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Article

Effects of Palm Kernel Shells (PKS) on Mechanical and Physical Properties of Fine Lateritic Soils Developed on Basalt in Bangangté (West Cameroon): Significance for Pavement Application

by
Verlène Hardy Njuikom Djoumbi
1,
Valentine Yato Katte
2,
Idriss Franklin Tiomo
1 and
Armand Sylvain Ludovic Wouatong
1,*
1
Department of Earth Sciences, Faculty of Sciences, University of Dschang, Dschang P.O. Box 67, Cameroon
2
Department of Civil and Mining Engineering (SEBE), University of Namibia, José Eduardo dos Santos Campus, Ongwediva P.O. Box 3624, Namibia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(15), 6610; https://doi.org/10.3390/app14156610 (registering DOI)
Submission received: 19 May 2023 / Revised: 9 July 2023 / Accepted: 11 July 2023 / Published: 29 July 2024
(This article belongs to the Section Earth Sciences)
Browse Figures
Versions Notes

Abstract

:
The utilization of an agricultural waste product known as palm kernel shells (PKS) combined with fine laterites (from basalt in Bangangté, West Cameroon) to produce low-cost and innovative materials with good bearing capacities for road pavement was investigated. Fine laterites from two soil profiles (BL31 and BL32) and made up of kaolinite, hematite, goethite, gibbsite, anatase, ilmenite and magnetite minerals were partially replaced with PKS at 15%, 25%, 35%, and 45% by weight. Physical and mechanical tests, including particle size distribution, Atterberg limits, unsoaked and soaked California Bearing Ratio (UCBR and SCBR), unconfined compressive strength (UCS), and tensile strength (Rt), were performed on the different mixtures. After the addition of PKS, a decrease in fine particle content (77 to 38%), liquidity limit (LL: 72 to 61%), plasticity index (PI: 30 to 19%), maximum dry density (MDD: 1.685 to 1.29 t/m3), and optimum moisture content (OMC: 27.5 to 24.0%) was noticed. Additionally, there was an increase in UCBR (16–72%), SCBR (14–66%), UCS (1.07–7.67 MPa), and Rt (2.24–9.71 MPa). This allows new materials suitable for the construction of base layers for low trafficked roads (T1–T2), as well as sub-base and base layers for high trafficked roads (T3), to be obtained. This newly formed material can be recommended locally for road construction works, though more in-depth studies are required.

1. Introduction

Lateritic soils are widely distributed worldwide in tropical and subtropical regions [1]. In Africa, particularly south of the Sahara, these soils are commonly used in road construction, both as support and fill materials [2,3,4,5]. In addition, laterites are more or less abundant depending on the site, locality, or region, and they are most often located near road alignments. These laterites are differentiated as lateritic gravels, fine laterites, lateritic shells, and lateritic hardpans [2,6,7,8,9,10,11]. Consequently, lateritic deposits provide different mechanical and physical characteristics, resulting in the different types of lateritic features [12,13,14]. Meanwhile, past and recent studies have revealed that lateritic gravels are the most sought-after lateritic materials for road construction works [2,6,15,16,17]. Lateritic gravel is becoming scarce due to its exhaustion, but other products of lateritization, such as fine lateritic soils, are abundantly available. As such, there is renewed interest in the utilization of fine or clay-rich laterites to optimize the cost of road projects [8,18,19].
Several studies have revealed that clay-rich or fine-grained laterites are not suitable materials for road construction due to their overall low California Bearing Ratio (CBR) [19,20,21,22]. The CBR is the most used parameter for the dimensioning of flexible pavements in tropical countries, and the bearing capacity of subgrade soil is important for the determination of the required pavement thickness [3,14,23,24]. Many studies have been carried out on the improvement of geotechnical properties of clay-rich laterites for use as sub-base course materials for pavements [25,26,27]. These studies revealed that bitumen, cement, lime, and other hydraulic binders perform as good stabilizers of fine-grained soils with a resulting significant increase in the bearing capacities [6,19,26,28,29,30]. Though these commonly used techniques are effective, they are still cost intensive and also have environmental challenges [2,27,31]. It becomes necessary, therefore, to adopt novel techniques to improve upon the mechanical characteristics so as to meet the relevant specifications [2,22,31]. Common applications of some innovative and lost-cost industrial waste materials (aluminum slag, crushed ceramic tiles, among others) for highway construction have been cited by [31,32,33,34,35,36].
In recent decades, a great number of alternative materials, for example, agricultural wastes such as coconut husks and shells, bamboo, corn cob, millet husk, palm kernel shells (PKS), and other fibers, have been introduced and used to optimize the properties of fine-grained lateritic soils [37,38,39,40]. It has been found from some of these studies [41,42,43] that the addition of bio-materials has shown positive effects in the improvement of some of the geotechnical characteristics of soils (decrease in liquid limit, plasticity index, soil permeability, swelling index and shrinkage limits, increase in CBR, unconfined compressive strength (UCS), maximum dry density) without adversely impacting the environment. More specifically, ref. [44] showed the use of pulverized PKS for road construction. Furthermore, ref. [45] revealed that PKS possesses excellent CBR potential to be used as a litho-stabilizer, and [44] showed that pulverized palm kernels induced a reduction in optimum moisture content (OMC) and an increase in soaked CBR and UCS values for stabilized soils. In Bangangté, an area in Western Cameroon, fine laterites are found associated with a thick lateritic cover derived from weathering and lateritization of basaltic rocks [46]. In recent years, the socioeconomic development has favored the construction of many main and secondary roads in this area. However, the materials used often come from other localities, with the use of local materials being accompanied by many structural failures. One consequence is, therefore, the high cost of road projects. In the meantime, there are no geotechnical studies of these abundant local materials. Thus, this paper firstly aims to provide the physical, mechanical, and mineralogical properties of fine laterites from two soil profiles in Bangangté. Secondly, this paper focuses on producing innovative and low-cost material of acceptable bearing capacity for pavement application with PKS as a partial replacement for fine laterites. Based on the results obtained from this study, the contribution of an agricultural waste in the stabilization of fine laterites for road construction projects has been evaluated.

