Ademilua Babatunde Oladipupo 4

Transcription

1 Geophysical Investigations for Subsurface Integrity Assessment and Its Implications on Existing and Proposed Buildings Around the Southwestern Part of the Ekiti State University Campus, Ado-Ekiti, Southwest Nigeria. Ademilua Oladimeji Lawrence 1, Eluwole Akinola Bolaji 2, Bawallah Musa 3, Ademilua Babatunde Oladipupo 4 1&2- Department of Geology, Ekiti State University, Ado Ekiti Nigeria. 3- Department of Applied Geophysics, Federal University of Technology, Akure Nigeria. 4- Department of Geology, Obafemi Awolowo University, Ile Ife Nigeria. Abstract Geophysical investigations involving the Very Low Frequency Electromagnetic (VLF-EM) and the electrical resistivity methods have been carried out in the southwestern part of the Ekiti State University, Ado-Ekiti with the aim of assessing the subsurface integrity of the area for construction purposes and also establishing the possible cause(s) of the structural distress that led to the demolition of the former faculty of the social sciences. VLF-EM data were taken at 10 m interval along fifteen (15) geophysical traverses. The VLF-EM data were interpreted both qualitatively and semi-quantitatively. The electrical resistivity method involved the Vertical Electrical Sounding (VES) technique, and the Schlumberger electrode array was adopted. Halfelectrode spacing (AB/2) varied from 1 to 100 m. VES data were presented as depth sounding curves and were interpreted quantitatively by partial curve matching and computer iteration. The anomalies diagnostic of subsurface conductors typical of clay, fractured basement and sheared zones that poses threats to engineering structures were identified on the VLF-EM profiles and the 2-D subsurface conductivity images. The H, A, HA, KH and HKH curve types were identified in the study area. A total of five geoelectric layers were delineated beneath the study area. These include the topsoil, the lateritic layer, the weathered/highly weathered layer, the partly weathered/fractured basement and the fresh bedrock. The resistivities of the layers ranged from ohm-m, ohm-m, ohm-m, ohm-m and ohm-m respectively. The thicknesses of the layers also varies from m, m, 5 16 m, m respectively while the fresh bedrock is boundless. The structural distress that led to the eventual demolition of the former faculty of social science was suspected to have been due to the very low resistivity (<50 ohm-m) nature of the weathered layer; the partly weathered/fractured basement and the thick overburden present in the vicinity of the demolished building. The subsurface integrity of the of the proposed site for the new faculty of science complex was described as intermediate, because the site straddles within both the competent and incompetent zones in same proportions. Key Words: electrical resistivity, subsurface integrity, structural distress, demolition, intermediate Lawrence,BolaJi,Musa,Oladipupo Page 1

2 1. INTRODUCTION Several structural engineering problems have been recorded within the Ekiti State University, Ado-Ekiti. These problems have manifested in different forms, ranging from minor foundation cracks to major cracks on buildings which in some cases have exhibited substantial distress and had to be consequently demolished as was the case of the defunct faculty of social sciences building situated within the sector under focus within the university campus (Figure 1). The problems have been addressed from the engineers point of view which attributed the building failures to defective structural designs or poor construction practices such as the use of inferior construction materials and the involvement of unskilled/unqualified personnel during construction. There have been recommendations for the redesigning and reconstruction of such buildings. No adequate measure have been adopted to unravel the reasons behind the building failures beyond the aforementioned engineers views. It is important to note that the nature of the soil upon which engineering structures such as buildings, dams, roads etc will be imposed determines their sustainability. Consequently, an adequate knowledge of subsurface integrity reduces the risk of wasting time and resources in constructing massive modern structures that will not last. Over the years, geophysical methods have been established to play complimentary roles in geotechnical studies. They have been applied as tools for pre-construction feasibility studies [[1];[2]], and for post-construction integrity assessment for buildings, dams and roads [[3];[4];[5];[6]]. The researchers have focused mainly on delineating subsurface stratifications and geologic structures such as fractures, faults, sheared zones etc which poses threats to engineering structures. The depth to competent bedrock and thickness of weathered layer have also been reported to play vital roles in subsurface integrity assessment. The southwestern part of the Ekiti State University has become a massive construction site in recent times, a portion of which has been earmarked for the construction of the proposed new Faculty of Science Complex. The former Faculty of Social Sciences which was recently demolished (see Figure 2) due to obvious evidences of distress also falls within the study area. This research therefore adopted integrated geophysical investigations involving the very low frequency electromagnetic method (VLF-EM) and the electrical resistivity method for the assessment of the subsurface integrity around the proposed site of the new faculty of science. Also, from a geophysical perspective, this paper provides the possible reasons for the distress that necessitated the eventual demolition of the former Faculty of Social Sciences. Figure 1: The Defunct Faculty of Social Sciences which exhibited distress and was consequently Demolished within the university campus. Lawrence,BolaJi,Musa,Oladipupo Page 2

