Available online at www.pelagiaresearchlibrary.com Advances in Applied Science Research, 2014, 5(2):199-209 ISSN: 0976-8610 CODEN (USA): AASRFC Integrated geophysical and geotechnical assessment of the permanent site of Adekunle Ajasin University, Akungba Akoko, Southwestern, Nigeria Oluwafemi O. and Ogunribido T. H. Tolulope Department of Geology and Applied Geophysics, Adekunle Ajasin University, Akungba-Akoko, Nigeria ABSTRACT An integrated geophysical and geotechnical investigations were carried out at the permanent site of Adekunle Ajasin University, Akungba Akoko, southwestern Nigeria to determine the suitability of the soil as foundation support for engineering structures. The geophysical methods used for the investigation were the Very Low Frequency Electromagnetic (VLF - EM) and the Electrical Resistivity methods (ER). The VLF-EM measurements were taken at an interval of 20m along six traverse lines. Sixteen vertical electrical sounding (VES) were carried out. Four major geoelectric layers were delineated: the topsoil, weathered layer, fractured layer and fresh basement with resistivity ranging from 70-708 Ωm, 26-300 Ωm, 124-783 Ωm and 1383 40272 Ωm respectively. The thickness varies from 1.7-4.0 m, 0.6-10.5 m and 3.0 27.0 m. The depth to the bedrock was less than 36.0m. Five soil samples were collected at depth of 1metre and were subjected to geotechnical tests such as compaction, consistency limits, unconfined compressive strength, specific gravity and California Bearing Ratio. Geotechnical properties indicates that the soil is good for engineering construction or as fill materials and geophysical investigation shows that the foundations of sky -scraper should be erected on piles to carry the loads to competent soil. Adequate measures should be taken since part of the study area is consisting of clay and clayey sands at shallow depths to avoid foundation failure or sudden collapse of engineering structures. Geotechnical problems could also be avoided, by the construction of drainage channels for run-off. Keywords: Very low electromagnetic, geoelectric section, foundations, geotechnical properties, fills materials INTRODUCTION Failures of engineering structures is a common phenomenon A pre-foundation study is essential before the design of foundation of any engineering construction. Environmental and engineering applications of surface geophysical techniques have gained wide interest in the last few decades. This is evidenced by the intensive research and exploration works using DC resistivity, ground penetrating radar and electromagnetic techniques [1]. These methods are quick, cheap and non-invasive means is used to provide information about the subsurface properties, depth to bedrock, location and distribution of conductive fluids, location and orientation of fractures [2]. Soil is indispensable material on which engineering structures are constructed. Therefore a detailed investigation of the subsurface materials of the proposed site is an important task needed to be considered before the erection of any structure to avoid foundation failure and undue loss of lives and properties. Factors responsible for failure of engineering structures include poor construction materials, non compliance to specifications, inadequate supervision and nature of the subsurface conditions of the ground on which the building is sited. To locate a successful site for construction in the sedimentary environment, a detailed subsurface study should be carried out. Site investigation is therefore required for a long term performance and stability of structures. The extent of such investigation depends on the type of foundation, the complexity of the soil conditions and available information on existing foundations on similar soils in order to ascertain the strength, bearing capacity and factor of safety of such soil. This has led to the geophysical and geotechnical assessment of the study area with the aim to investigate the shallow subsurface setting, 199
