University of Nigeria Research Publications

TAMUNOENE, K.S Abam

Aut

hor

PG_PhD_88_6711

Title

STABILITY OF RIVER BANKS IN THE NIGER DELTA

Facu

lty

PHYSICAL SCIENCE

Dep

artm

ent

GEOLOGY

Dat

e

DECEMBER,1995

Sign

atur

e

LI

STABILITY OF RIVER BANKS IN THE NIGER DELTA

TAMUNOENE K. S. ABAM

~ G / ~ h . ~ 1 8 8 / 6 7 1 1

, DEPARTMENT OF GEOLOGY UNIVERSITY OF NIGERIA

NSUKKA

December, 1995

STABILITY OF RIVER BANKS IN THE NIGER DELTA

TAMUNOENE K. S . ABAM PG/Ph.D/88/6711

T'FESIS SUBMITTED TO THE DEPARTMENT OF GEOLOGY, FACULTY OF PHYSICAL SCIENCES IN FUWmLMENT OF THE REQUIREMENTS FOR THE AWARD OF IHX DEGREE OF DOCTOR OF RHILOSOPHY IN ENGINEERING GEOLOGY OF THE UNIVERSITY OF NIGERIA, NSUKKA

December, 1995

CERTIFICATION

Mr. Tcvnunoene K.S. Abam, a post graduate student in the Department of Geology, University of Nigeria, Nsukka has satisfactorily completed the requirements for the award of the degree of Doctor of Philosophy (Ph.D. Engineering Geology).

The work embodied in this thesis is original and has not been submitted in part or full for any other diploma or degree of this or any other University.

.-- I Prof. C.O. Okogbue (Supervisor) Department bf Geology University of Nigeria Nsukka \

Prof. E.G. Akpokodje (External Examiner)

) -.A f -

Prof. C.O. Okogbue Head of Department Department of Geology University of Nigeria Ns~~kka

ABSTRACT

Processes of bank failure and erosion in the Niger Delta have been

characterised, leading to the identification of common factors affecting

the distribution of instability of river banks.

The rates of bank recession depend on season, soil type, stratigraphy,

bank height and inclination, The direction and rate of channel water

level fluctuation, flow velocity and relative location along the river

system have also been shown to have significant influence on bank

recession rates.

Bank failure events are episodic in nature and are concentrated at the

early stages of lowering of channel water level and are mainly caused

by the sensitivity of the banks to removal of passive resistance. High

ground 'water level accentuates seepage erosion which reduces bank

stability. Bank erosion occurs where ever the average channel flow

velocity is greater that 0.5 m/sec. However, banks with height greater

than 5 m and incfimtion steeper than 65" experience accelerated

recession. Depending on the part of the bank exposed to erosion, soil

removal can increase or decrease bank stability against rotational

failure.

Soil type and stratigraphy are identified as the major parameters that

determine the mechanism of bank failure. Stratified banks with

underlying sand strata easily developed overhangs which failed by

cantilever or sliding mechanisms depending on the overhang height.

Non cohesive banks were eroded piece-meal while cohesive banks

failed by mainly rotational mechanisms along the slip surface with a

factor of safety equal or less than unity at the highest water level. To

realistically predict the behaviour of river banks, the method of

stability evaluation must recognise the dynamic nature of the

controlling factors. The back analysis technique when combined with

knowledge of operating processes can lead to an interpretation of bank

development processes.

Analytical and quantitative methods, including charts, were developed

to facilitate the analysis of river bank stability+ The stability charts for

rotational failure of partidy submerged banks considered

l~ornogeneous soils and give conservative results. A model of river

bank profiles based on shear strength was described. The model which

can be applied to both homogeneous and stratified banks gives

reasonably good fit when cornpcved to natural river banks.

ACKNOWLEDGEMENT

I am indebted to Prof. C.O. Okogbue for supervising this work and for

his numerous contibutions to this research. I am also grateful to Profs.

John KniU, P.R. Vaughan, Drs. M.H, de-freitas, M. Rosenbaum, G.

Evans and G. Wharton all of the University of London, U.K. for their

active support and assistance at the initial stage of this research. I a m

very grateful to the Institute of Flood, Erosion, Reclamation and

Transportation (IFERT) of the Rivers State University of Science and

Technology, for placing her data bank at my disposal.

My special thanks go to the Association of C o m n w e a l t h Universities

for their Academic Staff Scholarship and to the Rivers State University

of Science and Technology, Port Hcucourt, for their financial support

towards the fieldwork aspect of this research.

I acknowledge with thanks the helpful advice of senior academics and colleagues including Profs. Mosto Onuoha, C.O. Ofoegbu, D.M.J.

Fubara, C.S. Teme and Drs. Uma Kaln, L. Mamah and H. Ezeigbo. I a m

sincerely grateful to Mr. N.I. Thomas for typing this work.

Finally, I wish to express my sincere thanks to Engr. E.A.J. George and

my family for inspiring me to complete this work and especially to my

wife Mrs. Barbara Kingdom-Abarn for the inconveniences she had to

endure in the course of this work.

T.K.S. ABAM December 1995

TABLE OF CONTENTS

Certification

Abstract

Acknowledgement

List of Figures

List of Tables

List of Appendices

CHAPTER ONE : Introduction

ProbIem

Objectives

Scope of the Study

Previous work

Fluvial aspects of bank stability

.The probSem of changing water level

Bank morphoIogy

Effective s&ss in partially saturated soils

CHAPTER TWO: Characteristics of the Study Area

2.1 Physico-geographical conditions of the Niger delta

2.2 Climate of the Niger delta and its effects on bank morphology

2.3 Distribution of soiI types in the Niger delta

CHAPTER THREE: Review of SIope Stability Analysis

3.1 Analysis of translational mechanism of failure

3.2 Analysis of rotational mechanism of failwe

ii

iii

v

xi

The friction circle method

The methods of slices

Other methods of stability analysis for rotational failure mechanism

Analysis of wedge mechanism of failure

Toppling mechanism of failure

Analysis of riverbank overhangs

Shear nmechanism of failure

Beam mechanism of fialure

Tensional mechanism of fialure

Probabilistic methods of analysis

CHAPTER FOUR: Research Me thodology

4.1 Introduction

4.2 Field measurements

4.3 Labor&ry tests

4 4 Analysis of riverbanks \

CHAPTER FIVE: Results

Field measurements

Water level

Riverbank profiles

Flow velocity

Vane shear strength

Results of pore pressure measurement using standpipe piezometer

Laboratory test results

Particle size distribution

Moisture contents and consistency lilntts

5.2,3 Bulk density 5.2.4 Shear strength

5,2.5 Permeability

CHAPTER SIX: Discussions 113

Seasonal distribution of bank failure

Areal distributoion of bank failure

Analysis of factors affecting riverbank stability

Influence of channeI water level

Influence of reIative location in river system

Influence of bank height

Influence of shear strength

Influence of unit weight

Influence of ground water IeveI

Influence of sim~dtaneous variation in bank height, water 'Ievel, unit weight and pore pressure 135 AnaIysis of processes of riverbank development and recession ' 140 Analysis of seepage erosion of alluvial river banks 140

Analysis of bank faiIure and recessiona1 mechanism 148

Determination of the roIe of fluvia1 erosion processes 179

Remedial rn-easures for riverbank stabilisation in the Niger delta 181

CHAPTER SEVEN: Development of stability charts and riverbank profile models 184

7.1 Introduction 184

7.2 RotationaI failure of partially submerged riverbank 186

7.3 Riverbank profile model deveIopment 194

Model development

Application of the model

Comparison between model prediction and field observation of riverbank profile

Design of stable riverbank profiIes

CHAPTER EIGHT: Summaries, Conclusion and Recommendations

Summaries

ConcIusions

Recommendations

References

Appendices

LIST OF FIGU'RES

Map showing the study area in relationship to Nigeria and Africa I1

Major rivers and creeks in the Niger DeIta 13

Montly variations in rainfaII, evaporation and temperature in the Niger delta 14

Distribution of annual rainfall in the Niger delta 16

A typical evapotranspiration anc concurrent precipitation graph for the Niger delta 17

Map showing dedirnentary environments and morphological features of present day Niger delta complex and adjacent areas

Borehole records from various geomorphic soil groups

Modified distribution of major soil groups in the Niger delta

Analysis of translational failure mechanism and it's modofication for tension crack

'Principle of the friction circle method of stability analysis

Geomeh.y.of lagarithrnic spiral slope failure mode \

Method of slices for rotational slope stability analysis

Geometrical considerations and correction factor for Janbu's method

Conditions for sliding and toppling of a block on an inclined plane

Shear failure mechanism

Beam failure mechcdsm

Tensional failure mechanism

Map of the Niger delta showing drainage pattern and study sites

Water gauge system

Typical velocity profiIe in a river channel

Monthly variation of water level at selected locations along River Niger and its distributaries

Variation in water level in Opobo town

Monthly variation in water level at a typical tidal boundary (Peremabiri) in the Niger delta

Regional stratigraphic models of riverbank in the study area

Riverbank profiIes in the study area (June, 1988)

