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8/4/2019 Slope Stability for Tar Sand Quarrying in Nigeria
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COMPUTER SIMULATION OF SAFETY FACTORSUSING GEOTECHNICAL PARAMETERS FOR TAR
SAND QUARRYING AT LODA VILLAGE,SOUTHWESTERN NIGERIA.
A PROJECT SEMINAR PRESENTED
By
BASSAGI, MOPA
95/041516
A SPECIAL PROJECT SEMINAR PRESENTED TO THE DEPARTMENT OF GEOLOGY AND MINERALSCIENCES, UNIVERSITY OF ILORIN, IN PARTIALFULFILMENT FOR THE AWARD OF MASTER OFSCIENCE DEGREE (M.Sc.) IN GEOLOGY(HYDROGEOLOGY AND ENGINEERING
GEOLOGY OPTION).
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OUTLINE
INTRODUCTION – Statement of the problem and objectives. – Justification for the Research. – Aims of the Research. – Geology of Dahomey Basin. – Previous works. – Location of the study area
METHODOLOGY – Field work. – Laboratory work. – Computer Simulation.
RESULTS OBTAINED AND INTERPRETATION DISCUSSION AND RECOMMENDATION
CONCLUSION
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INTRODUCTION
STATEMENT OF THE PROBLEM AND OBJECTIVES
Large deposit of tar sand occurs in Southwestern Nigeria along an East west belt and
span over 120km long from Ijebu – Ife in Ogun State, across Ondo State to the margin
of Edo State. Tar sand is the natural resource from which bitumen is extracted while,
Sulphur, Phenol, Ammonia, Nickel and Niobium are obtained as by products when it is
processed. Bitumen is useful for road construction, while it’s by products are useful as
base stock materials for petrochemical production. The production of synthetic crude isanother important use of tar sand (or oil sand). The open pit mining method is utilized
in mining oil sand or tar sand. Before excavation or mining takes place in any mine the
stability of slopes created in the course of mining must be ensured by the use of stable
slope angles. These slope angles are obtained after series of calculations. The angles
of inclination of the slope depend on the properties of the formation into which the slope
is cut.However, if correct slope angles are not used, the risk of slope failure occurring
increases within the mine. Slope failure can lead to loss of lives and earth moving
equipment (if no solution is proffered) thus reducing the productivity of the Mine.
Therefore it is necessary to calculate the slope angles at which the lithologies overlying
the tar sand deposit of Southwestern Nigeria can be mined. This study will do this,
using Loda village, Ondo State, southwestern Nigeria as a case study.
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INTRODUCTION (contd.):
JUSTIFICATION FOR THE RESEARCH
The mining of tar sand just like any other natural resource is capital intensive. The
fact that tar sand has to be processed after it has been mined makes its more
expensive than some other minerals. Due to this fact, necessary mine design methods
and procedures must not be sacrificed if profit is to be made. Billions of Naira would be
invested when large-scale mining of tar sand commences in Nigeria, therefore chances
must not be taken with standard pre-mining procedures.
However, due to the economic importance of tar sands in obtaining bitumen (as
material for road construction), synthetic crude and other chemicals (used for
production of petrochemicals), it is reasonable to ensure that the slopes cut into the
formation overlying the tar sand deposit are stable. One of the important ways of
ensuring that the slope are stable is by simulating the various slope angles that would
be cut into the formation and calculating the factor of safety values at each of theseangles. This would help to know the angle or angles at which slopes cut into the
formation are stable. When stable slope angles are used, the risk of slope failure in the
mine is reduced and production in the mine can be optimized for huge economic gain.
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INTRODUCTION (contd.)
AIMS OF THE THESIS.
a. To carry out a field study of the tar bearing sedimentsin Loda village, Southwestern Nigeria.
b. Sampling of tar bearing sediments and other lithologiesassociated with the tar sand deposit in Loda village,
Ondo state, Southwestern Nigeria.c. To carry out geotechnical tests on samples taken from
the study area that would help to establish the soilstrength parameters necessary for slope stabilityanalysis.
d. To relate geotechnical parameters obtained to miningoperations especially as it concerns open pit mining of tar sand and the slope design necessary for optimization of future mining operations, using Lodavillage as case study
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INTRODUCTION (contd.)GEOLOGY OF DAHOMEY BASIN
The Dahomey Basin, located in West Africa, runs parallel to the coastal margins of Ghana, Togo, the Republic of Benin and Southwestern Nigeria
Figure 1 Sketch map of Africa showing the location of Dahomey Basin B. Generalized Geological map of
Dahomey Basin (Adapted from Bankole et al, 2006).
