Alao poster-aeg2007(1)

David Afolayan

Mopa BASSAGI

ALAOand

COMPUTER MODELING OF SLOPE STABILITYIN 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. +2348037553842E-mail address: [email protected]

INTRODUCTION

LOCATION OF THE STUDYAREA

The Dahomey Basin located in West Africa runs parallel to the coastalmargins of Ghana, Togo, Republic of Benin and Southwestern Nigeria. Itis an extensive basin starting from Southeastern Ghana through Togo andthe Republic of Benin to Southwestern Nigeria where it is separated from itby the Niger Delta (Bankole et al., 2006). Tar sand occurrences in theDahomey Basin are restricted to the eastern portion of the basin and arecontained generally in Upper Cretaceous sediments. Extensivebituminous seepages and sediments impregnated with tarry oil define anarrow band about 5-8km wide between latitude 6 37′N and 6o48′Nstretching from east of Ijebu-Ode town (Ogun State) to the banks of thetributaries of the Siluko river at Ofosu village in Edo State an approximatedistance of 110km (MSMD Report, 2005).

A lot of work has been carried out on reservoir estimation (Adegoke etal., 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 whichrelate to the geotechnics of the Nigerian tar sand include the work of Alaoand Obielodan (2006).

However, Adegoke et al., (1980) gave an estimate of 30o to 35o forslope angles when mining of the tar sand deposit commences. This wasbased on approximations and estimations without actually carrying outlaboratory tests. Hence, the need for exact determination of thegeotechnical parameters (such as shear strength, unit weight and angle ofinternal friction) is necessary for bench face slope design. This paperseeks to examine the geotechnical properties of sediments associatedwith and overlying the tar sand deposit of Southwestern Nigeria using LodaVillage as a case study. This would be used in finding out the bench faceangle(s) safe for mining. Field activities consist of sediment description,identification and collection. Also core drill data was obtained from theBitumen Project Implementation Committee (BPIC) an arm of the NationalAgency charged with the responsibility of disseminating informationregarding tar sand exploitation and exploration. Geotechnical tests werecarried out on samples obtained and the results were utilized for computersimulation 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 ofOndo State, Southwestern Nigeria. Also, it is within the 120km long andabout 5-6km wide belt of tar sand outcrop which trends approximatelyEast-West (Adelu and Fayose, 1991). Location of the study area on the

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Figure 1

geologic map of Nigeria is shown in . is a Google Earthimage of Southwest Nigeria.

Loda Village, Ondo State, Southwestern Nigeria is made up ofundulating topography with rolling hills. The topography generally risesfrom the coastal area in the south to the rugged hills in the northeasternportion. The natural vegetation is high forest type, composed of manyvarieties of hardwood timber. The climate of the area is of the lowlandtropical rain forest type, with distinct wet and dry seasons. The meanmonthly temperature is 27 C with a mean monthly range of 2 C, whilemean relative humidity is less than 70%. Rainfall is common throughoutthe year but the months of November, December and January may berelatively dry. The mean annual total rainfall exceeds 2000 mm.

The sediments associated with and overlying the tar sand deposit atLoda village Southwestern Nigeria were sampled for geotechnicalanalyses. Lateritic soil samples and alluvial sand samples were obtainedfrom the study area. These two lithologies were observed to be overlyingthe tar sand deposit at the three locations examined 200 metres apart.

Samples were collected from three different points on a road cut at threelocations (L1 L2 and L3). Moreover, at each location, samples were pickedfrom the basal part of the road cut to the topmost part of it ( ).Lithologic sections of each of the three locations examined are shownbelow in a, b and c.

The unit weight, cohesion and angle of internal friction are essentialvalues necessary for factor of safety determination. For the SLOPE/Wsoftware this is also the case. The geotechnical test carried out on thesamples are grain size analysis, density determination and direct sheartest. All the samples were subjected to these tests and values wereobtained for each of these geotechnical test. The grain size analysis wascarried out using the mechanical dry sieve method in accordance with B.S1377, 1975. The density bottle was used to determine the density of thesamples while the direct shear test was carried out using the shear boxmachine which accommodates 60mm soil sample and is 20mm high (thesample compartment).

For the computer modeling, the Geostudio SLOPE/W software wasused courtesy of Geoslope International. A hypothetical bench dimensionof 10 metres and 20 metres was used separately with the same benchheight 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

GEOTECHNICAL TESTS

COMPUTER MODELING

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10 to 90 and Factor of Safety values was calculated using theMorgenstern-Price method of analysis. Groundwater levels of 1 metre, 4metres and greater than 40 metres was employed for each of the benchwidth used. This is necessary bearing in mind the prominent roleinundation plays in slope stability (Mandzic, 1992).

