The primary task of any foundation engineering study is to generate reliable geotechnical data to arrive at a safe design in order to avoid conflict of interest (Osammor, 2009). Hence, it is very crucial to obtain comprehensive information that reflect the true nature of the subgrade soil properties and ground conditions. This is more important especially in the design and construction of highways, where the sub-grade soils play an important role in imparting structural stability to the pavement structure as it receives loads imposed upon it by road traffic. However, their strength values is greatly influenced by the moisture content. The higher the moisture content, the lower the strength of the soils. This behavior is attributed to the variation in soil structure with varying moisture content. Gerrard (2000) affirms that bedrock influences soils through weathering process and afterward through the weathered materials. Adeyemi (1992) and Adebisi (2003) in their works all described the various degrees of weathering processes experienced by the parent rocks through the influence of water and other agents to form laterized profiles of soils with diverse thicknesses. According to Adeyemi (1995), textural and mineralogical characteristics of parent materials could be responsible for behavioral differences in the engineering properties of soils derived from genetically different parent rock material. CBR being sensitive to many parameters such as moisture content, grain size, density, soil types among others, IRC: SP 72(2015) recommends the use of dynamic cone penetration test (DCPT) to evaluate the subgrade for its in-situ CBR.

The Study Area

The study area covers parts of the F209 road within the northern environs of Akure, southwestern Nigeria. It stretches between Latitude 07°16.00’N-7°28.00’N and Longitude 005°10.00’E-005°17.00’E. It is bounded in the North by Iju/Ita-Ogbolu Local Government and in the South by Akure South Local Government both in Ondo State. The road passes through Iju/Ita-ogbolu Local Government. The study area contains brownish to reddish residual soils with fine to coarse-grained texture. The route is characterized by potholes, wavy and extensive surface failure. The texture is found to be fine to coarse grain. The laterite is sticky and well compacted together. The geological setting of the highway is underlain by rocks of the Precambrian basement complex which vary in type, color, nature, structure, mineralogical composition, and texture (Fig.1). The variation in properties of these rocks is primarily due to the effect of various orogenic and deformational episodes on the rocks at different times during their evolutionary history (Ademeso, 2010). Detailed geological mapping revealed the presence of that rocks such as coarse-grained charnockite, medium-to-coarse-grained granite, porphyritic granite. The rocks have also undergone various degrees of weathering.

Fig.1 Geological map of the study area (modified after Olarewaju, 1987)

Methods

In-situ geotechnical investigation for subgrade characterization was performed using a Dynamic cone penetration test (DCPT) as designated in ASTM D6951 / D6951M-09, (2015). This was performed at an average interval of 500m along the entire route since almost the whole road pavement had failed. A total of thirty-seven (37) test pits were established and dug for the collection of soil samples. Soil samples were taken at an interval of 450mm, 650mm, and 900mm respectively down the profile to examine moisture content variations and their effect along the profile, especially on the strength parameter (CBR). The collected soil samples were kept in polythene bags labeled, sealed, and brought to the laboratory for analysis. DCP readings are interpreted using DCP software package. The software uses an equation that relates Penetration Index with CBR. These tests were performed during the month of March, before the full onset of the rainy season.

The effect of moisture content conditions on the strength parameters was examined by applying the modified equation of reduction in CBR/increase in moisture content equations 1 and 2 (Adeyemi, 2022). The modified formulae make use of the highest and lowest CBR with their respective moisture content to derive a co-efficient of reduction properties instead of optimal moisture content and natural moisture content.

