Parameters Modeling and Probing of Highway Structural Deterioration: Case study of F-209 Segment of Ondo - Ore Pavement, Southwestern Nigeria

: The construction of highway route is influenced by geology, geotechnical, topography, and geomorphology of the terrain. However, for pavement that failed structurally, a critical investigation is required to ascertain the cause(s) of its failure, so that such study can assist during the rehabilitation/re-construction phase. Based on this, the incessant failed Ondo – Ore was studied using integrated methods. Electrical resistivity involving four vertical electrical sounding, ten dynamic cone penetration test, three coring by trial pits at 1.0 to 3.0 m offset from the edge of the highway at different chain-age, and laboratory geotechnical-geochemical analysis adopting standard procedures. The VES characterized the geological sequence within the highway alignment to be topsoil, subsoil, weathered layer, and basement rock. The topsoil/subsoil and the weathered layer on which the highway is founded is sandy clay and clay with resistivity less than 200 ohm-m. The depths to basement ranged from 33.5 to 45.1 m. The trial pits recordings are consistent with the results of the VES, geotechnical, and geochemical analysis, distinctly distinguishing the upper 1 m into clayey soil (sandy clay, clayey hardpan) laterite, and silt-clay-sand mixture. The engineering competence of the topsoil/subsoil on which the road is founded is poor, although is inactive SC-SM, A-7-5/A-7-6 lateritic soil type (silica-sesquioxide ratio of 1.67). The clay mineralogy is within the illite – montmorillonite group. The SNG, SN, and SNP contributions of the soil as subgrade, subbase, and base material are very low (<1.0). The regression models of all parameters correlated positively, although weak for RD and DCPI, in-situ CBR and MR, RD and in-situ CBR; while strong for soaked CBR and in-situ CBR, ER and MR. Consequently, based on the GI and CBR values, the expected average thickness of the highway should range between 191 mm (good segment) to 445 mm (for weak segment) (avg. 312 mm) which is far thicker than 274 mm measured along the highway alignment during reconnaissance survey. Therefore, it can be concluded that the failure of the highway is as a result of low soaked CBR/in-situ CBR values with low strength coefficient; and low design thickness across the highway. In addition, lack of drainage at the shoulders of the highway is also causative factor. Hence, the need for effective design of roads (to specification) and maintenance strategy was therefore advocated.


INTRODUCTION
The dismal status of most developing nations' road networks is one of their distinguishing qualities, and Nigeria is no exception. The majority of roads in Nigeria are flexible pavement that responds to loads through elasticity and normal deformation. Pavement can be classed as rigid, flexible, or composite in terms of structural performance (Bell, 2007;Wright, 1986;Yoder & Witczak, 1975). A layered structure with minimal flexural strength is what a flexible pavement is. As a result, the external load is predominantly transferred to the subgrade through the lateral distribution as depth increases (Putra Jaya et al., 2021). Because of its low flexural strength, the pavement deflects briefly under force but returns to its normal level when the load is removed (Paige-green & Zyl, 2019). The pavement thickness is chosen in such a way that strains on the subgrade soil are kept within it carrying capacity and excessive deformations are avoided. This suggests that the subgrade is vital in a flexible pavement since it carries the vehicle loads transferred to it through the pavement (Bell, 2007;Kadyali & Lal, 2005). The strength and smoothness of the pavement surface are more dependent on the subgrade's permanent Geologically, the highway is underlain by Precambrian Southwestern Basement Complex ( Figure  2) with migmatites, coarse porphyritic biotite granite, and shale -sandstone being the major rocks observed within the highway (Figure 3). The highway falls within the Ondo soil association type, which are weathered products of medium grained granites and gneisses, it is well drained, of medium to fine textured, orange brown to brownish red, fairly clayey soils overlying orange, brown and red mottled clay. Along the highway, no noticeable side drainage was observed, but the area is characterized by dendritic and trellised drainage systems. Data Acquisition and Analysis The Ondo -Ore was chosen for this study due to perennial failure of the highway, its importance in relation its connectivity to many important cities and towns. The incessant failure of the highway has led to increase in number of accidents, traffic congestion, extended journey time, increase in crime along the failed segments, and increase in faulty vehicles. The methods adopted for this study were divided into two stages: field work/survey and laboratory soil analysis (Murthy, 2007). Before beginning the stages, a literature review of