Poster presented on computer modelling of tar sands quarry in the Nigerian tar sands belt in SW Nigeria at the 50th AEG International Conference in Los Angeles.

1. David Afolayan
Mopa BASSAGI
ALAO
and
COMPUTER MODELING OF SLOPE STABILITY
IN MINING THE TAR SAND OF
DAHOMEY BASIN, SOUTHWESTERN NIGERIA
DEPARTMENT OF GEOLOGY AND MINERAL SCIENCES,
UNIVERSITY OF ILORIN
P.M.B. 1515, ILORIN, KWARA STATE, NIGERIA
Abstract
The need to have stable slopes in open pit mining of the tar sand deposit of the Dahomey Basin,
Southwestern Nigeria is emphasized in this study. In the Loda village, Southwestern Nigeria,
samples of the lateritic soil and alluvial sand which overlie the tar sand occurrence were subjected to
geotechnical tests. Computer simulation of bench face angles was carried out using the SLOPE/W
software to determine the bench face angle(s) with the least susceptibility to failure. Slope failure
might lead to loss of lives and valuable heavy equipment thus increasing overall expense of running
the mine.
Geotechnical tests carried out helped to determine the unit weight ( ), cohesion (c) and angle of
friction ( ) values. For the lateritic soil, = 25 kN/m3, c = 45 kPa and = 41 . Also, values of 18
kN/m , c = 0 kPa and = 34 were obtained for the alluvial sand. These values were inputted into the
software program to simulate different bench face angles that could be cut into the two lithologic
units. Factor of safety values were obtained for 10 to 90 bench face angles at 1 metre, 4 metres and
greater than 40 metres ground water level, respectively. 10 metre bench width and 6 metre bench
height were used while 20 metre bench width and 6 metre bench height were also utilized to make
room for heavier equipment. Factor of safety values ranging between 3.58 and 1.73 were obtained
for bench face angles between 10 and 30 making this range least susceptible to failure even when
inundation is considered.
Consequently when mining commences, a utilization of slope angles ranging from 10 to 30
coupled with adequate drainage conditions like digging of perimeter trenches and use of
submersible pumps would go a long way in ensuring optimum output for huge economic gain and
revenue generation. This will make Nigerian tar sand exploitation viable for sustainable growth of
the economy of the country in the foreseeable future.
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Corresponding author:Tel. +2348037553842
E-mail address: lomalao@yahoo.com
INTRODUCTION
LOCATION OF THE STUDYAREA
The Dahomey Basin located in West Africa runs parallel to the coastal
margins of Ghana, Togo, Republic of Benin and Southwestern Nigeria. It
is an extensive basin starting from Southeastern Ghana through Togo and
the Republic of Benin to Southwestern Nigeria where it is separated from it
by the Niger Delta (Bankole et al., 2006). Tar sand occurrences in the
Dahomey Basin are restricted to the eastern portion of the basin and are
contained generally in Upper Cretaceous sediments. Extensive
bituminous seepages and sediments impregnated with tarry oil define a
narrow band about 5-8km wide between latitude 6 37′N and 6o48′N
stretching from east of Ijebu-Ode town (Ogun State) to the banks of the
tributaries of the Siluko river at Ofosu village in Edo State an approximate
distance of 110km (MSMD Report, 2005).
A lot of work has been carried out on reservoir estimation (Adegoke et
al., 1976), physical and chemical characteristics (Adegoke and Ibe, 1972;
Enu, 1985), clay mineral suite (Enu et al 1981.), exploitation options
(Coker, 1990; Nwachukwu, 2003) and natural radioactivity (Fasasi et al.,
2003) of the Nigerian tar sand. Some of the recent works carried out which
relate to the geotechnics of the Nigerian tar sand include the work of Alao
and Obielodan (2006).
