Integration of VLF-EM and VES data for pavement failure investigation in a typical basement complex terrain of southwestern Nigeria

The VLF-EM measurements and the filtered graphs are presented in Figures 3–6. The in-phase profile shows positive peaks of different intensities, suggesting the presence of shallow and deep conductors [16, 17]. The 2D inversion model shows the variation of equivalent current density and change in conductivity with depth. It discriminates between conductive and resistive structures. Higher values of relative current density are indicative of conductive subsurface structures with varying low values of resistivity while lower values correspond to zones of low conductivity characterized by higher values of resistivity.

Figure 3:

Figure 4:

Figure 5:

Figure 6:

The 2D pseudosection indicates stable, partly failed, unstable and very unstable portions along the profile (Figure 6). The highly conductive zones, delineated as very unstable portions (distances of 150–260 m, 990–1200 m and 1600–1710 m) are underlain by conductive subsurface structures. These include linear geological structures (trending in different directions) which are indicative of major subsurface fractured/faulted zones with varying degrees of saturation and other conductive zones with lower positive amplitude suggesting the presence of clayey soil materials and/ or fractures at varying depths. Partly unstable zones (distances of 0–100 m and 260–820 m) are characterized by moderately low conductivity values.

Distances of 100–150 m and 1200–1260 m correspond to the stable zones and are composed of highly resistive materials. These portions are characterized by lower values of relative current density. The stable segments of the road are generally devoid of geological features [7, 9, 18].

A summary of the VES interpretation results is presented in Table 1. The curve types are H, A, Q, KQ, KH and QH; these are curves with occurrence of 33%, 17%, 8%, 8%, 17% and 17% respectively. They reflect the degree of weathering, fracturing and geoelectric complexity in the study area [8, 13, 14]. Figure 7 shows typical resistivity type curves in the study area.

Figure 7:

The H-type curve predominates the area of study. A stratigraphic sequence comprising the topsoil whose composition is variable alluvium, sand, lateritic clay or clay followed by clay/ sandy clay and the bedrock corresponds to the H-type curve. The bedrock could be weathered or possibly fractured basement. The intermediate layer is characterized by low resistivity. It is commonly water-saturated. The A-type curve presents subsurface conditions in which there is a persistent increase in the layer resistivity values from the topsoil to the basement rock (ρ₁ < ρ₂ < ρ₃). It suggests increasing subsoil strength and integrity [3, 9, 11].

The underlying geoelectrical units in QH-type curves consist of clay and clayey sand/ sand, respectively. The clayey sand and the weathered/fractured basement may jeopardize the stability of the pavement with varying degree of saturation. The KH-type curve has a succession of sand/sandy clay topsoil, lateritic clay, weathered basement and the fresh bedrock. The third layer is essentially clayey [7, 8].

The 2D geoelectric sections, Figures 8–10, reveal the subsurface geology and structural disposition of the study area. Figure 8 shows the 2D section beneath VES 1 to 4 (segment I). The first layer constitutes the topsoil with layer resistivity values ranging from 57.4 to 288.4 Ωm indicating materials of varying clay contents. The thin layer has an average thickness of 0.65 m except at VES 3 where composite clayey material with resistivity of 57.4 Ωm extends to a thickness of up to 6 m. This suggests lateral inhomogeneity and varying level of competence/stability [2, 8].

Figure 8:

Figure 9:

Figure 10:

The topsoil is underlaid by a clayey horizon of resistivity values ranging from 36.2 to 119.3 Ωm. The clayey substratum of about 14 m thickness delineated at VES 1 is inimical to the stability of the pavement as it is subject to flowing under traction. A clay fraction in a soil is generally expandable and compressible thereby promoting swelling and shrinkage. High clay contents thus impact weaknesses to the pavement structure [10, 20]. A lateritic cap with resistivity of 307.2 Ωm and average thickness of 1.2 m, observed at VES 4 may support stability at that zone.

The weathered/fractured basement is revealed at varying depths ranging from 4.0 to 15.0 m with a resistivity spectrum of 100.8 to 182.6 Ωm. This affirms a system of linear geological structures prescribed by the VLF-EM profile. The generally low resistivity values indicate low bearing capacity. Transmitting load to this layer may induce differential settlement of the pavement structure and precipitate failure [3, 7, 9].

Figure 9 reveals a 4-layer geoelectric sequence below VES 5–8 (segment II). The topsoil is clayey with resistivity values ranging from 88.7 to 177.8 Ωm and its thickness varies from 0.8 to 1.2 m. A lateritic crust forms the second layer with resistivity values ranging from 121.8 to 277.9 Ωm and thickness ranging from 2.1 to 4.0 m. This suggests varying levels of competence/stability giving rise to the partly unstable/stable zones along the profile. The average thickness of 2.98 m and varied composition may not absolutely fulfil the expected characteristics of a base course/sub-base course, particularly acting as the structural portion of the pavement and distributing the imposed stresses evenly [2, 9, 11].