2. Materials and Methods

2.1. General Setting of the Study Area

The study area is located in the Bangangté Sub-division between latitudes 05°06′ and 05°11′ North and longitudes 10°28′ and 10°37′ East, with a mean altitudinal level of 1337 m asl and a surface area of 1524 km2 (Figure 1a). The climate is Equatorial Guinean type with two seasons: a long rainy season of nine months (February to November) and a short dry season of three months (December to February) [46]. The mean annual temperature and precipitation are 21.5 °C and 1792 mm, respectively. According to [47], the natural vegetation is a strongly anthropogenized post-forestry savanna with remnants of persisting semi-deciduous forests in areas of inaccessible terrain. The main soils are Oxisols (about 80% of the area) on hillslopes, while the lowlands are occupied by Gleysols and Fluvisols [46]. The soil temperature regime is isothermic and the moisture regime is udic [46]. The study area is part of the Cameroon Volcanic Line (CVL) (Figure 1a,b), which is a megavolcano–tectonic structure extending from Pagalu island to Lake Chad [48,49,50]. It is made up of migmatites, granite and basalt (Figure 1c), with a migmatites outcrop in the form of slabs and points in the north and south-eastern part of the study area (Figure 1c), and granite outcrops in slab form in the south-western part. Basalts are dominant and cover almost 90% of the Bangangté study area [46,51,52].

2.2. Materials

The fine-grained laterites were obtained from road cut sections, where two spots, BG1 and BG2, were geo referenced, profiled as described by [54], and sampled as shown in Figure 2. BG1 (N05°06′02.3″, E10°37′04.6″, 1330 m asl) is a 380 m thick profile consisting of 10 cm of humic horizon A, having a dark red (10R 3/4) and clayey fine earth, with the presence of centimeter- to millimeter-sized rootlets (Figure 2). Horizon B is 370 cm thick and consists of a red (2.5YR 4/6) silty–clayey fine earth with a polyhedral structure. BG2 (N05°07′, E10°33′, 1341 m asl) is slightly similar to BG1 with a depth of more than 400 cm. Its B horizon is reddish-brown (2.5YR 5/4).
Twelve (12) samples of fine laterites of about 100 kg each were collected from BG1 and labeled as follows: BL15, BL17, BL23, BL24, BL26, and BL32. Similarly, the same procedure was carried out on BG2, and the samples were labeled as BL13, BL16, BL20, BL27, BL29, and BL31.
Meanwhile, about 350 kg of PKS was bought from a warehouse in Bangangté. The palm kernels are waste from palm oil production, which is a principal agricultural product from the area. The shells are initially spherical to ellipsoidal in shape when containing kernels (Figure 3a). After the crushing and extraction of the kernels, they are flaky, sub angular and angular. Some of the bigger particles are parabolic with convex and concave surfaces (Figure 3b). They are generally dark and about 5–7 mm thick.

2.3. Methods

2.3.1. Tests on Natural Materials

The mineralogical analysis was carried out on air-dried samples which were crushed into 63 µm fine powder. X-ray diffraction analysis was carried out using Brucker D8-Eco diffractometer emitting Cu K-alpha radiation wavelength (λ), 1.54 Å, energy of 40 kV and 45 mA, and 2θ from 0° to 70° at the AGES (Argiles, Géochimie et Environnement sédimentaire) laboratory of the University of Liege, Belgium. The mineral proportions were quantified from X-ray diffraction patterns using the Rietveld refinement method [55]. The geotechnical tests were carried out at the National Civil Engineering Laboratory of Cameroon (LABOGENIE), Yaoundé, Cameroon. Measuring the natural moisture content, which consists of recording successive weights of the soil samples before and after drying in an oven at 105 °C was carried out in accordance with the NF P 94-050 standard [56]. The specific gravity (γs) was obtained using the helium gas pycnometer method in accordance with the NF P94–054 standard [57]. The bulk density (γ) was determined using the hydrostatic method following the NF EN 1936 standard [58]. The particle size distribution was carried out using the sieve and hydrometer analysis based on the NF P 94-056 and NF P 94-057 standards [59,60]. The determination of the Atterberg limits was conducted to measure the plasticity of soil materials. It was carried out according to the NF P 94-051 standard [61]. The Atterberg limits test is represented by liquid limit (LL), plastic limit (PL), and plasticity index (PI). The obtained parameters allowed the classification of the studied materials according to the chart developed by the Highway Research Board (HRB) and the Casagrande chart [7]. The methylene blue test (MBT) was carried out in accordance with the NF P 94-068 standard [62]. The MBT enabled the quantification of the ionic absorption capacity of a soil by measuring the quantity of methylene blue necessary to cover the external and internal surfaces of the clay particles contained in the soil. The modified Proctor test was carried out in accordance with the NF P94–093 standards [63], which enabled the determination of the maximum dry density (MDD) and optimum moisture content (OMC) of the investigated soils.
The soaked CBR (SCBR) test was conducted on the soil sample according to the NF P 94-078 standard [64]. This test measures the resistance of soil materials under controlled moisture content and density by the penetration of a cylindrical plunger with a cross-sectional area of 1935 mm2. The SCBR was calculated as the ratio of the force required to push the plunger to a certain depth into soil materials and the force required to push the same plunger at the same depth into a standard sample of compacted crushed stone (2.5 mm and 5 mm of penetration) and calculated as shown in Equation (1).
C B R = A p p l i e d   l o a d S t a n d a r d   l o a d × 100

