E-ISSN : 2541-5794 P-ISSN :2503-216X Journal of Geoscience, Engineering, Environment, and Technology Vol 4 No 4 2019 248 Ojo, O. et al./ JGEET Vol 04 No 04/2019 RESEARCH ARTICLE Integrated Approach in Geophysical Investigation of Road Failure in Crystalline Basement Environment in South-western Part of Nigeria. Olabanji Ojo1*, Victor Adejumo2, Obaromi Olubunmi2 1Department of Geological Sciences, Osun State University, Osogbo, Osun State, Nigeria. 2Zibronel Geosciences, Akure, Ondo State, Nigeria * Corresponding author : adeolu.ojo@uniosun.edu.ng Tel.:+2348035626912. LinkedIn: OlabanjiOjo. Received: Oct 1, 2016. Accepted: Nov 20, 2019. DOI: 10.25299/jgeet.2019.4.4.2590 Abstract The result of the geophysical investigation carried out to access the cause of road failure and remediation measures along Ilesha-Osogbo expressway, Osun State, South-western Nigeria is presented. The study involved integrating the dipole-dipole technique of electrical resistivity method with the ground penetrating radar (GPR) method. Two dipole-dipole traverses, one long and 20 short GPR profiles were established across the failed section of the road. The dipole-dipole data were interpreted using the Diprowin software to produce the pseudo- section while the GPR data were interpreted using the RadProwin to produce the radargram. The result revealed that the failed road exhibit incompetent layer of resistivity values ranging between 17 Ωm to 171 Ωm lying between two competent layers of resistivity values greater than 200 Ωm. A combination of the dipole-dipole technique and the GPR techniques revealed the depth extend to failure to about 4.5 meters to 5 meters deep which overlie a basement rock of undifferentiated gneiss, a rock that is easily prone to weathering. The water level was observed to occur at shallow depth of about 2 meters and infiltrates the entire weathered layer. The shallow groundwater level coupled with the water-logged clayey layer derived from the weathered materials from basement rock were found responsible for the failure of this section of the road. The study recommends the excavation of the waterlogged clayey layer to a depth of about 5 to 6 meters deep into the subsurface and replacement first with heavy boulders of granitic materials and later overlaid with a thick layer of highly resistive landfill materials such as laterite. The result of the two techniques used in this work have proved to be supportive due to the integration of the double dipole technique with the GPR technique, a relatively new technique recently being introduced into road failure mapping. Keywords: Geophysical, road, failure, Idominasi, groundwater, clay 1. Introduction Failed roads generally constitute one of the major challenges to transportation especially in most developing nations of the world. It has been described as next to power supply in Nigeria (Ifabiyi and Kekere, 2013) and has been a major setback to growth and development other countries. Not only has it causes deaths of several thousands of people worldwide, it has also resulted to loss of properties worth millions of dollars annually. A greater percentage of developing nations are located in Africa and Asia. In Nigeria, this challenge is beginning to gain government attention due to concerted effort towards to putting an end to this menace. Reports show that out of 192 countries of the world, Nigeria rank 191 (FRSC, 2011) in terms of unsafe roads. Hence, death toll due to road crashes has been put at 162 deaths per 10,000 populations. Road failures in Nigeria come in form of bulges, potholes, cracks and depression, all of which makes road non-pliable for road uses who are involved in day to day delivery of goods and services. Governments at all the three tiers of government are coming up with concerted effort towards resolving these situations in the country. Roads in Nigeria has been classified into three. These include trunk A road which are highways that link state capitals together or dual carriage ways that link one part of the country to another. They constitute about 17% of the total national road network and the management of these types of road has been the responsibility of the federal government. The second, the trunk B roads which are intra-state roads which are managed by state governments constitute about 16% of the total road network of Nigeria (Ebhohimen and