This presentation summarizes the results of an integrated geophysical and geotechnical investigation of a proposed site for a Center of Excellence Laboratory at the Federal University of Technology in Akure, Nigeria. Resistivity and soil sampling methods were used to characterize the subsurface. The topsoil was found to be thin and unsuitable for foundations. Below this was a weathered layer and fractured basement rock, followed by fresh basement at depths from 2-15 meters. The basement rock was found to have an undulating surface and linear fractures. Based on the resistivity, thickness and soil properties, the basement rock was determined to be the most competent layer for foundations, using pile foundations to account for its uneven nature.

1. A PROJECT PRESENTATION
ON
DETERMINATION OF SUBSOIL COMPETENCE
USING INTEGRATED GEOPHYSICS AND
GEOTECHNIC STUDIES AT THE PROPOSED
CENTER OF EXCELLENCE LABORATORY SITE,
FUTA
BY
AKINGBOLA AKINSOLA EMMANUEL
AGP/08/3766
FEBRUARY, 20 1 4

2. PRESENTATION OUTLINE
• PROBLEM DEFINITION
• LOCATION AND SITE DESCRIPTION
• GEOLOGY OF THE STUDY AREA
• AIM AND OBJECTIVES
• METHODOLOGY
• RESULTS AND DISCUSSION
• CONCLUSION AND RECOMMENDATION

3. PROBLEM DEFINITION
THE MANAGEMENT OF THE FEDERAL UNIVERSITY OF
TECHNOLOGY, AKURE, RECENTLY ALLOCATED A SITE FOR A
PROPOSED CENTRE OF EXCELLENCE LABORATORY. THE SITE
IS LOCATED WITHIN THE BASEMENT COMPLEX AREA WITH
VARIABLE OVERBURDEN. THIS PROJECT INTENDS TO
PROVIDE INFORMATION ON THE SUBSURFACE SEQUENCE
AND STRUCTURAL DISPOSITION (NECESSARY FOR
FOUNDATION DESIGN), WHICH NECESSITATED AN
INTEGRATED GEOPHYSICAL AND GEOTECHNICAL
INVESTIGATION OF THE SITE.

4. LOCATION AND SITE DESCRIPTION
THE STUDY AREA IS A PROPOSED SITE FOR CENTER OF
EXCELLENCE LABORATORY SITED OPPOSITE THE POST
GRADUATE LABORATORY BUILDING, AND BESIDE THE
PROFESSOR JULIUS OKOJIE RESEARCH CENTER WITHIN
THE FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE. IT
LIES BETWEEN LATITUDES 0807116N – 0807122N AND
LONGITUDES 0735584E – 0735587E. THE AREA IS
CHARACTERIZED BY AN ABSENCE OF OUTCROPS WITH
REDDISH CLAY TOPSOIL. THE AREA IS ACCESSIBLE
THROUGH A GOOD NETWORK OF TARRED ROADS AS
SHOWN IN FIGURE 1.

5. FIG 1: LOCATION MAP OF THE STUDY AREA. (AFTER ABBEY, 2013)

6. GEOLOGY OF THE STUDY AREA
THE STUDY AREA WHICH IS LOCATED WITHIN THE FEDERAL
UNIVERSITY OF TECHNOLOGY IS UNDERLAIN BY ROCKS OF
THE PRECAMBRIAN BASEMENT COMPLEX OF SOUTHWESTERN
NIGERIA (RAHAMAN, 1976).
THE CRYSTALLINE ROCKS FOUND WITHIN THE CAMPUS ARE
PORPHYRITIC GRANITE, BIOTITE GRANITE, CHARNOCKITE,
QUARTZITE AND MIGMATITE GNEISS (FIGURE 2). MIGMATITE
GNEISS AND BIOTITE GRANITE ARE THE MAJOR OUTCROP,
WHILE CHARNOCKITE OCCURS AS A DISCRETE BODY IN
OTHER PART OF THE AREA.
THE PROPOSED SITE FOR THE CENTRE OF EXCELLENCE
LABORATORY IS UNDERLAIN BY MIGMATITE GNEISS AS SHOWN
IN FIGURE 2

7. FIGURE 2: GEOLOGICAL MAP OF THE FEDERAL UNIVERSITY OF TECHNOLOGY,
AKURE SHOWING THE STUDY AREA (AFTER KAREEM, 1997).

8. AIM AND OBJECTIVES
THE AIM OF THIS PROJECT IS TO DETERMINE THE COMPETENCE
OF THE SUBSOIL FOR ENGINEERING WORKS. IN ORDER TO
ACHIEVE THIS, THE OBJECTIVES ARE TO:
DELINEATE THE SUBSURFACE GEOLOGICAL SEQUENCE AND
DETERMINE THE GEO-ELECTRIC PARAMETER,
IDENTIFY EXISTING SUBSURFACE GEOLOGIC FEATURES SUCH AS
FAULTS, SINKHOLES AND CAVITIES IN AN AREA PRONE TO
SUBSIDENCE AND GEOLOGIC INSTABILITIES,
DETERMINE THE GEOTECHNICAL NATURE OF THE STUDY AREA,
AND
EVALUATE FROM THE ABOVE THE SUITABILITY OF THE
SUBSURFACE SOIL WITHIN THE STUDY AREA FOR THE PROPOSED