mapping of the fracture zones and any other geological structures that may contribute to the development of cracks or cause structural failure in any proposed engineering structures. Adekunle Ajasin University lies between latitudes 7 27' 21.6 N and 7 29' 14.46 N (825000 and 828500 N in the Universal Traverse Mercator scale, UTM), and longitudes 5 43' 33.84 and 5 44' 53.34 East (800900 and 803330 E in the Universal Traverse Mercator scale, UTM both in Datum 100 Minna-Nigeria and zone 31N 0 0 E to 6 0 E). The site occupies an area of about 2km 2. It is characterized by relatively gentle undulating terrain with elevations of between 326 and 350m. The site is surrounded in the north and south ends by hills and inselbergs. The hills trend between 090 0 and 125 0. The area lies within the tropical rain forest belt characterized by alternating wet and dry seasons with a mean annual rainfall of 1300mm and the drainage pattern is mainly dendritic. The area is characterized with fairly uniform temperature and high relative humidity [3]. The vegetation is densely evergreen consisting of palm trees, bamboo trees, thick grasses, moist deciduous trees, herbaceous plants and wooden shrubs. The study area is part of the Basement Complex of Southwestern Nigeria, underlain by migmatite- gneiss- quartzite complex with the granite gneiss and grey gneiss being the major units while the minor units include mafic, granodiorite, pegmatite, garnet-sillimanite gneiss and quartzite [4]. Based on field study area, the identified lithological units comprise of migmatite gneiss, charnokitic rocks and fine to medium grained biotite granite (Figure 1). Migmatite gneiss are dominant in the study area which are extensively weathered and fractured, with prominent direction of foliation lying between 176 0 and 184 0 with easterly dips of 44 0 and 76 0. They occur as ridges and hills which contain quartz veins, dykes, quartzo-feldspathic intrusion and pegmatitic veins usually very extensive. The granite forms the basement lithology in some other parts of the study area. These rocks general trend is N-S direction typical of the basement complex rocks. Structural features such as faults, joints, xenoliths, folds, dykes etc. characterize the rocks. The older granite of the study area is grey with a speckled appearance caused by the darker crystals. Figure 1: Geological Map of Akungba Akoko (Extracted from Geological Map of Ondo State) MATERIALS AND METHODS 1kg of each soil samples were collected from five locations in the study area from burrow pit below one meter depth. The soil samples collected were air dry for two weeks at the engineering Geology laboratory of the Federal University of Technology, Akure before they were subjected to the following tests: natural moisture content, 200
compaction, unconfined compressive strength, particle size, specific gravity, consistency limit and permeability test. Some researchers have worked on geotechnical properties of lateritic soils [5 9]. The very low frequency electromagnetic method (VLF EM) is a quick and powerful tool for the study of shallow conducting lineament features in the near surface earth [10]. It is an inductive technique, which uses electromagnetic signals of radio waves from remote military transmitters. These signals have frequency ranges between 15 and 30 KHz, and are propagated between the surface of the earth and the ionosphere causing the primary field. When there is a localized conductor, such as water-filled fractures, the primary field induces secondary currents inside that conductor, and these currents generate a secondary field which is different in phase and superimposed on the primary field. Thus, measuring the total field (primary and secondary) using VLF-EM receiver on the surface of the earth can help in detecting the conductive elongated structures, geological contacts like altered zones, faults and dykes [11]. The entire ABEM WADI system is portable equipment mounted on a belt worn by the user, which measures the inphase (Real) and quadrature (Imaginary) components of the induced vertical magnetic field as a percentage of the horizontal primary field. Six geophysical traverses were established within the survey area. The VLF-EM profiling was carried out using ABEM WADI instrument at 20 m interval along six traverses using the existing roads and footpaths with orientation at high angles to the direction of the transmitter. The WADI VLF-EM equipment detects the ratio (in