However, Adegoke et al., (1980) gave an estimate of 30o to 35o for
slope angles when mining of the tar sand deposit commences. This was
based on approximations and estimations without actually carrying out
laboratory tests. Hence, the need for exact determination of the
geotechnical parameters (such as shear strength, unit weight and angle of
internal friction) is necessary for bench face slope design. This paper
seeks to examine the geotechnical properties of sediments associated
with and overlying the tar sand deposit of Southwestern Nigeria using Loda
Village as a case study. This would be used in finding out the bench face
angle(s) safe for mining. Field activities consist of sediment description,
identification and collection. Also core drill data was obtained from the
Bitumen Project Implementation Committee (BPIC) an arm of the National
Agency charged with the responsibility of disseminating information
regarding tar sand exploitation and exploration. Geotechnical tests were
carried out on samples obtained and the results were utilized for computer
simulation using the Geostudio SLOPE/W software.
The study area located on longitude 4 55′N is Loda village, Ondo State,
Southwestern Nigeria. It exists within the Dahomey Basin of South-
western Nigeria ( ) and is about 150km fromAkure the capital city of
Ondo State, Southwestern Nigeria. Also, it is within the 120km long and
about 5-6km wide belt of tar sand outcrop which trends approximately
East-West (Adelu and Fayose, 1991). Location of the study area on the
o
o
Figure 1
geologic map of Nigeria is shown in . is a Google Earth
image of Southwest Nigeria.
Loda Village, Ondo State, Southwestern Nigeria is made up of
undulating topography with rolling hills. The topography generally rises
from the coastal area in the south to the rugged hills in the northeastern
portion. The natural vegetation is high forest type, composed of many
varieties of hardwood timber. The climate of the area is of the lowland
tropical rain forest type, with distinct wet and dry seasons. The mean
monthly temperature is 27 C with a mean monthly range of 2 C, while
mean relative humidity is less than 70%. Rainfall is common throughout
the year but the months of November, December and January may be
relatively dry. The mean annual total rainfall exceeds 2000 mm.
The sediments associated with and overlying the tar sand deposit at
Loda village Southwestern Nigeria were sampled for geotechnical
analyses. Lateritic soil samples and alluvial sand samples were obtained
from the study area. These two lithologies were observed to be overlying
the tar sand deposit at the three locations examined 200 metres apart.
Samples were collected from three different points on a road cut at three
locations (L1 L2 and L3). Moreover, at each location, samples were picked
from the basal part of the road cut to the topmost part of it ( ).
Lithologic sections of each of the three locations examined are shown
below in a, b and c.
The unit weight, cohesion and angle of internal friction are essential
values necessary for factor of safety determination. For the SLOPE/W
software this is also the case. The geotechnical test carried out on the
samples are grain size analysis, density determination and direct shear
test. All the samples were subjected to these tests and values were
obtained for each of these geotechnical test. The grain size analysis was
carried out using the mechanical dry sieve method in accordance with B.S
1377, 1975. The density bottle was used to determine the density of the
samples while the direct shear test was carried out using the shear box
machine which accommodates 60mm soil sample and is 20mm high (the
sample compartment).
For the computer modeling, the Geostudio SLOPE/W software was
used courtesy of Geoslope International. A hypothetical bench dimension
of 10 metres and 20 metres was used separately with the same bench
height of 6 metres for both. The bench face angles were increased from
Figure 2 Figure 3
Figure 4
Figures 5
GEOMORPHOLOGY, VEGETATIONAND CLIMATE
SELECTED LOCATIONSAND SAMPLE COLLECTION
GEOTECHNICALTESTS
COMPUTER MODELING
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2
10 to 90 and Factor of Safety values was calculated using the
Morgenstern-Price method of analysis. Groundwater levels of 1 metre, 4
metres and greater than 40 metres was employed for each of the bench
width used. This is necessary bearing in mind the prominent role
inundation plays in slope stability (Mandzic, 1992).