Luke, 2014) while the third, trunk C roads are locally constructed road that link the local communities together and serve the purpose of means of transportation for rural dwellers to move locally made goods and services from the rural area to urban markets. They constitute about 67% of the total road network in the nation. These types of roads are under the management of the third tier of the government, the local governments. The construction and ultimate maintenance of all Nigerian roads are sole responsibilities of these three tiers of government in Nigeria. Most of these roads do fail while construction is ongoing stage and also after the completion of the project. Investigation indicated that lapses exists at the design and construction level. It is unfortunate to note that most contractors fail to take into consideration a 249 Ojo, O. et al/ JGEET Vol 04 No 04/2019 number of factors during the design and construction of roads. These include geologic factors such as nature of soil and the near-surface geologic sequence, existence of geological structures such as fractures and fault, presence of cavities, existence of an ancient stream channels and shear zones. These concealed subsurface structures and zones of weakness are controlled by regional fractures and joint systems and in conjunction with leaching of silica could result in rock deficiency are known to contribute to failure of highways and rail tracks (Nelson and Haigh, 1990). Geomorphological factors have been related with the relief and surface/subsurface drainage systems which can be mapped by a combination of two or more geophysical and geotechnical methods (Olorunfemi et al., 1986; Olorunfemi and Mesida, 1987; Ojo and Olorunfemi, 1995). Biological factor such as the presence of surficial materials such as buried organics within the subsurface especially in locally made roads. Adegoke-Anthony and Agada (1980) as well as Ajayi, (1987) observed that road failure usually prevalent in basement complex of Nigeria are not only attributed to the design, construction and usage of the roads alone but also to lack of understanding of the role of the influence of geomorphology and geology during the design and construction phases. It also has to do with inadequate knowledge characteristic nature of residual soils underlying the roads. As a result, some roads are not capable to withstand stresses. Highway and road failure have become most noticeable almost all parts of Nigeria especially in the Western part of Nigeria seating on soils derived weathered materials from the migmatized and unmigmatized granite, schist, and rocks of the Pre-Cambrian Basement complex rocks. It is also prevalent in the Eastern part of the country sited in fairly competent and incompetent subsurface earth materials where intense erosion has washed away. This has serious obstructions and have been constituting serious economic setbacks to communities where they occur. The present study desire to integrate the use of electromagnetic method and electrical method in geophysics using the ground penetrating radar (GPR) and the dipole-dipole array spread to unravel the causes, characteristic nature of road failure in Idominasi community, along Ilesha-Osogbo expressway and to proffer a solution to the way out of the problem associated with the failed portion of the road. This community lies within the crystalline basement complex of south western Nigeria. This is necessary because it is the only road that links the state capital, Osogbo with the industrialized Ilesha Township. The road is a very important commercial pathway that links the economy of these two communities. The integration of these two methods becomes very necessary because the GPR technique would provide detail information depth to the failure, cause of the failure and the internal stratigraphy of the studied area. The dipole- dipole survey of the other hand would provide information about the resistivity variation of the subsurface rock of the study area. The results from these two methods would provide comprehensive information that is needed for tangible conclusions about the nature, the extent as well as probable remediation techniques needed in this failed portion of Ilesha Osogbo expressway. 