3 II. SITE DESCRIPTION AND GEOLOGY Ekiti State University is situated along Ado-Iworoko road in Ado-Ekiti, the capital of Ekiti State. The study area is within the southwestern part of the University (Figure 2). It exists within Latitudes 7 o and 7 o and Longitudes 5 o and 5 o The area currently hosts several Faculties among which are Faculty of Science, Faculty of Law, Faculty of Management Science, the Directorate of General Studies (GST) and other important buildings. Figure 2: Location Map and Field Layout of the Study Area The geology of the study area is that of the Precambrian Basement Complex described by [7], which comprises of the Migmatite Gneiss Quartzite Complex; the Slightly Migmatised to Unmigmatised Metasedimentary Schists and Metaigneous rocks; the Charnockitic, Gabbroic and Dioritic rocks; and the members of the Older Granite Suite. The study area has migmatite as its main rock unit (Figure 3). The area is associated with two distinct seasons (wet and dry seasons) typical of the tropical climate. The annual rainfall in the area is about 1300 mm [8]. Lawrence,BolaJi,Musa,Oladipupo Page 3

4 III. METHODOLOGY Fifteen (15) geophysical traverses with approximately West - East orientation were established within the available spaces in the study area (Figure 2). The reconnaissance stage of the geophysical investigation involved the Very Low Frequency Electromagnetic (VLF-EM) method. The method detects electrical conductors such as moderate to steeply dipping waterfilled fractures or faults, by utilizing radio signal in the 15 to 30 khz range. Figure 3: Geological Map of the Area Around Ado Ekiti Showing the Study Area (Adapted from Akure Sheet 56 of the Nigeria Geological Map.) VLF-EM data were acquired at an interval of 10 m along the established traverses with the aid of the ABEM WADI VLF-EM instrument. The instrument measures the interphase (real) and quadrature (imaginary) components of the induced vertical magnetic field as a percentage of the horizontal primary field. The data were processed using the Karous-Hjelt and Fraser filter (KHF Filt, [9]) program. The mode of interpretation was both qualitative and semi-quantitative. The qualitative interpretation involved the inspection of the raw real and filtered real data to identify points of inflexion and peak positive anomalies respectively, which are known to be indicators of the presence of subsurface conductors[10]. The subsurface conductors are usually associated with geologic structures such as fractured and faulted zones, which are inimical to civil engineering structures. The semi-quantitative interpretation involved the inspection of the 2-D subsurface conductivity models generated by the KHF Filt program to identify the positions and depth extents of subsurface conductive bodies. The positions where such conductors were observed were marked out as zones of potential threat and were noted for further detailed investigations. Lawrence,BolaJi,Musa,Oladipupo Page 4