One of the primary objectives of carrying out grain size analysis is to
provide a descriptive term for soil samples analysed. This is done by using
the particle size distribution graphs to estimate the range of sizes present
in the most representative fraction of the soil. Also, permeability values
could be obtained using Hazen's formula: K = CK(D10) m/s, where (D10)
is the grain size (in mm) and CK is a coefficient varying between 0.01 and
0.015 (Institution of Civil Engineers, 1976). All these calculations were
carried out on the readings obtained from grain-size graphs of the samples
analyzed.
From visual Inspection of the grain-size graphs, steep graph lines
indicate that a narrow range of particle sizes characterize the alluvium
samples thus the alluvium sample can be said to be poorly graded (Read,
1992). For the lateritic soil sample, a gentle line was obtained on the graph
indicating wide range of particle sizes present in the soil samples this
makes the sample well graded. The proportion of particle sizes shown in
is a confirmation of the interpretation from the graphs
Using the Hazen's formula the co-efficient of permeability values were
obtained for each of the samples using the grain size analysis data (
). Results obtained were compared with Lambe's (1951) classification to
indicate the degree of permeability ( ).
However, from the coefficient of permeability values obtained in ,
the alluvium samples (LD1A, LD1B, LD2A, LD2B, LD3A and LD3B) have
values ranging from 1.94x10 cm/s to 1.01x10 cm/s while the lateritic soil
samples (LD1C, LD2C and LD3C) have values ranging from 3.24x10 to
7.26x10 cm/s. Lambe's (1951) classification is shown in for the
samples in . Both the lateritic soil and the alluvium soil have
medium degree of permeability, showing the ease with which fluid will flow
through them. Relatively the alluvium sample has a higher permeability
value than the lateritic soil sample, this poses a threat when mining
commences especially when inundation is considered.
From the dry density determination table shown below ( ) the
alluvium samples (LD1A, LD2B, LD3A and LD3B) have values between
1.77 Mg/m and 1.90 Mg/m , which is low when compared with those
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2
-2 -1
-2
-2
3 3
RESULTS OBTAINEDAND DISCUSSION
GRAIN SIZEANALYSIS
DRYDENSITYDETERMINATION
Table 1
Table
2
Table 3
Table 2
Table 4
Table 2
Table 5
obtained for the lateritic soil samples (LD1C, LD2C and LD3C). The
lateritic soils have values ranging from 2.5 Mg/m to 2.62 Mg/m .
From , the lateritic soils have density values higher than the
alluvium soil showing a more compact grain size. Slopes cut into loose
soils are more susceptible to failure than denser soils if drainage
conditions, proper slope design and other factors are put in place. Adense
soil has high shear strength which makes the soil less susceptible to
failure.
Direct shear test were carried out on the alluvial soil and lateritic soil to
develop the relation between and .
The values obtained for angle of internal friction ( ) and cohesion (c) are
33 and 0kPa respectively for the alluvium soil. Also, 41 and 40kPa were
obtained for and c. Thus the values obtained confirms the fact that the
alluvial soil is cohesionless while the lateritic soil has a measure of
cohesion suggesting that it is relatively more stable than the alluvium soil,
when a slope is cut into them.
Schroeder and Dickenson (1996) observed that in order to estimate
Factor of Safety (which is the measure of stability or instability) for any
slope surface, the shear strength parameters, slope geometry and
groundwater level must be determined or defined. The Geostudio
SLOPE/W was used to model the different bench face angles cut into the
two lithologies overlying he tar sand deposit in the study area.
shows a summary of the geotechnical data inputed into the SLOPE/W
computer program.
Also, groundwater levels used were based on the two seasons that exist
in the study area. For the dry season which extends from November to
April, the groundwater level used for the modeling is 4m, while 1metre was
used for the wet season which runs from May to October of the subsequent
year. All these parameters and conditions were inputed and factor of
safety values were obtained for the different bench width dimensions and
bench face angles. Diagram showing a pass at bench face angle
determination and how excavation might be planned for a proposed open
pit mine at the study area is shown below in .