accessible geological, geotechnical, hydrogeological, roadway, and transportation articles or text pertinent to this subject was conducted (Emesiobi, 2000). Electrical resistivity survey utilizing vertical electrical sounding technique, in-situ dynamic cone penetrometer test, soil excavation in form pits and trenches, water table measurement of wells along the highway, and identification of spring/artesian well system were all part of the field activity. Figure 4 depicts the study's data acquisition map. The selection of the sample locations during the field survey was guided by geology, degree of failure, accessibility, and safety. Before, the commencement of the study relevant authority (such as ministry of Transport, ministry of works and housing) were briefed about the nature of the study; and necessary and satisfactory approval were obtained. The data collection methods were safe and environmentally friendly in accordance to American Standard of Testing and Materials (ASTM, 1990) and British Standard procedures (BS 1377) of 1990(British Standard Institution, 1990 . By detecting variations in the subsurface state, the geophysical inquiry aids in the detection of zones of abnormalities. By measuring certain physical qualities, they are used to ascertain the geological sequence and structure of subsurface rocks and soils (Telford et al., 1991;Attewell & Farmer, 1988). Density, elasticity, electrical conductivity, magnetic susceptibility, and gravitational attraction are the characteristics that are most often used in geophysical investigation (Williams, 1997;Vazirani & Chandola, 2009). In this investigation, vertical electrical sounding was used to measure electrical resistivity at four different points along the roadway (Aderemi & Adeola, 2021). By using two current electrodes to inject an electric current into the ground, this technique measures the difference in potential between two potential electrodes. Instead of measuring both current and voltage for this investigation, the resist-meter was able to measure the apparent resistance in ohms by performing automatic recording of both voltage 5 and current, stacks the results, computes the resistance in real time and digitally displays them. A 65 m half current spacing was employed with the Schlumberger array which progressively expands the current electrodes' spacing with fixed steps to enable sufficient penetration to the sub-surface earth and enhance structural responses. A bi-logarithmic graph of apparent resistivity value (i.e. the product of the geometric factor and the resistance recorded) obtained from the resist-meter against half the current electrode separation was generated. The data analysis comprised partial curve matching/curve fitting and modeling using WinResist software (Zhdanov & Keller, 1994) for the quantitative interpretation of the sounding curves. The results of the curve matching (layer resistivities and thicknesses) were used as starting model parameters for 1-D forward modeling using RESIST version 1.0. As a result, the electrode spacing at which the graph's inflection points appear provides information on the depth and thickness of the layers' interphases as well as their resistivity. The geoelectric segment of the roadway was built using the modeling results (VES interpretation results) in terms of resistivity and thickness. The DCPT was taken along the highway at a distance of between 1.0 and 3.0 m from the edge at both the stable and failed segments. The DCP is a straightforward mechanical device that can provide 45.5 joules of energy and is used for quick in-situ strength testing of roadway structural materials, particularly the subgrade and other unbound layers (Done & Samuel, 2006). When a standard force is applied, it measures the depth to which a standard cone penetrates. Along with the number of blows and depth of penetration, the standard hammer's DCP penetrative index in mm per blow is recorded. The conventional steel cone with a 20 mm diameter and a 60° angle was employed in this investigation. Ten (10) different places along the route were used to conduct the test. Due to the lack of security that typically characterizes failing highways, there were fewer tests conducted. For the analysis and interpretation of the gathered data, the UK DCP 3.1 software was employed. The data gathered at each location was adjusted for moisture content before being used to calculate the CBR using the Transport and Road Research Laboratory (TRL, 1990) relationship (Done & Samuel, 2006), as shown in Table 1. From Test No. 1 through Test No. 10, all of the test locations were given sequential numbers. The UK DCP 3.1 determined the strength coefficient of the test sites by first converting the penetration rate to the CBR value, then to the strength coefficient, and lastly to the structural number. The TRL equation was used for CBR calculation, as stated in equation 1. The strength coefficient of the subsoil for usage as the base and subbase layers is calculated using equations 2 (for base) and 3 (for subbase).