Changes in riverbank profile at Agbere

Changes in riverbank profile at Kaiama

Changes in riverbank profile at Yenagoa

Velocity profiles across Nun river at Agbere

Velocity profiles across Nun river at Agbere

Velocity profiles across River Nun at Sagbagreya

Velocity profiles across Oguobiri Creek near Amassoma

Velocity profiles across Egbedi creek

Vertical velocity profiles during a tidal cycle

Standpipe piezometer readings at three river bank sites near Opobo

',

Typical particle size distribution of soils in study area

Particle size distribution curves for some sites in the Niger delta

Particle size distribution of composite riverbak at Kaiama

Particle size distribution of composite riverbank at Agbere

Range of moisture content in major soil groups in the study area

Comparison of sediment shearing resistance paraIIeI and perpendicular to stratification

Result of triaxial test on specimens from Ndoni area

Result of triaxial test on specimens from Kaiama

Result of cyclic triaxial test showing changes in strain, axial stress and pore presswe on samples from Opobo area

Monthly distribution of bank failures in the study area

A comparison of water level changes and bank failure frequency in the Niger Delta

Graphical relationship between bank heights and inclination

Comparison of shear strength profile in Ndoni and Agbere of the Niger delta

A comparison of rainfall and bank failiue distribution in the Niger delta (1988)

Correlation of bulk unit weight with factor of safety of river banks

Predicted monthly variation of factor of safety of a riverbank

F1o.t~ nets after a given interval of time during recession of flood water in Ekole creek

'. '~ariatikh of hydraulic gradient with time in two riverbanks along Ekole creek

Variation sf flow rate with time in two river banks along Ekole creek

Variation of seepage force with time in two riverbanks along Ekole creek

Riverbank section within the light grey fine silty clay soil group in the Niger delta

AnaIysis of rotational failiue caused by recession of flood water

Riverbank overhang section in Okrika

Analysis of rotational failwe caused by infiltration

Rotational bank failure caused by tree of large biomass

Analysis of transIationa1 failure caused by oversteepening due to erosion

Analysis of muItiple retrogressive bank faiIure

Interpretation of riverbank failure process at Yenagoa

Illustration of criteria for interpreting bank stability

Analysis of criteria for recognising bank recession caused by erosion

Effect of local eroslon on. the stability of a riverbank against mass failure

A comparison of water table profiles in alluvial channel banks and earth dams

Stabili ty charts for partially submerged alluvial riverbanks

Stability charts for partially submerged alluvial riverbanks

Stability charts for partially submerged alluvial riverbanks

Equilibrium profile of a river bank

Comparison between observed and calculated bank profile

Comparison between observed and calculated bank profile c7 t Akinima

Relationship between bank inclination and stability number for various conditions of bank stability

LIST OF TABLES

Physical characteristics of some rivers in the Niger delta

Srunmary of physical and environmental characteristics of the major soil groups (modified from Akpokodje, 1987)

Classification and characteristics of soil slope instabilities (modified from Rib and Liang 1978) Water level variation in parts of the Niger delta for 1988

Average river discharge from selected cross-sections at different periods (Data from IFERT, 1983)

Summary of in-si tu vane shear strength results Summary of sensitivity classificati~n in soil groups in the study area

Maximum reduction in shear strength due to disturbance of soils in the Niger delta

Summary of natural moisture contents of soils in the study area

Summary of consistency limits of soils in the study area

~ u m m a j of plasticity index and linear shrinkage of soils in the stud area Y Summary of bulk density of soils in the study area

Summary of direct shear test results in LGTSC-3 soil group

Summary of angles of internal friction and cohesion of soils derived from undrained triaxial shear tests

Summary of permeability of soils in the study area

Monthly record of riverbank variables in Kaiama

Approximate exit hydraulic gradient and flow rates calculated from flow nets

Summary of results for test against seepage erosion criterion

Results of riverbank anaIysis and field observation of banks

Sensitivity coefficients of some factors affecting a partially submerged alluvial riverbank

Soil properties at different depths on a channel bank

Soil properties of an overhang

Sensitivity coeficients of variables affecting rotational beam failure of overhang

Hypothetical soil properties of a stratified riverbank

Appendix 1

Appendix 2

Appendix 3

Appendix 4

Appendix 5

Appendix 6

Appendix 7

Appendix 8

LIST OF APPENDICES

Depth to Neutral Axis

BASIC computer programme to calcrdate factor of safety of partially submerged bank using Bishop's simplified method

BASIC computer programme for calculating factor of safety against any of the three mudes of overhang failure of banks and sensitivity coefficients Plane shear failure analysis modified for partial submergence

BASI? computer programme to calculate factor of safety against plane shear failure of parfially submerged bank

A flow chart for generating stability chart parameters for partially submerged slopes

A flow chart for generating stability chart parameters for overhang failure in riverbanks

A BASIC computer programme for generating stability chart parameters for partially submerged slopes based on Bishop's simplified method

1.1 PROBLEM

Each year a vast amount of property and arable lcmd are lost as a resrdt

of river bank failure in various parts of the world. In 1988, for example,

the damage to property and arable land caused by bank failures in the

Niger delta was estimated at one hundred and six million ncaira

(Ministry of Works and Transport, Flood and Erosion Division 1991).

Bank fCdures also lead to a continuous adjustment of channel geometry

and this affects the utility of the rivers by man and increases the level of

risk in the commercial development of the rivers.

An essential reqeement towards reducing the losses arising &om the

instability of river banks is an understanding of the processes and

mechanisms of the failures and procedures for identifying instability.

Consequently, the main objectives of this study are to:

(i) locate and describe geographical occurrences of river bank

fcdures.

(ii) investigate the causes of river bank instability, identify and

critically evaluate some of the modes of failure and retreat of the

river beds.

(iii) develop analytical and qunntita tive methods including charts to

facilitate analysis of river bark stability.

1.3 SCOPE OF THE STUDY

Most studies of river bank inshbility have dealt with individual bank

failures or with those associated with a specific project and me thus not

able to give a regional interpretation of the causes of bank failure. The

consideration of river bank fail~ues on a regional basis has the

advantage of developing a broad overview of the distribution of bank

instability and the common factors affecting their occurence.

In order to achieve the above objectives, i t was considered nec- to

study the distribution . . patterns and engineering properties of the , .

geolgical materials making up the river banks. The influence of factors . \

such as geologic, ;tratigraphic and climatic features of the environment,

ground water and seepage, fluvial processes as

inundation were also investigated. On the basis

and adys i s the contributions of various factors

well as frequency of

of field observations

to the distribution of

river bank failure were assessed. Simplified methods of bank stability

evduation using stability charts were investigated for partially

submerged riverbanks. A criterion for seepage erosion of river banks

was also investigated.

1.4. PREVIOUS WORK

There is a vast amount of literahue on the stability of slopes but only a

few of these deaf strictly with river banks. These include: Thorne (1978))

Ponce (1978), Edil and Vallejo (1980)) Ilagerty et d (1981) and Thorne

and Torvey (1981). Other Literature on slope stability which relate to

riverbanks are those concerned with earth dams under conditions of

changing reservoir IeveI. Some of these are Feltenius (1936), Bishop

(1952), Janbu (1954) and Morgenstern (1963). Previous work on various

aspects of river processes and bank instability are discussed beIow.

1.4.1 Huvial Aspects of Bank Stability

There are many, factors which govern the forces that cause bank ',

instability. Char1 ton (1982) grouped these factors into two classes,

namely, those associated directly with the fluid flow in the channel and

those associated with conditions exterior to the channel. The fluvial

factors which affect bank, stability include, fluid flow velocity,

boundary shear stress, fluid lift force, secondary currents, fluctuating

discharge, migration of meander pattern in dynamic equilibrium,

fluctuating sediment discharge, changes in river Ievei, obstructions

deflecting CLIIT~ILLB L U W ~ U B cl and river traffic or wind with wave

generating capacity.

A qualitative evaluation of these factors in which the magnitude of the

forces acting on the river bank are expressed as a percentage of the

tractive force exerted on the bed of the channel by the flowing water

was provided by Simon and Li (1982). Thorne (1978) also recognised

various forces on a bank during his study of River Severn in the U.K.

First he recognised that soil material may be entrained directly from the

bank and transported downstream. Second, he recognised that the flow

may scour the.bed at the base of the bank (increasing bank angle and

height) to bring about gravitational. failure of an intact bank. These \

observations were later noted by Hooke (1979) as the two main river

bank processes.

1.4.2 The Problem of Changing Water Level:

One major feature of river channels is the periodic inundation of the

banks. The fluctuation of water level in a river channel leads to a

continuous adjustment in the balance of forces on the river bank. Desai

and Sherman (1971) showed, from an investigation of unconfined

seepage in sloping banks subjected to the effects of changing river stage

that, among other factors, pore pressures were induced within the earth

mass due to seepage. Several recorded cases of earth dam failures in

draw-down situations have been attributed to the role of undissipated

pore pressures. According to Vaughan (1987) undissipated pore

pressures arising from varying river stage or draw-down are hardly

correctly estimated. This is because for a fal l in river level, the free

water or the phreatic surface in the earth bank lags behind the f&ng

level of water in the river and this complicates the problem of stability

as it is.;then generally difficult to compute such a free water surface.