2.3.2. Tests on PKS

The PKS was subjected to particle size analysis, from which the fineness modulus (Mf) was determined. The flakiness index test was carried out according to NF EN 933–3 standards [65]. This test determines the percentage of flaky and elongated particles available in the total PKS sample. The specific gravity of the PKS was determined following the NF EN P94–054 standard [58], while the Los Angeles coefficient (LA) was performed following the NF P18-573 standard [66]. The Los Angeles coefficient test enabled the determination of the resistance of coarse PKS particles to impact in a rotating cylinder containing metallic spheres and this is expressed as shown in Equation (2).
L A = A − B A × 100
where A is the weight of the original test specimen to the nearest 1 g and B the weight retained on sieve number 12 after the specified number of revolutions to the nearest 1 g.
The thickness of PKS was measured using the Vernier caliper.

2.3.3. Preparation of the Soil–PKS Mixture

Soil–PKS mixture from the two soil profiles was carried out by mixing oven dried soil with sun dried PKS at a percentage of 15%, 25%, 35%, and 45% of the weight of dried soil (5000 g), respectively. This was labeled as follows: BL31 (100% BL31 + 0% PKS; 85% BL31 + 15% PKS; 75% BL31 + 25% PKS; 65%BL31 + 35%PKS; 55% BL31 + 45% PKS) and BL32 (100% BL32 + 0% PKS; 85% BL32 + 15% PKS; 75% BL32 + 25% PKS; 65% BL32 + 35% PKS; 55% BL32 + 45%PKS).

2.3.4. Tests on Soil–PKS Mixture

The PKS-improved fine laterite samples were prepared for geotechnical characterization such as particle size distribution, Atterberg limit test, compaction test, soaked CBR (SCBR), and unsoaked CBR (UCBR). Moreover, the UCS was performed on cylindrical specimens (diameter = 15 cm, height = 17 cm) in accordance with the NF P 94-422 standard [67]. The tensile strength (Rt) test was determined on cylindrical specimens (diameter = 15 cm, height = 15 cm) in accordance with the NF P 98 232-3 standard [68]. The specimens were air cured at 20 °C for seven, fourteen, and twenty-eight days before being subjected to the compressive and the indirect tensile tests, respectively. Five repetitions of each of the tests were performed for each mixture of the different samples, and the average values are reported and discussed in this manuscript.
The detailed methodology and analysis for the study are summarized in Figure 4.

3. Results and Discussions

3.1. Characterization of Fine Lateritic Materials

3.1.1. Mineralogical Composition and MBT

The XRD patterns and mineralogical semi-quantitative calculations of samples from profiles BG1 and BG2 are given in Figure 5 and Table 1. The XRD results show that the two lateritic soil profiles have almost the same mineral compositions with a slight variation in terms of proportions. These mineral compositions are dominated by newly formed secondary minerals which include kaolinite (28.6–47.0%), hematite (6.3–28.4%), goethite (10.5–24.0%), gibbsite (4.6–17.4%), anatase (0.9–11.7%), ilmenite (0.2–6.7%), and magnetite (0.0–5.9%) (Table 1). Quartz and feldspars are the residual primary minerals in the studied materials, with low proportions that range from 1.4% to 6.8% and from 1.3% to 4.4%, respectively (Table 1). The mineral compositions of the studied lateritic materials are found in the weathered products of basaltic rocks in the West Region of Cameroon [46,69].
The MBT of samples (Table 1) is very low and ranges between 0.2 and 0.6 g/100 g. These values are very close to those obtained by [41] (0.4–0.67 g/100 g) and allow the studied lateritic materials to be identified as silts [70,71]. Thus, these clay-rich lateritic soils are sensitive to water and are, therefore, not suitable for engineering purposes [72,73].

3.1.2. Water Content and Density Parameters

The values of water content and density parameters are given in Table 2. The lowest and highest values of water content (ω) which were obtained in the BG1 soil profile were 15.3 and 26.8%, respectively. According to [74], materials with 10 < ω < 30 are considered as silts but those with 20 < ω < 50 are rendered as clays. Therefore, the water content of the studied soil samples confirms the presence of silty and clayey particles as also revealed by the macroscopic description, mineral association, and grain size distribution. Moreover, there is a noticeable increase in water content with an increase in kaolinite proportions (Figure 6). Thus, the water absorption of these materials is significant and, therefore, unsuitable for the construction of pavement bases [75].
The bulk density and specific gravity values are also given in Table 2, and the values vary from 1.586 to 1.930 t/m3 and from 2.000 to 2.800, respectively. Thus, the studied laterites are considered good and satisfactory, as their specific gravity values range between 2.000 and 3.000 (as recommended for clayey soils) [76].