5 The detailed geophysical investigation involved the electrical resistivity method. Electrical resistivity measurements were taken via the R 50 DC resistivity meter at locations previously identified as conductive zones by the VLF-EM method. The Vertical Electrical Sounding (VES) technique involving the Schlumberger array was adopted, with half current electrode spacing (AB/2) varying from 1 to 100 m. A total of twenty (20) VES stations were occupied. The data were interpreted quantitatively by the analytical method of partial curve matching after having plotted apparent resistivity against half current electrode spacing (AB/2) on a bi-log graph. The geoelectric parameters obtained from the partial curve matching were smoothed using the WinResist 1.0 [11]. The smoothed resistivity parameters (layer thicknesses and resistivities) were thereafter used to generate 2-D geoelectric sections across sections A-B, C-D and E-F of the study area (Figure 5). A. VLF-EM PROFILES IV. RESULTS AND DISCUSSION The subsurface conductive zones identified around the study area are characterised by peak positive filtered real anomalies which falls within the yellowish to reddish colour bands of the 2- D pseudosections ( e.g. Figure 3). The pseudosections displays subsurface features of varying degree of conductivity (-20 20), trending in different directions. As an example, a major conductive zone is present between 100 m and 160 m on Profile 1 (Figure 3). A relatively conductive zone having a NW-SE trend within the major zone is characteristic of a subsurface fractured/faulted zone. The other conductive zone with lower positive amplitude present at 50 m may be a response to a clayey substratum. VES 1 and VES 2 were established at 100 m and 140 m for further investigation. Similarly, VES stations were established at the locations where significant conductive features were identified on the remaining profiles. A total of twenty (20) VES stations were so-generated around the study area. B SOUNDING CURVES AND GEOELECTRIC SECTIONS The sounding curve types that were obtained from the area are the 3-layer H and A types, the 4- layer HA and KH types and the 5-layer HKH type. The summary of the geoelectric characteristics of the VES points are presented in the appendix. The geoelectric sections (Figure 4a-c) beneath the study area delineated a total of five (5) geoelectric layers. The layers are the topsoil, the lateritic layer, the weathered/highly weathered layer, the partly weathered/fractured basement and the fresh bedrock. The topsoil has a thickness range of between 0.5 m and 1.4 m with a resistivity range of 108 ohm-m to 605 ohm-m. The lateritic layer is characterised by a thickness range between 1.0 m and 4.6 m with a resistivity range of 337 ohm-m to 970 ohm-m. The thickness of the weathered layer ranges between 5 m and 18 m while its resistivity is between 42 ohm-m and 225 ohm-m. The partly weathered/fractured basement has a resistivity range of 225 ohm-m to 365 ohm-m and its thickness is between 14.1 m and 16.7 m. the last layer is the basal fresh bedrock whose resistivity is generally above 1000 ohm-m and it has an infinite thickness. Lawrence,BolaJi,Musa,Oladipupo Page 5

6 The thickness and resistivity characteristics of the lateritic formation and that of the weathered layer have been reported in [6] as factors that influences the stability or instability of roads. Although, the load imposed on a road is dynamic while a building structure imposes a static load of greater magnitude on the subsurface, the findings of [6] guided the interpretation of the geoelectric sections. On Figure 4a the thickness of the lateritic layer is between 2.3 m and 4.2 m while the resistivity is between 337 ohm-m and 459 ohm-m. The resistivity range of the layer is not the same as the 600 ohm-m suggested by [6], but the moderate thickness above 1.5 m threshold may increase the competence of the layer. The weathered layer that underlies the lateritic layer has a clayey composition due to its relatively low resistivity of between 108 and 225 ohm-m. Clayey formations are porous and posses little or no permeability which makes them prone to differential settlement in the presence of imposed loadconsidering the static nature of the load. Figure 4: Typical VLF-EM Profile and Subsurface Conductivity Map Obtained from the Study Area Lawrence,BolaJi,Musa,Oladipupo Page 6

7 A-B (a) C-D (b) E-F (c) Figure 5: Geoelectric Sections Beneath VES Points along Sectoral Lines. (a) A-B, (b) C-D and (c} E-F Lawrence,BolaJi,Musa,Oladipupo Page 7

8 The lateritic layer beneath section C-D (Figure 5b) is generally thin, with an exception of that of VES 3. The weathered layer also exhibits relatively low resistivity characteristics ( ohm-m). The combined negative attributes of the thin lateritic layer as well as the low resistivity weathered layer are symptomatic of a doubtful subsurface competence potential. The basement ridges present beneath VES 7 and VES 4 are advantageous and could be a compensation for the incompetence attributed to thin lateritic layer and low resistivity weathered layer. Section E-F (Figure 5c) is the geoelectric section around the demolished former faculty of social sciences building. The lateritic layer beneath the section is characterised by a relatively favourable thickness which ranges between 1.5m and 2.4m and resistivity greater than 600 ohmm suggesting higher ironstone contents. The weathered layer displays high level of weathering because of its very low resistivity (<50 ohm-m) and this reduces the competence capability of the overlying lateritic layer. Also, the partly weathered/fractured basement beneath VES 18 and VES 19 could pose reasonable threat to building and related structures. The distress that led to the eventual demolition of the former faculty of social sciences can be attributed to subsurface incompetence arising from the highly weathered layer and the partly weathered/fractured basement layer. The relative structural stability of the Afe Babalola hall and the Moot Court around the same area may be due to their age (less than ten years of existence) and their simple bungalow design which have the tendency of imposing lesser stress on the earth load- bearing capacities. Other structures in the area are also relatively new, so there are no evidences of distress yet.therefore it is instructive that any additional design variation to the existing structures must not be encouraged without detailed competence evaluation and assessment. C. GEOELECTRIC MAPS 1. Thickness Map of the Lateritic Layer The thickness map of the lateritic layer around the study area ( Figure 5) indicates that the thickness of the layer varies between 0 and 4.5 m. The area where the proposed site for the new faculty of science complex is located is characterised by thickness of 1.8 m and above represented by the yellowish-reddish colour bands is considered suitable for building construction in terms of laterite thickness. On the other hand, zones having lower thickness (<1.8 m) that falls within the bluish-greenish colour bands are considered unfavourable. The current faculty of science building is situated within the relatively low laterite thickness zone, yet there are no evidences of distress on the building. This suggests that the resultant effect of other factors such as laterite ironstone contents as evidenced by the resistivity, coupled with overburden thickness and averagely weathered layer resistivity may have compensated for the negative effect of the thin lateritic layer. Meanwhile, the demolished former faculty of social sciences may have been distressed due to the thin lateritic layer around the area. 2. Resistivity Map of the Lateritic Layer The map (Figure 6) shows the resistivity distribution of the lateritic layer in the study area. The zones having resistivity lower than 300 ohm-m are considered incompetent while areas with resistivities greater than 300 ohm-m can be said to be relatively competent considering the high ironstone contents. However, the distress that led to the demolition of the former faculty of social science may not have been fully as a result of low iron contents laterite afterall, since its now Lawrence,BolaJi,Musa,Oladipupo Page 8