2. Location and Accessibility of the Study Area The study area, Idominasi, lies between Ilesha and Osogbo, two major towns in Osun state approximately between latitude 4° 40’E and 4°45’Eand between latitudes 7° 40’N and 7° 44’N north of the equator. The study area is surrounded by the following villages: Ijowa, Ibala, Iragun and Ipoye. Geologically, the study area is part of the Ilesha Schist belts in South western Nigeria. Rocks within the area include undifferentiated gneiss, granite gneiss andbanded gneiss (See Figure 1). Structurally, the area is divided into two; the Iwaraja faults to the eastern and Ifewara to the western part (Folami, 1992, Elueze, 1988). To the west of the fault is mostly amphibolites, amphibole schist, meta-ultramafites, and meta-pelites while to the east are units with minor meta-pelite, a major component of quartzites and quartz schist. Olusegun et al.,(1995) and Rahaman, (976) observed that “these entire assemblages are associated with migmatitic gneisses and are cut by a variety of granitic bodies”. The local geology of the study area is typical of the basement complex rock assemblage broadly grouped into gneiss-migmatite complex, mafic-ultramafic suite (or amphibolite complex), intrusive suite of granitic rocks and meta-sedimentary assemblages. A variety of minor rock types are also related to these units. The study area is made accessible by a network of roads that runs from Ilesha to Osogbo. Another road joins the expressway from Idominasi township. A major river (River Ora) running Fig. 2: GPR Profile Orientations along and across the failed portion of the Road 250 Ojo, O. et al./ JGEET Vol 04 No 04/2019 from east to the west and meandering southwards drains the entire area under study. Smaller river channels in the study area drains into River Ora. This study area possesses is a typical tropical climate having more wet season months than the dry season months. The wet season commences in April till October while thedry season commences from November to March. The Köppen-Geiger climate classification consider the climate around the study area as Aw. The average annual temperature in Ilesa is 25.6 °C. The average annual rainfall is 1317 mm. The least amount of rainfall occurs in January but precipitation reaches its peak every September with an average of 222 mm. On the average, the highest temperature is about 28.6 °C around March while the coldest month is about 23.9 °C on average around August every year. 3. Materials and Methods The ground penetrating radar (GPR) GSSI SIR 3000 monostatic equipment and the ABEM SAS 1000 resistivity meter were used for the exercise. Initial reconnaissance survey was carried out to map the geology and study the topographic layout of the studied area. This is followed by the establishment of a long traverse along the failed road using the global positioning system (GPS). Two dipole-dipole method of electrical resistivity technique were carried out along the failed portion of the road using ABEM SAS 1000 resistivity meter (terrameter). Results from this were interpreted using the Diprowin software to produce two pseudo-sections along the failed portion of the road. Zones of highly resistivity and low resistivity as well as depth to failure were identified from the pseudo-section. Also the electromagnetic wave was beamed into the subsurface using the (GPR SIR 3000) in order to view the subsurface depth extent to failure. The beginning and the end of the traverse were properly georeferenced. The GPR instruments was used to make 20 parallel traverses across the failed portion of the road. The operation was carried out using the geologic scan preset parameters configured into the TerraSirch mode as shown; T-Rate = 100, Rate = 80, Range = max, Gain = 5 point auto, Survey wheel calibration = 1024 sample per scan), frequency = 400 MHz, Collection mode = distance, sample per scan format = 16 bit (default).Radargram obtained from these activities were interpreted for the possible cause to road failure, depth to road failure, stratigraphy of the study area. The depth to water level and mud were computed from the analysis and subsequently used to construct a 3D model of the depth to ground water and depth to weathered layer/basement interface. Results from the two techniques were compared and inferences from these were used to proffer solution towards remediating the problem. 