Because of this difficulty, Morgenstern (1963) estimated slope stability \

dming draw-down by assuming that the pore pressure ratio B given

by:

where du = induced pore pressure

d a = change in stress

is unity and that no dissipation of pore pressure occurs. Desai (1972)

used the finite element technique to develop a procedure which

allowed the evaluation of an approximate free surface for gradual (and

sudden) draw-down in river banks and this provided data for more

precise stability analysis. However, because of variability in soil

properties for example, such rigorous approaches to bank stability

analysis may not necessarily be superior to the more common and

simplistic approach based on records of bank failure.

The historical records approach was used for the Ohio River by

Hagerty et al(1981) who concluded that bank failure and erosion on the

Ohio River was complex and episodic. They suggested that the

princip,al erosional mechanism was bank material removal by tractive

forces during flood events and internal erosion of bank materials by \

bank discharge following flood.

1.4.3 Bank Morphtrl ogy

Because bank erosion is not uniform, the bank face can take vcuious

and sometimes complex geometrical forms. Thorne and Torvey (1981)

described some bank face morphology as resembling cantilevers. Such

complex morphologies are common in large river net-works such as the

Niger Delta where Okagbue and Abcm (1986) have described failure

mechanisms of river banks as complex; ranging from simple slip failure

in high banks to shearing and toppling in low banks (less than 2.5m in

height). Okagbue and Abam (1986) further determined that the slip

circle model of stability evaluation described by Bishop (1955) was only

adequate for bank heights exceeding 3.5 m.

1.4.4 Effective Stress in Partially Saturated Soils

The occurence of complex slope Forms may be traced to the nature of

the soil and the effect of the e~wironment on the soil behaviour. By

exposing bank materids to long spells of dry season, high levels of

moisture deficit, can be created (Zurich 1985). This imparts partial

saturation to the soil where the void spaces are occupied both by water

and by air. For such par t icy saturated soils, Bishop (1955) found that

Terzaghi's equation for effective stress may be modified to read as

follows:

= u - [Ua - X (Ua- UiLI)] 1.2 where (T = total stress

Ua = pore air pressure U,,, = pore water pressure X = fraction of unit, of cross-setional area of soil

occupied by water. This parameter is related to the degree of saturation and by implication soil suction

For saturated soils X = I, and for dry soils X = 0. Fredlund e t al. (1978) investigated the contribution of suction

to the shear strength and discovered that suction can be most

conveniently expressed in terms of cohesion C by:

where C, = suction or apparent cohesion of the soil Ct = true cohesion of the soil

A combination of equation 1.2 and 1.3 would thus lead to a more general expression for shear strength in the form:

> _ . L ' - \

In unsaturated soils, shear deformation has a significant effect

on soil strength. According to Lumb (1975) dilatant shearing of soils increases suction, while campre$sive shearing decreases

suction.

2.1 I'IIYSICO - GEOGRAPHTCAL CONDTTIIONS OF THE NIGER DELTA

The Niger delta is situated in the coastal sedimentary basin area of

Southern Nigeria (Fig. 2.1). It covers an area of 36,270 km2 constituting

roughly 3.9% of the land area of Nigeria. Its formation is attributed

primarily to the structural movement of the earth's crust and the

operations of physic0 - geographical processes of erosion and

sedimentation (Short and Stauble, 1967; Whiteman 1982). This led to

the establishment of nn extensive s&hentary flatf criss-crossed by

several rivers and creeks, the Nun River and Forcados River being the :,

two principal rivers (fig. 2.2). \

The River Niger bifurcates near A m a b i r i into the Nun and the

Forcados which flowed westward as the main stream of the Niger. The

Forcados gave rise on it's left side to the Sagbama River, the Bomadi

Creek and the Nikoro Creek. The Forcados River is joined from the

right by the Ase River, the &ow Creek and the Oreri Creek.

The Nun River on the other hand is joined by the Taylor Creek from it's

left and bifurcates into the Ekole Creek, which debouches into the Brass

River. The Egbedi Creek takes off from the right of the Nun. After

some 2-4 krn in its meandering course, it meets the Sagbama River, a

left distributary of the Forcados.

River Orashi is joined by Ndoni Creek near Aboh and gives rise to

Egorobiri Creek, Saka creek and Kugbo Creek. The rivers have varying

lengths and widths. Table 2.1 summcvises the length, width and height

of some of the rivers and creeks.

TabIe (2.1) Physical characteristics of some rivers in the Niger

Name of River or. creek" 1

Width of water

channel Cm)

Nun Forcados Sagbama

Ekole Egbedi Orashi

Ndoni

Height from water level to ground at dry

season (m)

Slope of River

2 104 N/A

1.23 x lo4

N/ A N/ A

6 x 1.16 x 10"

Station

Kaiama Pa tani

Tungbo Y enagoa Egbedi Mbiama

Ndoni

The Nun and the Forcados have more distributaries than tributaries,

consequently, they are narrower in width in their lower reaches

G L/ L F O F G U I N E A

Fig. 2.1 May showing the study area in relationship to Nigeria an

Africa

(NDECO 1961). They have many meanders and variations in width,

attributable to the modest slope (averaging 6.3 x l o 5 at Bomadi), and

slow speed as well as the effect of tides.

2.2 CIirnate of The Niger DeI fa and it's Effects on Bank Morphohgy

The Niger delta is characterised by two distinctive seasons, namely: dry

season (November to March) and wet season (April to October), (fig.

2.3). The dry season is generally characterised by fairly high

temperatures usually greater than 26°C and a low monthly average

rainfall usually less than 200 mm. The combination of high . ., ,

temperhres and low rainfall resulted in high evaporation (monthly . \

average of about500 mm). The excessive evaporation results in the

depletion of soil moisture, shrinkage and finally soil cracking.

At the inception of the wet season, the average rainfall in the delta

increases gradually and reaches a peak (up to 500 mm/month) in Jdy.

A slight decrease is normally experienced from August to October . The

rainfall drops sharply in November (fig. 2.3). As a result of the high

rainfall during the wet season, soil moisture is recharged.

Fig. 2.2 Major Rivers and Creeks in the Niger Delta

RAINFALL 450 EVAPORATION 1

#

--*' @.??ahfoil ( m m l A Ewaporatlon ( m m )

( m m 1 wo 1 ( A 1 PORT HARCOURT ff Temp(bC3 A

TEMP.

[OC

I 1 1 I I I I I 1 4 J F M , A M J J A S O N 0

T I M E I m o n t h s )

Fig. 2.3 Monthly varia t i o m in rainfall, evaporation and te~nperatrrre in

the Niger Delta (Data from IFERT, 1988)

The mean annual rainfall is reduced farther away from the coast Line

(fig. 2.4). Consequently, the areas south of Opobo, and the estuaries of

the Pennington River are the wettest areas in the Niger Delta with

about 4,000mm annual average rainfall, while the middle Delta has

3,000mm and the upper delta 2 ,000m.

The alternation of dry and wet season causes the soils in the Niger

Delta to undergo volume changes without necessarily being subjected

to any external load. Such v o l ~ m e changes =arise from changes in the

soils water content (fig. 2.5) and thus effective stress (Head, 1986). h

the field, large volume changes are manifested as shrinkage and tension ' , %

cracks within the acfected soils . When the duration of the dry season is '?

long, extensive dessication occurs leading to partial saturation of the

soil and the development of a net-work of shrinkage cracks which

extend over large areas, producing a blocky structure in the soil

(W olman 19.59).

The wetting of compact or stiff partially saturated soils results in

softening arising mainly from reduction in cohesion with little effect on

the angle of internal friction (Bishop and Henkel, 1967)

Fig. 2.4 Distribution of atmud rainfall in the Niger Delta (BERT 1983)

0

PRECfPlTATlON /EVAPORATION ( m m )

Avwaqe monthly precipitation and evaporation ( 1960 - 198 0 ) Port Harcourt . I

Fig. 2.5 A typical evapotranspiration and conc~vren t precipitation

graph for the Niger Delta (IFERT 1988)

2.3 Distribntion of Soil Types in the Niger Delta

The principal geomorphic soil groups in the Niger delta were described

by Short and Stauble (1967). Those described included the coastal plain

sands, meander belt, fresh water and back swamp deposits (fig. 2.6).

Others are mangrove sw~vnp and .abandoned coastd beaches. These

soil groups are differentiated by their mode of origin rather than by

their engineering properties. Typical vertical soil profiles obtained

from boreholes drilled within the different geomorphic soil groups are

presented in fig (2.7).

Based on diffe~ences in engineering behaviour, Akpokodje (1987)

reclassified the soils in the Niger delta into four major groups namely: i

(I) Reddish brown sandy clay loam soil of low to medium plasticity

(RBSCL-I); (2) Brown sandy clay of mednm to high plasticity (BSC-2);

(3) Light grey, slightly organic, fine sand and silty day (LGFSC-3); and

(4) dark organic/peaty clay of high to extremely high plasticity

(DOPC-4). Following extensive fieldwork in the area, the writer has

refined this classification in both geographical extent and composition

as shown in fig 2.8.