The SNG which is referred to as subgrade structural number i.e. the contribution of the subsoil as subgrade material to structural number of a pavement (Done & Samuel, 2006). It is usually derived from CBR just like the base and the subbase layers. The relationship between SNG and CBR is presented shown in equation 4. SNG = 3.51 Log 10 (CBR) − 0.85 Log 10 (CBR) 2 − 1.43 The relative densities of each subsoil layering were derived using DIN 4094 (1980) model (equation 5, where n10 is the number of blows for every 10 cm). The resilient modulus (using Lockwood et al., 1992;George & Uddin, 2000;and Chen et al., 1999 models, as shown in equations 6 -8 respectively) and Young modulus were obtained from each site along the highway alignment using equation 9.
From the results of models, important correlations and parameters modeling were obtained between soaked CBR and in-situ CBR, MR and ER, MR and CBR, DCPI and relative density, and CBR and relative density (Chen et al., 2005;Uz et al., 2015;Mohammad et al., 2007).
Three trial pits were dug along the highway to study the ground conditions, as it gives opportunity to assess directly the weathered rocks (Attewell & Farmer, 1988). The holes were dug with a digger by repeatedly dropping the tool into the ground. The depths range of the trial pits are within the upper 1.0 m, and no groundwater table was encountered. In addition, ten soil samples were taken at different chainage along the study highway as shown in Figure 4. Drilling was done using flight auger. The diameter of drilled boreholes was (15 cm). The disturbed samples were collected from the cutting of the auger at depths between 1 -2 m. The disturbed soil samples (10 kg each) were collected at each of the locations within the site. The samples were put inside black polythene bags, labelled and packed 7 under controlled temperature to prevent the escape of moisture. The samples were subjected to geotechnical and geochemical tests at Applied Geology Department, Federal University of Technology, Akure, Nigeria. The geotechnical tests were conducted using ASTM methods/procedures (1990), and these included California Bearing Ratio (D-1883), compaction test (D-1557), particle size analysis (D-422), Atterberg limits (D-4318), moisture content (D-2216) and specific gravity (D-854; D-5550). The geochemical test was only analyzed for mineral oxides of SiO2, Fe2O3, and Al2O3 using X-ray fluorescence and Atomic Absorption Spectrophotometer (AAS). The sample were initially sieved using 2 mm sieve and 2 g of the sieved sample was taken, after which they were put into digesting tube and digested using and HCl, then with HClO4 and H2O2. The samples were heated to dryness and make up with distilled water in a 100 ml volumetric flask. The resultant solution was analyzed using X-ray fluorescence and Atomic Absorption Spectrophotometer (AAS). Subsequently, the silica/sesquioxides (se) ratio (Adeyemi, 1995;Falowo & Dahunsi, 2020;Quadri et al., 2012) was calculated to know the type of the soil and classified it, if laterite (se < 1.33), lateritic (1.33<se>2.0) and non-laterite (se>2.0). Traffic survey (classified volume counts) was conducted for seven days taking records of all vehicles plying the highway per day (Brown, 1996;Kadyali & Lal, 2005). It was conducted by noting the number of various classes of vehicles that pass the count point in each direction, hence average of daily traffic was used in estimating design thickness for highway pavement.