The third geoelectric layer, a clay/clayey horizon extending to a depth of about 12.5 m at VES 5 is geotechnically significant. The layer resistivity values ranging from 26.5 to 64.7 Ωm indicate varying degrees of saturation owing to capillary effect. The material in place lacks a basic requirement of sub-base course to facilitate the drainage of free water that may accumulate below the pavement. Clayey formations are prone to differential settlement under prolonged vehicular traffic/axle loading owing to high porosity and practically little or no permeability [5, 21]. This fragile layer is underlain by a weathered/fractured basement with resistivity values ranging from 113.9 to 208.2 Ωm. The degree of weathering and fracturing permits groundwater accumulation and the effect of rising water table. The bearing capacity of the layer is corresponding low [3, 16, 20].

The geoelectric section of segment III encompassing VES 9 to 12 presents a 3-layer geoelectric succession (Figure 10). The resistivity of the topsoil ranges from 94.4 to 289 Ωm; indicating materials of varying clay contents, with thickness varying from 0.4 to 9.9 m. The generally thin layer is delimited by a massive clayey material (resistivity of 94.9 Ωm) stretching from VES 9 to VES 11 and extending to 9.9 m at VES 10. It is underlaid by a clayey substratum with resistivity value as low as 33.4 Ωm at a depth of 0.9 m around VES 11.

A weathered/fractured basement forms the third layer with resistivity values ranging from 62.1 to 377.9 Ωm. The layer is closer to the surface at VES stations 9 and 11, being at 2.3 m and 3.0 m depth, with associated resistivity values of 62.1 and 149.0 Ωm, respectively. High contrasts of resistivity values confirm the presence of linear features, mainly fractures/faults, lithologic contacts and near-surface lateral inhomogeneity [9, 17].

Adverse effects of groundwater can be expected across the third layer as the features often store groundwater in the basement complex terrain [16, 20]. This geoelectric layer constitutes a layer of low bearing capacity. It is structurally weak and may not be able to support heavy wheel loading structures [6, 18, 22].

The basic structure of a pavement represents four layers of surface: wearing course, base course, sub-base course and the subgrade. A flexible pavement surface reflects the entire behaviour of the subgrade layer. The characteristics of the soil bed over which the entire pavement system rests are indicative of the geotechnical properties of the pavement [21, 22]. Engineering design should be in line with the anticipated traffic load and ultimate strength of the pavement layers to ensure the stability and durability of the pavement structure [2, 18, 23].

The crystalline basement terrain of southwestern Nigeria presents challenges in geotechnical investigations due to the heterogeneous nature of the subsurface and the existence of localized geologic features [13, 20]. In this study, the subsurface geology indicates substantially clayey subsoil with relatively low resistivity values and presence of geological features including fractures, faults and lateral inhomogeneity. The presence of the linear features in the area suggests lithological contacts and/or fault/fracture zones along the road alignment occurring within the coarse-grained biotite and biotite hornblende granite and coarse-grained charnockitic rocks associations [13, 14].

According to Akintorinwa & Oluwole [11] and Asif et al. [23], the electrical resistivity of subsoil increases with increase in compaction and decrease in porosity. Higher moisture content decreases the value of subsoil resistivity. Compact subsoil is characterized by reduced porosity and moisture content with consequent increase in electrical resistivity. Diastrophic features and lithological contacts are associated with typically high porosity and permeability which enhance groundwater accumulation and transmission. The presence of these tectonic features readily permits the flow of water to the overlying clayey materials. The subgrade at some portions of the road might be wholly or partly immersed in the zone of saturation, particularly during the wet season of the tropics when groundwater table rises, leading to low resistivity and bearing capacity [16, 24]. This geological scenario ultimately promotes heaving, differential settlement and reduction in the load bearing capacity of the near surface subsoil [10, 22, 25].

The failure of the overlying pavement becomes apparent due to the plastic nature of the clayey soil and high moisture content [2, 7, 9]. Clays characterized by low resistivity (usually less than 100 Ωm) are regarded as incompetent materials as they tend to flow under stress. This result agrees with Akintoriwa & Oluwole [11] and Falowo [26], whose studies indicate an inverse relationship between apparent resistivity and moisture content, clay and silt content, liquid limit, plastic index, linear shrinkage, and unconfined compression shear strength. The specific gravity, maximum dry density and California Bearing Ratio (CBR) were shown to exhibit a direct relationship with apparent resistivity.

In an earlier study, Adams et al. [1] adopted conventional geotechnical methods to study the pavement indices influencing failure along the road under investigation, the Ado-Ikere Road, in southwestern Nigeria. The work provides a basis for comparison and validity of the present study. The study classified most of materials in place as clayey soil under group A-2-6 and A-7, rated as fair to poor material for road use according to the AASHTO classification system. The compaction and CBR tests indicated unsuitable base course materials. The soil has a high water retention capacity, with the natural moisture content ranging between 7.7 and 14.5% for base course, between 10.5 and 17.5% for sub-base and between 12 and 19% for subgrade, for all failed segments considered. The liquid limit showed that the majority of the failed section exceeded the allowable limit for base course materials.

The study of Adams et al. [1] significantly affirmed the presence of clayey formation underlying the pavement and effect of a rising water table, as reported in the present geophysical investigations. Application of the geophysical methods enabled delineation of linear geological structures including fractures, faults and lateral inhomogeneity beneath the pavement. The presence of these features renders geomaterials at such regions geotechnically weak with resultant pavement failures [9, 20, 26]. Incessant failure of the pavement under investigation has been promoted by the effect of the rising water table, heterogeneity of the subgrade materials, presence of expansive clays and linear features such as fractures, faults and geological contacts.