Computer modeling was carried out for 10metre bench width and
20metre bench width separately. Also, a bench height of 6metre was used
for both bench width dimensions based on the suggested bench height of
4-8 metres given by Peters (1978). Diagrams obtained for the 10metre
bench width and 6metre bench height are shown below in
for 10 , 40 and 90 bench face angles respectively. Diagrams obtained
for the 20 metre bench width and 6metre bench height are shown below in
3 3
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Table 5
Table 6
Figure 6
Figures 7, 8 and
9
SHEAR STRENGTH TEST
MODELING OF BENCH FACEANGLES
shear stress (kPa) normal stress (kPa)
f
f
Figures 10, 11 and 12 Tables 7
and 8
Tables 7 and 8
Figures 7, 8 and 9
Figures 10, 11 and 12
Tables 7 and 8
Tables
7 and 8
. Factor of Safety values are summarized in
respectively for various bench face angles.
It is obvious from that the factors of safety values
decrease as bench face angles become steeper for both the 10 and
20metre bench width respectively. From the factor of
safety values obtained for the 10metre bench width and 1metre ground
water level are 3.37, 1.35 and 0.35 for 10 , 40 and 90 respectively.
For the 20metre bench width in the factors of
safety values are 3.58, 1.66 and 0.59 respectively. The factor of safety
values for the 4metre ground water level and greater than 40metres level
also decrease as the bench face angles increase or become steeper.
However, the great influence that groundwater level or groundwater
pressure has on open pit mine stability is expressed in the values obtained
for various ground water levels used in . It can be observed
that the factor of safety values are higher for very deep ground water level
conditions (which is safer) than those of shallow groundwater condition. In
open pit mining practice more than 40% of slope instability risk depends on
ground water conditions in the slope (Mandzic, 1992). Also, Duncan et al.,
(2004) gave a factor of safety range of 1.2 to 1.4 for rock slopes in open pit
mines. Thus, for soil slopes such as those in the area higher factor of
safety values must be proposed.
Consequently upon this the factor of safety values obtained from
for bench face angles of 10 to 30 are safer than those above 30 .
This range is chosen to give room for groundwater level fluctuations due to
inundation.
The two lithologic units overlying the tar sand deposit at Loda village
were sampled with geotechnical tests carried out on them. These tests
were carried to determine the parameters necessary for slope stability
analysis as it relates to future mining operations on the tar sand deposit of
Southwestern Nigeria using Loda village Ondo State Southwestern
Nigeria as a case study.
However, the crucial engineering problem of slope instability or stability
has been simulated to determine the bench face angle that would be safe
for excavating the lithologies overlying the tar sand deposit at the study
area. Therefore, for a proposed open pit mine at Loda village,
Southwestern Nigeria. The following measures need to be put in place in
the design and operation of the mine;
1. Installation of piezometers in investigative boreholes to measure
the water pressure constantly.
2. Benches must be surveyed regularly to see if small movements
are taking place.
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RECOMMENDATIONS
3. The use of state-of-the art monitoring equipment like
extensometers is suggested.
4. Digging of wide perimeter trenches which would help to provide
good drainage network for he flow of surface water from rainfall and
groundwater from the subsurface in the proposed mine.
5. The use of high capacity submersible pumps for dewatering.
6. The need to carry out core drilling to have a large data base for
detailed and accurate calculations of safety factors.
Earlier works carried out on the tar sand deposit of Southwestern
Nigeria made observations based on approximations of geotechnical
parameters. Detailed geotechnical tests that relate to slope stability
analysis were not carried out. This paper has gone a bit further by carrying
out geotechnical tests on the lithologies overlying the deposit at a
designated location. Also computer software is used to model the bench
face angles appropriate for excavation.
