Engineering Site Investigation and Shallow Foundation Design in Ore Area of Ondo State, Nigeria

Shallow Foundations are usually located no more than six feet below the lowest finished floor [1, 2, 3]. A shallow foundation system generally used when the soil close the ground surface has sufficient bearing capacity, and underlying weaker strata do not result in undue settlement. The shallow foundations are commonly used most economical foundation systems. Footings are structural elements, which transfer loads to the soil from columns, walls or lateral loads from earth retaining structures. In order to transfer these loads properly to the soil, footings must be design to: prevent excessive settlement; minimize differential settlement; and provide adequate safety against overturning and sliding. Deep foundations are usually used when the conditions of the upper soil layers are weak and unable to support the superstructure loads [4]. Piles carry these superstructure loads deep in the ground. Therefore, the safety and stability of pile supported structures depend on the behavior of piles [5–6]. Most soil deposits in Ore are soft in nature.

There are different methods commonly used in assessing the in-situ geo-mechanical properties of the soil; the use of geophysical techniques, such as electrical resistivity method (VES) or seismic method; direct probing using static or dynamic penetration techniques and or boreholes. The success in the applicability of geophysical techniques depends on so many factors. The most important one is that there must be significant and detectable contrast between the physical properties of the different units in the subsurface, such as velocity, electrical resistivity, conductivity, density, acoustic properties, subsurface geology and the environmental conditions. Among these techniques, electrical resistivity method has a lots of applications: evaluation of temperature of soil and water content, evaluation of soil salinity, groundwater and mining survey and geotechnical investigation and geological mapping [7, 8, 9]. In addition, the 1D electrical method had been improved to a two dimensional imaging of the subsurface [10–11]. More recently, D-C electrical resistivity methods had been used for environmental studies [12] soil characterization for engineering purposes [13] and mapping of growth fault.

In addition cone penetration test (CPT) is considered the most frequently used method for characterization of geomedia [14, 15, 16]. The CPT is basically advancing a cylindrical rod with a cone tip into the soil and measuring the tip resistance and sleeve friction due to this intrusion. The resistance parameters are used to classify soil strata and to estimate strength and deformation characteristics of soils. The CPT is a simple, quick, and economical test that provides reliable in situ continuous soundings of subsurface soil [17, 18, 19]. Due to the soft nature of soil deposits in Ore, Ondo State, Nigeria the CPT is considered a perfect perfect tool for the area's site characterization. In subsurface exploration, the CPT in conjunction with SPT or in correlation with laboratory analysis of samples have effectively used to identify and classify soils and to evaluate the undrained shear strength. Implementation of the CPT can drastically decrease the number of soil borings and reduce the cost and time required for subsurface characterization. Following the standardization of test procedure [20] and improvement on the method of data interpretation [21], its reliability is found to be excellent. Mechanical cone, electric cone, and piezocone are the devices commonly used in cone penetration testing. The mechanical type is least efficient and least sensitive to changes in soil conditions, sensitive to changes in soil conditions. The goals of this research is to identify and estimate the bearing capacity of the subsoil layering through geophysical, cone penetration test data, and laboratory sampling analysis, for design of foundation and bases [22] in the study area.

The VES curves identified in the area are H, QH, and KH types with three to four geoelectric layer combinations. The QH curve type predominates, constituting 50% of the total, the KH curve type constitutes 33.3% and H curve type constitute 16.7%. The geoelectric section along Traverse 1 in Odigbo (Figure 4) identified maximum of four geoelectric/geologic subsurface layers comprising the topsoil, weathered layer, partly weathered/fractured basement/fresh basement. The topsoil varies in composition from clay to clayey sand, sandy clay and lateritic clay with resistivity values ranges from 68 to 689 ohm-m and thickness varies from 0.9–2.5 m. The resistivity of this layer has average of 200 ohm-m whose geoelectric characteristic is typical of sandy clay. The weathered layer resistivities are generally within the range of 46 and 890 ohm-m, typical of clay, clayey sand, and laterite. Resistivity in the range of 40–200 ohm-m is the most dominant, signifying a clayey weathered layer, The thickness is moderately thick with values varying from 12.7 m to 20.5 m and generally. The partly weathered/fractured basement/fresh basement has layer resistivity values vary from 882–1464 ohm-m. The depth to (overburden thickness) this layer is in between 15.2–22.1 m. Information gathered from existing hand dug well and boreholes shows that the groundwater level varies from 7.4 to 9.5 m. Therefore the weathered layer could be stabilized prior construction to improve the geotechnical properties since it's predominantly clayey. This would reduce consolidation settlement usually associated with clay. The thickness of the topsoil is very thin to accommodate civil engineering foundation structure because the upper 1 m is usually removed during construction to guide against undue settlement arising from buried/decayed plants and animals. The groundwater level ranges between 4.5–6.2 m.

