Geophysical and Geotechnical Characterisation of...

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (5): 864-870 (ISSN: 2141-7016)

864

Geophysical and Geotechnical Characterisation of Foundation

Beds at Kuchiyaku, Kuje Area, Abuja, Nigeria

1E.T. Faleye and 2G.O. Omosuyi

1Physical and Earth Science Department, Wesley University of Science and Technology, Ondo, Nigeria.

2Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria

Corresponding Author: E. T. Faleye ___________________________________________________________________________ Abstract The geoelectrical and geotechnical parameters of foundation soils in Kuchiyaku, Kuje area, Abuja, Nigeria were determined. The exercise was aimed at evaluating the competence of near surface geomaterials to bear civil engineering loads. The study combined 20 Schlumberger Vertical Electrical Soundings (VES), in-situ tests involving 19 Standard Penetration Tests (SPT), 15 Cone Penetration Tests (CPT), and multiple Atterberg’s laboratory tests (liquid and plastic limits). The VES interpretations delineated topsoil, weathered basement and the fractured/fresh bedrock within the study area. The layer resistivity ranges from 199 to 1947 Ohm-m for topsoil, 32 to 540 Ohm-m for weathered basement and 495 to 16986 Ohm-m for fresh/fractured bedrock. High resistivity values (>500 Ohm-m) suggest geotechnical competence. Maximum depth to bedrock is about 31m. The cone penetration probed maximum depth of 6.4m at CPT 13 and 10m depth was reached by the standard penetration test via sample holes. The geophysical interpretation results correlated well with the results of CPT, SPT and laboratory Atterberg’s tests in the study area. High CPT and SPT values have direct correlation with high resistivity values where there is no much differential in soil fluid content, while areas with high liquid and plastic limits correlate with resistivity low zones, apparently suggesting high groundwater saturation. The Atterberg’s limits observed from borehole samples revealed useful knowledge of the soils’ engineering properties. The investigation reveals that the near surface foundation materials in the area is generally geotechnically competent. __________________________________________________________________________________________ Keywords: foundation beds, geotechnics, standard penetration test, cone penetration test, depth sounding __________________________________________________________________________________________ INTRODUCTION The study area is a proposed residential estate at Kuchiyaku, Kuje, near Abuja, Nigeria. News of collapse of one building or the other has become a recurring decimal in virtually all parts of Nigeria. Many reasons have been adduced for its frequency: improper foundation (Beckmann, 1994), faulty planning, insincere monitoring by the regulatory authorities, and development control lapses. Often, existing civil and other engineering structures are located over anomalous subsurface zones which are significantly incompetent to bear the load of the structures (Soupios et al., 2007). All engineering structures (buildings, bridges, airport runways and roadways) rest directly or indirectly on the ground. Consequently, the primary consideration is the foundation and the capacity of the foundation bed to sustain both the dead and live load imposed by the overlying structure. Hence the stability of the foundation and the superstructure supported by the foundation depends on the bearing capacity of the geologic materials underlying the site (Oyedele, 2009). In any foundation studies, large or small scale, it is desirable to investigate the detailed foundation conditions at the site (Hawkins, 1971). These studies

should include determination of depth to the bedrock, the geotechnical integrity of the bedrock, the physical properties of foundation geomaterials and the groundwater condition of near-surface materials (Peck et al., 1973; Bowles, 1984; Sharma, 1997; Venkatranmaiah, 2006). Most engineers consider drilling of exploratory holes to obtain subsurface materials for identification and subsequent laboratory analyses as the ultimate. However, Ako (1996) believes that the integration of geophysical methods have become indispensable in general foundation studies, since they provide subsurface information at reasonable cost, locate critical areas for test drilling and thus eliminate less favorable sites. Consequently, an efficient sequence of foundation investigation should therefore embrace surface geological survey, subsurface geophysical investigation and the traditional insitu engineering/laboratory tests, in order to design the most suitable foundation for the proposed structure. Resistivity images have been used in studies that include soil and bedrock property characterization,

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (5): 864-870 © Scholarlink Research Institute Journals, 2011 (ISSN: 2141-7016) jeteas.scholarlinkresearch.org


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detection of bedrock voids and fractures among other types of investigation (Adesida and Omosuyi, 2005). This study integrated the electrical resistivity geophysical data with those of Cone Penetration Test (CPT) and the Standard Penetration Test (SPT) in order to characterize the subsurface and establish the competence of the geologic materials underlying the study area to sustain the load of the proposed engineering structures.