3.1.3. Particle Size Distribution (PSD) and Atterberg Limits

The PSD of lateritic soils in the study area is summarized in Table 2. Five (05) granulometric classes are given for these materials and include clays (40.0–61.1%), silts (11.3–27.9%), fine sands (5.7–22.3%), coarse sands (0.7–20.9%), and gravels (0–15.1%). According to Figure 7, PSD curves for the samples are very close to each other. Therefore, these lateritic materials are classified as well graded to poorly graded laterites. The results also indicate that the studied soil materials are found in the A-7-5 and A-6- classes in accordance with the AASHTO classification system. These soil materials display low to tolerable performance in road construction. These poor performances are linked to their high fine content greater than 35%, which is the maximum percentage recommended by [72] for pavements carrying a low volume of traffic.
The liquid limit (LL) values of the studied fine laterites range from 48.2 to 71.8% (Table 2). These values are similar to those obtained on soil from Sangmelima-Mengong [3]. However, they confirm the abundance of silts and clays in all samples [41,77]. Therefore, the high values of the LL (>35%) indicate that the soil is susceptible to shrinkage or cracking [44]. Accordingly, these soils require improvement before an optimal use in subgrade construction.
The values of the plasticity index are presented in Table 2, with both lowest and highest values being observed in the BG1 soil profile, with respective values of 19.5 and 35.4% for samples BL26 and BL23. According to [42], these values are greater than 15%, which is the maximum percentage recommended for the base layer. With the exception of samples BL23 and BL13, all the other samples had plasticity index values less than 30%, which corresponds to the maximum value required for the base layers of pavements. Furthermore, the high plasticity of these samples can be linked to the high content of the clay fraction as well as the clay minerals [15,78]. Based on the Casagrande plasticity chart, BL26 can be designated as inorganic clays of medium plasticity, and the rest of the samples as inorganic silts of high compressibility and organic clays (Figure 8). With regard to the [59] classification, all the studied soil samples belong to the A class of the A2 and A3 groups. According to [79], these materials display low to tolerable performance in road construction. Their consistency index (CI) varies from 0.92 to 2.28, which is an indicator of the firmness of the soil. As the value increases, so does the shear strength.

3.1.4. Compaction Characteristics

The maximum dry density (MDD) and OMC values determined from the Proctor curves are presented in Table 2. The maximum dry density values exhibit a very slight variation between 1.6 and 1.8 t/m3, with both lowest and highest values reported in the BG1 soil profile (Table 2). Meanwhile, the OMC values vary between 14.8 and 27.5%. These minimum and maximum values are also reported in the BG2 soil profile (Table 2). However, Refs. [72,80] recommended the following variation of MDD and OMC for subgrade layers of roadways: 1.9 t/m3 < MDD < 2.1 t/m3 and 7 < OMC < 13%. Additionally, 1.90 t/m3 is the minimum value recommended for the use of materials as a sub-base layer [81]. Therefore, the studied clay-rich laterites, with their MDD lower than 1.90 t/m3 and their OMC greater than those mentioned above, also require a treatment or a correction before use, both as subgrade and sub-base layer materials.
The CBR indices at 95% of MDD after 4 days of immersion of the studied clay-rich laterites are summarized in Table 2. As indicated by [82,83], CBR appears to be one of the most important criteria for the selection of natural materials in road construction. Therefore, [72] prescribed some uses of soil materials in road pavements based on their specific values of the CBR. Consequently, since all fine-grained laterites have a CBR > 10, they can be conveniently utilized for subgrades (Table 2). However, some materials from BG1, namely BL13 and BL27, had CBR values greater than 30%, as shown in (Table 2), and were classified as the S5 bearing class and suitable as a sub-base for high traffic (T2/T3) loadings or as a base course or wearing course for pavements with a low volume of traffic [72]. The other samples from BG1 had a CBR lower than 30% (Table 2). Consequently, they will have a low to tolerable application as a sub-base for pavements unless they are treated. Of all the samples tested, BL31 and BL32 had the lowest CBR values (Table 2) and were, therefore, chosen for soil stabilization with PKS powder.

3.2. Physical and Mechanical Characterization of Palm Kernel Shells (PKS)

The particle size curve is presented in Figure 9. The maximum diameter of particles (Dmax) is 16 mm with skeleton content (≥2 mm) equal to or greater than 94% (Table 3). The coefficient curvature (Cc) and uniformity coefficient (Cu) values are 1.4 and 4.8, respectively. This indicates that the particle size of the studied materials is spread out and well graded [84]. Consequently, they can be used as highway sub-base and sub-grade materials [85]. The fineness modulus indicates the average size of particles in the coarse aggregate. A value of 5.7 was obtained for PKS (Table 3). Furthermore, the value of flakiness index of PKS is reported in Table 3 and shows the abundance of flat particles. The values of the Los Angeles coefficients on the 6.3–10 mm and 10–14 mm fractions of PKS are summarized in Table 3 and are 4.5 and 4.1%, respectively. These low values show that these two PKS fractions are very resistant [86]. Thus, PKS display some favorable characteristics, allowing their use for the mechanical stabilization of the studied clay-rich laterites.

4. Geotechnical Characteristics of the PKS-Improved Fine Laterites

4.1. Particle Size Distribution and Atterberg Limits

BL31 and BL32 were chosen as the representative samples for the two lateritic soil profiles and were then stabilized by the addition of PKS. The results are summarized in Table 4 and Table 5.
The stabilized BL31 is accompanied by a decrease in fine particle content (d < 0.08 mm) from 77 to 38% (Figure 10). However, these values remain greater than 35%, which is the maximum percent recommended by [72] for pavements. It is also characterized by an increase in sand fraction (d ≥ 2 mm) from 77 to 95% (Figure 10). In addition, a similar pattern with a slight variation is reported on stabilized BL32 (shown in Figure 10; Table 5). According to the specifications in [72], this indicates the improvement of natural materials with low performance and the production of new materials with tolerable performance.
The evolution of plasticity parameters of the stabilized soil samples is presented in Figure 10 and Table 4 and Table 5. It is noticed that the addition of PKS leads to a slight decrease in the LL values (from 72 to 61%). This decreasing trend is concomitant with a decrease in PI values (from 30 to 19%; Figure 10). Nevertheless, the new PI values remains greater than the minimum value (12%) recommended by [72] for the use of materials in pavement construction. Moreover, the particle size distribution of the stabilized fine laterites ranges from silty clay to clayey sand, which is suitable for subgrade layers [72]. The evolution of these parameters is correlated to many other litho-stabilization studies [2,5,22,87].

