The results of the tests conducted on the soil samples with and without OPFF are presented and discussed in this section. These results have been discussed based on the behavior of the materials as subgrade materials.
3.1 Grain size analysis
The particle size distribution analysis was conducted on the soil samples to determine their grading and suitability for pavement construction. Figure 4 shows the curves for the grain size analysis for the three samples.
The particle size analysis for the 3 lateritic samples shows that the cumulative percent retained on No. 200 BS sieve (0.075 mm) were in the range of 74.64% and 86.03%. It was observed from the sieve analysis that the Ugwuoba sample has the highest amount of sand (86.03%), next to Nawfia (75.87%) and least is Okpuno (74.64%). The cumulative percentages passing on No. 200 BS sieve are in the range of 13.97–25.36%, with Okpuno having the highest amount of fines (silt and clay) as 25.36%, next to Nawfia which has 24.13% of fines and the least is Ugwuoba with 13.97% of fines. Based on these fine contents, the lateritic soils are suitable for pavement subgrades even in their stabilised state (Bello & Adegoke, 2010). This is in accordance with the Federal Ministry of Works and Housing (FMWH, 1997) specification which recommends samples with less than 35% fines as suitable subgrade materials. Consequently, the Ugwuoba soil is the most suitable with regards to this.
From the curves, it can be seen that all three samples have no D10 (particle size such that 10% of the size is finer than this size). D50 for all 3 soil samples range from 0.21–0.25. For Ugwuoba soil samples, 50% of the particles are larger than 0.25 and 50% is less than 0.25. For Okpuno soil sample, 50% of the particle sizes are larger than 0.25, and 50% are less than 0.25. While for Nawfia soil sample, 50% of the particle sizes are larger than 0.21, and 50% are less than 0.21
3.2 Consistency limits
The Atterberg’s Limits tests were conducted on the three soil samples used for this experiment. Figure 5 shows the plots for determination of the liquid limits for the three soil samples. The moisture content corresponding to 25 blows is reported as the liquid limits. The difference between the liquid limits and plastic limits gives the plasticity indices of the soils as shown in Fig. 6.
The plasticity indices of the soils are; 10.82% (Okpuno), 8.92% (Nawfia) and 2.75% (Ugwuoba). These are within the 55% maximum limit set by FMWH for subgrades and fill materials. The value of plasticity indices gives an idea on the amount of clay content contained in a soil sample. The plasticity increases with increase in the fine content of the soil. The presence of fines influence the consistency of the soil. Highly plastic soils are not desirable as they are prone to mud pumping and heaving. Granular materials are more stable for engineering applications.
3.3 Soil classification
According to the USCS, the soil samples may be classified as thus; since all the soils have more than 50% of the soil retained on No. 200 BS sieve, the soils are coarse grained, and the soils are identified as sand (since more than 50% of the coarse grained fraction pass No. 4 sieve (4.75 mm). Also since Okpuno and Nawfia samples have plasticity indices greater than 7%, they are referred to as inorganic clays with slight to medium plasticity and group name SC (silty clays). Ugwuoba soil sample has plastic index less than 4 and is classified as inorganic silts with no plasticity and group name SM (silty sands)
According to AASHTO, all the soil samples are granular materials since they all have 35% or less of the soil passing through No. 200 BS sieve (0.075 mm aperture). All soil samples are classified under A-2 since they all have a maximum of 35% passing 0.075 mm sieve. Nawfia and Ugwuoba soil samples are classified as A-2-4 since they have plasticity indices less than 10% and liquid limits less than 40%. Okpuno soil sample falls in to group A-2-6 since it has liquid limit value less than 40% and plasticity index above 11%.
3.4 Compaction of the soils
The compaction tests were performed to determine the Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) for each of the soil samples. Three compaction efforts were adopted, the BSH, WAS, and BSL. The curves are presented in Fig. 7, Fig. 8, and Fig. 9 for Ugwuoba, Okpuno, and Nawfia samples respectively.