One of the primary objectives of carrying out grain size analysis is toprovide a descriptive term for soil samples analysed. This is done by usingthe particle size distribution graphs to estimate the range of sizes presentin the most representative fraction of the soil. Also, permeability valuescould 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 and0.015 (Institution of Civil Engineers, 1976). All these calculations werecarried out on the readings obtained from grain-size graphs of the samplesanalyzed.

From visual Inspection of the grain-size graphs, steep graph linesindicate that a narrow range of particle sizes characterize the alluviumsamples 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 graphindicating wide range of particle sizes present in the soil samples thismakes 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 wereobtained for each of the samples using the grain size analysis data (

). Results obtained were compared with Lambe's (1951) classification toindicate the degree of permeability ( ).

However, from the coefficient of permeability values obtained in ,the alluvium samples (LD1A, LD1B, LD2A, LD2B, LD3A and LD3B) havevalues ranging from 1.94x10 cm/s to 1.01x10 cm/s while the lateritic soilsamples (LD1C, LD2C and LD3C) have values ranging from 3.24x10 to7.26x10 cm/s. Lambe's (1951) classification is shown in for thesamples in . Both the lateritic soil and the alluvium soil havemedium degree of permeability, showing the ease with which fluid will flowthrough them. Relatively the alluvium sample has a higher permeabilityvalue than the lateritic soil sample, this poses a threat when miningcommences especially when inundation is considered.

From the dry density determination table shown below ( ) thealluvium samples (LD1A, LD2B, LD3A and LD3B) have values between1.77 Mg/m and 1.90 Mg/m , which is low when compared with those

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RESULTS OBTAINEDAND DISCUSSION

GRAIN SIZEANALYSIS

DRY DENSITY DETERMINATION

Table 1

Table2

Table 3

Table 2

Table 4Table 2

Table 5

obtained for the lateritic soil samples (LD1C, LD2C and LD3C). Thelateritic soils have values ranging from 2.5 Mg/m to 2.62 Mg/m .

From , the lateritic soils have density values higher than thealluvium soil showing a more compact grain size. Slopes cut into loosesoils are more susceptible to failure than denser soils if drainageconditions, proper slope design and other factors are put in place. Adensesoil has high shear strength which makes the soil less susceptible tofailure.

Direct shear test were carried out on the alluvial soil and lateritic soil todevelop the relation between and .

The values obtained for angle of internal friction ( ) and cohesion (c) are33 and 0kPa respectively for the alluvium soil. Also, 41 and 40kPa were

obtained for and c. Thus the values obtained confirms the fact that thealluvial soil is cohesionless while the lateritic soil has a measure ofcohesion 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 estimateFactor of Safety (which is the measure of stability or instability) for anyslope surface, the shear strength parameters, slope geometry andgroundwater level must be determined or defined. The GeostudioSLOPE/W was used to model the different bench face angles cut into thetwo lithologies overlying he tar sand deposit in the study area.shows a summary of the geotechnical data inputed into the SLOPE/Wcomputer program.

Also, groundwater levels used were based on the two seasons that existin the study area. For the dry season which extends from November toApril, the groundwater level used for the modeling is 4m, while 1metre wasused for the wet season which runs from May to October of the subsequentyear. All these parameters and conditions were inputed and factor ofsafety values were obtained for the different bench width dimensions andbench face angles. Diagram showing a pass at bench face angledetermination and how excavation might be planned for a proposed openpit mine at the study area is shown below in .

Computer modeling was carried out for 10metre bench width and20metre bench width separately. Also, a bench height of 6metre was usedfor both bench width dimensions based on the suggested bench height of4-8 metres given by Peters (1978). Diagrams obtained for the 10metrebench width and 6metre bench height are shown below in

for 10 , 40 and 90 bench face angles respectively. Diagrams obtainedfor the 20 metre bench width and 6metre bench height are shown below in

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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)

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Figures 10, 11 and 12 Tables 7and 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 inrespectively for various bench face angles.

It is obvious from that the factors of safety valuesdecrease as bench face angles become steeper for both the 10 and20metre bench width respectively. From the factor ofsafety values obtained for the 10metre bench width and 1metre groundwater level are 3.37, 1.35 and 0.35 for 10 , 40 and 90 respectively.

For the 20metre bench width in the factors ofsafety values are 3.58, 1.66 and 0.59 respectively. The factor of safetyvalues for the 4metre ground water level and greater than 40metres levelalso decrease as the bench face angles increase or become steeper.

However, the great influence that groundwater level or groundwaterpressure has on open pit mine stability is expressed in the values obtainedfor various ground water levels used in . It can be observedthat the factor of safety values are higher for very deep ground water levelconditions (which is safer) than those of shallow groundwater condition. Inopen pit mining practice more than 40% of slope instability risk depends onground 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 pitmines. Thus, for soil slopes such as those in the area higher factor ofsafety values must be proposed.