4.2. Mechanical Parameters

The compaction characteristics and the CBR values of the stabilized materials are given in Table 4 and Table 5.
The results of the modified Proctor test produced the curves as given in Figure 11. The addition of PKS to the studied soils is characterized by a decrease in maximum dry density (MDD) and optimum moisture content (OMC) values (Figure 11a; Table 4 and Table 5). The MDD decreased slightly from 1.685 to 1.3 t/m3 (from BL31 + 00%PKS to BL31 + 45%PKS) and from 1.59 to 1.29 t/m3 (from BL32 + 00%PKS to BL32 + 45%PKS). Likewise, the OMC also decreased slightly, from 24.4 to 24.0% and 27.5 to 24.8%, respectively (Figure 11c; Table 4 and Table 5). The decrease in MDD indicates that the compaction energy is less than what is obtained in the natural state [88]. This may be due to the addition of a lighter material to a much denser one. Furthermore, the decrease in MDD values could be related to the decrease in fine particle content of the new material and, more importantly, to the volume occupied by PKS particles, which are much lighter than the clay-rich laterites [45,89,90,91].
Meanwhile, the decrease in OMC values could be due to the absorption of water by the PKS particles, which, according to [44], leads to a gain in the strength of the lateritic soils. It may further be due to the fact that compaction reduces voids in the mixture and makes it less permeable [45]. However, [90,92] mentioned that over certain values of water absorption (10.23%), the pores of PKS behave as a water transmitter and convey water between the particles of fine laterites. Overall, the decrease in both MDD and OMC values is indicative of an improvement in treated materials.
It was also noticed that there is a continuous increase in SCBR and UCBR values with an increase in PKS content. This pattern is in contrast to the decrease in maximum dry density values. According to [45], this concludes that the bearing capacity does not only depend on dry density after compaction, but also on the amount of the solids fractions they contain.
Additionally, the UCBR values are higher than those of SCBR (Figure 12). The UCBR values range from 16 to 72% (from BL31 + 00%PKS to BL31 + 45%PKS) and from 18 to 65% (from BL32 + 00%PKS to BL32 + 45%PKS) (Figure 13). This behavior confirms the fact that the new materials have good trafficability. SCBR values rise from 14 to 66% (from BL31 + 00%PKS to BL31 + 45%PKS) and from 16 to 54% (from BL32 + 00%PKS to BL32 + 45%PKS) (Figure 13). The increase in these parameters associated with the decrease in OMC is in accordance with the study of [92,93]. As mentioned by [91,94], the CBR value increments are related to the increase in shear strength and density. Accordingly, in this study, the characteristics of UCBR, SCBR, and MDD of PKS-improved fine laterites disagrees with the previous findings.
Indeed, a minimum value of CBR accepted in subbase and base layers for high pavement traffic (T3) is equal to or greater than 30 with S5 bearing class and 300 MPa of elastic moduli [72]. In accordance with this specification, and despite their low γd max, the mixtures from BL31 + 25%PKS to BL31 + 45%PKS and from BL32 + 25%PKS to B32 + 45%PKS are suitable for sub-base and base layers for high pavement traffic (T3). Moreover, BL31 + 00%PKS, BL31 + 15%PKS, BL32 + 00%PKS and B32 + 15%PKS can be used as base layers for roads with low pavement traffic (T1–T2). These are in accordance with S3, 75 MPa (BL31 + 00%PKS) and S4 bearing class and 150 MPa of elastic moduli [72].
The results of the unconfined compressive strength (UCS) and tensile strength (Rt) of BL31 improved specimens are presented in Table 6 and Figure 14.
The UCS values increase progressively with the curing time and with increasing proportions of PKS. This means that the UCS varies from 1.07 MPa for natural material at 7 days of curing to 7.67 MPa for the stabilized material 65%BL31 + 45%PKS after 28 days [31,91,94]. A similar trend is observed for the Rt values. However, Rt is more sensitive than UCS. In fact, a natural material displays an Rt value of 2.24 MPa after 7 days of curing, while a value of 9.71 MPa is recorded for the optimal mixture (65%BL31 + 45%PKS) after 28 days. According to [44], the increasing values of UCS and Rt with the addition of PKS suggests a good adhesion between the soil and PKS. Consequently, the studied materials can be utilized for sub-base courses [16,79].

5. Conclusions

The present study investigates the effects of PKS on fine laterites derived from basalt in Bangangté to address the problem of low bearing capacities of soils for pavement application. It also addresses environmental protection by reducing the use of industrial materials (cement and lime among others), as well as the effective use of locally available agricultural waste products. It consists essentially in the partial replacement of fine laterites with PKS at 15%, 25%, 35%, and 45% by weight. Based on the obtained results, the addition of this selected agricultural waste as a stabilizer was effective in enhancing the soil properties, particularly fine particle content, skeleton fraction, LL, PI, OMC, UCBR, SCBR, UCS, and Rt. The highest performance of the improved materials was obtained with 45% PKS partial replacement of fine laterites with PKS. This newly formed material was therefore suitable for the construction of base layers for low trafficked roads (T1–T2), as well as sub-base and base layers for high trafficked roads (T3). Although some other properties, such as fine particle content, LL, PI and MDD, were significantly improved, they remain slightly below or above the recommended values. However, considering that the newly formed materials are low-cost and environmentally friendly, they can already be recommended locally for road construction works. Meanwhile, further in-depth studies are necessary for optimal use of these materials, such as cyclic loading conditions, and other parameters such as traffic benefit ratio (TBR), settlement reduction factor (SRF), and resilient modulus.