percentage) between the vertical and horizontal components of the EM signal. Therefore, the in-phase and quadrature values in percentages were plotted against station positions using the Microsoft Excel Program. Qualitatively, the varying amplitude from this anomaly profiles is a measure of the conductivity changes in the subsurface. To produce 2-D models along the traverses in the study area, the Karous-Hjelt and Fraser filter [12] program was used to perform Karous-Hjelt and Fraser filtering on the VLF-EM data. The anomaly inflections appear as peak positive anomalies and false VLF anomaly inflections as negative anomalies of the profiles. 828500 828000 827500 2 PERMANENT SITE ROAD 10 11 9 12 15 1 16 PERMANENT SITE 8 14 13 7 INSTITUTE 6 827000 OF 5 826500 VC'S COMPUTER BUILDING OFFICE CENTRE BUILDING 826000 825500 825000 SOC. & MGT. SC. OF SCIENCE I.P.T.P ROAD 3 4 FACULTY OF PIONEER HOSTEL ROAD LEGEND MAJOR ROAD VLF TRAVERSES VES STATIONS 824500 OF 800000 800500 801000 801500 802000 802500 803000 803500 Figure 2: Geophysical Field Layout Map of the Study Area. In this study, vertical electrical sounding (VES) using Schlumberger array was adopted in order to study the variations in the resistivity distribution of the soil with depth. In this technique, vertical variations in the ground 201
apparent resistivity were measured with respect to a fixed centre of array by gradually expanding/increasing the electrode spacing. The equipment employed for the resistivity field data measurements was the ABEM 1000 SAS resistivity meter. Sixteen Vertical Electrical Resistivity Soundings were conducted at selected locations based on the results obtained from the VLF surveys along the traverses. Half current electrode spacing (AB/2) varying from 1 m to 100 m was used to be able to determine the depth to bedrock of the study area. The apparent resistivity values obtained from the vertical electrical sounding (VES) were plotted against electrode spacing (AB/2) on a bi-logarithmic graph paper. Visual inspection of these curves gave qualitative interpretation of the subsurface resistivity variations in order to determine the number and nature of the layering. This variation in resistivity is due to change in rock type, weathering characteristics and fluid content [13]. Quantitatively the sounding curves were interpreted by partial curve matching techniques [14] using a 2-layer master curves and the corresponding auxiliary curves. The geoelectric parameters from this manual interpretation were improved upon through the use of computer iteration technique using the computer algorithm RESIST Version 1.0 [15]. The interpretation results (layer resistivities and thicknesses) obtained was then presented as geoelectric sections which were used to generate layer parameter histogram. RESULTS AND DISCUSSION The double plots of the raw real and filtered real components enable qualitative identification of the top of linear features i.e. points of coincident of crossovers and positive peaks of the real and filtered anomaly. From these plots minor linear features suspected to be faults/fractured zones were identified which were used to pick points for VES survey. The varying amplitude which is a measure of the anomaly changes in the subsurface vary in the study area, indicating variable conductivity changes of the subsurface materials. The K-H pseudo-section of the profile which is a measure of conductivity of the subsurface as a function of depth is shown in figures 3 to 8. This conductivity is shown as colour codes with response increasing from left to right (i.e. from negative to positive). The 2-D model section of the EM filtered real profiles reveal points showing conductive zones. Major conductive features of varying degree of conductivity trending in different directions were delineated on the section, for instance, between distances 120 200 m and 260 370 m conductive features are shown along traverse 1. Similarly, between distances 180 240 m and 300 420 m are other conductive bodies along traverse 2, all trending in the SW-NE directions. The same process of qualitative interpretation was adopted for the remaining pseudo-sections. The identified conductive zones are suspected to be fault/fracture zones and sheared zones within the bedrock. The fractured zones are considered as weak zones liable to cause foundation failure and total collapse of structures erected on it because it is characterized by low bearing capacity, and are consequently geotechnically incompetent to support the weight of heavy civil engineering structures. Based on this interpretation, the points of interest marked as conductive zones were among the points that were further investigated using the Vertical Electrical Resistivity Sounding (VES). W E Figure 3: Karous and Hjelt pseudosection along Traverse 1 202