However, increasing the slope angle decreases the stability of slope
although this depends on the geologic structure, soil and rock properties
involved. An attempt has been made in designing the best bench face
angles for excavating the lithologic units overlying the shallow tar sand
occurrence at Loda village Southwestern Nigeria.
Not only is an attempt made, a range of bench face angles has been
suggested for future mining operations on the tar sand deposit of
Southwestern Nigeria. Adherence to slope angles ranging between 10
and 30 with controlled ground water conditions would go a long way in
ensuring safe mine slopes with minimum instability risk, thereby optimizing
the operations of the proposed open pit mine for maximum economic gain.
The authors express their sincere appreciation to Mr. Femi for the
geotechnical tests carried out at the soil laboratory of the Department of
Civil Engineering, University of Ilroin; to Mr. Ola Sayomi for downloading
the Geostudio SLOPE/W Software; to Mrs. Sayomi for typing the
manuscript and to all reviewers for their constructive comments that
helped to improve the quality of this paper.
Adegoke, O.S., Ako, B.D., Enu, E.I, Petters, s.W., Adegoke, A.C.W.,
Odebode, M.O. and Emofurieta, W.O. (1976). Tar sand Project Phase
II; Estimation of Reserves, Materials Testing and Chemical Analyses.
Unpublished report, Geological Consultancy Unit, Department of
Geology, University of Ife, Ile-Ife, 75p.
CONCLUSION
ACKNOWLEDGMENT
REFERENCES
o
o
Adegoke, O.S., Ako, B.D., Enu, E.I., Petters, S.W., Adegoke A.C.W.,
Odebode, M.O. and Emofurieta, W.O. (1980). Geotechnical
Investigations of the Ondo State Bituminous Sands. Report by the
Geological Consultancy Unit, Department of Geology, University of Ife,
Ile-Ife, 25p.
Adegoke, O.S. and Ibe, A.C. (1982). The Tar sand and Heavy Crude
Resources of Nigeria. Proceedings of the 2nd International Conference
onTar sands and Heavy crude, pp 280-285.
Adelu, R.D. and Fayose, E.A. (1991). Development Prospects for the
Bituminous Deposits in Nigeria. In: R.F. Meyer (Ed.) 5th UNITAR
International Conference on Heavy Crude and Tar sands, Edmonton,
AOSTRAtechnical reports, pp 509-515.
Alao, D.A. and Obielodan, J.B. (2006). Hydrocarbon Potential of Nigerian
Tar Sand and its Application in Construction: Call for a New Policy. A
paper presentation at the Southern Interdisciplinary Roundtable on
African Studies (SIRAS), Kentucky State University Frankfort, Kentucky
U.S.A. 20p.
Bankole, S.I. Shrank, E., Erdtmann, B.D. and Akande, S.O. (2006).
Palynostratigraphic Age and Paleoenvironments of the newly exposed
section of the Oshosun Formation in the Shagamu quarry Dahomey
basin, Southwestern Nigeria. NAPE Bulletin, Vol.19 No.1, pp 25-34.
Bitumen Project Implementation Committee Report (2002). Report on
Appraisal wells drilled along the Nigerian Tar sand belt. Unpublished
Report, 30p.
Coker, S.J.L. (1990). Exploration of he Nigerian Oilsand Deposits. In:
B.D. Ako and E.I. Enu (Eds). Occurrence, Utilization and Economics of
Nigerian tar sands. NMFS, Ibadan Chapter publication, pp 30-49.
Duncan, C.W. and Wyllie, C.W. (2004). Rock Slope Engineering. 4th
edition, spon press, UK. 480p
Enu, E.I. (1985). Textual Characteristics of the Nigerian Tar sands.
Sedimentary Geology vol. 44, pp 65-81.
Enu, E.I, Adegoke, O.lS., Robert, C., Ako, B.D., Ajayi, T.R., Adediran, S.A.
and Oriyomi, A. (1981). Clay mineral Distribution in the Nigerian tar
sand sequence. In: Van Olphen and F. Variable (Eds.). International
Clay Conference 1981. Development in Sedimentology, 35. Elsevier
Amsterdam, pp 321-323.