9. EQUIPMENT USED
• HAMMER
• ELECTRODES
• MEASURING TAPE
• OHMEGA RESISTIVITY
METER
• CONNECTING CABLES
• CROCODILE CLIPS
• PREMARKED STRINGS
• GLOBAL POSITIONING
SYSTEM (GPS)
• SOFTWARE PACKAGES
• RECORDING SHEET

10. METHODOLOGY
• THE VERTICAL ELECTRICAL SOUNDING (VES) TECHNIQUE WAS
ADOPTED USING THE SCHLUMBERGER CONFIGURATION ALONG
WITH THE COMBINED HORIZONTAL PROFILING (HP) AND VES
TECHNIQUE USING THE DIPOLE-DIPOLE CONFIGURATION FOR
DETAILED GEOPHYSICAL STUDY. SOIL SAMPLES WERE ACQUIRED
AT DIFFERENT LOCATIONS ON THE SITE FOR GEOTECHNICAL
STUDIES.
• DATA INTERPRETATION WAS PERFORMED USING BOTH MANUAL
AND COMPUTER PACKAGES FROM WHICH CURVES, COLUMNAR
SECTIONS AND PSEUDO SECTIONS WERE DERIVED.

11. FIGURE 3: GEOPHYSICAL AND GEOTECHNICAL DATA ACQUISITION MAP OF THE

12. FIELD PROCEDURE
• THE DIPOLE-DIPOLE PROFILING AND SOUNDING WAS PERFORMED
WITH 2M ELECTRODE SPACING AND EXPANSION FACTOR OF N, VARIED
FROM 1 - 5 ON ALL SEVEN TRAVERSES. DATA WAS ACQUIRED IN THE
WEST-EAST DIRECTION ON TRAVERSE 1 TO 4, WHILE ON THE
CONTROL TRAVERSE 5 TO 7, IT WAS ACQUIRED IN THE SOUTH-NORTH
DIRECTION.
• VERTICAL ELECTRICAL SOUNDING USING SCHLUMBERGER
ELECTRODE ARRAY WERE CONDUCTED ALONG TRAVERSES 1, 2 AND 3
(WITH THE ELECTRODE SPACING VARYING FROM 1 - 100 M),
• EIGHT SOIL SAMPLES WERE COLLECTED AT DIFFERENT LOCATIONS
WITHIN THE SITE AT DEPTHS EXCEEDING 0.5M AS SHOWN IN FIGURE 3.
THESE SAMPLES WERE PRESERVED IN POLYTHENE BAGS AND
TRANSPORTED TO THE LABORATORY THE NATURAL MOISTURE
CONTENT OF THE SAMPLES COLLECTED FROM THE FIELD WAS
DETERMINED IN THE LABORATORY WITHIN A PERIOD OF 24 HOURS
AFTER COLLECTION. THE TESTS INCLUDE NATURAL MOISTURE

13. RESULTS AND DISCUSSION

14. DATA PRESENTATION
• THE DIPOLE-DIPOLE DATA ARE PRESENTED AS PSEUDO-
SECTIONS SHOWN IN FIGURE 4-10. APPARENT RESISTIVITY
VALUES ARE PLOTTED AT THE POINTS OF INTERSECTION OF 45
DEGREES INCLINED LINES FROM THE MID POINTS OF THE
CURRRENT AND POTENTIAL DIPOLES AND WERE CONTOURED.
THIS IS DONE USING THE DIPPRO SOFTWARE. THE SECTION
SHOWS APPARENT RESISTIVITY DISTRIBUTION IN TWO
DIMENSIONS AND THUS A STRUCTURAL SIGNIFICANCE.
• THE VES DATA ARE PRESENTED AS SOUNDING CURVES- PLOTS
OF THE APPARENT RESISTIVITY AGAINST THE ELECTRODE
SPACING ON A BI-LOG GRAPH PAPER. COLUMNAR SECTIONS ARE
ALSO GENERATED. TYPICAL DEPTH SOUNDING CURVES
OBTAINED FROM THE SITE ARE SHOWN IN FIGURE 14.