Figure 4

Along Traverse 2 in Ore metropolis, the geoelectric section (Figure 5) also delineates maximum of four geoelectric/geologic subsurface layers comprising the topsoil, weathered layer, partly weathered/fractured basement/fresh basement. The topsoil has resistivity values ranging from 69 to 228 ohm-m and thickness varies from 0.9–2.0 m. It varies in composition from clay to clayey sand. The weathered layer resistivities are generally within the range of 16 and 215 ohm-m, typical of clayey soil. The thickness is moderately thick with values varying from 17.5 to 32 m. The partly weathered/fractured basement/fresh basement has layer resistivity values vary from 991–1200 ohm-m. This is the major aquifer in the area especially where the fracture basement is extensive with high fracture density/lineament interception. The depth to (overburden thickness) this layer is in between 19.5–32.9 m. The groundwater level measured from an existing well along this Traverse under VES 4/VES 5 records 12.3 m. Consequently this water level may not pose any/serious threat to foundation structure in the area. Therefore the weathered layer thickness is thick enough to distribute structural load to underlying soil/basement rock. But appreciable degree of stabilization (especially mechanical stabilization) to improve the geotechnical properties since it's predominantly clayey. This would also reduce consolidation settlement usually associated with clay soil material.

Figure 5

The summary of the geotechnical results is shown in Tables 1 and 2.

The % gravel ranges from 0.1–0.2, sand varies in between 22–44.6%, % silt varies from 20.8–63.3, and clay ranges from 8.4–36.8%. Generally the soil is dominated by sandy silt (SM). The average clay content in the soil is less than 35% which falls within 35% recommended for subsoil material that are good for civil engineering foundation construction. The specific gravity of the soil recorded values in the range of 2.65 (sand)–2.70 (clay). The average percentage passing 0.075 m is 33.3% which falls within 35% recommended for subsoil material that are good for construction. The engineering parameters of the soil samples are within the federal ministry of works and housing [40] specification for civil engineering building foundation construction. The analyzed soil samples at both locations shows liquid limits of 27.2–57.3% (avg. 39.7%), plastic limits of 19.4–37.4% (avg. 24.57%), plasticity index of 7.80–21.61% (avg. 15.18%) and shrinkage limits of 6.3–11.3% (avg. 9.1%) indicating moderate soil quality [41–42]. Generally, the lower the linear shrinkage, the lesser the tendency for the soil to shrink when desiccated [41–42]. The natural moisture content ranges from 6.3–18.7% (avg. 12.7%) which is moderately low. The FMWH [40] recommends liquid limit of 50% maximum, plastic limit of 30% maximum, plastic index of 20% maximum and 8% maximum for foundation material. Hence, the soil can be adjudged as a good foundation soil. The results of cone resistance with depth shows increase in cone resistance and sleeve resistance with depth (Tables 1 and 2), range from 20–85 kg/cm² and 35–120 kg/cm² at CPT 1, and 20–100 kg/cm² and 30–140 kg/cm² at CPT 2 respectively. The friction ratio ranges from 1.13–1.75 (CPT 1) and 1.40–1.50 (CPT 2). The Robertson [43] soil chart classification (Figure 6) shows two dominant zones of 6 to 7 corresponding to sandy silt to clayey silt and silty sand to sandy silt respectively (Figure 7). The plots of cone resistance and sleeve resistance against depth (Figure 8) showed two geological succession of sandy silt to clayey silt (0–0.4 m) and silty sand to sandy silt (0.4–1.4 m) at CPT 1 and three geologic sequence in CPT 2, namely sandy silt to clayey silt (0–0.4 m), and silty sand to sandy silt (0.4–1.0 m). Consequently, at least depth of 1.0 m would be appropriate as founding depth for design and construction of shallow foundation in the area.