RESULT AND DISCUSSION Electrical resistivity geophysical survey
The summary of the VES is presented in Table 2, while the geoelectric along the highway is shown in Figure 5. The curve types obtained from the highway alignment are H, KH, and HK denoting three-four-layer sequences. Geologically, it is made of topsoil, subsoil, weathered layer, fracture basement/basement rock. The topsoil has resistivity ranging from 98 -366 ohm-m (avg. 184 ohm-m) and thickness varying from 0.8 -1.2 m (avg. 0.95 m) and composed of clay, and sand clay (using interpretation Table 3). The subsoil resistivity is between 45 -302 ohm-m. The weathered layer has resistivity ranging between 64 ohm-m and 406 ohm-m (avg. 159 ohm-m) indicating clayey weathered layer. The depths to basement rock varied from 33.5 -45.1 m, indicating high weathering profile. Consequently, the topsoil, subsoil, and weathered layer are generally composed of clayey soil material, which can be regarded as incompetent/moderately competent soil material to support the pavement structure. It is observed that the basement relief is valley-like between VES 2 and 4 based on its configuration, this segment could aid flow of water or impound of water especially during the wet season, which can damage the road structure.   Geochemical Analysis Highway stability and serviceability performance depends on the mineralogical make-up of the soil (Kézdi & Rétháti, 1988;Bell, 1993). The result of chemical analysis (oxides) of the major elements (SiO2, Fe2O3, and Al2O3) contained in the soil samples, and silica-sesquioxide ratio is presented in Table  4. The samples are well dominated (in descending order) by SiO2 -Fe2O3 -Al2O3, with an average value of 57.76 %, 17.95 % and 16.71 % respectively. All the samples are very rich in iron oxide (Fe2O3) which can attribute to chemical weathering of mafic mineral composition of the parent rock and ferruginization of Fe-bearing minerals, while enrichment of Al2O3 can be attributed to the weathering alteration of feldspar to clay mineral causing leaching of Al2O3 by infiltrating acid rain/recharge water into the ground (Bell, 2004;Bell, 2007). Silica-sesquioxide (Se) ratio of the ranges from 1.58 to 1.73 (avg. of 1.67), hence lateritic soil type. Trial Pit Section Trial pits can be used for all soil types. It is the cheapest way of site exploration, and do not require any specialized equipment (Brink et al., 1992). In this method a pit is manually excavated and soil is inspected in the natural condition. Five geologic units were observed from the three sites investigated ( Figure 6) comprising sandy clay, clayey hardpan, laterite, clay, and silt-clay-sand mixture. At the northern axis of the highway notably at sites 01 and 02, (the geology is not different), thereby the weathering profile in the upper 1 m are the same consisting of sandy clay and clayey hardpan with corresponding depths of 0 -0.5 m and 0.5 -1.0 m; and 0 -0.15 m, and 0.15 m -0.70 m respectively. At site 02, laterite was observed at depth range of 0.70 -1.0 m. Clay and silt-clay-sand mixture constitute the weathering profile within 1.0 m at Site 03 with depth range of 0 -0.8 m, and 0.8 -1.0 m. Consequently, this result compliment the ER which delineated clayey topsoil along the highway alignment. Table 5 presents the summary of the geotechnical results. The natural moisture content varied from 18.2 to 23.8 % (avg. 20.71 %). Grain size analysis can be used to characterize the subsoil material for engineering foundation, which can serve as a guide to the engineering performance of the soil type and also provides a means by which soils can be identified quickly. The gravel and sand contents vary from 0 -1 % (avg. 0.2 %) and 36.4 -45.8 % (avg. 40.1 %) respectively. The % silt and clay contents ranged from 21.5 to 29.5 % (avg. 26.4 %) and 26.6 to 39.6 % (avg. 33.4 %). The %fines ranged from 54.2 to 63.6 (avg. 59.7). The composition of the soil is dominated by sand, clay, and silt (SC-SM). The plasticity chart (Figure 7) shows that the fines in the samples is dominated by clay of intermediate plasticity/compressibility (Figure 7a). All the soil samples plotted above the A-line. Most of the soil samples are plotted within the Illite clay mineralogy group (Figure 7b). Like kaolinite, illite also may be of hydrothermal origin. The development of illite, both under weathering and by hydrothermal processes, is favoured by an alkaline environment (Bell, 2007;Chapman, 1981). The activity ranged from 0.52 to 0.78 (avg. 0.66) signifying an inactive clay type. The values of specific gravity of the samples ranged between 2.65 -2.74 (avg. 2.69). According to Wright (1986), the standard range of value of specific gravity of soils lies between 2.60 and 2.80; these values are considered normal. Specific gravity is known to correlate with mechanical strength of soil and may be used as a basis for selecting suitable highway pavement construction materials particularly when used with other pavement construction materials. The liquid limit (LL) values ranged between 42.5 to 50.1 % (47.2 %), plastic limits (PL) ranged between 22.4 to 27.6 % (avg. 25.1 %) and plasticity index (PI) is between to 19.8 to 25.1 % (avg. 22.2 %). The Federal Ministry of Works and Housing (1997) recommends LL of 50% (max.), PI of 20% as (max.), plastic limit of 30 % (max.) and % Fines of 35 maximum for highway subgrade soil. Hence the soils partially satisfied this requirement as subgrade material, since the PI is marginally above 20 % specification. The linear shrinkage ranged between 9.4 to 13.5 % (avg. 11.8 %).