Location and Geologic Setting The study area, Kuchiyaku layout, Kuje area, Abuja, Nigeria is predominantly underlain by the pre-Cambrian basement complex rocks. The local lithological units in the study area are Migmatite-gneiss, granite, and schistose gneiss. The migmatite-gneiss is the most wide spread rock unit. The granite occurs in several locations. They are porphyritic and of medium-coarse-grained texture, Granites mostly occur as intrusive, low-lying outcrops around the gneiss. They are severely jointed and fairly incised by quartz veins.

Fig. 1: Base map of the Study Area (Inset-Map of Nigeria) MATERIALS AND METHODS Twenty (20) vertical electrical soundings, using the Schlumberger electrode array, were conducted in the area. The Soil Test Conductivity Meter was used for the resistivity measurement. The field curves were at first manually interpreted, using the conventional partial curve-matching technique of Koefoed (1979), with the help of master curves (Orellana and Mooney, 1966). From the manual interpretation, initial estimates of the resistivity and thickness of the various geoelectric layers at each VES location were obtained (Zohdy et al., 1974). The field curves were preliminarily manually interpreted, using the conventional partial curve matching (Koefoed, 1979; Patra and Nath, 1998), to determine the thickness and resistivity of the layers. The model derived from manual interpretation was interactively adjusted (Vander Velpen, 1988) to get a better fit in each case. The fit in all cases is within the error limit of 4.0%. Cone Penetrometer Tests (CPT) was conducted at fifteen locations across the study area. The test involved pushing an instrumented cone tip into the ground at a controlled rate of 2cm/sec, until the

maximum capacity (250 Kg/cm2) of the testing machine was reached or the ground anchorage of the machine attained yielding point. Resistance to penetration was measured at intervals of 0.2m. The resolution of the CPT enabled the delineation of stratigraphic layers in the area. Successive cone resistance readings plotted against appropriate depths, form a resistance profile which indicates the strata sequence penetrated (Bell, 2007). The Standard Penetration Test (SPT) was conducted inside 19 boreholes across the area. In each test, a soil sampler (split spoon) of about 35mm internal diameter, connected to the drill rod, was driven 450mm into the soil by repeated blows from a hammer weighing 65Kg and falling a free height of 760mm. The SPT resistance, (or N-value), representing the number of blows required to drive the sampler through a full distance of 300mm after an initial penetration of 150mm, was determined/noted. The N-value gives an empirical measure of the soil competence or consistency, and also can be used to provide a guide to the relative strength of weathered

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rock. SPT was conducted in all type of soil deposits within each borehole, to observe the variation in soil stratification by comparing the N-values. A detailed log of each of the boreholes was prepared as per the relevant standard specification. RESULTS AND DISCUSSION Geoelectric and Lithologic Characterization The results of interpretation of the depth soundings are shown in table 1. The table shows layer resistivity and thickness, and inferred lithology of each geoelectric layer. The VES curves are mostly the H-type (Fig. 2), characterized by three geoelectric/lithologic layers consisting of sand or sandy clay (low resistivity) and resistive or fresh bedrock. The electrical resistivity contrasts existing between geoelectric layers in the area enabled the delineation of lithologic units, occurring at varying depths with variable thicknesses. The results of the interpretation were used to construct three sections AB, CD and EF, taken in the south-north, west-east and southwest-northeast directions (Fig.3). The sections show three-four geoelectric/lithologic layers. Resistivity ranges from 199 Ohm-m to 1947 Ohm-m while thickness varies from 0.4m to 2.4m in the first layer (topsoil). In the second and third layers, resistivity varies from 32 to 540 Ohm-m and 67 Ohm-m to 16986 Ohm-m respectively while layer thickness ranges between 0.3m to 12.1m and 10.8m to 26.7m respectively. In the fourth layer, the presumed bedrock resistivity is over 495 Ohm-m. Resistivity in the range of 1-100 Ohm-m suggests clays while those in the range of 50 – 150 Ohm-m suggest lateritic clay (Reynolds, 1997). Table 1: The ranges of layer thickness and resistivity across the geoelectric sections