15. FIGURE 4: PSEUDO-SECTION OF TRAVERSE 1

16. FIGURE 5: PSEUDO-SECTION OF TRAVERSE 2

17. FIGURE 6: PSEUDO-SECTION OF TRAVERSE 3

18. FIGURE 7: PSEUDO-SECTION OF TRAVERSE 4

19. FIGURE 8: PSEUDO-SECTION OF TRAVERSE 5

20. FIGURE 9: PSEUDO-SECTION OF TRAVERSE 6

21. FIGURE 10: PSEUDO-SECTION OF TRAVERSE 7

22. FIGURE 11: STACKED PSEUDO-SECTIONS OF TRAVERSE 1 TO 4
W- E

23. FIGURE 12: STACKED PSEUDO-SECTIONS OF TRAVERSE 5 TO 7
S-N

24. (A) (B)
FIGURE 13: VES CURVES FROM THE STUDY AREA (A) KH CURVE TYPE, (B)
HKH CURVE TYPE,
(C) KH CURVE TYPE
(C)

25. FIGURE 14: COLUMNAR SECTIONS OF VES 1, 2 AND 3

26. DATA INTERPRETATION
• THE VES CURVES WERE QUANTITATIVELY INTERPRETED BY
CURVE MATCHING THE FIELD CURVE SEGMENT BY SEGMENT
USING THE MODEL CURVES AND COMPUTER-AIDED TECHNIQUES.
• INTERPRETATION OF THE DIPOLE-DIPOLE PSEUDO-SECTION IS
QUALITATIVE BUT PROVIDES ADDITIONAL INFORMATION ON THE
SUBSURFACE GEOLOGY.

27. TABLE 1: INTERPRETED GEO-ELECTRIC PARAMETERS FROM THE
SOUNDING CURVES.
VES
STATION
LAYERS RESISTIVITY
(ohm-m)
THICKNESS
(m)
DEPTH
(m)
INFERRED
LITHOLOGY
CURVE
TYPE
1 1 210 2.3 2.3 Topsoil KH
2 486 5.3 7.6 Weathered
layer
3 171 6.2 13.8 Fractured
Basement
Rock
4 7829 ---- --- Fresh
Basement
Rock
2 1 101 0.7 0.7 Topsoil HKH
2 54 1.3 2.0 Clayey layer
3 953 5.4 7.4 Weathered
Basement
Rock
4 1346 7.0 14.3 Fresh
Basement
Rock
5 404 --- --- Slightly
Fractured
Basement Rock

28. TABLE 2: INTERPRETED GEO-ELECTRIC PARAMETERS
FROM THE SOUNDING CURVES. (CONTD.)
3 1 132 1.9 1.9 Topsoil KH
2 1580 4.8 6.6 Weathered
layer
3 38 2.1 8.7 Fractured
Basement
Rock
4 28680 --- --- Fresh
Basement
Rock

29. TABLE 3: SUMMARY OF THE GEOTECHNICAL RESULTS
SAMPLE NATURAL
MOISTURE
CONTENT
(%)
LIQUID
LIMIT
(%)
PLASTIC
LIMIT
(%)
PLASTICITY
INDEX
(%)
LINEAR
SHRINKAGE
(%)
A 21.3 61.1 21.5 39.60 7.7
B 18.2 43.8 22.5 21.35 9.6
C 10.4 30.2 19.1 11.10 12.0
D 14.1 31.1 19.3 11.85 11.5
E 16.5 39.7 19.9 19.80 10.1
F 16.2 47.0 20.3 26.75 9.1
G 14.4 37.4 20.7 16.70 9.6
H 6.5 24.5 19.1 5.45 12.5

30. DISCUSSION OF RESULTS
• FROM GEOPHYSICAL RESULTS, THE TOP SOIL IS THIN AND NOT COMPETENT ENOUGH FOR A
CIVIL STRUCTURE. RESULTS FROM THE COMBINED HP AND VES SHOW THE UNDULATING
NATURE OF THE BEDROCK AND THE RESISTIVITY DISTRIBUTION. THE VES RESULTS REVEALED
FOUR GEO-ELECTRIC SEQUENCES WITHIN THE STUDY AREA WHICH COMPRISES OF TOPSOIL,
WEATHERED LAYER, FRACTURED BASEMENT AND FRESH BASEMENT. THE TOP SOILS ARE
GENERALLY THIN (< 2.5 M) AND MAJORLY COMPOSED OF SANDY CLAY/ CLAYEY SAND WHILE
THE WEATHERED LAYER IS COMPOSED OF SAND/LATERITES. THE BASEMENT ROCK IS
CHARACTERIZED BY LINEAR STRUCTURES SUCH AS FRACTURES AND FAULTS THAT MIGHT AID
STRUCTURE SUBSIDENCE. THE COLUMNAR SECTIONS SHOW DEPTH OF ROCK HEAD OF
BETWEEN 2 M AND 15 M.
• OVERBURDEN THICKNESSES ARE GENERALLY LESS THAN 10M AND DEPTH TO BASEMENT
ROCK VARIES FROM 2 – 14 M FOR MOST PARTS OF THE AREA. THE EASTERN PARTS OF THE
STUDIED AREA POSSESSES HIGH OVERBURDEN THICKNESSES. LINEAR STRUCTURES SUCH AS
FRACTURES AND FAULTS OCCUR AT A DEPTH OF ABOUT 10 M.
• ENGINEERING COMPETENCE OF SOIL CAN BE QUALITATIVELY EVALUATED FROM LAYER
RESISTIVITY, THICKNESS AND GEOTECHNICAL PARAMETER I.E. THE HIGHER THE LAYER
RESISTIVITY VALUE, THE HIGHER THE COMPETENCE OF A LAYER, HENCE FROM THE POINT OF
VIEW OF RESISTIVITY VALUE THEREFORE; LATERITE IS THE MOST COMPETENT OF THE