Figure 6

Figure 7

The ultimate and allowable bearing capacity estimated from the cone resistance using Meyerhof [35] equation are presented in Table 3. The calculated bearing capacities could be used in determining the foundation type for structures. The allowable bearing of the soil varies between 49 to 208 kPa for CPT 1, and 49–245 kPa for CPT 2 and ultimate bearing capacity of 147 to 625 kPa and 147 to 735 kPa respectively. Consequently an average allowable bearing capacity of 200 kPa (ultimate bearing capacity of 600 kPa) is recommended and would be appropriate for design of shallow foundation in the area, at a depth not less than 1.0 m in location 1 and 1.4 m at location 2.

Settlement and bearing capacity are the major factors that govern foundation design. The commonly accepted basis of design is that the total settlement of a footing should be restricted to about 25 mm [34], [42, 43, 44] as by so doing the differential settlement between adjacent footings is confined within limits that can be tolerated by a structure. The settlement analysis for foundation width of 0.6–3.0 m at three depth levels of 1 m, 2 m and 3 m produces values between 30.06–45.92 mm (Table 4). But foundation width above 1.5 m produces settlement less than 25 mm (Table 5) recommended by Bell [42] as it ranges between 9.03–21.33 mm. Although according to Meyerhof [35], Schmertamnn [36] total settlement limits of 60 mm (clay) and 50 mm (granular soil) are still tolerable [45]. Therefore foundation width not less than 1.5 m for depth not less than 0.6 m is still feasible and appropriate. The calculation of bearing capacities for strip and square foundation is shown in Table 5. For strip foundation, the appropriate (recommended) ultimate bearing and allowable bearing capacity for depth levels of 1–3 m vary from 1403–2666 kPa and 468–889 kPa, while square footing varies in between 1956–3489 kPa and 652–1163 kPa respectively (Table 6).

Subsoil evaluation and shallow foundation design have been carried out in Ore area of Ondo State, Nigeria for civil engineering structure using geophysical and geotechnical method of investigations. The investigation was able to provide information on the stratigraphy, nature, structural disposition, competence of the subsoil. It also recommended appropriate foundation bearing capacities and corresponding expected settlements for different footing sizes and founding depths. The VES curves identified in the area are H, QH, and KH types with three to four geoelectric layer combinations. The QH curve type predominates, constituting 50% of the total, the KH curve type constitutes 33.3% and H curve type constitute 16.7%. The investigation delineated four geologic layers which include the topsoil, weathered layer, partially weathered/fractured basement/fresh bedrock. The groundwater level measured from an existing well and borehole ranged from 4.5–12.3 m. Consequently this water level may not pose any/serious threat to foundation structure in the area. Therefore the weathered layer thickness is thick enough to distribute structural load to underlying soil/basement rock. But appreciable degree of stabilization (especially mechanical stabilization) to improve the geotechnical properties since it's predominantly clayey. This would also reduce consolidation settlement usually associated with clay soil material. All the determined geotechnical parameters of the subsoil fall within the specification recommended for foundation material by FMWH. In view of this, an average allowable bearing capacity of 200 kPa (ultimate bearing capacity of 600 kPa) is recommended and would be appropriate for design of shallow foundation in the area, at a depth not less than 1.0 m in location 1 and 1.4 m at location 2. This would produce settlement values ranging from 9.03–48.20 mm depending on the width of the foundation. The ultimate bearing and allowable bearing capacity for depth levels of 1–3 m vary from 1403–2666 kPa and 468–889 kPa for strip footing while square footing varies in between 1956–3489 kPa and 652–1163 kPa respectively.