Estuaries

i Fluuia -Mar ine a Beochta and barn Traoaltionol Environment Marina Includiog Lower Oelloic Ploin 8 CaoStai belt Mar in r Envlronmenl Fig. 2.6 Map showing morphological soil groups in present day Niger

delta Complex (Short & Stauble, 1967)

M: nqravm worn p Gmul

Fig. 2.7 C o m p ~ i s o n of borehole records drilled at various genrnorphic

units (Data from Enmh George Associates 1990)

These soil groups and their hydraulic properties are sumrnarised in

table (2.2). Within these major soil groups, however, other soil types

occur and this results in variability in the surface distribution and

stratification. Stratigraphic sections obtained from various borehole

logs and from exposed river bank surfaces show that generally, the soil

layers are irregularly stratified and show a tendency for coarsening

upwards. In the upper reaches of the Niger delta, the exposed river

bank faces are dominated by sandy soils while silty and clayey soils

predominate towards the coast.

Fi. 2.8 Mo.dified distribution of major soil groups in the Niger

Delta (Modified from Akpokodje, 1987)

nE G E NL

1 Reddish b r o w n r a n d y c lay loam ( R B S C L - I ) IT1 Brown sandy c lay ( B S C - 2 1 1wJ Light grey to d a r k s l i g h t l y organic t i ne r a n d a n d s i l t y cloy ( L - G F S C - 3 )

Dark t o d a r k brownish o r p a n k a n d pea ty c lay of high plast ic i ty ( D O P C - i d I -. -

.-

Fi. 2.8 Modified distribution of major soil groups in the Niger

Delta (Modified from Akpokodje, 1987)

Table (22) Summary of Physical and Environmental Characteristics of the Major

Soil Groups (Modified from Akpokodje 1987)

Major Soil Group

Reddish brown sandy day loam RBSCL-I

Brown sandy clay BSC-2

.... . - , < Light grey slightIy .. organic fine ' sand and silt cIay LGFSC-3

Dark organid peaty day DOPC-4

In-Situ Condition

Very loose when wet to very dense when dry

- - - - - -.

Loose to dense when dry

Hard with abundant s hrinkase cracks when dry

Hard with abundant shrinkage cracks when dry but very soft to highly compressible when wet

Geologic Unit

Coastal plain sand

Coastal plain sand and Sornbrciro- Warri deltaic plain

Backswamp and fresh water swamps and meander belts

Mangrove swamps and salt water LMcksnmnps

Geomorphic and Hydraulic Properties

Dry, flat to subhorizontal land, generally rmt affected by seasonal floods, g d drainage conditions, with water table 3 5 m, marshes are scarce. Dry, flat to subhorizontal sloping land with prominent seasonal fresh water n~arshes, poor to good drainage, water table generally between 3-5 m.

Mmos t totally submerged during et season, with exception of naturaI drainage problems with seasonal and temporary flooding due to rainfall and rise in groundwater tabIe. Groundwater table generally between 0-3 m. Frequently totally submerged during high tides a n d seasonal floods; very severe drainage problems, water table genenily between 0-2 m.

3. REVIEW OF SLOPE STA'BTLX'IY ANALYSIS

Introduction

As a result of the need to understand and correctly predict slope

behaviour, several methods of assessing the stability of slopes have

been developed. These slope stability methods are based on five

mechanisms of failure, namely: translational, rotational, wedge,

toppling, and flow slides. Rib and Linng (1978) distinquished these

various types of slope instability (table 3.1) and suggested guidelines

for recogrusing each type in the field. Rib and Liang's guidelines have

been extended to accommodate characteristic features observed in . . I

channel bank failures. Whereas slope failures of the transla tional and .. \ \

rotational types are generally amenable to one of the simple techniques

of analysis, toppling, and flow slides have rarely been confidently

analysed in terms of a factor of safety.

3.1 Analysis of Translational Mechanism of Failure

A translational failure mechanism usually involves the displacement of

the unstable soil or rock mass along a continuous weak plannar surface

as illustrated in fig (3.1).

Culrnan's Method of Analysis

A method for analysing translational failure mechanism was first

suggested by Culman (1886). Hx method involves the resolution of

forces parallel and normal to the weak planner surface (fig. 3.1)

resulting in the foLlowing expressions:

where C = cohesion (kN/m2) L = length of plane of weakness (m)

W = weight of unstable soil mass (kN) O = angle of internal friction (degree) j3 = slope angle (degree)

.< - . \ . < The ratio FI/F2 is taken as a measure of the factor of safety (FS) of the

- \ slope. The slop: is considered unstable if the factor of safety is less or

equal to unity. The resolution of forces in this way implicitly assumes

that aH forces on the sliding mzss including it's gravitational weight

and forces due to the pressure of water act through the centroid of the

sliding mass. The implication of this assumption is that rotational

moments are ehinated although these contribute to instability.

Strictly, this assumption is not true for acixal slopes, but according to

Hoek and Bray (1981), the errors introduced by such moments can be

ignored without greatly altering the accuracy.

FI = C , L t N Tan N b

( a A N A L Y S I S OF T R A N S L A T I O N A L FAILURE

Z o = Oeplh of Tension crack Zt = Llmi t lng depth of Tentl la Z o n e

2 C ' y + T a n ( 4 5 t @/2 1

( bl CULMAN'S ANAUIS15 MODlFlEO TO TAKE A C C O U N T O F TD(SION CRPI(

Fig. 3.1 Analysis of translational fCail~u.e mechanism and its

modification for tension crack

f I Parts Surrounding Slide Main Scam

Has no cracks

s flow Has few cracks

tf, 1''

fiow May Rave a few cracks

f l~w; Has few cracks

[S h m l shaped at angle of rcpose

Typically has wrraeed or V-shaped upper part; is long and narrow, bare and commonly striated

-- Its concave towards slide, in some types is nearly circular and slide issues through narrow orifice Is steep and concave toward slide; may have variety of shapes in outline: nearly straight, gentle arc, circular or bottle shape

Flanks

3ave cantinubus. xrve into main ..' ;cam

steep and rregular in uppe: pa* m y bhve levels built in lower parts

Arc cumed; have steep sides

Commonly diverge in direction of movement

Parts that Wave Moved Head

Usually has none

May haw none

Commonly consists of a slump block

Is generally under wa ti? r

Body

Is mnEcal hmp of soil, equal in vrslume to head Cmsisrs of large blocks pushed along in a matrix of finer material; has flow h e s

form. Flows drainage ways a d Can make sharp turns; is very Tong compared to length Js broken into small pieces, shows flow structure

Spreads out on underwater floor

Foot

Has none

Has none

Has none

In a critical condition where F l / F 2 = 1, the maximum height that a

slope can sustain beyond which the translational mechanism of fadure

would be expected to occur is given by:

4G'sin u cos 0 ?'" = 1' (I - cos(p - 8 ) )

where I-IC = criticaI height of dope Y = unit weight (kN/rn3) ,O = slope angle (degree O = angle of friction (degree)

C = cohesion (kN/m2)

As the slope angle decreases, plane failure becomes much less Likely

and the Culmcm's analysis seriously overestimates bank stability

(Thorne 1978).

Correction for Tension Crack \

The non-provision for tension cracks in Culman's original relationship

may seriously d e b t its application. Tension cracks reduce the

effective length of the potential failure surface and decrease bank

stability but according to Thorne and Torvey (1981) they do not

invaLidate the stability analysis provided that the depth of tension

cracking is smcd compared with the bank height. In order to

accomodate the effect of tension, Carson (1971) considered the depth of

the zone of tension in relation to the length of the fdu re surface. The

maximum depth to which tension cracks may develop can be predicted

from the engineering properties of the soil (Terzaghi and Peck 1969).

Within the tensile zone, the shear strength of the soil is zero. Thus the

strength in this area can be discounted from that which would be

mobilized in a slope that has no tension crack.

3.2 AnaIysis of Rotational Mechanism of Failure

Two techniques of analysis have been applied to rotational failure

mechanisms, namely, finite element and Limit equilibrium techniques.

The finite element technique determines the normal and shear stresses

along a failure surface by considering the elastic properties of the soil

in terms of Young's modulus and poissons ratio. Although this

technique gives a useful indication of the influence of stress

distribution on the stability of the slope, it is not widely used owing \

primady to the highly ma thema tical procedures required for obtaining

solutions. Besides, the accuracy of this technique in practical problems

is not superior to those of the limit equilibrium technique (Duncan and

Wright 1980).

The Limit equilibrium methods on the other hand define stability with

respect to the limiting strength of the soil. All Limit equilibrium

methods have two principal features:

(1) They define the factor of safety (FS) in the same way,

in which S = shear strength and T = shear stress required to induce

equilibrium.