Layer

Least value

Maximum value

Inferred lithology

Resistivity (Ohm-m)

1 2 3 4

199 32 495 1231

1947 540 1484 16986

Lateritic, Clayey/Sandy Weathered Basement Fractured Basement Fresh Basement

Thickness (m) 1 2 3 4

0.4 4.3 Undefined Undefined

2.4 26.7 Undefined Undefined

Fig. 2: Typical Schlumberger Depth Sounding Curves from the Study Area (VES 7 and 8) An important factor often considered in foundation design is water table and water table fluctuation (Bowles, 1984; Coduto, 1998). The aquifer resistivity map (figure 4) presents the level of water saturation condition in the study area. The aquifer resistivity ranges between 32 - 686 Ohm-m. The high resistivity depicts competent geologic materials, such as sand or clayey sand formation. Very low resistivity suggests clay or sandy clay materials, or water saturated materials, often less competent to support the stability of heavy engineering structures. The depth to the aquifer unit ranges between 0.5 m and 4.8 m in the area. Soils below the groundwater tables are generally saturated (Coduto, 1998). In addition, a raised water table may create a wet basement or foundation, and consequently engender instability of the overlying structure (Bowles, 1982; Othman, 2005). The resistivity distribution at the basement or bedrock ranges between 1000-m at VES 18 (F-7) and over 16000 -m at VES 5 (B-6). The highest resistivity values occur around the southwestern


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portion of the area, thus suggesting reliable degrees of competence of the near surface geologic materials in that zone. It was however noted that some of the bedrock underlying some VES points could not be probed, possibly owing to the very thick clayey overburden materials, as interpreted in VES 20 (F-1) (Figure 3).

VES 17 (E-7)

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SW NE

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Ohm-m

Ohm-m

Vertical Exaggeration = 39

(c) Fig. 3: Geoelectric sections along: (a) North-South, (b) West-East, and (c) Southwest-Northeast traverses across the study area.

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Fig. 4: Aquifer Resistivity Map of the Study Area.

Fig. 5: Bedrock relief map of the study area. The significance of bedrock relief map (Fig 5) is to reveal the bedrock head topography and structural disposition. It pictures the lows (depressions) and the highs (ridges). The bedrock relief map of the area shows that the northeast region is characterized by high relief, while depressions characterize the southern and western segments of the area. Should there be a need for excavation of subsurface materials; the amount could be assessed using the bedrock relief map. Geotechnical Characterization The stability of any engineering structure is primarily dependent on their supporting foundation which is largely a function of the nature and condition of the underlying soil materials. The cone penetration test plot is shown in Fig 6a. The plot shows that from depth of 3.0m, about 60% of the probed depths are made of competent materials. The penetration