% Reduction in CBR= Unsoaked CBR−Soaked CBR/x100%……………………………………………….1 

Unsoaked CBR                                                                                

% Increase in MC = Moisture content of unsoaked sample−OMCx100% ……………………….2                                  OMC

Results and Discussion

The properties of lateritic soil deposits vary from place to place because of the differences in the geological arrangements, prevailing climatic conditions, and the type of mineral present (Adnan et al., 1994). However, the degree of change (reduction) in the CBR values is directly proportional to the amount of moisture content absorption. The result of moisture content variations on FCBR values and co-efficient of reduction of studied soils are presented in Table 1 and 2: Table 1 and Table 2 respectively. The mean moisture content values ranged from 13.67 – 24.96%, 7.52 – 15.67% and 11.38 – 17.21% for Ch, OGp, and OGe-derived soils respectively. While while the mean FCBR values ranged from 7 – 19%, 13 – 53% and 7 – 37% for Ch, OGp, and OGe investigation points respectively. Generally, Ch derived soils give rise to soil with the highest moisture content and lowest FCBR, OGp-derived soils give rise to soil with the lowest moisture content with the highest FCBR while and OGe derived soils give rise to soil with the intermediate moisture content with corresponding FCBR values. This may be attributed to the percentage of fines, amount of clay content, and types of clay minerals of the derived soils. However, the results of the ratio values ranges range from 0.71 to 6.15 with an average of 1.85 for Ch derived soils, from 0.27 to 2.78 with an average of 1.47 for OGp derived soils and 0.67 to 6.36 with an average of 1.67 for OGe derived soils. This implies that all the derived soils are susceptible to ingress of water with a consequential reduction in strength which is the characteristic of the lateritic soils. Meanwhile, the degree of susceptibility differed. The result showed that Ch derived soils have the highest reduction rate, next by OGe derived soils and OGp-derived soils have the lowest reduction properties. This is because the water absorbing capacity of the Ch-derived soil is greater than that of the kaolinite dominated granite derived soils. Rate of loss of shear strength could contribute to the failure observed along the entire derived soil section along the study area. A good subgrade or sub-base soil should not suffer high reduction in CBR as a result of soaking. The co-efficient of reduction in strength parameter values of the derived soils differ on account of the type of mineral present and textural composition. The results showed that increase or decrease in the moisture content values give rise to decrease or increase in the CBR values respectively, either vertically along the soil’s profiles or laterally across the soil’s horizons.

Table 1: Moisture Content Variations on FCBR values of Studied Soils

 Table 2: Rate of Reduction in CBR and Increase in MC of Studied Soils

     HCBR=highest CBR, HMC= highest MC   LCBR = lowest CBR, LMC= lowest MC

Conclusion

This study gives insight into the influence of moisture content variations on the California bearing ratio with respect to local geology characteristics of sub-grade soils. The study areas was underlain mainly by charnockites and granites. The variations in mineralogical composition of the rocks give rise to the difference in the mineralogical and textural properties of their derived soils and the corresponding engineering characteristics of the soils in the study area. The granites derived soils have kaolinite as predominant clay mineral while Charnockites derived soils have vermiculite and kaolinite as predominant clay minerals. This give rise to different water absorption characteristics of the derived soils in study area. Ch derived soils have highest moisture content while OGp derived soils have the lowest moisture content.

The results revealed that Ch derived soils have the highest Penetration Index values, followed by OGe derived soils and OGp derived soils have the least value.The indicates that Ch derived soils have the lowest strength properties, followed by OGe derived soils and OGp derived soils have the highest strength properties. This may be attributed to the percentage of fines content and types of clay minerals of the derived soils which depend on the parent rocks. The results further showed that increase or decrease in the moisture content values give rise to decrease or increase in the CBR values respectively. The degree of change (reduction) in the CBR values is directly proportional to the amount of moisture content absorption.

Results of co-efficient of reduction of the CBR values revealed that Ch derived soils are more susceptible to ingress of water and high reduction in strength characteristics in comparison with OGe and OGp derived soils respectively of the lateritic soils.

Recommendations

Geological factors such as lithological and water content conditions should be taken into consideration whenever structural infrastructures such as dam and road are being considered. Also in-situ studies of the soils can be carried out at different climatic conditions to further elucidate the effect of seasonal variations on the soil engineering properties.

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