(2) They make the implicit assump tion that the same values of shear

strength may be mobilized over a wide range of strains along the slip

surface. This assumption is a weakness of the h n i t equilibrium

tecluuques since strength is strongly dependent on stmin. However in

comparison with the finite element analysis technique, the Limit

equilibrium methods do not in general requhe as much iterative

computations and as a result are widely used. The various limit

equilibrium' methods are now discussed.

3.2.1 The Ffictioh Circle Method

The ecuIiest limit equilibrium technique is perhaps the "Friction Circle"

method originally developed by Taylor (1937) for analysing circular

failures. The method considers the stability of an entire sLiding mass as

a single unit fig (3.2). At Limiting equilibrium the method assumes that

the resultant of the normal and frictional force is tangent to a

hypothetical circle "Friction Circle" &awn at the centre of the circular

surface and with radius (r) given by:

where R -= radius of the circular failure surface 0 = angle of int,ernal friction of soil.

The disadvantage of the "Friction Cirde'method is that it can only be

applied to a homogeneous slope with single values of cohesion and

angle of internal friction and unfortunately, most slope materids are

not homogeneous. Also, in using the friction circle method, the

distribution of forces along the failure surface must be arbitrarily

assumed. This in itself is a source of error. This difficulty is overcame

by the logarithmic spiral method in which the resultant of dl the \

normal cvld frictional forces pass though the origin of the spiral no

matter their magnitude. Consequently, when a moment is taken about

the origin, the combined effect of normal and frictional forces is nill

(Huang 1983), and only the weight and the cohesion moments need be

considered. The problem with the log spiral method, however, is that

the geometry of most failure surfaces do not bear much resemblance to

the log spiral. The log-spiral method is schematically represented in fig

3.3

TRIAL C I R C U L A R F A I L U R E S U R F 4 CE

Fig. 3.2 Principle of the friction circle method of stability analysis

C d T a n Q R = R o C

R = I n s t a n t r a d i u s o l 0 f r o m R W h e r e

R o t R a d l u s o t e n t r a n c e p o l n l

E x p i e = b a s e o f n o l u r o l l o p a r i t h m s

Fig. 3.3 Geometry of logarithmic spiral slope fcdrue mode

0.J

3.2.2 The Methods of Sfires

Unlike the friction circle m d log spiral methods, the methods of slices

involve the division of the potentidy unstable soil mass into slices (fig

3.4a) and can thus accomodate complex slope geometries, variable soil

and water pressure conditions. However, the division into slices gives

rise to intersLice forces (fig 3.4b) which may affect bank stability.

Fellenirms's Method of Slices

The Fellenius method was the earliest of the methods of slices. This

method Like the friction circle method deals exclusively with circular

failure surfaces. Here, the factor of safety is defined in terms of moment

equilibrium and is expressed numerically by:

where P V = bulk weight of soil in slice a = imlinatiorl of slice L = width of dice W = pore water pxessure at the base of she slice O = angle of internal Friction of soil

C =. cohesion of the soil

The imylication of this definition is that the factor of safety is the same

in each slice a situation which implies mutual support between

adjacent slices. To achieve equihbrium in each slice in the direction

normal to the base of the slice, the method further assumes that the

resultant of all forces on the sides of the slice acts parallel to the

bottom of the slice. In actuality, however, this assumption is not true

for ail slices (Whitman and Bailey 1967), and as a result, some of the

slices will not be in equilibrium. This lack of equilibrium in the balance

of forces in the slope is one source of error. The others are associated

with problem solving when large pore water pressures are developed

and for situations of deep circrdar failures. Where large pore pressures

are involved and the inclination of the slip surface is steep, the UL term

in equation (3.5) becomes greater than the W. cos (a) term suggesting

condition of net uplift at the base of the slice. The method therefore

underestimates t+e stability of natural slopes and can not thus be used

to estimate long-term stability.

Bishop's Method of Slices

The sources of error in Fellenius method are eliminated in the Bishop's

method due chiefly to the indusion of interslice forces in the equations

of equilibrium. In this method, the factor of safety is expressed by:

where CL = effective cohesion 0 = effective angle of internal Friction b = width of sKce U = pore water pressure ai = inclination of base of slice Xi and Xi+l are slide forces on a slice

When (Y - Xi+,) in equation 3.6 is equated to zero, the solution to the problem becomes greatly simplified and the method is then referred to

as Bishop's simplified method. Because the difference in the side forces

may not be- zero, Bishop's simplified method does not satisfy all

equilibrium conditions. Inspite of this, however, it has been shown by

severd workers '(Whitman and Bailey 1967, Duncan and Wright 1980,

Huang 1983) that the method can give satisfactory results, especially

where failure surfaces can be approximated by a circle. Bishop (1955)

compared the safety factors obtained from the simplified method with

those from the more rigorous method in which all equilibrium

conditions are satisfied. He found that the vertical interslice force could

be assumed zero without introducing significant errors (typically less

than 1%).

do

. - -

I

Channel Water Level

Q

Hydrostatic Pressure

( a \ CIRCULAR FAILURE MECHANISM O F f ' A R T I A w SUBMERGE0 SLOPE

( b ) FORCES ON A TYPICAL SLICE IN A CIRCULAR FAILURE MECHANISM .

Fig. 3.4 Method of slices for rotational slope stability analysis

Duncan and Wright (1980) also compared Bishop's simplified method

with more accurate methods. The Bishops simplified method was

shown by these workers to be accrua te within 5% of the methods which

satisfy all equilibrium conditions. Due to it's simplicity and accuracy,

the Bishops simplified method has become one of the most widely used

methods for the analysis of circular failures (Bromhead 19%). By

modrfying the distribution of unit weight below and above water level,

Bishops method cnn also effectively deal with problems of partial

submergence (fig. 3.4b). The weight of the submerged part of a slice is

computed by multiplying the bouyant unit weight of the soil by the

area of the pqrt of the slice below water level. The consideration of

partial submerg~nce in this way modifies equation (3.6) to:

1 P S = C[C'.O+ tan^(^^ + C V ~ - [J . b +

C ( W I + W 2 ) sin a sec CY

f (Xi - Xi+l))I L+tang , t ana FS

where VV1 = weight of part of sfice above water level IY2 = weight of part of slice below water level

A si~nplification of the above equation as in Bishop's simplified method leads to the expressiorl:

1 F S = [ ~ ' ~ t t a n B ( ( W ~ + W ~ ) cos a-tJ-h-sec a)]

(W1 + 1V2) sin a 3.8

Janbu (1957) had given a rather simplistic and approximate

approach to partial submergence in terms of total stress. Using

his approach the factor of safety of a partially submerged

homogeneous slope is expressed by:

where IVJ , ,u. are constants depending on the angle of internal friction of the soil

C = cohesiou (kN/m2) y = bulk unit weight of soil (kN/m3) y, = unit weight of water ( k ~ / m ~ ) 15 = slope height (m) H, = channel water levels (m)

3.3 Other Methods of Stability Analysis for Rotational Failure Mechanism

For slip surfaces of irregular shapes, limit equilibrium methods such as

Morges tern ilnd Price (1965), Spencer (l967), Janbu(1973) and Sarma

(1979) are usually used, The basic concept in these methods is the same,

the difference lies in the assumption of the interslice forces. Huang

(3983) noted that if both moment and force equilibrium are satisfied the

assumption of interslice force should have only small effects on the

safety factors obtained by any other method. The only problem that

would naturally zuise from analyzing irregulnr surfaces is that because

of the irregular shape of the surface, forces normal to that surface do

not meet at a single point. Consequently, the convenience of a single

point thro~igh which a number of force components act and are

therefore lost from a moment equation based on that point is no longer

available. The problem is eliminated in Janbu (1973) by resolving the

forces on a slice vertically and assuming the sum of the vertical forces

to be zero. This res~dts in the expression of the factor of safety as:

z(C'.b + (W - i.J . b t d r ) tan01 secZ a F S = C W . t anu I+t,nn antan 0 3.10

FS

where 10 = ccorection factor depending on soil type and dope geometry (fig. 3.5) and defined nurnericalIy as fo!lows:

for C > O m d 0 > 0; I. = 1 + 0 . 5 ( d / L - 1 . 4 ( d / ~ ) ~ ) for C = 0, = 1 + 0.31 ( d / L - ~ . 4 ( d / ~ ) ~ )

C = effectiveness cohesion (kNl/rn2) b = width of slice m)

, u = pore pressure I kx/rn2) 14' = bulk weight of slice (kN) dz = resultant interslice foi-cc jkN) '0 = angle of internal friction (clegwe) L = span of potentially unst,able soil mass (m) d = relative depth of potentially unstabIe soif mass (m)

Some of the geometrical considerations of Janbu (1973) are illustrated in

fig.(3.5a). The solution to equation (3.10) requires an iterative procedure

in which successive values of FS are substituted until;

Slip Surface

a ) Explanatary dlaqram of peamelrlcal mrr ldmra l i o n a tn Jan but Method

C o r r s c ? l o n toc tor char t tar dan b u i Me1 hod

0 0.2 0 3 0.4

R A T I O d j L

Fig. 3.5. Geometrical considerations and correction factor for

Janbu's method

The correction factor (fo) can result in an increase of 13'' in the factor of

safety (Bromhead 1986). Landslides with small (d/L) ratio are

thus more accurately analysed. In riverbank, L is generally small and

the (d/ L) ratio is often large. Therefore Janbu's method is not very

suitable for riverbanks.