Figure 4: Karous and Hjelt pseudosection along Traverse 2 Figure 5: Karous and Hjelt pseudosection along Traverse 3 Figure 6: Karous and Hjelt pseudosection along Traverse 4 203
Figure 7: Karous and Hjelt pseudosection along Traverse 5 Figure 8: Karous and Hjelt pseudosection along Traverse 6 The resistivity sounding curves identified range from A, H, K, KH, HA and KA varying between three to four geoelectric layers with the H as the predominant curve type (Figure 9). The geoelectric sections along the six traverses (Figures 10a to 10c) show maximum of four geologic subsurface layers comprising the top soil, weathered layer, fractured basement and fresh bedrock with the shallow subsurface composed of a relatively low resistivity formation relatively soft and correspond to areas saturated with water. The top soil with resistivity values varying from 70-708 Ωm and thickness of between 1.7-4.0 m is composed of clay, sandy clay/clayey sand/laterite. The low resistivity and loose soil are not suitable for use in shallow foundations except the sandy clay/clayey sand. Areas within the topsoil with resistivity values higher than 300 Ωm are considered competent for engineering construction especially the high rise structures. The weathered layer composed of clay and sandy clay has the resistivity values ranging from 26-300 Ωm and thickness of between 0.6-10.5 m. Clay may cause foundation failure because of the varying environmental conditions on them annually. Based on this, adequate and valuable measures must be taken especially on the major parts of the study area consist of clay and clayey sands at shallow depths as this might cause foundation failure and even sudden collapse on engineering structures. The fractured basement has resistivity values varying from 124-783 Ωm and thickness of between 3-27m and the fresh basement ranging in resistivity from 1383 40272 Ωm with the depth to bedrock generally less than 36m. The areas with fairly thick overburden are considered to be incompetent for any engineering structures when composed of geotechnically weak overburden materials. 204
Figure 9: Histogram of the VES curve types in the study area Depth (m) 0m VES 1 VES 4 VES 6 VES 7 VES 9 181 165 165 187 153 469 783 455 149 34 164 139 4m 164 139 783 455 26 34 469 300 8m 12m SCALE 10m 16m 20m 1420 4m 24m 124 7529 LEGEND TOP SOIL 28m WEATHERED LAYER 32m 4967 2007 565 FRACTURED LAYER FRESH BASEMENT 36m 2007 Figure 10a: Geoelectric section across VES stations 1, 4, 6, 7, and 9 205
Depth (m) 2m 4m 6m VES 2 VES 3 VES 5 VES 8 VES 10 VES 11 90 156 235 135 182 708 115 115 128 1558 598 81 40272 8m 3688 53 53 10m 12m 4546 23523 6131 28 14m 194 SCALE 10m LEGEND TOP SOIL WEATHERED LAYER 2m FRESH BASEMENT Figure 10b: Geoelectric section across VES stations 2, 3, 5, 8, 10, and 11 0m 2m VES 12 VES 13 VES 14 VES 15 VES 16 179 137 70 153 455 132 Depth (m) 4m 6m 35 35 75 32 139 8m 10m 1501 98 1876 1383 1528 LEGEND SCALE 10m TOP SOIL WEATHERED LAYER 2m FRESH BASEMENT Figure 10c: Geoelectric section across VES stations 12, 13, 14, 15, and 16 Table 1: Summary of geotechnical properties of the soil samples Geotechnical properties Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Specific gravity 2.65 2.65 2.68 2.69 2.69 Natural moisture content % 12.35 10.10 13.30 15.35 15.10 Liquid limits % 33.4 33.9 41.8 48.8 48.3 Plastic limits % 19.4 20.1 21.4 23.1 22.0 Plasticity index % 14.0 13.8 20.4 25.7 26.3 Optimum Moisture content % 12.5 12.6 15.4 16.6 16.6 Maximum dry density Kg/ m 3 1879 1885 1845 1825 1823 UCS KN/ m 2 210 190 260 320 314 206