Author Contributions

Conceptualization, V.H.N.D., I.F.T. and A.S.L.W.; Methodology, V.H.N.D., V.Y.K., I.F.T. and A.S.L.W.; Software, V.Y.K. and I.F.T.; Validation, V.Y.K., I.F.T. and A.S.L.W.; Formal analysis, V.H.N.D., V.Y.K., I.F.T. and A.S.L.W.; Investigation, V.H.N.D. and V.Y.K.; Resources, V.Y.K., I.F.T. and A.S.L.W.; Data curation, V.H.N.D., V.Y.K., I.F.T. and A.S.L.W.; Writing—original draft, V.H.N.D., V.Y.K., I.F.T. and A.S.L.W.; Writing—review & editing, A.S.L.W.; Visualization, V.H.N.D., V.Y.K., I.F.T. and A.S.L.W.; Supervision, A.S.L.W. 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

No data was used for the research described in the article.

Acknowledgments

This paper is a part of the first author’s Ph. D work carried out in the Department of Earth Science of the Faculty Sciences, University of Dschang. The authors express their sincere gratitude to Ivana Bajic and four anonymous reviewers who helped to significantly improve the quality of work presented in this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geological map of the study area (from [48]). (a) Location of the CVL with other major African features. (b) Location map of the CVL within the map of Cameroon [46]. (c) Geological settings of the Bangangté study area (extracted from [53]).
Figure 1. Geological map of the study area (from [48]). (a) Location of the CVL with other major African features. (b) Location map of the CVL within the map of Cameroon [46]. (c) Geological settings of the Bangangté study area (extracted from [53]).
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Figure 2. Clay-rich lateritic soil profiles in the Bangangté study area. A, humiferous horizon; B, mineral-rich horizon.
Figure 2. Clay-rich lateritic soil profiles in the Bangangté study area. A, humiferous horizon; B, mineral-rich horizon.
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Figure 3. Selected photographs of the studied palm kernel shells. (a) Whole palm kernel shells; (b) crushed palm shells.
Figure 3. Selected photographs of the studied palm kernel shells. (a) Whole palm kernel shells; (b) crushed palm shells.
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Figure 4. Synoptic diagram of study method.
Figure 4. Synoptic diagram of study method.
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Figure 5. XRD patterns of the Bangangté clay-rich laterites. Kln: kaolinite; Qtz: quartz; He: hematite; Goe: goethite; Gibs: gibbsite; An: anatase; Fds: feldspar; Il: ilmenite; Mag: magnetite.
Figure 5. XRD patterns of the Bangangté clay-rich laterites. Kln: kaolinite; Qtz: quartz; He: hematite; Goe: goethite; Gibs: gibbsite; An: anatase; Fds: feldspar; Il: ilmenite; Mag: magnetite.
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Figure 6. Water content variation and kaolinite proportions in the fine lateritic soils of Bangangté.
Figure 6. Water content variation and kaolinite proportions in the fine lateritic soils of Bangangté.
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Figure 7. Particle size distribution of fine lateritic materials in Bangangté.
Figure 7. Particle size distribution of fine lateritic materials in Bangangté.
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Figure 8. Casagrande chart classification [7] of the soil samples.
Figure 8. Casagrande chart classification [7] of the soil samples.
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Figure 9. Particle size distribution curves of the PKS collected in Bangangté.
Figure 9. Particle size distribution curves of the PKS collected in Bangangté.
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Figure 10. Particle size distribution and plasticity parameters of the PKS - improved fine laterites.
Figure 10. Particle size distribution and plasticity parameters of the PKS - improved fine laterites.
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Figure 11. Modified Proctor curves of the PKS—improved fine laterites. (a) BL31 sample; (b) BL32 sample. (c) Variation of MDD and OMC.
Figure 11. Modified Proctor curves of the PKS—improved fine laterites. (a) BL31 sample; (b) BL32 sample. (c) Variation of MDD and OMC.
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Figure 12. UCBR versus MDD in the PKS-improved fine laterites. (a) BL31 sample; (b) BL32 sample; and SCBR versus MDD in the PKS-improved fine laterites. (c) BL31 sample; (d) BL32 sample.
Figure 12. UCBR versus MDD in the PKS-improved fine laterites. (a) BL31 sample; (b) BL32 sample; and SCBR versus MDD in the PKS-improved fine laterites. (c) BL31 sample; (d) BL32 sample.