4. Results and Discussions Figure 2 shows the GPR profile orientations along failed road segment while Figure 3 shows the pseudo-section obtained using the dipole-dipole electrical method. Figure 4 to 15 shows the radargram obtained from GPR profiles. Figure 16 and 17 is the pictorial situation of the failed portion of the road. Fig. 2: GPR Profile Orientations along and across the failed portion of the Road The results obtained from the dipole-dipole investigation reveals that the first part of the road is relatively stable (indicated in red colour in Figure 3) around Idominasi junction but progressively becomes unstable towards the south eastern direction (indicated in blue colour). This is exemplified by the low resistivity values obtained within the middle portion of the traverse. The earlier part is underlain with thick lateritic material of about 20 meters’ thickness with rubble of unweathered quartzite having resistivity values of between 325 Ωm and 621 Ωm. As the profile continues, failure begins to be prominent and the resistivity begin to reduce to as low as between 17 Ωm to 46 Ωm (Figure 3) The 2-D resistivity pseudosection reveals relatively low resistivity values in the range of 46 and 69 Ωm at a depth range between 0.6 and 5.0 m typical of clay material. The extent of the failure is prominent at the top 5 m while, the failure reduces gradually at relatively deeper depth. However, at about 10 m depth, the effect was not noticeable. As shown in Figure 3, the depth to failure is about 2 m deep while beyond this depth, there seem to be a relatively competent layer underlying the muddy interval. Stratigraphically, the area under investigation is underlain by clay to a depth of about 4 m to 44 m and basement below the strata. Within the failed portion, the entire basement is almost weathered and almost lacking as the resistivity is as low as 17-117 Ωm. About 20 short GPR profiles were established during the course of the survey with the aim of revealing the water level, subsurface stratigraphy and the disturbed layer. Analysis of the GPR profiles (Figures 4-15) reveal that the cause of the failed portion of the road is the closeness of the water level to the surface coupled with the low resistivity clay material. This extends to a depth of about 0 to 4.5 or 5 meters. This result is similar to that obtained using the dipole-dipole technique. The material that constitute the overburden is rich in clay and varies in thickness from about 4 – 5 meters. The depth to the water level also varies from 1.40 meters in Figure 4 to 2.0 m in Figure 5. This result is also confirmed in the dipole-dipole that was run in the area which reveals the depth of 4.5 to 5 meters to the incompetent less resistive layer having resistivity values of 46 and 89 Ωm in Figure 4 and resistivity values of 14 and 171 Ωm in Figure 5. This observation is also obtained in Figures 6 to 15. 251 Ojo, O. et al/ JGEET Vol 04 No 04/2019 Fig. 3: Pseudosection of the failed portion of the from North to South. (Red = competent highly resistive layer; Blue = low resistivity incompetent failed portion) Fig. 4: GPR Radargram for Profile 1 Fig. 5: GPR Radargram Profile 2 252 Ojo, O. et al./ JGEET Vol 04 No 04/2019 Fig 6: GPR Profile Traverse 3Fig 7: GPR Profile Traverse 4 Fig 8: GPR Profile Traverse 5 Fig 9: GPR Profile Traverse 6 Fig 10: GPR Profile Traverse 7 Fig 11: GPR Profile Traverse 8 253 Ojo, O. et al/ JGEET Vol 04 No 04/2019 Fig 12: GPR Profile Traverse 9 Fig 13: GPR Profile Traverse 10 Fig 14: GPR Profile Traverse 11 Fig 15: GPR Profile Traverse 12 Figure 16 and 17 shows the ongoing geophysical investigation using the GPR instrument as well as the dipole-dipole electrical resistivity instrument respectively. Figure 16: Picture of the failed portion of the road and investigation using the GPR instrument Figure 17: Picture showing ongoing dipole-dipole geophysical survey on the failed portion of the road Table 1: Table showing First Dipole-Dipole Array Field Data 254 Ojo, O. et al./ JGEET Vol 04 No 04/2019 TRAVERSE ONE RESISTIVITY FIELD RECORD (DIPOLE-DIPOLE ARRAY) Date: Observer: Instrument Used: ABEM SAS 1000 terrameter Traverse Azimuth Traverse No. One (1) Electrode Spacing: 5 Site Description: Number of n = 5 Electrode Position C1 C2 P1 P2 Geometric Factor (K) Resistance R (Ω) Apparent Resistivity (Ω) 0 1 2 3 94.2478 4.3446 409 3 4 376.9911 1.0698 403 4 5 942.4778 0.38497 363 5 6 1884.9556 0.16367 309 6 7 3298.6723 0.098464 325 1 2 3 4 94.2478 