Slope Stability for Tar Sand Quarrying in Nigeria

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).



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



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.



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.



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



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




Figure 2. Map of Nigeria showing the Dahomey Basin (Adapted from

Bankole et al, 2006)..

B A

Study Area




Table 1: Stratigraphic setting of the Dahomey Basin, Southwestern Nigeria (Modifiedform Idowu et al, 1993)



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




Figure 4. Geological and Minerals Map of Ondo State, Southwestern Nigeria (Adapted from the

Geological Agency of Nigeria Digital Map, 2005).




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’



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.



METHODOLOGY( Contd.)

Figure 6: Picture of the road cut exposed at Loda Village, Ondo State,Southwestern Nigeria



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).


L t it

L t it



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)




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




RESULTS OBTAINED AND THE INTERPRETATION

70

80

90

100



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




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



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




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




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




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




Figure 12: Graph of Shear Stress (kPa) versus Normal

Stress (kPa) for Direct Shear test carried out


0

100

200

300

400

500

600

0 100 200 300 400 500 600

Strain (%)

ShearStre

ss(KPa)

c = 40 kPa

θ = 41O

θ



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




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



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




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.




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




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


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.




2.160


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


the corresponding factors of safety values fordifferent ground water levels.




1.099


0 5 10 15 20 25 30 35 40 45

D e p t h ( m )

0

5

10

15

20

25

30

35


the corresponding factors of safety values fordifferent round water levels.




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.


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



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.


SLOPE AN



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



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.

Slope Stability for Tar Sand Quarrying in Nigeria

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