B A
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INTRODUCTION (contd.)
Figure 2. Map of Nigeria showing the Dahomey Basin (Adapted from
Bankole et al, 2006)..
B A
Study Area
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INTRODUCTION (contd.)
Table 1: Stratigraphic setting of the Dahomey Basin, Southwestern Nigeria (Modifiedform Idowu et al, 1993)
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INTRODUCTION (contd.)LOCATION OF THE STUDY AREA.
The study area is Loda Village, Ondo State, Southwestern Nigeria. The study area exists within the Dahomey Basin of Southwestern Nigeria (Figure 5). It is within the 120km long and 5 – 6km wide belt of tar sand outcrop, which trends
approximately East-West from Ogun State, through Ondo State to parts of Edo State (formerly called Bendel State)
(Adelu and Fayose, 1991). The study area, Loda Village, Ondo State, Southwestern Nigeria is located on Longitude
4o55'E and Latitude 6o40'N (Figures 6a and b). Tar sand seepages are observed within the study area
Figure 3: Map of Nigeria showing the study area, Loda Village, Ondo State Southwestern, Nigeria( Modified from Kogbe, 1989)
120N
80E
90N
Phanerozoic Sediments
Pre-Cambrian basement Complex
0 300Km
120E40E
30N
Study Area
Loda
Cretaceous –Recent Sediment
Abuja
Akure
Lagos
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INTRODUCTION (contd.)
Figure 4. Geological and Minerals Map of Ondo State, Southwestern Nigeria (Adapted from the
Geological Agency of Nigeria Digital Map, 2005).
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INTRODUCTION (contd.)
Fig 5. An enlarged version of the Geological and Minerals Map of Ondo State, Southwestern
Nigeria Showing Loda Village, Ondo State, Southwestern Nigeria (modified from The
Geological Survey Agency of Nigeria Digital Map of Ondo state, 2005)
N
0 1 2 3Km0 1 2 3Km
4040’ 5000’7000’
6040’ 6040’5000
7000’
4040’
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METHODOLOGY: FIELD WORK
Field work took place in the month of April, 2005. The
Field work involved examination of the various
sediments associated with the tar sand deposit located
within Loda village, Ondo state, Southwestern Nigeria.A road cut (Ore to Ode-Irele road) was examined at
three different points, measurements and samples were
also taken a these different points. Soil profiles were
also drawn for these three locations. A total of nine
samples were obtained from the three different pointsacross the road cut.
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METHODOLOGY( Contd.)
Figure 6: Picture of the road cut exposed at Loda Village, Ondo State,Southwestern Nigeria
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Figures 7(A – C): Profile of Various Soil types associated with the tar sand deposit andsampled at three different points Location 1(LD 1), Location 2 (LD 2) and
Location 3 (LD 3).
METHODOLOGY( Contd.)
L t it
L t it
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32.61m
2.44m
Loose sand
Laterite
32.61m
2.44m
Loose sand
Laterite
Figure 8: Diagram of Core-log drilled at Loda Village, Southwestern
Nigeria. (Bitumen Project Implementation Committee
Report, 2002)
METHODOLOGY( Contd.)
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LABORATORY WORK
GRAIN SIZE ANALYSIS.
DRY DENSITY DETERMINATION.
DIRECT SHEAR TEST.COMPUTER SIMULATION
DRAWING SLOPE PROFILE
ENTERING STRENGTH PARAMETERS
PIEZOMETRIC LINESIMULATION OF FAILURE MODE
FACTOR OF SAFETY IS CALCULATED BY SLOPE/W SOFTWARE
METHODOLOGY( Contd.)
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RESULTS OBTAINED AND THE INTERPRETATION
70
80
90
100
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RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
Table 2: Proportion of particle sizes present in each of the 9 samples analysed (as obtained from each of thegrain size analysis graphs).