Geotechnical Analysis
Compaction is concerned with relationships between moisture content, applied effort and density. Compaction is undertaken on the road to enhance the mass density and hence the strength, rigidity and durability of placed materials (FHWA, 2006).  In the laboratory compaction testing is undertaken to predict moisture density responses of a material to applied effort and to provide a reference with which to control on-site compaction during construction (Bell, 2004). The maximum dry density (MDD) for the soil samples varied between 1699 and 1878 kg/m 3 at standard proctor compaction energy while the optimum moisture content (OMC) ranged between 22.2 and 26.9 %. An important part of the grading of the site often includes the compaction of fill. All the soil samples have moderately high MDD at moderate OMC.
The California Bearing Ratio (CBR) is an empirical test employed in road engineering as an index of compacted material strength and rigidity, corresponding to a defined level of compaction (Bell, 2004;Ampadu, 2007). All compacted samples show soaked CBR values ranging between 5 and 15 % (avg. 9.8 %), with corresponding in-situ values obtained from DCPT ranging from 4 to 19 (avg. 9.7). The Federal Ministry of Works and Housing (1997) recommends a California Bearing Ratio of greater than 10% for subgrade materials. Therefore, using Table, the soils are rated as low as pavement subgrade material. The Group Index (GI) values obtained ranged from 9 to 15 (avg. 11) corresponding to poor subgrade soil. The result shows that the California Bearing Ratio values of the soils are marginally lower than 10%. Using Table 6, the soil can be regarded as subgrade soil with medium strength classification. Based on the GI and CBR values, and the traffic count carried out which placed the highway as Class-E, the recommended thickness of the basement should range from 191 mm (good segment) to 445 mm (for weak segment) (avg. 312 mm) as shown in Figure 8, which is far higher than 274 mm measured along the highway alignment during reconnaissance study. Extremely weak Geotextile reinforcement and separation layer with a working platform typically required 1 % -2 % Very weak Geotextile reinforcement and/or separation layer and/or a working platform typically required 2 % -3 % Weak Geotextile separation layer and/or a working platform typically required 3 % -10 % Medium 10 % -30 % Strong Good subgrades to sub-base quality material >30% Extremely strong Sub-base to base quality material Figure 8. The CBR Chart adopted for determine the recommended thickness across the highway alignment

DCPT Analysis
The result summary of the DCPT is presented in Table 7, while subsoil layering in relation to its depth and in-situ CBR are shown Figure 9. In Table 7, the level of penetration ranged from 951 -988 mm, with cumulative number of blows ranging from 24 to 36. The penetrative index or rate ranged between 7.67 mm/blow (site 4 at 926 mm depth) -82 mm/blow (site 2 at 759 mm depth). All the sites are characterized by low -moderate cumulative number of blows in the upper 1 m investigated, signifying a loose/medium soil material. Along road alignment, one layer (sites 3, 9, 10), to two layers (sites 1, 2, 5, 8), and three layers (sites 4, 6, 7) were delineated. The obtained CBR ranged from 4 -19 %. The most competent layers in terms of the obtained CBR are generally between 408 mm to 956 mm.
The estimated relative densities (RD) gives consistencies of the soil either very dense, dense, medium, loose or very loose (Ilori, 2015). Table 8 showed that the soil is generally loose (with relative density of 0.320) in the upper 800 mm except at site 4 where the soil become consistent of medium soil material (with density of 0.429) at depth of 500 mm. However, the obtained layering not totally consistent with those observed from DCPI except at sites 3, 4, 8, and 10. The SNG contribution of the soil as subgrade material ranged from -0.34 to 0.88. This range of values is less than 1.0 SNG strength coefficient for subgrade pavement layer. Sites 3 and 10 are characterized with very poor subgrade material as shown in their SNG (Table 9).