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Figure 13. SCBR and UCBR in the PKS-improved fine laterites.
Figure 13. SCBR and UCBR in the PKS-improved fine laterites.
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Figure 14. Strength characteristics of the PKS-improved fine laterites. (a) UCS; (b) Rt.
Figure 14. Strength characteristics of the PKS-improved fine laterites. (a) UCS; (b) Rt.
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Table 1. Mineralogical composition (wt. %) and MBT (g/100 g) of the samples from BG1 and BG2 soil profiles.
Table 1. Mineralogical composition (wt. %) and MBT (g/100 g) of the samples from BG1 and BG2 soil profiles.
ProfileSamplesKaoliniteHematiteGoethiteGibbsiteAnataseFeldsparsQuartzMagnetiteIlmeniteMBT (g/100 g)
BG1BL1539.612.317.17.17.42.71.45.66.70.4
BL1747.07.123.64.611.61.33.41.10.20.4
BL2344.37.810.510.910.83.94.73.13.90.4
BL2444.38.112.56.910.83.95.73.13.90.6
BL2628.617.824.09.07.24.93.23.91.30.5
BL3237.39.312.114.411.74.13.74.92.30.5
BG2BL1339.328.414.110.50.93.62.50.00.60.3
BL1639.317.215.110.66.22.94.41.55.50.4
BL2033.118.113.217.44.93.05.52.12.10.4
BL2732.717.811.115.73.94.94.35.73.60.2
BL2939.013.711.68.78.74.26.85.90.50.5
BL3138.015.912.68.78.33.45.52.84.70.6
Table 2. Geotechnical properties of fine laterites in the Bangangté study area.
Table 2. Geotechnical properties of fine laterites in the Bangangté study area.
ProfilesBG1BG2
Geotechnical parametersBL15BL17BL23BL24BL26BL32BL13BL16BL20BL27BL29BL31
Water content (ω) (%)25.326.826.122.315.321.522.325.118.618.222.622.5
Specific gravity (Gs) 2.792.092.0922.112.032.0212.0582.0572.0772.0832.0942.025
Bulk density (γ) (t/m3)1.931.581.911.611.611.5981.911.681.771.7111.651.667
Gravel0.02.9515.1112.220.142.60.00.715.110.04.992.4
Coarse sand0.7912.313.620.918.213.42.878.86.729.9613.896.6
Fine sand7.546.548.8311.9721.9215.811.4820.635.7122.38310.7515.7
Silt27.3224.8511.3414.8920.1624.227.9827.0911.3423.42712.9727.5
Clay57.4546.5461.1240.0048.3450.054.0040.4561.1241.6757.447.8
Atterberg limits
LL (%)69.0159.876.85948.2571.759.265.560.461.668.371.8
PL (%)40.3130.464135.1528.7841.735.3831.136.7637.445.0241.7
PI (%)28.729.435.823.8719.4730.023.8234.423.6324.223.2830.0
CI2.010.921.641.861.81.01.751.362.281.972.371.0
Modified Proctor
OMC (%)24.92114.82019.227.517.417.625.52118.224.4
MDD (t/m3)1.631.631.831.731.601.581.701.701.581.651.691.63
CBR at 95% of MDD (%)2225.529232916.0322723352714
Classification of soils samples
Highway Research Board (AASHTO)A-7-5-(19)A-7-5-(17)A-7-5-(20)A-6-(14)A-6-(13)A-7-5-(20)A-7-5-(17)A-7-5-(20)A-7-5-(18)A-7-5-(18)A-7-5-(20)A-7-5-(20)
Earthworks Road Guide (GTR)A3tsA2tsA2tsA3tsA3tsA2tsA2tsA2tsA3tsA3tsA2tsA2ts
USCSSilty clay (CH)
USCS: Unified Soil Classification System; HBR: High Board Road; GTR: Guide des Terrassements Routiers.
Table 3. Physical and mechanical properties of PKS.
Table 3. Physical and mechanical properties of PKS.
PropertiesPKS of Bangangté
Variety of shellsDura
Crushing modeMechanical
ɤ (t/m3)1.09
Water absorption after 48 h immersion20.78
Thickness (mm)5–7
Particle size distributiond < 0.08 mm0.00%
d ≥ 2 mm94.60%
Maximum size (mm)16.00
Uniformity coefficient4.80
Curvature coefficient1.40
Fineness modulus5.70
Flakiness index (%) FC34.60
Los Angeles (%)6.3/104.00
010/144.1
Table 4. Geotechnical parameters of the BL31 material and its various mixtures with PKS. Max, Maximum; Min, Minimum; SD, Standard Deviation.
Table 4. Geotechnical parameters of the BL31 material and its various mixtures with PKS. Max, Maximum; Min, Minimum; SD, Standard Deviation.
MixturesStatistical Data of Each ParameterParticle Size DistributionAtterberg LimitsModified ProctorCalifornian Bearing Ratios
0.08 mm2 mmLL (%)LP (%)PI (%)CIMDD (t/m3)OMC (%)SCBR (%)UCBR (%)
100%BL31 + 0%PKSMin75.95474.98069.94040.47028.7701.0001.58923.85013.95015.020
Max78.01079.97073.73043.09031.5701.0901.68625.03015.00017.010
Mean76.87177.40071.81041.74030.0101.0101.62824.40014.00016.000
SD14.9048.4042.8201.9043.4000.0610.0811.3021.1300.995
85%BL31 + 15%PKSMin49.78065.98062.72042.81018.7801.0101.59022.40015.90018.460
Max51.05068.27065.66045.50020.0201.5001.64223.45017.20019.970
Mean50.70266.41064.35044.41019.9401.1601.56022.60016.00019.400
SD14.6018.1002.5001.5003.6000.0610.1171.1001.2001.200
75%BL31 + 25%PKSMin41.68070.61063.02042.90019.0001.1001.39723.60031.74036.190
Max44.13073.04064.45044.14021.1701.1001.42224.87033.01037.760
Mean43.68072.30063.57043.35020.1601.1001.44124.20032.00037.000
SD14.4238.2002.7001.5003.3000.0610.1171.1001.1001.300
65%BL31 + 35%PKSMin40.02076.01063.04043.07020.0221.0901.40023.00047.00049.010