Summary of geotechnical results are presented in Table 1 and the compaction graph of the soil samples are presented in Figures 11 15. Compaction and specific gravity Soil sample 3 has the highest maximum dry density (1885 kg/ m 3 ) and this high maximum dry density (MDD) might be due to high clay content and the highest optimum moisture content (OMC) was 16.6%. Federal ministry of Works and Housing recommend that for soil to be use as fills, MDD must be greater than 470kg/ m 3 and OMC less than 18%. This indicates soil samples in the study are can be use as fill materials (see Figures 11 to 15). The specific gravity ranges between 2.65 and 2.69 this also shows that the soil samples are good construction materials Consistency limits The liquid limits, plastic limits and plasticity index for soil sample 1, 2, 3, 4 and 5 in the study area were 33.4, 19.4 and 14.0; 33.9, 20.1 and 13.8, 33.9, 20.1 and 13.8, 41.8, 21.4 and 20.4, 48.8, 23.1 and 25.7, 48.3, 22.0 and 26.3 respectively. The liquid limits are less than 50%, the plasticity ranges from low to intermediate. This indicates that the soil has good geotechnical properties; therefore the soil is good for construction purposes. Figure 11: Compaction graph for soil sample 1 \ Figure 12: Compaction graph for soil sample 2 207
Figure 13: Compaction graph for soil sample 3 Figure 14: Compaction graph for soil sample 4 Figure 15: Compaction graph for soil sample 5 Unconfined compressive strength From the unconfined compressive strength, the consistency of a clayey soil in the samples can be grouped as very stiff for soil samples 1, 3, 4 and 5 and stiff for soil sample 2 [16]. 208
CONCLUSION The integrated geophysical and geotechnical investigation carried out in this research are useful in characterizing the earth material that underlies the study area. The geophysical results revealed the presence of four subsurface geoelectric layers within the study area. These include topsoil, weathered layer, fractured basement and the fresh bedrock. Anomaly responses due to conductive clayey material and weathered layer/fracture zones were shown on the 2-D VLF-EM models. Shallow foundation is considered unsuitable for the proposed buildings because of the presence of incompetent materials hence, foundations of high rise building within the study area should be erected on piles which will carry the super-structural loads to competent soil, this will prevent foundation failure or sudden collapse of sky scrapers. The results of geotechnical properties of the soil samples shows that the soil are good as fill materials as well as for other engineering construction purposes. REFERENCES [1] Schoor, M. V, Journal of Applied Geophysics, 2002, 50, 393-399. [2] Reynolds, J. M, An introduction to Applied and Environmental Geophysics, West Sussex, England: John Wiley and Sons Ltd., 1997, pp 796. [3] Nigeria Meteorological Agency, (NIMET), Daily weather forecast on the Nigerian Television Authority, Oshodi, Lagos, 2007 [4] Rahaman, M. A, Geological Survey of Nigeria, 1988, 11 41. [5] Ogunribido, T.H.T International Journal of Pure and Applied sciences and Technology, 2012, 10 (1), 10 16. [6] Akpah, F. A, Onimisi, M, Lekdun, M.O, Global Journal of geological sciences, 2009, 7(2), 131 141. [7] Jegede, O, Construction and building materials, 2000, 14, 311-315. [8] Badmus, B.S, Research journal of soils and water Management, 2010, 1(1), 10-14. [9] Mustapha, A.M, Alhassan, M, EJGE, 2012, 17, 1505 1513. [10] Telford, W. M., King, W. F, Becker, A, Geological Survey of Canada, 1977, pp76. [11] McNeill, J. D, Labson, V. F, Electromagnetic Methods in Applied Geophysics, 2, B, Tulsa, Oklahoma: Society of Exploration Geophysicists, 521-640, 1990. [12] Pirttijarvi, M, KHFfilt Program, A Geophysical Software for Karous-Hjelt and Fraser Filtering on Geophysical VLF (Very-Low-Frequency) Data, Geophysics Division Department of Geosciences, University of Oulu: Oulu, Finland, 2004. [13] Arong, T. O., Oghenero, A. E. Advances in Applied Science Research, 2013, 4(5): 24. [14] Keller, G.V, Frischknecht, F. C, Electrical method in Geophysical Prospecting. Pergamon Press, Oxford, 1966, 523. [15] Vander Velpen, B. P. A. M.Sc. Research Project, ITC, Delft, (Netherland, 1988). [16] DAS, B.M Fundamental of Geotechnical Engineering, 4th edition, Thomson Learning, USA Frias M, Cement and Concrete, 2000 209