Consequently, relating the CBR and SNG, the depths of 475 and 408 mm will be appropriate for sites 1 and 2 respectively, 926 mm (site 4) , 476 mm (site 5), 956 mm (site 6), 941 mm (site 7), 938 (site 8), 926 mm (site 9), and 937 (site 10). The strength coefficient of the soil as subbase and base ranged from 0.02 -0.09, and 0.01 -0.05, with respective SN and SNP ranging from 0.68 to 2.18 and 0.78 to 1.54; and 0.56 to 0.97 and 0.56 to 0.97. From the values, the strength coefficient is generally low for subbase and base material. The Young modulus and resilient modulus was estimated from Lockwood et al. (2000), Chen et al. (1999), and George & Uddin (2000) Lockwood et al. (2000) and George & Uddin (2000) showed closely overlapping values, while Chen et al. (1999) showed a wide variation (Table 10).

Parameters modeling and correlations
The obtained soaked CBR from the laboratory was correlated with in-situ CBR obtained from processing of DCPT data, the plot gives positive correlation coefficient (R 2 ) of 0.9507 (Figure 10a), and linear regression model (equation 10): CBR (in-situ) = 1.0432x + 5.0393 In this relationship, x = CBR (soaked).
The relative density values obtained from "DIN 4094" equation was plotted against in-situ CBR and DCPI. This gives a regression model of equations 11 and 12, with weakly positive correlations (R 2 ) of 0.0023 (Figure 10b) and 0.0259 (Figure 10c) respectively. This also showed the same trend.
The relationship between ER derived from "DIN 4094" and average MR calculated from expressions proposed by Lockwood et al. (1992), Chen et al. (1999), and George & Uddin (2000) is shown by the regression model in equation 13, with R 2 of 0.9612 (Figure 10d). MR = 74.397 ln (x) -223.08 (13) Where x is modulus of elasticity.
The correlation between in-situ CBR and average MR derived from the expressions of Lockwood et al. (1992), Chen et al. (1999), and George & Uddin (2000) to give equation 14, with correlation coefficient of 0.0796 ( Figure 10e); while the plots of the in-situ CBR against each of this authors give R 2 of 0.0739, 0.0848, and 0.0838 (Figure 10f). All the models follow the same trend. The variation in the coefficients is marginally as showed weak positive correlations. The model expressions for these relationships are presented in equations 15 -17.

Synthesis of results and Summary
The geological layers under the highway consist of clayey topsoil and weathered layer, with appreciable depth to basement rock. The relief of the basement is valley -like in some of the segments especially at failed segments which could aid flow of water or impound of water especially during the wet season, which can damage the road structure. This phenomenon was observed during the reconnaissance survey that surface water and ground water did not drain freely and quickly away from the road causing soil erosion, weaken pavement, destruction of road shoulders and wash out the embankments. This observation was also pointed out in Adams & Adetoro (2014), Adetoro & Akinwande (2014), Ifada et al. (2015), Adlinge & Gupta (2010). Therefore, no matter how beautifully, and anesthetics a highway appears, without taking care of the topographical low sections favourable for water impoundment, the highway would fall short of its design life expectancy.
The composition of the topsoil and weathered layer can attribute to chemical weathering of mafic mineral composition of the parent rock and ferruginization of iron bearing minerals; and alteration of feldspar to clay minerals. This was also verified in the trial pit inspected, as the weathering profile in the upper 1 m consisted of sandy clay, clayey hardpan, laterite, and silt-clay-sand mixture. However, it was clayey subsoil that dominated in all the trial pits, as supported by high fine contents (59%) obtained during the grain size analysis with intermediate compressibility.
The implication is that clayey formation is not good as subgrade because of its high porosity and very low permeability hence it tends to absorb water and swells, this result in high conductivity or low resistivity and differential settlement of the layer which subsequently lead to road failure. Therefore, the deterioration frequently experienced at many segments along the highway could arise from the differential settlement of the subgrade clay. This clayey occurrence has been recorded in many articles as one of the causes of road failure in the basement complex of Nigeria (Aderemi & Adeola, 2021;Ibitomi et al., 2014;Adeyemo & Omosuyi, 2012;Daramola et al., 2015;Daramola et al., 2018;Meshida, 2006;Jegede, 1997Jegede, , 1998Jegede, , 2004Adiat, 2009;Adesola et al., 2017;Falowo & Dahunsi, 2020).  0  3  3  3  3  3  3  3  -----Penetration (mm)  28  166  292  448  591  766  852  965  ----   It is recommended that for any road construction purpose, the clayey materials should be excavated away because of their unstable characteristics and re-filled with more stable materials. Even though, the silica-sesquioxide ratio of the samples signified lateritic soil. But there's possibility that the laterization process of the soil is still ongoing. Most of the soil samples are plotted within the Illite (inactive) clay mineralogy group. Consequently, some degree of expansion is expected from illite dominated clay but might not be as severe as montmorrilonite (Okunlola et al., 2015;Obaje, 2017).