9 obvious that the resistivity within the vicinity of the demolished structure is relatively high (>600 ohm-m), suggesting high iron contents. The situation therefore could be due obviously to other reasons as would be further considered in the course of the study. 3. Weathered Layer Resistivity Map The resistivity of the weathered layer is determined by the amount of residual clay associated with the layer. As earlier mentioned, zones with low weathered layer resistivity (<100 ohm-m) are characteristic of high clay content. On the weathered layer resistivity map (Figure 7), zones within the bluish colour band where the proposed site for the new faculty of science complex is located are considered unfavourable to building structures. This is confirmed by the presence of the demolished structure in such zone. On the other hand, zones within the greenish to yellowish colour band can be said to be fairly competent. Figure 6: Thickness Map of the Lateritic Layer Figure 7: Resistivity Map of the Lateritic Layer Lawrence,BolaJi,Musa,Oladipupo Page 9

10 4. Overburden Thickness Map The overburden thickness map (Figure 8) was generated from the depths to rock head obtained from all the VES points. The basement topography which is dependent on the overburden thickness also plays vital role in soil stability. Areas with thin overburden resulting to shallow depth to bedrock possess better stability potential. On Figure 8, the zone within the bluish colour band is considered geotechnically competent. This is because of the relatively thin overburden (<12 m) observed in the zone. The basement topography around the zone is typical of a basement ridge and this may have played a good role in the stability of the faculty of science. On the other hand, zones within the yellowish/brownish/reddish colour bands are associated with basement depressions whish poses threat to civil engineering structures. The bedrock around the demolished structure is deep seated and this also may have contributed to the distress that led to its eventual demolition. V. CONCLUSIONS The southwestern part of the Ekiti State University, Ado-Ekiti has been investigated using integrated geophysical methods in assessing the subsurface integrity of the area, for construction purposes. The Very Low Frequency Electromagnetic (VLF-EM) data were acquired along fifteen (15) traverses. Anomalies associated with conductive zones resulting from clay and geological structures such as fractured and sheared zones were delineated. The points where such conductive zones were identified were investigated further with the Vertical Electrical Sounding (VES) technique of the electrical resistivity method. Twenty VES stations were occupied in the study area. Some factors, including laterite thickness, laterite ironstone contents / resistivity, weathered layer resistivity and overburden thickness (depth to bedrock) were identified as part of the factors that determines the competence/incompetence of the subsurface. The structural distress that led to the eventual demolition of the former faculty of social science was suspected to have been due to intense weathering of the weathered layer as evidenced by the very low resistivity (<50 ohm-m) nature of the weathered layer; the partly weathered/fractured basement and the thick overburden present in the vicinity of the demolished building.in terms of laterite thickness; laterite resistivity; weathered layer resistivity and overburden thickness, the subsurface integrity of the proposed site for the new faculty of science complex can be said to be intermediate. This is because the site extends and falls within both the zones considered competent as well the other incompetent zones in almost equal share of surficial proportions. VI. RECOMMENDATIONS Building structures already existing around the zones that have been characterised as geotechnically incompetent should be adequately reinforced to prevent future manifestation of subsurface-induced structural distress. The proposed new faculty of science complex should be constructed on the competent portion of the site with the suitable type of foundation. To forestall future geotechnical problems in the University, pre-construction geophysical assessments should be carried out before the construction of buildings. This will guide the engineers on the type of Lawrence,BolaJi,Musa,Oladipupo Page 10