3.4 Analysis of W ~ d g e Mechanism o f Failure

In the wedge mechcanism of fd~u-e , the unstable soil mass is assumed

to be approximately bound by two or three intersecting planes of

weakness. The wedge analysis gives a satisfactory estimate of the safety

factor (Lcmbe and Whitman 1969) although the method of slice ccm be

used for solution of such problems. The wedge mechanism is generdy \r

suitable o d y for blo&y soil rock masses.

The shear resistance along the segments of the failure surface is

expressed in terms of the applicable strength parameters and a safety

factor which is the same for all segments.

3.5 Toppling Mechanism of Failure

Toppling by definition is the overturning of columns or blocks of earth

mass about some fixed base. According to Zcvuba and Mencl (1969) the

earliest reported recognition of toppLing of geological materials in

scientific literature was probably in the 1950's. de-Freitas and Watters

(1977) reported several field examples of toppling failures. Ashby

(1971), Teme (1982) and Jarvis (1985) used physical models to study

toppling failure. Ashby's work considered the simple case of a rigid

rectangular block on a steeply dipping phne and established the

relationship between the slope of the plane and the dunensions of the

block (fig 3.6)' such that equilibrium exists when:

. .. where d-breadthofblock

h = height of block a ='dip of the base plane

when b/h > t.an a , the block is stable when tan > tan 0, sliding occurs when b/h < t,an a and 0 .: a toppling and sliding occur simultaneously

While Jarvis (1985) extended Ashby's criteria to cope with

non-rectangular blocks (produced essentially by non-parallel joints),

Teme (1982) investigated the behaviour of blocky geological materials

a ure. undergoing toppling f il

3.6 Analysis of Riverbank Overhangs

Thorne (1978) identified three mechanisms of failwe of bank overhangs

and proposed methods for their analysis. These mechanisms include

shear, beam and tensile f d w e of overhang soil masses (figs 3.7 to 3.9).

3.6.1 ShearMechanismof Failure

The shear failure involves displacement along a vertical plane such as

AA (fig 3.7). Such failures will be expected if the shear stress exceeds

the shear strength along AA. Numericdy this condition may be

expressed by:

wher; 1.V = weight of overhang (kN) T, = shear st-rength per unit lerlgth (kIV/m2) H = height of overhang (m) d t = depkh of crack (m)

A factor of safety against shear failure can be defined as the ratio of shear strength to the shear stress

Ts F S , = -

S s

where S, = shear stress (k8/m2). Since shear sbress is. due entirely to the weight of t,he ovehang, it can be equated to:

T but T3 = 9 wl~ere I-r, = compressive strength [k~/m') . Therefore

Substituting 3.14 into 3.15 yields

where Y = m i t weight of soil ( k ~ / n ~ ~ ) B = widt.h of overhang (m)

Equation 3.16 is an expression of factor of safety against overhang shear

fLdure in terms of measurable p~ammeters,

3.6.2 Beam Mechanism of Failure

Beam mechanism of failure results from the rotational moment due to

the self weight of the overhang (fig 3.8), overcoming the moment due to

cohesion along AA. In this failure mechanism, the overhang fails by

rotation of the block forward into the channel. The cohesion within the

overhang should be strong enough to preclude any form of

disintegration during this failure process.

GEOMETRY OF BLOCK ON INCLINED PLANE

10 2.0' 30' 4 0 5 0 60 70 80 90

BASE PLANE ANGl F 'b - nFGRFFS

Fig. 3.6. Conditions for sliding and toppling of a block on an

inclined plane af(nfter Hoek and Bray 1977)

( a UNFA ILED CONOlTlON ( b) SHEAR FAILURE

Fig. 3.7. Analysis of shear failure overhang (after Thcvne 1978)

C Z I Compressive Zone) "143- Nwtrol axis I

Fig. 3.8. Analysis of rotational beam (after Thorne 1978)

Fig. 3.9. Analysis of tensional failure of overhang (after Thorne

1978)

In the analysis of a beam mechanism of failure, Thorne (1978) used the

concept of neutral axis common to beams in structural mechanics.

Above the neutral axis, it is assumed that the overhang is in tension

while below it the overhang is in compression. An approximate

location of the neutral axis can be calculated from a Mohr-Coulomb

diagram of the soil material (Appendix I).

At the Limiting condition of a beam failure, both force and moment

equilibrium are satisfied. These conditions are expressed numerically

first by resolving forces horizontcaUy with respect to AA which results

in: , , '.

i

where Tt = tensile strength (kN/m2) T Z = length of tensile zone (nl) Tc = compressive strength (kiY/m2)

C Z = length of compressive zone (m)

and second by tCzking moments about the vertical or horizontal axis

which yields:

where TS, = tensile stress.

Since failure by the beam mechanism is considered to occur when the

tensile stress overcomes the tensile strength, the factor of safety against

failure may be defined as:

substituting equation 2.18 into 2.19 yields:

\

3.6.3 Tensional Mechanism of Failure

Tensional mechanism of failure occ~us across a horizontal plane such

as a bedding plane at the outer fibre below the neutral axis when the

self weight of the ccmtilever overcomes the material cohesion (fig 3.9).

At f d u r e , the part of the overhang below the weak plane is detached

and translates vertically downward. Numerically, this failure

condition is expressed (Thome 1978) by:

where Tr. = tensile strength (kPl/rn2) B = width of overhang (m)

Y = bulk unit weight of soil (k3/rn3) d = thickness of underpart of overhang involved in

tensional failure (rn)

A factor of safety against failure by tensional mechanism is thus

defined as:

The weakness in Thorne's analysis is the implicit assumption that the \

entire overhang is involved in the failure process. This is not correct for

soils in general and as is exemplified in the Niger delta where gradud

disintegration of overhang is common. The part of river bank overhang

that is involved in failure is determined by the location, depth and time

of development of tension crack.

3.7 Probabilistic Methods of Analysis

N1 the stability andysis methods described so far are deterministic, in

that the shear strength of soils, the loadings applied to the slope and

the factor of safety are assumed to have fixed values although in real

field situations, large variations in shear strength and in loading

frequently exist. The probabilistic method which evaluates the factor of

safety in terms of a probabilty of failure attempts to account for the

nahual variability in property. However, because a large number of

tests is normally required to ascertain the variability of the shear

strength, the probabilistic method is rarely used in practice and is also

considered expensive for a developing country like Nigeria.

4. RESEARCH M l i ~ O D O L O G Y

4.1 htrodnction

This research specifically examined bank failures along Rivers Niger,

Nun, Forcados, Orashi and h o and a few of their distributaries (fig.

4.1). These rivers traverse the entire Niger Delh and so provide a

regional perspective.

4.2 Field Measmrnents

Fieldwork to idenbfy soil types and. their spatial distribution was

carried out by land and sea routes to the study area. This led to the

modification of the engineering soil map by Akpokodje (1987).

Based on the modified engineering soil map, the Niger delta was

sub-divided into four zones. These zones correspond to the soil groups

differentiated in fig. 2.8. Typical sites were then selected from each

zone for detailed observation and measurement. These sites include

Ndoni, Akitlima, Agbere, Port Harcorut, Opobo and O w a .

The measurements carried out include soil pro+rties (cohesion, angle

of internal friction, bulk tmit weight), recession rates of riverbanks and

flow velocity. The physical process of bank erosion along a 250x11

Fig. 4.1 Map of the Niger Delta showing drainage pattern

stretch of river bank in each site was carefully observed. The

monitoring of recession was based on indicator pegs installed exactly

10m from the immediate bank at each of the sites. The pegs were

wooded stakes (50 x 50 x 300mn1) driven into the ground. A similar

approach was adopted by Hooke (1979,1980), Thorne and Tovey (1981)

and Hagerty et al (1983). The approach enabled some quantitative

measure of material removal or deposition at the site to be readily

determined.

Site inspection to locate areas of bank instability was carried out on

speed boat and by canoeing along the rivers. Active inactive bank

slides were mapped. h evaluation was made regarding the type of

failure cmd whether or not soil type played a significant role in

inducing failure. The incidence of toe erosion was assessed by

considering the seepage characteristics of the soil, while the location of

each slide was assessed by considering the l~ydrodyn&tics of the river ,'

systems. ?.

Twenty five riverbank sites within the study sea were selected for

detailed studies. The selection of each specific site was governed by the

presence of recent riverbank failures. However, an attempt was made

to select sites that represented alI sub-environment within the large

depositional environment of the delta to observe changes in the pattern

of erosion and bank recession. This was necessary to high-light

specific environmental influences on the problem. In some of these

sites, monitoring schemes were set up and observations made on a

regular basis during the study period.