The deterioration of the highway could be also as a result of low Group index and soaked CBR values; and corresponding DCPT values, as they are generally less than the standard recommendation of the Federal Ministry of Works and Housing (1997) recommends of minimum of 10% and 5% maximum respectively for subgrade materials. The most competent layers in terms of the obtained CBR are generally between 408 mm to 956 mm. The subgrade structural number (SNG) contribution of the soil as subgrade, the strength coefficient of the soil as subbase and base material are very low. This range of values is less than 1.0. Therefore, poor geotechnical properties of the subsoil is a factor be considered critically during the rehabilitation/reconstruction of the highway, since clayey soil (which is the commonest subsoil material that characterized the basement complex Nigeria) are mechanically unstable in terms of shear strength, bearing capacity, and compressibility. This assertion was also supported by Jegede & Olaleye (2013), Aghamelu & Okogbue (2011), Ubido et al. (2021, Adetoro & Abe (2018), Adiat, 2017Obaje, 2017).
The expected thickness of the highway structure should range from 191 mm (good segment) to 445 mm (for weak segment) (avg. 312 mm). Hence, this average value is above 274 mm measured along the highway alignment. Definitely, the existing thickness may not be able to sustain the haulage and high axle loads plying the highway presently. The estimated relative densities (RD) gives consistencies of the soil are of medium soil material (with density of 0.429) at depth of 500 mm. However, the strength of the subgrade can be improved when subjected to stabilization measures; or better still, failed segments of the highway with clayey or silty materials should be scooped out from the subsurface to a reasonable depth of at least 1 m from the topsoil and re-fill with competent materials. This should be put into consideration during the reconstruction and rehabilitation of the highway. Other contributing factors to the failure of some the highway segments, such as inadequate drainage system, paucity of maintenance, poor pavement coating of the road, and substandard construction materials are factors should be investigated by the federal government, most especially periodic and seasonal maintenances. On the issue of poor drainage facilities, proper drainage should be constructed to discourage the accumulation of runoff and thereby expose the subsurface material to erosion.

CONCLUSION
The structural deterioration of F-209 Segment of Ondo -Ore pavement had been investigated, and modeling of its geoengineering parameters. The main findings from the study showed that the subsoil on which the highway is constructed are characterized by clayey soil material of low soaked CBR/in-situ CBR values, low strength coefficient; and low design thickness across the roadway are the causes of the highway's collapse. The absence of drainage on the highway's shoulders is another contributing cause. The significance of the findings of this study for engineering practice, policy making, scientific knowledge, is that comprehensive or adequate site evaluation must be carried out prior to design and construction highway; in addition, on-site assessment of in-situ materials must be done, for use of such materials for construction. Thus, where inadequacies have been identified during the site assessment, this must be incorporated into the design; and where subsoil are geotechnical -poor or weak (as observed in this study), proper stabilization techniques must be adopted as may be required to achieve compliance with specifications. Nevertheless, it should be noted that, the influence of geology or the parent rock on engineering properties and the position of the horizon within the soil profile determine the engineering properties and behaviour of residual soils.
The limitation of this study was that the VES carried out along highway was insufficient to give an overall geologic profile of the highway, layer thickness and the properties of different pavement layers. Hence, it's advocated that Ground Penetrating Radar should be deployed for further studies. The GPR method provides a high-resolution image of subsurface features in the form of a cross section view that is essentially a map of the variation in ground electrical properties. This can be linked with physical changes such the subsurface layering, soil bedrock boundary, the boundaries between asphaltic layers, and water table.

ACKNOWLEDGEMENT
The Tertiary Education Trust Fund (TETFund) of Nigeria.