4.5423 428 4 5 376.9911 1.0976 414 5 6 942.4778 0.34256 323 6 7 1884.9556 0.17121 323 7 8 3298.6723 0.11068 365 2 3 4 5 94.2478 3.6199 341 5 6 376.9911 0.71980 271 6 7 942.4778 0.27639 260 7 8 1884.9556 0.14664 276 8 9 3298.6723 0.092391 305 3 4 5 6 94.2478 2.4770 233 6 7 376.9911 0.58185 219 7 8 942.4778 0.25172 237 8 9 1884.9556 0.14695 277 9 10 3298.6723 0.11324 374 4 5 6 7 94.2478 2.3010 217 7 8 376.9911 0.48245 182 8 9 942.4778 0.17720 167 9 10 1884.9556 0.10213 193 10 11 3298.6723 0.070471 232 5 6 7 8 94.2478 2.0330 192 8 9 376.9911 0.38671 146 9 10 942.4778 0.14869 140 10 11 1884.9556 0.083077 157 11 12 3298.6723 0.051189 169 6 7 8 9 94.2478 1.4265 134 9 10 376.9911 0.30677 116 10 11 942.4778 0.12170 115 11 12 1884.9556 0.062223 117 12 13 3298.6723 0.035221 116 7 8 9 10 94.2478 1.3773 130 10 11 376.9911 0.30058 113 11 12 942.4778 0.12363 117 12 13 1884.9556 0.065282 123 13 14 3298.6723 0.046602 154 8 9 10 11 94.2478 1.2367 117 11 12 376.9911 0.25734 97 12 13 942.4778 0.065953 62 13 14 1884.9556 0.048762 92 14 15 3298.6723 0.035587 117 9 10 11 12 94.2478 1.45220 137 12 13 376.9911 0.25038 94 13 14 942.4778 0.10360 98 14 15 1884.9556 0.065972 124 15 16 3298.6723 0.043294 143 10 11 12 13 94.2478 1.2113 114 13 14 376.9911 0.22586 85 14 15 942.4778 0.11383 107 15 16 1884.9556 0.071055 134 16 17 3298.6723 0.054921 181 11 12 13 14 94.2478 1.0519 99 14 15 376.9911 0.22983 87 15 16 942.4778 0.11715 110 16 17 1884.9556 0.080555 152 17 18 3298.6723 0.07866 259 12 13 14 15 94.2478 0.81797 77 15 16 376.9911 0.15770 59 16 17 942.4778 0.077111 73 17 18 1884.9556 0.053673 101 18 19 3298.6723 0.043151 142 13 14 15 16 94.2478 0.72319 68 16 17 376.9911 0.16589 63 17 18 942.4778 0.087615 83 18 19 1884.9556 0.063142 119 19 20 3298.6723 0.039343 130 14 15 16 17 94.2478 0.67862 64 17 18 376.9911 0.18749 71 18 19 942.4778 0.10817 102 19 20 1884.9556 0.060088 113 20 21 3298.6723 15 16 17 18 94.2478 0.71850 68 18 19 376.9911 0.19061 72 19 20 942.4778 0.089627 84 20 21 1884.9556 21 22 3298.6723 16 17 18 19 94.2478 0.91152 86 19 20 376.9911 0.20725 78 20 21 942.4778 21 22 1884.9556 22 23 3298.6723 17 18 19 20 94.2478 1.3760 130 20 21 376.9911 21 22 942.4778 22 23 1884.9556 23 24 3298.6723 Table 2: Table showing Second Dipole-Dipole Array Field Data TRAVERSE TWO RESISTIVITY FIELD RECORD (DIPOLE-DIPOLE ARRAY) Date: Observer: Instrument Used: ABEM SAS 1000 terrameter Traverse Azimuth Traverse No. Two (2) Electrode Spacing: 5 Site Description: Number of n = 5 Electrode Position C1 C2 P1 P2 Geometric Factor (G) Resistance R (Ω ohm) Apparent Resistivity (Ω ohm) 0 1 2 3 94.2478 0.69290 65 3 4 376.9911 0.22664 85 4 5 942.4778 0.014776 14 5 6 1884.9556 0.061431 116 6 7 3298.6723 0.050204 167 1 2 3 4 94.2478 0.45880 43 4 5 376.9911 0.13538 51 5 6 942.4778 0.087540 83 6 7 1884.9556 0.063269 119 7 8 3298.6723 0.088032 290 2 3 4 5 94.2478 0.51966 49 5 6 376.9911 0.16736 63 6 7 942.4778 0.12739 120 7 8 1884.9556 0.15163 286 8 9 3298.6723 0.13716 452 3 4 5 6 94.2478 0.39549 37 6 7 376.9911 0.26276 99 7 8 942.4778 0.25160 237 8 9 1884.9556 0.25199 475 9 10 3298.6723 0.52869 1744 4 5 6 7 94.2478 1.0900 103 7 8 376.9911 5.2541 1981 8 9 942.4778 0.34672 327 9 10 1884.9556 2.8150 5306 10 11 3298.6723 0.13563 447 5 6 7 8 94.2478 1.0305 97.1 8 9 376.9911 0.37437 141 9 10 942.4778 1.2457 1174 10 11 1884.9556 0.096081 181 11 12 3298.6723 0.066031 218 6 7 8 9 94.2478 1.2515 118 9 10 376.9911 3.6771 1386 10 11 942.4778 0.29211 275 11 12 1884.9556 0.18016 339.6 12 13 3298.6723 0.045060 149 7 8 9 10 94.2478 0.014504 1.41 10 11 376.9911 0.88606 334 11 12 942.4778 0.43357 409 12 13 1884.9556 0.12729 239.9 13 14 3298.6723 0.10456 345 8 9 10 11 94.2478 1.9173 181 11 12 376.9911 0.68693 259 12 13 942.4778 5.6166 5294 13 14 1884.9556 0.097445 184 14 15 3298.6723 0.088045 290 9 10 11 12 94.2478 7.5819 715 12 13 376.9911 1.5697 592 13 14 942.4778 0.60218 568 14 15 1884.9556 0.51181 965 15 16 3298.6723 0.39240 1294 10 11 12 13 94.2478 1.1978 113 13 14 376.9911 0.17760 67 14 15 942.4778 0.11088 105 15 16 1884.9556 0.079292 149 255 Ojo, O. et al/ JGEET Vol 04 No 04/2019 16 17 3298.6723 0.074019 244 1112 13 14 94.2478 0.34964 113 14 15 376.9911 0.17750 67 15 16 942.4778 0.098327 93 16 17 1884.9556 0.068025 128 17 18 3298.6723 0.058611 193 1213 14 15 94.2478 1.1306 107 15 16 376.9911 0.19492 73.5 16 17 942.4778 0.083223 78 17 18 1884.9556 0.077026 145 18 19 3298.6723 