Max42.43079.80064.31035.78021.2301.1401.46024.19050.00050.910
Mean41.34678.80063.35043.64020.7101.1101.43023.90049.00050.000
SD14.2108.4012.5011.7113.4100.0610.1171.0001.6031.000
55%BL31 + 45%PKSMin37.98089.01058.39046.79018.6701.3001.31023.00164.00369.660
Max39.53090.11062.75049.58021.3101.3301.38026.00067.00174.040
Mean38.89089.09361.10048.81019.2901.0301.33024.00066.00072.000
SD14.2128.5212.3031.7013.4200.1010.1001.4121.4321.902
Table 5. Geotechnical parameters of the BL32 material and its various mixtures with PKS. Max, Maximum; Min, Minimum; SD, Standard Deviation.
Table 5. Geotechnical parameters of the BL32 material and its various mixtures with PKS. Max, Maximum; Min, Minimum; SD, Standard Deviation.
MixturesStatistical Data of Each ParameterParticle Size DistributionAtterberg LimitsModified ProctorCalifornian Bearing Ratios
0.08 mm2 mmLL (%)LP (%)PI (%)CIMDD (t/m3)OMC (%)SCBR (%)UCBR (%)
100%BL32 + 0%PKSMin75.65093.65068.51038.83029.0101.0101.50526.01014.89017.000
Max78.98095.92072.84044.61030.0001.0101.59627.90017.70020.210
Mean78.20395.06071.70041.70030.0001.0101.57527.50016.00018.000
SD13.6037.1002.6001.8003.7000.0000.1170.9961.5001.600
85%BL32 + 15%PKSMin61.11067.15063.90043.01019.3201.0001.50123.50015.98018.780
Max64.72069.25065.71046.00221.2101.2001.57326.40018.50020.500
Mean62.59068.89364.40044.41019.9901.1601.55024.00017.00020.000
SD13.4037.3102.1001.8003.7000.1060.1171.5001.5001.900
75%BL32 + 25%PKSMin47.98069.24062.50042.55019.1401.1001.41125.30038.96039.850
Max49.78071.10065.90044.53020.9101.1001.46226.93041.60042.190
Mean49.01270.00063.51043.35020.1501.1001.42526.70040.00041.000
SD13.5007.5002.0101.7003.4000.0000.1171.1001.4001.200
65%BL31 + 35%PKSMin43.15081.70067.00942.79023.5601.0901.34326.10136.60042.000
Max44.98084.79070.50244.96026.3401.2401.37127.10137.80044.000
Mean44.68084.45769.90344.71025.1301.1401.35326.80037.00043.000
SD13.3207.4012.2201.5003.4100.1100.1171.5021.6011.010
55%BL32 + 45%PKSMin39.71094.21070.11043.64026.1901.1101.22124.70053.95063.990
Max41.69095.81071.98045.12027.3101.1301.32124.90055.53065.701
Mean40.98095.06471.10044.50026.5001.1201.29524.80054.00065.000
SD13.3007.2002.2001.1003.5000.0600.1171.2001.9031.100
Table 6. Evolution of unconfined compressive strength (UCS) and tensile strength (Rt) of the BL31 material and its various mixtures with PKS. Max, Maximum; Min, Minimum; SD, Standard Deviation.
Table 6. Evolution of unconfined compressive strength (UCS) and tensile strength (Rt) of the BL31 material and its various mixtures with PKS. Max, Maximum; Min, Minimum; SD, Standard Deviation.
UCS (MPa)Rt (MPa)
Parameters7 Days14 Days28 Days7 Days14 Days28 Days
100%BL31 + 0%PKSMin0.6751.2401.9802.0902.2502.720
Max1.1821.5602.5102.2502.6003.070
Mean1.0701.5412.1222.2412.5213.001
SD0.3630.1580.2720.0760.1750.156
85%BL31 + 15%PKSMin1.0001.7402.4402.4103.0103.550
Max1.1101.8202.5902.5003.2103.700
Mean1.0711.7922.5312.4623.1743.653
SD0.0950.0620.0730.0330.1090.087
75%BL31 + 25%PKSMin1.0801.9903.0203.1104.0005.010
Max1.7432.8303.3103.4104.3106.130
Mean1.4032.6003.1603.2044.0805.800
SD0.3420.1920.3180.4600.3830.783
65%BL31 + 35%PKSMin1.8902.4505.0903.9705.7507.010
Max1.8902.8305.7134.8906.5108.560
Mean1.8902.6005.6304.4206.0607.983
SD0.0000.1910.3280.4600.3180.878
55%BL31 + 45%PKSMin3.1603.9366.8914.2306.0438.980
Max3.1604.9507.9125.0207.9609.931
Mean3.1604.7107.6704.9016.9629.711
SD0.0000.7290.5120.4250.9690.492
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Njuikom Djoumbi, V.H.; Yato Katte, V.; Tiomo, I.F.; Wouatong, A.S.L. Effects of Palm Kernel Shells (PKS) on Mechanical and Physical Properties of Fine Lateritic Soils Developed on Basalt in Bangangté (West Cameroon): Significance for Pavement Application. Appl. Sci. 2024, 14, 6610. https://doi.org/10.3390/app14156610

AMA Style

Njuikom Djoumbi VH, Yato Katte V, Tiomo IF, Wouatong ASL. Effects of Palm Kernel Shells (PKS) on Mechanical and Physical Properties of Fine Lateritic Soils Developed on Basalt in Bangangté (West Cameroon): Significance for Pavement Application. Applied Sciences. 2024; 14(15):6610. https://doi.org/10.3390/app14156610

Chicago/Turabian Style

Njuikom Djoumbi, Verlène Hardy, Valentine Yato Katte, Idriss Franklin Tiomo, and Armand Sylvain Ludovic Wouatong. 2024. "Effects of Palm Kernel Shells (PKS) on Mechanical and Physical Properties of Fine Lateritic Soils Developed on Basalt in Bangangté (West Cameroon): Significance for Pavement Application" Applied Sciences 14, no. 15: 6610. https://doi.org/10.3390/app14156610

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