31. DISCUSSION OF RESULTS (CONTD.)
• THE GEOTECHNICAL PROPERTIES OF THE TOPSOIL ARE RELATIVELY GOOD AS
MOST OF SOIL SAMPLES TAKEN WITHIN THE TOPSOIL FALL WITHIN FEDERAL
MINISTRY OF WORKS AND HOUSING, 1972 RECOMMENDATION.
• THE GEOTECHNICAL RESULTS SHOW THAT THE SOILS ARE GENERALLY OF
RELATIVELY LOW NATURAL MOISTURE CONTENT. IT HAS RELATIVELY LOW CLAY
CONTENT. THE PLASTIC INDEX OF THE SOILS WITHIN THE AREA ARE LESS
THAN 20%, (EXCEPT FOR SAMPLES A,B AND F) THE SOIL CAN BE ADJUDGED TO
BE LOW TO MEDIUM PLASTICITY, HENCE, THE SOILS ARE EXPECTED TO
EXHIBIT LOW TO MEDIUM SWELLING POTENTIAL. THE LINEAR SHRINKAGE OF
THE SOILS ARE GREATER THAN 8%, INDICATING ACTIVE AND EXPENSIVE
NATURE OF THE SOIL.

32. CONCLUSION
• A CIVIL STRUCTURE IS NOT ADVISED TO BE PLACED ON THE
WEATHERED LAYER BECAUSE OF THE FRACTURED BASEMENT
THAT UNDERLIES IT WHICH MAY BE SEISMICALLY UNSTABLE AND
THE EASE WITH WHICH IT CAN BE SATURATED WITH FLUID. DUE TO
THE UNEVEN BASEMENT TOPOGRAPHY IN THE AREA, FOUNDATION
MAY BE PLACED ON THE BASEMENT ROCK USING PILE
FOUNDATION. THE WESTERN REGION OF THE STUDIED AREA
APPEARS TO BE THE BEST FOR ANY FOUNDATION WORK BECAUSE
IT CONSTITUTES BASEMENT ROCK THAT ARE RELATIVELY CLOSER
TO THE SURFACE.
• THE DEDUCTION FROM THE ABOVE IS THAT, THE SUBSOIL ON OR
WITHIN WHICH ENGINEERING STRUCTURES WILL BE FOUNDED
WITHIN THE STUDY AREA IS NOT COMPETENT. THE FOUNDATION OF
THE PROPOSED CIVIL STRUCTURE CAN BE HOSTED BY THE
BASEMENT ROCK FORMATION USING PILE FOUNDATION. THE
UNEVEN NATURE OF THIS LAYER HAS TO BE CONSIDERED IN THE
DESIGN OF THE FOUNDATION.

33. RECOMMENDATION
• THE IMPORTANCE OF INTEGRATED GEOPHYSICAL TECHNIQUES
AND GEOTECHNICAL INVESTIGATIONS IN PRE-FOUNDATION
STUDY HAVE BEEN SHOWN AND THEREFORE, IT IS
RECOMMENDED THAT THE FOUNDATION OF THE PROPOSED
STRUCTURE MAY BE PLACED ON THE BASEMENT BEDROCK
USING PILE FOUNDATION WITH PILE LENGTH RANGING BETWEEN
2 M AND 15 M. ALSO, INTEGRATED GEOPHYSICAL TECHNIQUES
ARE RECOMMENDED IN ADDITION WITH GEOTECHNICAL
INVESTIGATION FOR FOUNDATION STUDY.
• THE RESULTS OF THIS STUDY WILL ALSO ENHANCE THE
EVALUATION OF THE SITE FOR ENGINEERING WORKS AND THIS
CAN SERVE AS REFERENCE MATERIAL FOR FURTHER RESEARCH

34. THANKS
FOR
LISTENING