SampleNo
Coarse(silt (%)
Finesand (%)
Mediumsand (%)
Coarsesand (%)
Finegravel
(%)
Mediumgravel
(%)
LD1A - 5 90 5 - -LD1B - 30 60 5 5 -
LD1C - 30 40 15 15 -
LD2A - 5 90 5 - -
LD2B - 20 70 5 5 -
LD2C - 50 30 35 30 -
LD3A - 5 90 5 - -
LD3B - 30 60 5 5 -
LD3C - 15 20 40 20 5
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RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
Table 3: Coefficients of permeability for the 9 samplesanalysed, using Hazen’s formula
SampleLabel
Permeability valuesobtained from Hazen’s
formula(m/s)
Equivalent values in(cm/s)
LD1A 6.76 x 10-4 – 1.01 x 10-3 6.76 x 10-2 – 1.01 x 10-1
LD1B 1.96 x 10-4 – 2.94 x 10-4 1.96 x 10-2 – 2.94 x 10-2
LD1C 4.84 x 10-4- 7.26 x 10-4 4.84 x 10-2 – 7.26 x 10-2
LD2A 6.25 x 10-4 – 9.375 x 10-4 6.25 x 10-4 – 9.375 x 10-2
LD2B 1.96 x 10-4 – 2.94 x 10-4 1.96 x 10-2 – 2.94 x 10-2
LD2C 3.24 x 10-4 – 4.86 x 10-4 3.24 x 10-2 – 4.86 x 10-2
LD3A 1.96 x 10-4 – 2.94 x 10-4 1.96 x 10-2 – 2.94 x 10-2
LD3B 1.94 x 10-4 – 2.94 x 10-4 1.94 x 10-2 – 2.94 x 10-2
LD3C 3.24 x 10-4 – 4.86 x 10-4 3.24 x 10-2 – 4.86 x 10-2
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Table 4: Classification of soilpermeability
(from Lambe, 1951)
Permeability
(cm/s)
Degree of
permeability
>10-1 High
10-1 – 10-3 Medium
10-3 – 10-5 Low
10-5 – 10-7 Very low
<10-7 Impermeable
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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Table 5: Comparison between permeability valuesobtained for each of the samples from the study areawith Lambe’s (1951) classification indicating theirdegree of permeability. Sample
NoRange of Permeability
values obtained(cm/s)
Degree of Permeability(Based on Lambe’s
(1951) Classification)
LD1A 6.76 x 10-2 – 1.01 x 10-1 Medium
LD1B 1.96 x 10-2 – 2.94 x 10-2 Medium
LD1C 4.84 x 10-2- 7.26 x 10-2 Medium
LD2A 6.25 x 10-4 – 9.375 x 10-2 Medium
LD2B 1.96 x 10-2 – 2.94 x 10-2 Medium
LD2C 3.24 x 10-2 – 4.86 x 10-2 Medium
LD3A 1.96 x 10-2 – 2.94 x 10-2 Medium
LD3B 1.96 x 10-2 – 2.94 x 10-2 Medium
LD3C 3.24 x 10-2 – 4.86 x 10-2 Medium
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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Table 6: Dry Density values of 9 samples obtained fromLoda Village, Ondo State, Southwestern Nigeria withtheir corresponding, calculated unit weight values.
SampleNo
Dry Density Values(mg/cm3)
LD1A 1.77
LD1B 1.82
LD1C 2.62
LD2A 1.85
LD2B 1.88LD2C 2.56
LD3A 1.89
LD3B 1.90
LD3C 2.54
DRY DENSITY RESULTS
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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DIRECT SHEAR TEST RESULTS
600
Figure 11: Graph of Shear Stress (kPa) versus Normal Stress
(kPa) for Direct Shear test carried out on sampleLD 1A
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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Figure 12: Graph of Shear Stress (kPa) versus Normal
Stress (kPa) for Direct Shear test carried out
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
0
100
200
300
400
500
600
0 100 200 300 400 500 600
Strain (%)
ShearStre
ss(KPa)
c = 40 kPa
θ = 41O
θ
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Table 7: Table showing the summary of data used for the SlopeSimulation using the SLOPE/W computer program.
Average unit
weight values(kN/m3)
Average
cohesionvalues (kPa)
Average
angle of internalfrictionvalues
{In degrees(°)}
Layer 1 –laterite
25 45 41
Layer 2 –Alluvium(loose sand)
18 0 34
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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Figure 13: Diagram showing a pass at slope angledetermination and how excavation might occur for anopen pit mine using Loda Village southwestern Nigeria
as a case study.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)m0
θ
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COMPUTER SIMULATION FOR VARIOUS SLOPE ANGLES USING THE
SLOPE/W SOFTWARE FOR A 10-METRE BENCH WIDTH AND 6-METREBENCH HEIGHT.