Consequently upon this the factor of safety values obtained fromfor 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 toinundation.

The two lithologic units overlying the tar sand deposit at Loda villagewere sampled with geotechnical tests carried out on them. These testswere carried to determine the parameters necessary for slope stabilityanalysis as it relates to future mining operations on the tar sand deposit ofSouthwestern Nigeria using Loda village Ondo State SouthwesternNigeria as a case study.

However, the crucial engineering problem of slope instability or stabilityhas been simulated to determine the bench face angle that would be safefor excavating the lithologies overlying the tar sand deposit at the studyarea. Therefore, for a proposed open pit mine at Loda village,Southwestern Nigeria. The following measures need to be put in place inthe design and operation of the mine;

1. Installation of piezometers in investigative boreholes to measurethe water pressure constantly.

2. Benches must be surveyed regularly to see if small movementsare taking place.

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RECOMMENDATIONS

3. The use of state-of-the art monitoring equipment likeextensometers is suggested.

4. Digging of wide perimeter trenches which would help to providegood drainage network for he flow of surface water from rainfall andgroundwater 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 fordetailed and accurate calculations of safety factors.

Earlier works carried out on the tar sand deposit of SouthwesternNigeria made observations based on approximations of geotechnicalparameters. Detailed geotechnical tests that relate to slope stabilityanalysis were not carried out. This paper has gone a bit further by carryingout geotechnical tests on the lithologies overlying the deposit at adesignated location. Also computer software is used to model the benchface angles appropriate for excavation.

However, increasing the slope angle decreases the stability of slopealthough this depends on the geologic structure, soil and rock propertiesinvolved. An attempt has been made in designing the best bench faceangles for excavating the lithologic units overlying the shallow tar sandoccurrence at Loda village Southwestern Nigeria.

Not only is an attempt made, a range of bench face angles has beensuggested for future mining operations on the tar sand deposit ofSouthwestern Nigeria. Adherence to slope angles ranging between 10and 30 with controlled ground water conditions would go a long way inensuring safe mine slopes with minimum instability risk, thereby optimizingthe operations of the proposed open pit mine for maximum economic gain.

The authors express their sincere appreciation to Mr. Femi for thegeotechnical tests carried out at the soil laboratory of the Department ofCivil Engineering, University of Ilroin; to Mr. Ola Sayomi for downloadingthe Geostudio SLOPE/W Software; to Mrs. Sayomi for typing themanuscript and to all reviewers for their constructive comments thathelped 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 PhaseII; Estimation of Reserves, Materials Testing and Chemical Analyses.Unpublished report, Geological Consultancy Unit, Department ofGeology, University of Ife, Ile-Ife, 75p.

CONCLUSION

ACKNOWLEDGMENT

REFERENCES

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Adegoke, O.S., Ako, B.D., Enu, E.I., Petters, S.W., Adegoke A.C.W.,Odebode, M.O. and Emofurieta, W.O. (1980). GeotechnicalInvestigations of the Ondo State Bituminous Sands. Report by theGeological Consultancy Unit, Department of Geology, University of Ife,Ile-Ife, 25p.

Adegoke, O.S. and Ibe, A.C. (1982). The Tar sand and Heavy CrudeResources of Nigeria. Proceedings of the 2nd International Conferenceon Tar sands and Heavy crude, pp 280-285.

Adelu, R.D. and Fayose, E.A. (1991). Development Prospects for theBituminous Deposits in Nigeria. In: R.F. Meyer (Ed.) 5th UNITARInternational Conference on Heavy Crude and Tar sands, Edmonton,AOSTRAtechnical reports, pp 509-515.

Alao, D.A. and Obielodan, J.B. (2006). Hydrocarbon Potential of NigerianTar Sand and its Application in Construction: Call for a New Policy. Apaper presentation at the Southern Interdisciplinary Roundtable onAfrican Studies (SIRAS), Kentucky State University Frankfort, KentuckyU.S.A. 20p.

Bankole, S.I. Shrank, E., Erdtmann, B.D. and Akande, S.O. (2006).Palynostratigraphic Age and Paleoenvironments of the newly exposedsection of the Oshosun Formation in the Shagamu quarry Dahomeybasin, Southwestern Nigeria. NAPE Bulletin, Vol.19 No.1, pp 25-34.

Bitumen Project Implementation Committee Report (2002). Report onAppraisal wells drilled along the Nigerian Tar sand belt. UnpublishedReport, 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 ofNigerian tar sands. NMFS, Ibadan Chapter publication, pp 30-49.

