cc from mc, E50, and Sp at variable moisture content ranging from 20% (unsaturated phase) to 32% (wet phase). Cc predicted by Equations (1) and (2) fall within 10% of the line of equality (1:1), inferring the high precision of the proposed models.

3.7. Cost-Benefit Analysis

Cost-benefit analysis is the key aspect of soil stabilization at any geotechnical project [58]. The use of waste fibers for stabilization of clay subgrade can bring economic benefits to flexible pavement construction [12]. The comparison of improved geotechnical parameters with the cost of fiber inclusion has been shown in Figure 20 for the feasibility of fiber mixing in high plastic clay subgrade. Cost analysis covers additional expenditures on waste polyester and banana fibers, mixing/spreading expenditures, and the cost of roller compaction to achieve optimum E50, Sp, and CBR per ton of clay. According to market rates, the average cost of waste polyester and banana fibers is $11/Kg and $4/Kg, respectively. It is observed in Figure 20 that P (1.2%), B (0.9%), and PB (1.2% P + 0.9% B) showed a good level of enhancement in E50, Sp, and CBR at comparatively lower costs, presenting an eco-economical and cost-effective use of a polyester-banana mix for the amelioration of expansive clay subgrade based on optimum moisture content and maximum dry density values determined from a modified compaction test. It is observed that the cost of polyester fibers is about three times that of banana fibers, whereas the peak strength contribution of P (1.2%) is just 1.15 times that of B (0.9%). Hence, the use of banana (B) fibers in conjunction with polyester (P) fibers is cost-effective as compared to performance in clay stabilization, which is the basic parameter for banana fiber inclusion criteria in this research.

3.8. Statistical Analysis

The Analysis of variance (ANOVA) statistical tool was applied to the datasets of mc, Es, Sp, and Cc parameters used in developing models for the evaluation of Cc for the CPB mix. ANOVA validated the good health of data sets as no outlier was identified, inferring high precision and confidence (as per the F and p values mentioned in Table 7 for the application of this research to geotechnical problems during the stabilization of high plastic clays).

4. Conclusions

The goal of this study was to attain sustainable, environmentally friendly green amelioration of the high-plastic expansive subgrade using agro-synthetic (i.e., polyester-banana) waste fibers to evaluate secant elastic modulus (E50), peak strength (Sp), compression index (Cc), California bearing ratio (CBR), and swell potential. The following conclusions are drawn from the results of this study:
(1)
Results show that the optimal mix, i.e., 1.2% polyester (P) plus 0.9% banana (B) fibers in high plastic clay at OMC and ϒdmax, augmented the secant elastic modulus (E50), peak strength (Sp), and California bearing ratio (CBR) by 2.5, 2.43, and 2.7 times, respectively, whereas an increase in moisture content (mc) from 20% to 32% caused a considerable decrease in Sp from 212 kPa to 56 kPa and E50 from 8.42 MPa to 2.16 MPa.
(2)
It is observed that the swell potential and compression index (Cc) parameters of the optimal CPB mix are reduced by 48% and 42.8%, respectively. Enhancement in elastic-compressive characteristic ratio, i.e., E50/Cc for CP, CB, and CPB, is found to be 24.7, 17.6, and 53.7 MPa, respectively, as compared with 12.2 MPa of clay (C).
(3)
The data are validated by Analysis of Variance (ANOVA) and found to be in good health with no outliers, and correlations are developed for the optimal CPB mix for prediction of Cc from E50, Sp, and initial moisture content (mc) for the evaluation of the feasibility and performance of clay-fiber blends in wet seasons as follows:
Cc = −0.0280 + 0.0086 × mc + 0.0024 × E50
Cc = 0.0339 + 0.0069 × mc − 0.0428 × Sp