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
D e p t h ( m )
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Figure 14: Simulation of 100
slope angles to determine thecorresponding factors of safety values for different
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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1 .82 1
X -S ec t io na l L eng th (m )
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0
D e p t h
( m )
0
2
4
6
8
1 0
1 2
1 4
1 6
1 8
2 0
2 2
2 4
2 6
2 8
3 0
3 2
3 4
Figure 15: Simulation of 300 slope angles to
determine the corresponding factors of safetyvalues for different ground water levels.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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1.099
X-Sectional Length (m)
0 10 20 30 40 50 60 70
D e p t h ( m
)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Figure 16: Simulation of 900 slope angles to determine
the corresponding factors of safety values for
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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COMPUTER SIMULATION FOR VARIOUS SLOPE ANGLES USING THE SLOPE/WSOFTWARE FOR A 20-METRE BENCH WIDTH AND 6-METRE BENCH HEIGHTDESIGN
1
2
1 23
4 5
6
7
8 9
10
4.203
1 23
4 5
6
7
8 9
10
X-Sectional Length (m)
0 10 20 30 40 50 60 70 80 90 100
D e p t h ( m )
0
4
8
12
16
20
24
28
32
Figure 17: Simulation of 100 slope angles to determine thecorresponding factors of safety values for different
ground water levels.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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2.160
X-Sectional Length (m)
0 10 20 30 40 50 60 70
D e p t h ( m )
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
Figure 18: Simulation of 300 slope angles to determine
the corresponding factors of safety values fordifferent ground water levels.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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1.099
X-Sectional Length (m)
0 5 10 15 20 25 30 35 40 45
D e p t h ( m )
0
5
10
15
20
25
30
35
Figure 19: Simulation of 900 slope angles to determine
the corresponding factors of safety values fordifferent round water levels.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
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Table 8: Table showing the factor of safety values obtained from computersimulation using different slope angles for a bench width of 10 metresand bench height of 6 metres cut into the two layers overlying the tarsand deposit at Loda Village, Southwestern Nigeria.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
SLOPE ANGLE (INDEGREES)°
MORGENSTERN-PRICE METHOD OF ANALYSIS
1m Piezometriclevel
4m Piezometriclevel
Very deep Piezometric level(O metre )
10 3.37 3.91 4.20
20 2.03 2.26 2.44
30 1.56 1.61 1.82
40 1.35 1.39 1.58
50 1.18 1.37 1.51
60 1.05 1.24 1.35
70 0.79 1.22 1.25
80 0.65 1.07 1.15
90 0.35 1.02 1.09
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Table 9: Table showing the factor of safety values obtained from computersimulation using different slope angles for a bench width of 20metresand bench width of 6metres cut into the two layers overlying the tarsand deposit at Loda Village, Southwestern Nigeria.
RESULTS OBTAINED AND THE INTERPRETATION(Contd.)
SLOPE AN
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RECOMMENDATION
For a proposed open pit (strip) mine at Loda Village, Ondo
State, Southwestern Nigeria, the following measures must be put in
place to guard against slope instability.
1.Installation of piezometers in investigative boreholes to
measure the water pressure constantly. A rise in water pressure
would indicate that heavy equipment may have to be moved off the
benches and heavy submersible pumps used, to lower the level of
the water table.
2.Benches must be surveyed regularly to see if small
movements are taking place.3.The use of state-of-the art monitoring equipment like
extensometers is suggested.
4.Digging of wide perimeter trenches would go a long way in
providing a good drainage network for the flow of surface water from
rain fall and ground water in the proposed mine.
RECOMMENDATIONS
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CONCLUSION
In the design of a typical open pit, increasing the slope angle
decreases the stripping and/or increases the recoverable ore. However,
increasing the slope angle decreases the stability of the slope. Because
of the variability of geologic structure, soil and rock properties, there is
not a unique angle below which there is no slope instability and above
which massive failure occurs. An attempt has been made in designingthe range of slope angles safe for excavating the lithologic units
overlying the shallow tar sand occurrences at Loda Village Ondo State,
Southwestern Nigeria. The tar sand deposits at Loda Village which is
part of the X- horizon mentioned by other authors is part of the
enormous tar sand deposit in the Dahomey Basin, Southwestern
Nigeria.However, bench face angles less than 300 with controlled
groundwater conditions would go a long way in ensuring safe slopes
with minimum instability thereby optimizing the operations of the
proposed open cast (strip) mine for maximum economic gain.