Duncan, C.W. and Wyllie, C.W. (2004). Rock Slope Engineering. 4thedition, 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 tarsand sequence. In: Van Olphen and F. Variable (Eds.). InternationalClay Conference 1981. Development in Sedimentology, 35. ElsevierAmsterdam, pp 321-323.

Fasasi, M;K., Oyawale, A.A., Mokobia, C.E., Tchokossa, P., Ajayi, T.R. andBalogun, F.A. (2003). Natural Radioactivity of the Tar sand Deposits ofOndo State, Southwestern Nigeria. Nuclear Instruments and Methodsin Physics Research,A. 505, pp 449-453.

Geological Survey Agency of Nigeria, (2005). Geological and MineralsMap of Ondo State. Produced by the Geological Survey Agency ofNigeria.

Institution of Civil Engineers (1976). Manual of Applied Geology forEngineers. 1st Edition, Institution of Civil Engineers, London 378p.

Lambe, T.W. (1951). Soil Testing for Civil Engineers, John Wiley andSons, New York.

Mandzic, E.H. (1992). Mineral water Risk in Open pit slope Stability. Minewater and the Environment, Vol. Ii No.4, pp 35-42.

Ministry of Solid Minerals Development, (2005). Nigerian tar sands andBitumen; 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 ofSouthwestern Nigeria. NMGS, Ibadan Chapter Special Publication, pp67-74.

Peters, W.C. (1978). Exploration and Mining Geology, 1st edition, JohnWiley and Sons Inc. New Jersey, 694p

Read, K.H. (1992). Manual of Soil Laboratory Testing Volume 1, 2ndEdition, Pentech Press, London, 380p.

Schroeder, W.L. and Dickenson, S.E. (1996). Soils in Construction, 4thEdition, Prentice-Hall, New Jersey, U.S.A.. 317p

Figure 1. Location of the study area, Loda village, Southwestern Nigeriawithin 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


Whitish loose sand(Alluvium) (LD2A)


4.5m

0.8m

1.0m

4.5m

0.8m

0.8m

Tar sand seepages


2.6m

0.5m

0.8m

Whitish loose sand(Alluvium) (LD1A


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 angleto determine the corresponding factor of safetyfor 10m bench width and 6m bench height

o

1.589

X-Sectional Length(m)

0 10 20 30 40 50 60 70

Dep

th(m

)

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34



GWL 4m

GWL 1m

Figure 8: Modeling of 40 bench face angleto determine the corresponding factor of safetyfor 10m bench width and 6m bench height

o

Figure 6: Diagram showing a pass at how excavation might occurfrom different bench face angles at the study area when miningcommences. (Modified from Bitumen Project ImplementationCommittee Report, 2002).

70.44m

57.91m

32.61m

2.44m0

Basement rock

Shale

Bituminous sand

Loose sand

Laterite

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Figure 10: Modeling of 10 bench face anglesto determine the corresponding factor of safetyfor 20m bench width and 6m bench height

o

1

2

1 234 5

6

7

8 9

10

4.203

1 234 5

6

7

8 9

10


0 10 20 30 40 50 60 70 80 90 100 110 120

Depth

(m)

0

4

8

12

16

20

24

28

32



GWL 4m

GWL 1m

Figure 11: Modeling of 40 bench face angleto determine the corresponding factor of safetyfor 20m bench width and 6m bench height

o

1.859


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



GWL 4m

GWL 1m

Figure 12. Modeling of 90 bench face angleto determine the corresponding factor of safetyfor 20m bench width and 6m bench height

o

1.099


0 5 10 15 20 25 30 35 40 45

Depth

(m)

0

5

10

15

20

25

30

35



GWL 4m

GWL 1m

Figure 7: Modeling of 10 bench face anglesto determine the corresponding factor of safetyfor 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

Dep

th(m

)

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34



GWL 4m

GWL 1m

70.44m

57.91m

32.61m

2.44m0

Basement rock

Shale

Bituminous sand

Loose sand

Laterite

70.44m

57.91m

32.61m

2.44m0

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 sampleswith 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 samplesobtained 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 obtainedfor different models of bench face angles using abench 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 obtainedfor different models of bench face angles using abench 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/WComputer program for the calculation of factor of safety valuesat 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 andAdegoke 1981

LITHOLOGY

Ilaro formation Ilaro formation SandstoneEOCENE

Oshosunformation

Oshosun formation Shale

TE

RT

IAR

Y

PALEOCENE Ewekoroformation

Ewekoro formation Limestone

Araromi Shale

Afowo Sandstone andshale

Ise Sandstone

CR

ET

AC

EO

US MAASTRICHT IAN

TURONIAN

BERREMIAN

ABEOKUTAFORMATION

AB

EO

KU

TA

GR

OU

P

PRECAMBRIAN BASEMENT COMPLEX

Stratigraphic correlation chart