For the three soil samples, the BSH compaction gave the highest value of MDD, followed by WAS, then BSL was the least. The Ugwuoba sample achieved MDD of 2080 kg/m3 for BSH, 2002 kg/m3 for WAS, and 1940 kg/m3 for BSL. The OMCs were 9.5%, 9.5%, and 10.2% for BSH, WAS, and BSL respectively. The Okpuno sample achieved MDD of 2076 kg/m3 for BSH, 1983 kg/m3 for WAS, and 1905 kg/m3 for BSL. The OMCs for this sample are 11.8%, 12.7%, and 13% for BSH, WAS, and BSL respectively. The Nawfia sample achieved MDD of 1884 kg/m3 for BSH, 1792 kg/m3 for WAS, and 1750 kg/m3 for BSL. The OMCs are 14%, 14.2% and 15% for BSH, WAS, and BSL respectively. Generally, higher compaction energies achieved higher MDD and lesser OMCs. This is because higher compaction energies less amount of water to achieve the necessary densification of the soils (Nwakaire et al., 2015). The compaction energies were computed in accordance with Eq. 1 and a plot of their relationship with MDD is shown in Fig. 10. It is a clear indication that compaction plays a key role in densifying and strengthening engineering soils. For this reason, compaction is an important aspect of earthworks in road construction.
Compaction energy (CE) = …………………..…… (1)
Where b = number of blows; l = number of layers; w = weight of the rammer (kg), h = height of fall (m), g = acceleration due to gravity (9.81 m/s2), and V = volume of the mould. Based on Eq. 1, the CEs for BSL, WAS, and BSH are 0.60 MN/m3, 1.0 MN/m3, and 2.7MN/m3 respectively.
As compaction energies increase, the MDD also increased. The BSH with the highest compaction energy achieved the highest densification of the soil. This is also the situation in actual practice, the weight of the compactor, intensity of the compaction, and rate of compaction both play important roles in ensuring effective pavement material densification for proper performance.
3.5 California Bearing Ratio (CBR)
The results of the CBR conducted for the three soil samples without inclusion of OPFF are reported in Table 1, together with the summary of other index properties already discussed. The highest CBR was recorded for the Ugwuoba soil sample (24%), next to the Okpuno soil sample (6%) and is least for the Nawfia sample (4%). The index properties of these soil samples are summarised in Table 1. They are soil materials with different geo-mechanical properties and satisfactory subgrade parameters. They were found to be suitable for this study as the objective is to monitor the improvements or reductions in these geo-mechanical properties of theses soils with the inclusion of OPFF.
Table 1
Index and compaction properties of the natural soils
SOIL SAMPLES | UGWUOBA | OKPUNO | NAWFIA |
Liquid Limit (%) | 16.6 | 32.8 | 30.2 |
Plastic Limit (%) | 13.85 | 21.98 | 21.291 |
Plasticity Index (%) | 2.75 | 10.82 | 8.91 |
Plasticity | Non plastic | Slightly plastic | Slightly plastic |
Sand (%) | 86.03 | 74.64 | 75.87 |
Silt + Clay (%) | 13.97 | 25.36 | 24.13 |
COLOUR | Reddish brown | Reddish brown | Reddish brown |
Coeff. Of Uniformity (Cu) | NIL | NIL | NIL |
Coeff. Of Curvature (Cc) | NIL | NIL | NIL |
AASHTO Classification | A-2-6 | A-2-6 | A-2-4 |
USCS Classification system | SM | SC | SC |
Specific Gravity | 2.60 | 2.57 | 2.55 |
MDD BSL (Kg/m3) | 1940 | 1905 | 1750 |
OMC BSL (%) | 10.2 | 13 | 15 |
MDD BSH (Kg/m3) | 2080 | 2076 | 1884 |
OMC BSH (%) | 9.5 | 11.8 | 14 |
MDD WAS (Kg/m3) | 2002 | 1980 | 1792 |
OMC WAS (%) | 9.4 | 12.7 | 14.2 |
CBR (48 hours. soaked) (%) | 24 | 6 | 4 |
General rating as subgrade | EXCELLENT | FAIR | POOR |
The CBR is the major discriminating parameter for pavement construction materials. However, all the physical properties are included in Table one for a quick evaluation of how these properties influence the CBR of the soil. In a nutshell, soils with higher specific gravities and MDD are expected to achieve higher CBR. The minimum requirements for 48 hours soaking by the FMWH specification for different layers of a road pavement are listed in Table 2.