Fasasi, M;K., Oyawale,A.A., Mokobia, C.E., Tchokossa, P.,Ajayi, T.R. and
Balogun, F.A. (2003). Natural Radioactivity of the Tar sand Deposits of
Ondo State, Southwestern Nigeria. Nuclear Instruments and Methods
in Physics Research,A. 505, pp 449-453.
Geological Survey Agency of Nigeria, (2005). Geological and Minerals
Map of Ondo State. Produced by the Geological Survey Agency of
Nigeria.
Institution of Civil Engineers (1976). Manual of Applied Geology for
Engineers. 1st Edition, Institution of Civil Engineers, London 378p.
Lambe, T.W. (1951). Soil Testing for Civil Engineers, John Wiley and
Sons, NewYork.
Mandzic, E.H. (1992). Mineral water Risk in Open pit slope Stability. Mine
water and the Environment, Vol. Ii No.4, pp 35-42.
Ministry of Solid Minerals Development, (2005). Nigerian tar sands and
Bitumen; Exploration Opportunities. Unpublished Report, 6p.
Nwachukwu, J.I. (2003). Exploitation of the Bitumen Deposits of Nigeria.
In:A.A. Elueze (Ed.). Prospects for Investment in Mineral Resources of
Southwestern Nigeria. NMGS, Ibadan Chapter Special Publication, pp
67-74.
Peters, W.C. (1978). Exploration and Mining Geology, 1st edition, John
Wiley and Sons Inc. New Jersey, 694p
Read, K.H. (1992). Manual of Soil Laboratory Testing Volume 1, 2nd
Edition, Pentech Press, London, 380p.
Schroeder, W.L. and Dickenson, S.E. (1996). Soils in Construction, 4th
Edition, Prentice-Hall, New Jersey, U.S.A.. 317p
Figure 1. Location of the study area, Loda village, Southwestern Nigeria
within the Dahomey Basin. (Modified from Bankole et al., 2006)
120N
80E
90N
Phanerozoic Sediments
Pre-Cambrian basement Complex
0 300Km
120E40E
30N
Study Area
Loda
Cretaceous –Recent Sediment
Abuja
Akure
Lagos
120N
80E
90N
Phanerozoic Sediments
Pre-Cambrian basement Complex
0 300Km
120E40E
30N
Study Area
Loda
Cretaceous –Recent Sediment
Abuja
Akure
Lagos
Figure 2: Location of the study area Map of Nigeria (modified from Kogbe, 1989)
Figure 4: Picture of one of the locations (L2)
1.5m
Tar sand seepages
Reddish brown loose sand
(Alluvium) (LD3B)
Whitish loose sand
(Alluvium) (LD3A
Lateritic overburden
(LD2C)
2.5m
0.4m
1.0m
Tar sand seepages
Reddish brown loose sand
(Alluvium) (LD2B)
Whitish loose sand
(Alluvium) (LD2A)
Lateritic overburden
(LD2C)
4.5m
0.8m
1.0m
4.5m
0.8m
0.8m
Tar sand seepages
Reddish brown loose sand
(Alluvium) (LD1B)
2.6m
0.5m
0.8m
Whitish loose sand
(Alluvium) (LD1A
Lateritic overburden
(LD1C)
2.6m
0.5m
Figures 5a, b and c: Lithologic section of the three Locations L1 L2 and L3.