Author Contributions

Conceptualization, A.Z. and K.F.; Data curation, A.Z.; Formal analysis, A.Z., Z.F. and K.F.; Funding acquisition, K.F.; Investigation, A.Z.; Methodology, A.Z. and Z.F.; Project administration, Z.M. and H.M.; Resources, K.F. and H.M.; Software, A.Z.; Supervision, H.M.; Validation, A.Z. and Z.M.; Visualization, A.Z. and K.F.; Writing—original draft, A.Z. and K.F.; Writing—review and editing, Z.M., H.M. and A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors express their gratitude to the Geotechnical Engineering Laboratory, Department of Civil Engineering, University of Engineering and Technology, Lahore, Pakistan, for conducting this study. Also, this study is partially funded/supported by Future University in Egypt (FUE).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Clay, polyester, and banana fiber materials were used in this study.
Figure 1. Clay, polyester, and banana fiber materials were used in this study.
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Figure 2. Typical samples of clay (C), clay polyster (CP), clay-banana (CB), and clay-banana-polyster (CBP) before failure.
Figure 2. Typical samples of clay (C), clay polyster (CP), clay-banana (CB), and clay-banana-polyster (CBP) before failure.
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Figure 3. Particle size distribution curve for clay.
Figure 3. Particle size distribution curve for clay.
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Figure 4. Dry density vs. moisture content.
Figure 4. Dry density vs. moisture content.
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Figure 5. Typical crack pattern after a triaxial (CU) test on C, CP, CB, and CBP.
Figure 5. Typical crack pattern after a triaxial (CU) test on C, CP, CB, and CBP.
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Figure 6. Typical scattering of fibers in C + 1.2% P.
Figure 6. Typical scattering of fibers in C + 1.2% P.
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Figure 7. Interlocking of fibers in an optimal mix (C+ 1.2% P + 0.9% B).
Figure 7. Interlocking of fibers in an optimal mix (C+ 1.2% P + 0.9% B).
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Figure 8. Typical triaxial (CU) curves at OMC and γdmax (at matric suction = 25 kPa).
Figure 8. Typical triaxial (CU) curves at OMC and γdmax (at matric suction = 25 kPa).
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Figure 9. Typical triaxial curves expanded for E50 evaluation (at matric suction = 25 kPa).
Figure 9. Typical triaxial curves expanded for E50 evaluation (at matric suction = 25 kPa).
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Figure 10. Typical triaxial stress-strain curves for optimal CPB at various mc.
Figure 10. Typical triaxial stress-strain curves for optimal CPB at various mc.
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Figure 11. Triaxial curves were expanded for E50 for optimal CPB at various mc.
Figure 11. Triaxial curves were expanded for E50 for optimal CPB at various mc.
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Figure 12. Typical consolidation curves of C, CP, CB, and CPB.
Figure 12. Typical consolidation curves of C, CP, CB, and CPB.
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Figure 13. Cc versus E50 for C, CP, CB, and CPB blends.
Figure 13. Cc versus E50 for C, CP, CB, and CPB blends.
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Figure 14. Relation of Cc vs. mc for optimal CPB mix.
Figure 14. Relation of Cc vs. mc for optimal CPB mix.
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Figure 15. Relation of Cc vs. E50 for optimal CPB mix.
Figure 15. Relation of Cc vs. E50 for optimal CPB mix.
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Figure 16. Relation of Cc versus mc and E50.
Figure 16. Relation of Cc versus mc and E50.
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Figure 17. Relation of Cc with Sp for optimal CPB mix.
Figure 17. Relation of Cc with Sp for optimal CPB mix.
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Figure 18. Relation of Cc versus mc and Sp.
Figure 18. Relation of Cc versus mc and Sp.
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Figure 19. Measured versus predicted Cc (at mc = 20% to 32%).
Figure 19. Measured versus predicted Cc (at mc = 20% to 32%).
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Figure 20. Cost versus E50, CBR and Sp. (C + 1.2% P + 0.9% B is the optimal mix).
Figure 20. Cost versus E50, CBR and Sp. (C + 1.2% P + 0.9% B is the optimal mix).