Table 2
Specifications for road pavement materials (FMWH, 1997)
S/NO. | Pavement structural component | Minimum values of CBR (%) |
1 | Base course (natural or unstabilized soil material) | 80 |
2 | Base course (cement stabilized soil) | 180 |
3 | Sub- base | 30 |
4 | Subgrade and/or foundation soil | 5 |
Part of the project aim lies in estimating the effect of OPFF on subgrade soil of a pavement, hence the project centres around the subgrade. Comparing the values of the CBR gotten form the experiment conducted and the minimum values of the CBR for subgrade as specified by the FMWH, it can be deduced that the soil sample gotten from Ugwuoba and Okpuno borrow pits satisfy the minimum requirements and are thus suitable for use as subgrade material. Whereas, the soil sample gotten from the Nawfia laterite does not meet the minimum requirements and would therefore require stabilization. Nevertheless, how the incorporation of the OPFF will affect the compaction properties and CBR of the three samples were investigated and reported in the next section of this paper.
3.6 Effect of OPFF on the compaction of the soil samples
Compaction was carried out on the three soil samples with varying inclusion OPFF within the range of 0–3%. The following results were gotten from the compaction experiments. One compaction energy was used to ensure uniformity of the results gotten. The BSL was chosen because of its lesser compaction energy. It would introduce the least effect from compaction and elaborate more on the effect of OPFF inclusion. Figure 11–13 show the results of the compaction tests with OPFF inclusion.
From the figures it can be observed that the MDD decreases and OMC increases with increasing percentage of oil palm fruit fibre (OPFF). The reduction in MDD for the Nawfia samples ranged from 0.29–5.31% as the OPFF increased from 0.5–3%. The MDD of the Okpuno sample also reduced within the range of 1.25–9.51% whereas the reduction for the Ugwuoba soil is within the range of 1.55% and 9.79%. The trend for the three soils suggest that higher inclusion of the OPFF will lead to a much higher reduction in MDD. The reason can be traced to the fact that MDD depends on the physical properties of the material. The specific gravity of the material affects their density. Yusoff (2010) gave the range of specific gravity for OPFF as 1.2–1.45. This is significantly less than the specific gravity of the soil. More so, the fibres are biodegradable in nature. They were not stiff enough to provide additional rigidity to the soil samples. The MDDs decreased linearly with increase in OPFF as shown in Fig. 14. Consequently, the OMC of the samples increased with increase in OPFF as shown in Fig. 15.
3.7 Effect of OPFF on California Bearing Ratio of the soils
The results of the CBR experiments are presented in Fig. 16. Inclusion of OPFF reduces the value of the CBR after 48 hours soaking.
From these results, there was a remarkable drop in CBR values. The reduction was very significant that the soils could become unsuitable as subgrade or fill material. With up to 1% OPFF, the CBR tends to zero. The Federal Ministry of Works specification (FMWH, 1997) state that the minimum values of CBR for subgrade in a highway pavement is 5%. It was observed that the Ugwuoba soil sample failed to meet this requirement when the OPFF percentage was in excess of 1.8%. This means that above 1.8% inclusion of the fibre, the Ugwuoba soil sample was no longer suitable for use as a subgrade material. Also, the Okpuno soil sample failed to meet this criterion when OPFF in percentages as little as 0.22% was added to the soil. This means that at percentage as little as 0.22% of the fibre, the Okpuno soil sample was no longer suitable for use as a subgrade material. This sends a very important signal, that instead of considering the fibre from oil palm seed pericarp as a stabilising material, it is best to be considered a deleterious material which should be cleared from any site before compaction is commenced. Since the fibre is biodegradable, it could have some potential feasible reuse alternative in the agricultural industry. However, in line with previous studies, OPFF can be burnt to ash in an electric kiln. The ash which is pozolanic in nature can bring about significant improvement in soil properties (Otoko et al., 2016). Mahato et al. (1993) also described a form of chemical treatment for OPFF to stiffen the fibre before incorporation into soil for improvement of properties. Suroso et al (2013) reported that a combination of OPFF and cement can improve the geotechnical properties of weak soils. This report suggests that the improvement must have been as a result of the cement stabilisation since the OPFF alone failed to exhibit any form of soil improvement potential based on the observations from this study.