0.044675 147 1314 15 16 94.2478 0.49428 47 16 17 376.9911 0.11667 44 17 18 942.4778 0.085506 81 18 19 1884.9556 0.046120 87 19 20 3298.6723 0.043888 145 1415 16 17 94.2478 0.38620 36 17 18 376.9911 0.20279 76 18 19 942.4778 0.10907 103 19 20 1884.9556 0.10323 195 20 21 3298.6723 1516 17 18 94.2478 0.45114 43 18 19 376.9911 0.17232 65 19 20 942.4778 0.15002 141 20 21 1884.9556 21 22 3298.6723 1617 18 19 94.2478 0.44504 42 19 20 376.9911 0.23666 89 20 21 942.4778 21 22 1884.9556 22 23 3298.6723 1718 19 20 94.2478 0.60064 57 20 21 376.9911 21 22 942.4778 22 23 1884.9556 23 24 3298.6723 Highway structures constructed on top of subgrade soils are supposed to be strong enough to support heavy loads on them. (Momoh, et. al., 2008) observed that subgrade soils underlying a stable highway should possess highly resistive and sufficient geotechnical strength to withstand stress. Such soil must have good drainage and permeability characteristics and not shrink or swell excessively (Adeleye, 2005, Oladapo, 1998). However, the result obtained from this work does not conform to the ideal due to the fact that the soil underlying the study area has been affected by the presence of water such that they could shrink and swell at any time. Probably, the study area is underlain by typical expansive clay materials. The stable segment which also falls within the earlier part within the first pseudo-section is underlain by weathered basement that is not waterlogged. Hence, there exist no failure in this segment and this segment is devoid of any geological features or structures that could aid and abet failure. The top soil and the subgrade soil here is purely lateritic with a significant thickness of about 20 meters. This means this area is thick enough to support any impose wheel load. As the traverse progresses, failure begin to be noticeable due to the presence of water level located closer to the surface, coupled with the fact that the section is made up of weathered incompetent layers made up of mud/clay materials. 5.0 Conclusion and Recommendation This research has been carried out in order to investigate the cause of road failure at Idominasi, along Ilesha-Osogbo expressway using an integrated approach that combines the use of the dipole-dipole technique of electrical resistivity method with the electromagnetic ground penetrating radar (GPR) method. The cause of the road failure was found to be the presence of a low resistivity (weak and incompetent) lithology localized and sandwiched between two competent layers. The water level was observed to be located very close to the surface. The combination of these two has been the root cause of the failed portion of the road. In order to proffer a solution to the problem on ground, the depth to water table and the depth to mud were extracted from the GPR radargrams for each profile of the radargram carried out in the study area. From the results obtained, since the clay is localized within a small portion of the road sandwiched within competent lateritic layers, towards the north western side and towards the south eastern part, the possible solution to the problem is to excavate the entire clayey layer in the study area, and replace with landfill material which can resist and withstand imposed wheel load. Excavation would be done to a depth of about 6 m to 7 m deep. Landfill materials to be used should be composed of homogenous and highly resistive lateritic material which is devoid of any significant geological features that could aid the development of swells and shrinking. This will go a long way to resuscitate the road and support heavy wheel load imposed on it. 6. Acknowledgement The authors would like to say that this work was funded by research grant award by the TETFund. We wish to appreciate the effort of members of staff of Geological Sciences of Osun State University, Osogbo during the field work as well as the University NEEDS Assessment Fund for the purchase of the GSSI SIR 3000 machine. We also wish to appreciate all the anonymous reviewers for their contribution towards the review of the manuscript. We are highly indebted to all. 7. References Adegoke-Anthony, WC.And O.A. Agada, 1980. Geotechnical characteristics of some residual soils and their implications on road design in Nigeria. Technical lecture. Lagos, pp:1-16. 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