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resistance ranged between 34kg/sqcm at CPT12 and 250kg/sqcm at CPT 5, 6 and 15. Very low penetration values are indicative of materials of low competence. The SPT investigation was undertaken between 1.5m and 10.0m depth range (Fig 6b). At depth of 1.5m, the SPT number of blows (N) was as low as 9 and with gradual increase around the central segment and towards the southeast extreme end of the study area. The SPT blows ranges between 9 and 66. Low SPT blows are mostly typical of clay soils, even those which are not pre-stressed, i.e. when they are not under a long-term process of compression and stiffening (gradual reduction in pore fluid pressure and increase of the effective stress), are more susceptible to subsidence. They are also affected by environmental conditions (besides others), particularly temperature and moisture, which vary irregularly throughout the year (Soupios et al., 2007). The results of the geotechnical boreholes showed that the specific type of clay is very soft (easy penetration), supporting our considerations about the nature of the clay in the site. At depth of 4.5m, the trend of the SPT number of blows are high and low at some point of investigation, which show proportional rate of penetration with the nature and hardness of rock encountered when correlated with geoelectric parameters. CPT 5

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(b) Figs. 6: CPT and SPT Plots from the Study Area Liquid limit determination constitutes geotechnical parameter from which soils plastic limit range con be determined. Maps produced for liquid limit at depth of 1.0m and 4.5m are shown in figure 7). The liquid limit map at depth of 1.0m shows an increasing liquid limit values from 40% at point F-6 to 65% at point C-7. This suggests that the soil material at this depth has averagely high moisture content. It is suspected that the stream channels around these points may be responsible for this. This suggests that the soil here may not serve as good foundation base. Liquid limit map at 4.5m (figure 7) presents a depth terrain still of high moisture content as the liquid limit values vary between 36% and 60%. Since this depth correlates with area delineated to be weathered basement in the depth sounding data interpretation, it therefore follows that the high moisture content range is a significant pointer to the presence of moisture/water along the rock interstice. The plastic limit maps were generated in order to visualize the spread of moisture content below which the soil across the study location is non-plastic at specific depth. Plastic limit map at depth of 1.0m reveals that the limit increased from 17% at point (E-6) to 40% at point (D-6). The increase is visible at the central part and down to the southwestern section. This reflects that the region with high plastic limits is associated with high proportion of clay fraction, as observed in Burnett and Fookes (1974), Bell (1994b). This in turn influences the amount of water attracted in the soil particles. This directly relates with the geoelectrically delineated subsurface materials at this depth (top soil), which is mainly lateritic to clayey material and sparsely sandy materials. At depth of 4.5m of the plastic limit map (figure 7), a significant change in trend was noticed, showing a range between 21% at borehole (E-5) and 38% at borehole (F-4). Correlations with the corresponding or adjacent geoelectric parameters suggest that the weathered


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material is likely sandy or clayey in texture (see geoelectric sections).

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(b) Fig. 7: Liquid Limit and Plastic Limit Maps at 4.5m depths in each case in the Study Area CONCLUSION AND RECOMMENDATION This study combined geophysical survey with field and laboratory geotechnical investigations as pre-construction foundation studies at Kuchiyaku, Kuje area, Abuja, Nigeria. The correlation of the geophysical results with those of cone penetration and standard penetration investigations and laboratory tests show agreement in the interpretation results. High CPT and SPT values have direct correlation with high resistivity values where there is no much differential in soil fluid content. It was however noted that the depth of investigation of other field tests, that is, CPT and SPT, is about one-fourth to one-third when compared to that of vertical electrical sounding. Thus CPT and SPT methods of investigation are shallow depth probing investigation tools. Hence, vertical electrical sounding method is