Water Level Measurement

Channel water level was measured at eight locations I

area as shown in (fig. 4.1). The measurement of water level

commenced with the setting up of Temporary Bench Marks (TBM) and

gauge station (fig. 4.2). Thereafter the water level was measured by

reference to the scale of the gauge on a regular basis. These tasks were

carried out jointly with the Flood and Erosion Division of Rivers State

Ministry of Works and Transport, Port Harcourt. f

Riverbank Profile Measurement ',

Riverbank profiles were measured using regular surveying instruments

including a theodolite, an optical clinometer, a geological compass,

BENCH MARK /'

( b ) IRON

BENCH M A R K

SCALE 1- 1 : 25

WATER GAUGE

Fig. 4.2. Water Gauge System -

tape and ranging poles. The optical clinometer was used to estimate

average bank height and inclination where riverbcanks were

inaccessable. Because it is difficult to detect recessions with this

technique only, measurement was complimented with the use of

reference pegs (Jacob's rods) which consisted of easily identifiable thin

rods with yellow flags. The geological compass was used to measure

the inclination of bank slopes in places where the banks were

accessible. The tape was used for h e a r measurements including the

areal extent of riverbank failures.

C h m d Cross-Section Measurement

The profile of the river channel was determined by echo-sounding from

the water surface at a sufficient number of points along the breadth of

the river. Echo-sounders are electronic devices which measure the

travel t ime of accoustic waves. The principle of the echo-sounder is

based upon:

(i) water being a good medium to propagate dound waves.

(ii) sound waves being very well reflected by the riverbed.

During field use, the echo-sounder was callowed to hang over the side

of an out-board engine boat. As the boat travels across the river

channel the echo-sounder generates transmits accoustic waves to

the riverbed which reflects these waves. The arrival of the reflected

waves on the surface is recorded by a hydrophone. The time lapse

between transmission and reception of an accoustic pulse is a measure

of the depth to the riverbed. From these depth measurements, the

cross-sectional area of the channel can be calculated.

Flow Velocity Measurement

Flow velocities were measured using the propeller-type current meters.

The current meter was introduced into the river and readings on the

meter taken at. the surface and at least three intervals on the vertical to

the riverbed. The procedure was repeated for other verticals in the

cross-section with the same current meter. From these measurements,

a mean velocity (V) was determined using the .4rapezoidd rule

expressed numerically by:

where the parameters are as illustrated in a typical velocity profile (fig.

4.3).

The product of the average velocity (11) and the cross-sectional area

gives the dischc?xge.

So3 Sampling

The selection of a sampling technique in the riverbanks was determined

by the soil type and quantity desired for testing. Both disturbed and

relatively undisturbed samples were takgn for laboratory shear and

permeability tests. Here disturbance has been qualified, for even the

best s a m y h g technique causes some mechanical disturbance (Bishop

1966. Three techniques were used in obtaining relatively trndishubed

samples.

(1) A U-4 sample tube was driven by a scampler head to an

appropriate depth excavated to the surface. The tube was then cleared

up, seded and transported to the testing laboratory.

(2) The 3 8 m diameter cylindrical s m P h tubes with piston

sampler were also employed. In order to avoid the development of

0

'I Fig. 4.3.

f d i .

A

Typical velocity profile in a river channel

large tensile stresses as the sampler was prdled out, two precautions

were observed:

(i) a rest period was doeved, so that f i d i adhesion and friction

between the soil and tube codd develop.

(ii) the scvnyle tube was turned at least two revolutions to shear the

sbil at the bottom end of the tube. OnIy tubes with sufficiently sharp

and tappered cutting edges were employed in order to minimize

sample disturbance.

(3) Rectangular blocks of samples were excavated and trimmed.

This technique was applied mainly to surface sediments. Excavation

of samples was done in order to obtain samples that were minimally

disturbed and whose tested properties would very much replicate

properties of the in-situ soils. Bishop (1966) concluded that although

no specimen may be regarded as entirely undisturbed, carefdly

excavated and trimmed block samples are often the least disturbed.

Field Tests

A number of field tests were carried out mainly at locations where

collection of relatively undisturbed samples was difficult and where

knowledge of the soil properties was necess~wy. The field tests carried

out included: (i) field vane shear tests and (ii) field pore pressure

measurement with- stand pipe piezometer.

Field Vane Shear Test

The field vane shear test was required for the measurement of

undmined strength and the determination of the loss of sbength due to

disturbance or remoulding (sensitivity)

after failure in order to obtain both peak and residual shear strength

values. After each test, the vane was thoroughly cleaned of all clay

adhering to the vane surfaces. This minimised the disturbance effects

for subsequent tests.

The results obtained from the vane test were subjected to a correction

for friction losses in the system. TlGs was carried out by inserting the

torque rods to a typical test depth (but without the vane affixed) and

measuring the torque required for rotation. From the pec& and

residual values of vane shear strength, the sensitivity of the soil (SN)

was calculated using the relationship:

Peak vcme shear strengkh SN = 4.2

Residual mane shear

The values of SN were compared with standard sensitivity values,

developed by Rosenqvist (l953), thus enabling the soil to be classified

in terms of sensitivifiy. High sensitivity values ( ~ 2 . 5 ) imply that the soil

would lose strength sapidly upon disturbance. Nsq, from the vcme /

results, the maximum reduction in shear strerikth upon disturbance

(SR%) was derived using the relationship: I ,'

(v, - v,) . l o o S R =

%J where V,, = peak shear strength

I/, = rcsidual shear strength

Stand-Pipe Pore Pressure Measurement

The set-up for this consisted of a stand-pipe at the end of which was

fitted a porous stone. The stand-pipe was W e d with water coloured

with Potassium Permanganate dye to ensue visibility from a distance

of about 20m (at high water an observer is required to retreat about

20m for safety). The reading of the external water level and the water

level within the stand-pipe was made simultaneously at intervals of 15

minutes. However, at peak flood, observation was terminated because

external water level exceeded the water level, within the tube thus

obscuring i t .

4.3 Laboratory Tests

Laboratory tests to characterise the soils were, m k e d out. These ,

include, index tests comprising Liquid and plastic Limits and natural I.'

moisture content. Others include determination of bullc unit weight,

particle size distribution, shear strength and permeability tests.

Triaxial Shear Strength Tests

Shear strength tests were carried out in a txiaxial and a shear box

apparatus. The triaxlal apparatus used has a capacity of five tons and

consists of a compression load frrlme with a multi-speed drive. The

five ton capacity was considered suitable for normally consolidated

alluvial sediments.

The load and strain did gauges together with the pore pressure system

were linked with transducer and connected to a 24-channel data

logging device enabling automatic recording of readings. These

trcmducers were first calibrated to derive transducer constants which

enabled computer processing of the test results. The BS.1377: 2975, was

the standard adopted in the testing programme. Depending on the

type of test, the gear setting was adjusted to produce a suitable

machine speed as follows:

Quck undrained tests; 0.744 mm/minrrte x

Drained tests; 0.00119 mrn/minute ,

Consolidated undrained tests; 0.744 mm/dnute .

The direct shear test was carried out on a shear box apparatus

equipped with a drive unit, shear box carriage and a load hanger. A

specimen size measuring about 6 c m x 6 cm was used consistently. A

gear combination of 100/50, producing a machine speed of 0.372

mm/rninute was used in all the tests.

The test procedures adopted follow those specified in ASTM.3080.

Readings of the shear and vertical displacement together with the load

dial guage were recorded at intends of 1.0 minute. Where rapid

changes in deformation were noticed, the interval was reduced to

enable an accurate picture of the behaviour of the specimen to be

obtained.

Particle Size Distribution

The pca.rticle size distribution, permeability, moisture content and

Atterberg Limits were determined in accordance with the British

Standards (BS 1377,1975). K

Bulk Density x>

The density of the sediments was determined by the core cutter method

which involves measuring the mass (M) of a fixed volume of cylindrical

core samples of sod.

4.4 Analysis of Everbanks

Both intact and fcziled bank sections were studied and their modes of

failure assessed as either flow slides, rotational, translational, toppling

or a combination of these. The approximate area of each slide was

measured on ground and this provided a basis for assessing the overall

slide activity in a pcuticular reach of the river. On the basis of field and

analytical evidence a probable cause of instability was proposed.

After a mechanism of bank failure was id.entified, the sensitivity of the

bcmk to various factors was analysed using a deterministic technique.

Estimating The ImporE;ance Of Factors Affecting Channel Bank Failure

The imp~rt~ance of various factors czffecting the stability of channel

banks can be estimated from a sensitivity analysis. Rmachandran

lczrge number of tests or observations are made. Thus the reliability

approach is uneconomical, especially in a third world country like

Nigeria. For these reasons, a deterministic approach was preferred in

this study.

Using the deterministic approach, the effect on the factor of safety of

hdividual pczrarneters was determined by partial differentiation of the

factor of safety (FS) (equation 3.8) with respect to each pczrarneter. This

will result in a number of equations expressed in terms of observable

and measurable parameters such as cl~annel water level (K.), angle of

internal friction @, cohesion (C), bank height (H), and pore pressure (U).