1.099
X-Sectional Length(m)
0 10 20 30 40 50 60 70
Depth(m)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Layer 1 – Laterite
Layer 2 – Alluvium (Loose sand)
GWL 4m
GWL 1m
Figure 9. Modeling of 90 bench face angle
to determine the corresponding factor of safety
for 10m bench width and 6m bench height
o
1.589
X-Sectional Length(m)
0 10 20 30 40 50 60 70
Depth(m)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Layer 1 – Laterite
Layer 2 – Alluvium (Loose sand)
GWL 4m
GWL 1m
Figure 8: Modeling of 40 bench faceangle
to determine the corresponding factor of safety
for 10m bench widthand 6m bench height
o
Figure 6: Diagram showing a pass at how excavation might occur
from different bench face angles at the study area when mining
commences. (Modified from Bitumen Project Implementation
Committee Report, 2002).
70.44m
57.91m
32.61m
2.44m
0
Basement rock
Shale
Bituminous sand
Loose sand
Laterite
? ?
Figure 10: Modeling of 10 bench face angles
to determine the corresponding factor of safety
for 20m bench width and 6m bench height
o
1
2
1 2
3
4 5
6
7
8 9
10
4.203
1 2
3
4 5
6
7
8 9
10
X-Sectional Length (m)
0 10 20 30 40 50 60 70 80 90 100 110 120
Depth(m)
0
4
8
12
16
20
24
28
32
Layer 1 – Laterite
Layer 2 – Alluvium (Loose sand)
GWL 4m
GWL 1m
Figure 11: Modeling of 40 bench faceangle
to determine the corresponding factor of safety
for 20m bench widthand 6m bench height
o
1.859
X-Sectional Length (m)
0 10 20 30 40 50 60 70 80 90
Depth(m)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Layer 1 – Laterite
Layer 2 – Alluvium (Loose sand)
GWL 4m
GWL 1m
Figure 12. Modeling of 90 bench face angle
to determine the corresponding factor of safety
for 20m bench width and 6m bench height
o
1.099
X-Sectional Length(m)
0 5 10 15 20 25 30 35 40 45
Depth(m)
0
5
10
15
20
25
30
35
Layer 1 – Laterite
Layer 2 – Alluvium (Loose sand)
GWL 4m
GWL 1m
Figure 7: Modeling of 10 bench face angles
to determine the corresponding factor of safety
for 10m bench width and 6m bench height
o
1
2
1 2
3 4
56
7
8 9
10
11 12
3.364
1 2
3 4
56
7
8 9
10
11 12
X-Sectional (m)
0 10 20 30 40 50 60 70
Depth(m)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Layer 1 – Laterite
Layer 2 – Alluvium (Loose sand)
GWL 4m
GWL 1m
70.44m
57.91m
32.61m
2.44m
0
Basement rock
Shale
Bituminous sand
Loose sand
Laterite
70.44m
57.91m
32.61m
2.44m
0
Basement rock
Shale
Bituminous sand
Loose sand
Laterite
Diagram of Core-log drilled at Loda Village, Southwestern Nigeria.
(Bitumen Project Implementation Committee Report, 2002
Table 1. Proportion of Particle sizes present in each of the samples analyzed.