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Table 1. Physical properties of experimental clay.
Table 1. Physical properties of experimental clay.
Property/ConstituentValue
Maximum dry density (g/cm3) [44]1.83
Optimum moisture content (%) [44]16.3
Plasticity index (%) [45]34
Specific gravity (Gs) [46]2.70
Clay (%) 64
Silt (%)34
Sand (%)2
Table 2. Index properties of polyester fibers.
Table 2. Index properties of polyester fibers.
PropertyValue
LusterBright
Melting point277 °C
Ultimate tensile strength (103 N/mm2)0.35–0.76
Density (g/cm3)1.41
Ultimate strain (%)6.5–7.1
Specific gravity 1.29
Length (mm)65–68
Colourwhite/brown
Moisture (%)11–13
Elasticity (breaking extension), %39.1
Strength (Tenacity), Dry4.5 gm/den
Strength (Tenacity), Wet0.69 × (dry strength)
Table 3. Index properties of banana fibers.
Table 3. Index properties of banana fibers.
Constituent/PropertyValue
Diameter (mm)0.138–0.289
Natural moisture content (%)62
Elongation at break (%)1.65–3.48
Cellulose content (%)73
Density (g/cm3)1.12
Length (mm)43–51
Ultimate strain (%)3.4–4.8
Specific gravity1.14
ColourBrown
Table 4. Geotechnical parameters of C, CP, and CB (average of triplicate tests).
Table 4. Geotechnical parameters of C, CP, and CB (average of triplicate tests).
Sr No.Clay/Fiber MixturesTriaxial
(Consolidated Undrained)		Compression Index (Cc)E50/Cc
(MPa)		Soaked CBR (%)
E50 (MPa)Sp
(MPa)		E50/Sp
1Clay (C)3.440.10831.850.2812.293.1
2C + 0.4% P3.870.12530.960.2714.334.1
3C + 0.8% P4.410.16826.250.2716.334.6
4C + 1.2% P5.930.20928.370.2424.715.1
5C + 1.6% P4.480.17725.310.2716.594.7
6C + 0.3% B3.570.10733.360.2713.223.4
7C + 0.6% B3.720.13727.150.2614.314.2
8C + 0.9% B4.400.17824.720.2517.604.7
9C + 1.2% B3.840.14925.770.2614.774.3
Table 5. Geotechnical parameters of CPB (average of triplicate tests).
Table 5. Geotechnical parameters of CPB (average of triplicate tests).
Sr No.Clay/Fiber MixturesTriaxial
(Consolidated
Undrained)		Compression Index (Cc)E50/Cc
(MPa)		Soaked CBR (%)
E50 (MPa)Sp (MPa)E50/Sp
1C + 0.4% P + 0.3% B5.960.21728.620.2524.845.4
2C + 0.4% P + 0.6% B6.120.22827.540.2426.175.5
3C + 0.4% P + 0.9% B6.250.23627.840.2328.576.1
4C + 0.4% P + 1.2% B6.110.23026.570.2326.575.7
5C + 0.8% P + 0.3% B6.370.22728.060.2228.955.9
6C + 0.8% P + 0.6% B6.590.23428.160.2131.386.4
7C + 0.8% P + 0.9% B6.820.23928.540.2034.106.8
8C + 0.8% P + 1.2% B6.760.22430.180.2132.196.0
9C + 1.2% P + 0.3% B7.270.23529.280.1936.216.5
10C + 1.2% P + 0.6% B7.650.24129.630.1839.676.9
11C + 1.2% P + 0.9% B8.600.26332.700.1653.758.4
12C + 1.2% P + 1.2% B8.360.25133.310.1846.448.0
13C + 1.5% P + 0.3% B7.110.23130.780.1937.427.1
14C + 1.6% P + 0.6% B7.250.23930.330.2036.257.8
15C + 1.6% P + 0.9% B7.370.24629.960.2036.857.9
16C + 1.6% P + 1.2% B6.920.22331.030.2132.956.8
Table 6. Chemical composition of expansive soil (C) (from X-ray fluorescence analysis).
Table 6. Chemical composition of expansive soil (C) (from X-ray fluorescence analysis).
ConstituentsPercentage, %
SiO2—Silica 47.1
Al2O3—Aluminium Oxide15.3
Fe2O3—Ferric Oxide9.2
MgO—Magnesium Oxide5.1
CaO—Calcium Oxide13.6
K2O—Potassium Oxide0.6
Na2O—Sodium Oxide2.7
Other 6.4
Table 7. ANOVA results for geotechnical parameters for the CPB mix.
Table 7. ANOVA results for geotechnical parameters for the CPB mix.
Sr. No.Statisticsmc, %E50 (MPa)Sp (MPa)Cc
1F387.913219.615188.05693.952
2p2.5 × 10−165.3 × 10−134.2 × 10−122.6 × 10−10
(p-value of all measurements in Table 7 indicates a significant difference at p < 0.05).
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Zubair, A.; Farooq, Z.; Farooq, K.; Masoud, Z.; Mujtaba, H.; Mohamed, A. Prediction of Compression Index from Secant Elastic Modulus and Peak Strength of High Plastic Clay Ameliorated by Agro-Synthetic Waste Fibers for Green Subgrade. Sustainability 2023, 15, 15871. https://doi.org/10.3390/su152215871

AMA Style

Zubair A, Farooq Z, Farooq K, Masoud Z, Mujtaba H, Mohamed A. Prediction of Compression Index from Secant Elastic Modulus and Peak Strength of High Plastic Clay Ameliorated by Agro-Synthetic Waste Fibers for Green Subgrade. Sustainability. 2023; 15(22):15871. https://doi.org/10.3390/su152215871

Chicago/Turabian Style

Zubair, Ayesha, Zainab Farooq, Khalid Farooq, Zubair Masoud, Hassan Mujtaba, and Abdullah Mohamed. 2023. "Prediction of Compression Index from Secant Elastic Modulus and Peak Strength of High Plastic Clay Ameliorated by Agro-Synthetic Waste Fibers for Green Subgrade" Sustainability 15, no. 22: 15871. https://doi.org/10.3390/su152215871

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