For example, the effect of cohesion on the factor of safety against

rotational failure of channel bank will be given by:

d F - - L ac (rv, c W2) sin CY

To obtain the effect of pammeters which are not

equation 3.8, the equation is re-expressed in a

/

directly reflected in

way, such that the

parameter can be related to the factor of safety directly. For example,

to derive the effect of bank height and channel water level on the factor

of safety, the term (W, + W,) in equation 3.8 is replaced with

(H - LVL) LI; Yl + \,IfL . C* , Y2 where H = bank height

f,; = width of slice Y1 = unit weight of soil above water b e 1 Yz = unit weight of soil below water level

iVL = channel w ~ t e r level

A sensitivity coefficient for each such variable can then be obtained.

Correlation and interdependences between any factors were taken

account of by multiplying the sensitivity coefficient by an appropriate

correlation coefficient. Because the calculated sensitivity coefficients

may vary widely, it is usually convenient to normalise them+to range

beteween I and 0. This enabled graphical comparisons to be carried

out since d coefficients wiU then be of the same order of magnitude,

The method of normalisation employed expressed the relative

sensitivity ( p ) in the form: /

A sensitivity coefficient may be positive or negative. A negative

coefficient implies that an increased value of the variable will reduce

the factor of safety. Conversely, a positive sensitivity coefficient

suggests that an increased value of variable wiU enhance stability. The

higher the .sensitivity coefficient, the more important the variable is to

the stabiLity of the river bank. According to Akpokodje (1995) a

comparable resuIt can be achieved using step-wise multi-variate

regression analysis.

5. RESULTS

5.1 FlXLD MEASUREMENTS

5.1,1 Water Level

A seasonal vcviation of water level was recorded for the rivers studied

in the Niger Delta. The water level began to rise soon after the on-set of

the rainy season in April and reached a peak about the f i s t half of

October (fig. 5.1). During this period, the rate of channel water rise

varied slightly and averaged 0.083 m/day. The dates at which the

water level reached its peak in 1988 varied between 15 October to 17

October (table 5.1). From the second hcdf of October, the water level

began to fall, attaining a m j n i m r m value t o ~ w d s the end of January of

the following year. Thereafter, the water level remained fairly constant

until about April of the following year. The average rate of fal l is 0.25

m/day which is approximately 3 times the rate of rise (table 5.1).

h tidal areas, e.g. Opobo, a semidiurnal tidal cycle is usually

experienced in which the water level attains two peaks and a low

within 24 hours (fig, 5.2). Two tidal seasons (the spring and neap

tides) are distinguished. The water level at sprhg tide varied over a

range of approximately 2m, whereas at neap, the tidal range is reduced . /

to about 1.2m. The durations of rise and fal l of water level in the tidal

areas are fairly equal.

Monthly variation of water level at selected locations dong

River Niger and its distributaries (IFERT)

WATER LEVEL'^.' REFERENCE

TO h lSL( rn ) 0.8

Fig. 5.2. Variation in water level in Oyobo Town (Data from

NEDECO, 1980)

Table (5.1) Water Level Variation in Parts of the Niger Delta For 1988

Rived Maximum Grcck Wxtcr

Abovc

River Niger

River Nun

River Xun

Ki vcr Nun

EkoIe Greek

Ekole Creek

River

6 Ocr..

15 Oct.

15 Oct.

9 Oct.

17 Oct.

17 Oct .

16 Oct.

15 Jan .

20 Jan.

20 Jan.

7 Jan.

21 Jan .

21 J a n .

20 Jan.

Fate of iicc c ~ k n j/w Oct. & 30 J;m. (M/d:ry)

0.110

0.271

0.296

0.100

0 -229

0.221

0.250

At the boundary of the tidal zone with the fresh water zone, there are

periodic reversals of flow direction and water level (fig. 5.3). Between

January and August, the tidal influence on water level is significant,

with flow directions regularly reversed. From September to P-

November, the reversal of flow was no longer experienced due to the \

overwl~eelming impact of the flood water from the upstream of the river

system.

5.1.2 Riverbank Profiles

Measurement of riverbank profiles was carried out in locations dong

three major river routes in the Niger Delta, namely; Rivers Niger, Nun

and Orashi (fig. 4.1). The regional stratigraphy along the river routes is

as shown in (fig. 5.4). Various riverbank forms were observed (fig 5.5).

The riverbanks range in height from 1.2m to about 15m. The highest

baxk occur upstream. Towards the sea, the bank height is reduced. In

Ndoni for instance, some banks measure 15m in height whereas in the

most southerly parts of the delta, e.g. Bonny, the bcmks are generCdy

less than 3.5m in height.

A common feature of the riverbanks irrespective .. of the

sub-environment is their steep bank angles which are generally above

45 degrees. The top sections of the bank profiles are nearly always

vertical. Sometimes, the banks are obkse in inchnation as illustrated

by the slope profiles in Ndoni (fig 5.5a), Agbere (fig 5 .5~) and #-

Agudma (fig 5.5d). To a large extent, the Gank profiles me ':

geologiccaUy and stratigraphically controlled as was earlier reported by

Henkel(1867). The granular and more erodible &ata tend to be gently

inclined while the cohesive layers tend to be steeply inclined.

Fig.5.6. Changes in Riverbank profile at Agbere

Fig.5.7. Changes in Riverbank profile at Kaiama

Y E N A G O A

Fig. 5.3. Monthly variation in water levei at a typical tidal

boundary (Perernabiri) in the Niger Delta (IFERT)

Fig. 5.4 Regional stratigraphic models of riverbank in the study

area

The bank faces and crest are frequently cracked. Cracks as deep as

1.2m are common. Most bank faces have accu~xundations of debris

arising from bank failures. The slide areas range from 29m2 to 81m2.

The toe area of the banks are gener'dly steep (usually between 80 and

90 degrees) due to constant presence of wave activity. Profiles of three

bank sections located in Agbere, Kaicvna and Yenagoa monitored

between 15 November, 1988 and 30 November 1990 are shown in figs

(5.6-5.8). Clearly, recession is apparent in each of the banks. However

the amount of recession is uneven although these bculks are located

dong the same river, and have comparable soil type and bmk height.

This is due to differences in the soil properties, pore pressure,

seconbcuy currents and boundqr shear stress associated with the

meandering nature of the rivers.

Bank profiles appear to reflect the combined actions of all geomorphic

processes in operation in the area. A dose exahination of the

geornorphic processes in relation to bank fahure have led to the

identification of toe erosion, soil type, high current velocity as some of

the pertinent factors causing recession. Although granular and less

cohesive strata within bank profiles tend to be more erodible, factors

other than soil type clearly contributed to observed recession. This is

exemplified by one of the monitored bank profiles (fig, 5.7) in which

the preferential erosion of the bottom sandy strata resulted in

undermining = ~ d consequent mass failure and retreat of the bank.

5.1.3 Flow VPl~city

Flow velocity and flow depth were measured by IFERT (1983) in

Agbere, Sabagreya, Amassoma and Egbecli. Shdtaneous

measurement of velocity and flow depth was necessary to calculate

discharge as well as investigate any possible relationship between the

~ M T O variables. Discharge values cdcdated for these locations at

different times are presented in table 5.2.

At Agbere, four sets of flow velocity measurements were made across

the river cros+section (fig. 5.9). The highest velocity was recorded just

before yec& flood on 11 September 1983. Although tKe flow velocities

varied, the average flow velocity for each measurement was

consistently higher than 0.75 m/sec. ,,

Fig. 5.8 Changes in riverbank profile in Yenagoa

The result of flow velocity measurement at Sabagreya showed wide

variations in time and across the channel cross-section (fig. 5.10). The

highest velocities occur at the steep concave banks. Towards the

convex bank, the velocities are reduced at average rate of about 2.2 x

lo5 m per second. As the water level increased, the flow velocities and

discharge dso increased. Average flow velocities increased from 0.62

m/sec by 9 July to 1.10 m/sec by 7 October at peak flood and fell to

1.01 m/sec by 18 October, 1983.

At Amassoma, the average flow velocity varied with time from 0.41

m/sec by 7 July to a peak of 1.09 m/sec by 5 October (fig. 5.11). The

flow velocity fell thereafter to 0.73 m/sec by 19 October. .. Velocity

across the channel at any given t h e was however fairly constant.

At Egbedi creek, the flow velocity was somewhat evenly distributed

across the channel (fig. 5.12) with the maximum velocities occuring

during peak flood. At low water, velocity estimates obtained by timing

the displacement of floating objects show averag'e values of 0.22 m/sec.

,.'

DISTANCE FFlOU L E F T R l h S

Fig. 5.9. Velocity profiles a c m s Nun River at Agbere (Dah from IFERT , 1983)

b 9 J u l y ,983

Fig. 5.10. Velocity profiles across River Nun at Sabagreya (Data from WERT ,1983)

Fig. 5.11. Velocity profiles across Oguobiri Creek near hmassoma (Data from I F ~ R T ,1983)

I I I 1 I i I I 20 4 0 60 80 100 I 2 0 140

DISTANCE FROM L E F T BANK

Fig. 5.12. Velocity profiles across Egbedi Creek (Data from IFERT , 1983)

Table (5.2): Average River Discharge From Selected Cross-Sections at Different

periods (Data from IFERT, 1983)