Sample No
LD1A
LD1B
LD1C
LD2A
LD2B
LD2C
LD3A
LD3B
LD3C
Coarse
Silt (%)
—
—
—
—
—
—
—
—
—
Fine
Gravel (%)
—
5
15
5
30
5
20
—
—
Medium
Gravel (%)
—
5
—
—
—
—
—
—
—
Fine
Sand (%)
5
30
30
5
20
50
5
30
15
Medium
Sand (%)
90
60
40
90
70
30
90
60
20
Coarse
Sand (%)
5
5
15
5
5
35
5
5
40
Table 4: Comparison of permeability values obtained for each of the samples
with Lambe's (1951) classification to indicate the degree of permeability
Sample No
LD1A
LD1B
LD1C
LD2A
LD2B
LD2C
LD3A
LD3B
LD3C
Degree of Permeability (Based on
Lambe’s (1951) Classification)
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Range of Permeability
Values in (10 cm/s)
6.76 to 10.1
1.96 to 2.94
4.84 to 7.26
6.25 to 9.38
1.96 to 2.94
3.24 to 4.86
1.96 to 2.94
1.94 to 2.94
3.24 to 4.86
-2
Table 2: Coefficients of permeability for the samples analysed, using Hazen's formula
Sample No
LD1A
LD1B
LD1C
LD2A
LD2B
LD2C
LD3A
LD3B
LD3C
Permeability Values Obtained
from Hazen’s Formula (10 m/s)
6.76 to 10.1
1.96 to 2.94
4.84 to 7.26
6.25 to 9.38
1.96 to 2.94
3.24 to 4.86
1.96 to 2.94
1.94 to 2.94
3.24 to 4.86
-4
Equivalent Values
in (10 cm/s)
6.76 to 10.1
1.96 to 2.94
4.84 to 7.26
6.25 to 9.38
1.96 to 2.94
3.24 to 4.86
1.96 to 2.94
1.94 to 2.94
3.24 to 4.86
-2
Table 3: Classification of soil permeability (from Lambe, 1951)
Permeability (cm/s)
>10
10 to 10
10 to 10
10 to 10
<10
-1
-1 -3
-3 -5
-5 -7
-7
Degree of Permeability
High
Medium
Low
Very Low
Impermeable
Table 5: Dry density values of samples
obtained from the study area
Sample No
LD1A
LD1B
LD1C
LD2A
LD2B
LD2C
LD3A
LD3B
LD3C
Dry Density (Mg/m )
1.77
1.82
2.62
1.85
1.88
2.56
1.89
1.90
2.54
3
Table 8. Table showing factor of safety values obtained
for different models of bench face angles using a
bench width of 20metres and bench height of 6metres.
Piezometric
Level 1 m
3.58
2.41
1.73
1.66
1.52
1.19
0.96
0.83
0.59
Piezometric
Level 4 m
4.20
2.66
1.85
1.51
1.30
1.42
1.28
1.18
1.02
Piezometric
Level >40 m
4.51
2.72
2.16
1.85
1.82
1.59
1.40
1.25
1.09
Slope Angle
(degrees)
10
20
30
40
50
60
70
80
90
Morgenstern-Price Method of Analysis
Table 7. Table showing factor of safety values obtained
for different models of bench face angles using a
bench width of 10metres and bench height of 6metrres.
Piezometric
Level 1 m
3.37
2.03
1.56
1.35
1.18
1.05
0.79
0.65
0.35
Piezometric
Level 4 m
3.91
2.26
1.61
1.39
1.37
1.24
1.22
1.07
1.02
Piezometric
Level >40 m
4.20
2.44
1.82
1.58
1.51
1.35
1.25
1.15
1.09
Slope Angle
(degrees)
10
20
30
40
50
60
70
80
90
Morgenstern-Price Method of Analysis
Table 6: Summary of data inputted into the Geostudio SLOPE/W
Computer program for the calculation of factor of safety values
at various bench face angles
SLOPE/W
Layer
Layer 1
Lateritic Soil
Layer 2
Alluvium
(loose sand)
Average Unit
Weight (kN/m )
25
18
3
Average Cohesion
Intercept (kPa)
45
0
Average Friction
Angle (degrees)
41
34
Figure 3. Google Earth image of southwest Nigeria.
FORMATIONAGE
Ako et al., 1980 Omatsola and
Adegoke 1981
LITHOLOGY
Ilaro formation Ilaro formation SandstoneEOCENE
Oshosun
formation
Oshosun formation Shale
TERTIARY
PALEOCENE Ewekoro
formation
Ewekoro formation Limestone
Araromi Shale
Afowo Sandstone and
shale
Ise Sandstone
CRETACEOUS
MAASTRICHT IAN
TURONIAN
BERREMIAN
ABEOKUTA
FORMATION
ABEOKUTA
GROUP
PRECAMBRIAN BASEMENT COMPLEX
Stratigraphic correlation chart