not only suitable for foundation studies but is also more proficient in giving adequate subsurface information useful for foundation investigation. In no small way will it help to provide necessary soil bearing capacity factors useful to the civil/structural engineer in the right choice of foundation designs. The in-situ Penetration tests and Atterberg’s analysis reveal the possible tendencies of environmental effects (majorly temperature and moisture) on foundation soils. The investigation reveals that the near surface foundation materials in the area is generally geotechnically competent. From the afore-mentioned, it is recommended that geophysical methods of investigation should essentially be made to complement the traditional engineering methods of foundation investigation. Consequently, geoscientists should be party to the regulatory agencies and be involved in site supervisions to ensure that what was approved in the planning offices is built by owners. This is envisaged to forestall the recent trend of collapse of buildings across the nation. ACKNOWLEDGEMENT Some members of staff of Engineering Department, Federal Capital Development Authority (FCDA), Abuja, Nigeria, were involved in the data acquisition. The authors gratefully acknowledged this. REFERENCES Adesida, A.and Omosuyi, G.O. 2005. Geoelectric Investigation of Bedrock Structures in the Mini-campus of the Federal University of Technology, Akure, Sowthwestren Nigeria, and the geotechnical significance. Nig. Jour. Pure Appl. Phys Vol. 4 pp 32-40. Ako, B.D. 1976. An integration of geophysical and geological data in damsite investigation- The case of Opa Dam. Jour. of Min. and Geology vol. 13 (1), pp. 1-6. Beckmann, P. 1994. Structural Aspect of Building Conservation, McGraw – Hill International Series in Civil Engineering. Bell, F.G. 1994b. The Speeton Clay of North Yorkshire, England: an investigation of its Geotechnical properties. Engineering Geology, 36, 259–266. Bell, F.G. 2007. Engineering Geology 2nd Edition, Butterworth-Heinemann, Elsevier 593p. Bowles, J.E. 1984. Physical and Geotechnical Properties of Soils. McGraw-Hill, London. Burnett, A.D. and Fookes, P.G. 1974. A regional engineering geological study of the London Clay in the London and Hampshire Basins. Quarterly Journal Engineering Geology, 7, pp. 257–296.

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Coduto, S.A. 1998. Geotechnical Engineering: Principles and Practices. Prentice Hall Inc. pp 759 Hawkins, L.V. .1971. Seimic Refraction Surveys for civil Engineering. ABEM Geophysical Memo No. 2/69. 12p. Koefeod, O. 1979. Geosounding Principles, 1- Resistivity sounding measurements. Elsevier Scientific Publishing Comp., Amsterdam, pp 275. Orellana, E. and Mooney, H.M. 1966. Master Tables and Curves for Vertical Electrical Sounding over Layered Structures. Interciencia, Madrid, 34p. Othman, A.A.A. 2007. Construed Geotechnical Characteristics of Foundation Beds by Seismic Measurements. J. Geophys. Eng. 2, pp. 126-138. Oyedele, K.F. 2009. Engineering Geophysical Approach to Progressive or Sudden Collapse of Engineering Structures in Lagos, Nigeria. The Journal of American Science, 5(5); 91-100. Pattra, H.P. and Nath, S.K. 1998. Schlumberger Geoelectric Sounding in Groundwater. Principles, interpretation and Application. A.A. Balkema Publishers, USA. Pp 153. Peck, R.B., Hanson, W.E. and Thornburn, T.H. 1973. Foundation Engineering 2nd Edition, John Willey & Sons Inc., 514p. Reynolds, J.M. 1997. An Introduction to Applied and Environmental Geophysics. Wiley, Chichester. Sharma, P.V. 1997. Environmental and Engineering Geophysics. Cambridge University Press. Pp. 475. Soupios, P.M., Georgakopoulos, P., Papadopoulos, N., Saltas, V., Andreadakis, A., Vallianatos, F., Sarris, A., and Makris, J.P. 2007. Use of Engineering Geophysics to investigate a Site for a building foundation. Journal of Geophysics and Engineering Vol. 4, pp. 94-103. Vander Velpen, B. P. A. 1988. Resist Version 1.0, M.Sc. Research Project, ITC. Delft, Netherlands. Venkatranmaiah, C. 2006. Geotechnical Engineering. New Age International Limited Publisher. Pp. 541-603. Zohdy, A. A. R., Eaton, G. P. and Mabey, D. R. 1974. Application of surface geophysics to groundwater investigations: Techniques of water resources investigations of U.S. Geol. Survey: Book 2, Chapter DI, U.S. Government Printing Office, Washington, pp.66.

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