11 foundation and the structural design suitable for each site. Engineering geological studies can also be carried out in the study area to complement the geophysical investigations. Figure 8: Weathered Layer Resistivity Map Figure 9: Overburden Thickness Map Lawrence,BolaJi,Musa,Oladipupo Page 11

12 REFERENCES [1].A.P. Aizebeokhai and A.I.Olayinka, Application of 2-D and 3-D Geoelectrical Resistivity Imaging for Engineering Site Investigation in a Crystalline Basement Terrain, Southwest Nigeria. Journal of Environment Earth Science, 2010, 61(7): [2]. N.D. Andrews, A.A. Aning and R.M. Noye. Geophysical Investigations at the Proposed Site of the KNUST Teaching Hospital Building Using the 2D and 3D Resistivity Imaging Techniques. International Research Journal of Geology and Mining. 2013, 3(3), pp [3]. M.O. Olorunfemi, J.S. Ojo, F.A. Sonuga, J.O.Ajayi and M.I. Oladapo. Geophysical Investigation of Karkaku Earth Dam Embankment. Global Journal of Pure and Applied Science, 2000 Vol. 6(1), pp [4]. M.O.Ofomola, K.A.N. Adiat, G.M. Olayanju and B.D. Ako. Integrated Geophysical Methods for PostFoundation Studies, Obanla Staff Quarters of the Federal University of Technology, Akure, Nigeria. Pacific Journal of Science and Technology (2), pp [5]. A.B.Eluwole and M.O. Olorunfemi. Time-Lapsed Geophysical Investigation of the Mokuro Earth Dam Embankment, Southwestern Nigeria, for Anomalous Seepages. Pacific Journal of Science and Technology (1), pp [6]. J.O. Fatoba,Geoelectric, Geotechnical and Remote Sensing Investigations of Flexible Highway Pavement Failure along Shagamu-Benin Expressway, Southwestern Nigeria. Unpublished Ph.D. Thesis of the Obafemi Awolowo University, Ile-Ife, pp. [7]. M.A. Rahaman. Recent advances in the study of the Basement Complex of Nigeria. Editedby Nigeria Geological Survey, pp [8]. Nigeria Meteorological Agency, (NIMET). Daily weather forecast on the Nigerian Television Authority. Nigerian Metrological Agency, Oshodi, Lagos, [9]. M. Pirttijärvi 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, Finland [10]. J.M.Reynolds. An introduction to Applied and Environmental Geophysics. Published by John Wiley and Sons Ltd. Baffins Lane. Chichester, West Sussex PO191UD, England, pp. [11]. Vander Velpen, B. P. A., 1988, Resist Version 1.0. M.Sc Research Project, ITC, Delft Netherlands, Lawrence,BolaJi,Musa,Oladipupo Page 12

13 APPENDIX : Summary of the Geoelectric Characteristics of the Study Area VES Stn. No. of Layers Resistivity (Ohm-m) / / Curve Type Thickness (m) / / Depth (m) / / /432/112/994 KH 0.6/4.2/ /4.9/ /165/361/98/950 HKH 0.5/1.0/2.9/ /1.5/4.4/ /227/94/1233 KH 1.0/4.6/ /5.6/ /108/2267 H 4.0/ / /185/2257 H 2.7/ / /459/166/2563 KH 0.9/2.3/ /3.2/ /60/824/52/2682 HKH 0.5/0.4/1.6/ /0.9/2.5/ /433/128/2696 KH 0.5/2.3/ /2.8/ /502/119/1718 KH 1.0/1.4/ /2.4/ /1545/52/5310 KH 1.4/2.1/ /3.5/ /577/152/2466 KH 0.7/1.0/ /1.7/ /225/2910 H 0.5/ / /192/7547 H 0.6/ / /165/1674 H 0.5/ / /85/970/42/2223 HKH 0.5/0.3/1.4/ /0.7/2.2/ /652/53/6713 KH 1.4/2.4/ /3.8/ /861/42/973 KH 1.3/1.8/ /3.0/ /368/5333 A 0.8/ / /225/4128 A 2.3/ / /28/376/3276 HA 0.5/0.2/ /0.6/22.7 Lawrence,BolaJi,Musa,Oladipupo Page 13