geo-risk analysis of slopes bounding a deep gully erosion ... · pdf filegeo-risk analysis of...

CASE STUDY

Geo-risk analysis of slopes bounding a deep gully erosion sitein Uyo, Akwa-Ibom State, Southeastern Nigeria

Abidemi O. Ilori1 • Muyeed A. Wadud2 • Eric E. Ese2

Received: 8 December 2016 / Accepted: 28 March 2017 / Published online: 5 April 2017

� Springer International Publishing Switzerland 2017

Abstract The stability of slopes around a deep gully ero-

sion site is the subject of this article. The site geology is the

Coastal Plain Sands. To carry out stability analysis, the

slopes were first characterized by deployment of cone and

standard penetration tests (CPT and SPT) equipment. The

slopes are highly stratified, and consist of numerous layers

up to the depth of investigation which is about 37.0 m. The

slope is made up of five layers of materials up to the gully

valley floor. The soils of the five layers in succession from

the top are clayey sand (SC), clayey/silty sand (SC–SM),

gravelly clay (GC), poorly graded gravel with well graded

sand mixture (GP–SW), and well graded sand (SW). Two

locations on the eastern side of the of the gully site which

poses serious risk to lives and properties were chosen for the

analysis. The first position with the highest slope of 16.0 m

gave a critical factor of safety less than 1.5 based on sim-

plified Bishop method of analysis indicating unstable slope,

while the second location results in a factor of safety greater

than 1.5 suggesting a more stable slope, although there is a

potential of sheet erosion on the slope at this location.

Keywords Uyo � Gully � Erosion � Stability � Slopes � SPT

Introduction

Nigerian Southeastern regional area has natural valleys

whose depths are breathtaking relative to the surroundings

area. Some of these valleys are active erosion sites

[1, 2, 13, 27], while others are not. The formation of these

deep gullies is still a subject of active research. Uyo city

is the capital of Akwa-Ibom State, one of the states in

Southeastern Nigeria, which has three deep gully erosion

sites all located in the Northern half of the city. On to the

floor of one the gullies, empties a concrete channel. This

channel serves as drainage outfall to a number channels

carrying storm waters from nearby streets and settlements.

The slopes bounding this gully are being gradually eroded

and the stability of the nearby structures and facilities like

roads is being threatened. A number of civil works pro-

gramme were proposed to mitigate the threat to stability

of the surrounding settlements. These include three

retaining walls at different locations along the gully, a

stilling basin, and drainage channel on a rim of one the

slopes that will gradually transport runoff down the slope.

Sand bags were placed on the rim bounding western side

of the slopes to keep it stable. The vertical height of this

rim which is from the crown of the slope to the valley

floor is about three to four meters. The other rim of the

slope (the Eastern side) is bounded by built areas con-

sisting of residential, commercial and religious buildings,

and is such that the crown of the slope to the valley floor

is at an average height of about 16.0 m. The slope on this

side cannot be worked on without affecting some of the

structures; thus, the stability of these bounding slopes at

the near and far end of the gully is the object of this

Electronic supplementary material The online version of thisarticle (doi:10.1007/s41062-017-0056-9) contains supplementarymaterial, which is available to authorized users.

& Abidemi O. Ilori

[email protected]; [email protected]

Muyeed A. Wadud

[email protected]

Eric E. Ese

[email protected]

1 Department of Civil Engineering, University of Uyo, Uyo,

Akwa-Ibom State, Nigeria.

2 Nigerpet Structures Limited, G32 Ewet Housing Estate, Uyo,

Akwa - Ibom State, Nigeria.

123

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article. Some of these can be seen in Fig. 1, which shows

a general layout of the site.

A number of studies have been carried out on erosion

and associated deep gully. This includes: Afegbua et al.

[2], who reports on gully erosion in Auchi, Nigeria. The

area affected by gully erosion was mapped using satellite

imagery, and slopes adjoining the gully (some with resi-

dential settlements) were judged to be unstable. Some part

of the buildings has slid into the gully, while others are

highly threatened. The depths of the gullies were not stated.

There was no indication of determination of factor of safety

for the slopes with threatened houses, and there was no

forecast or prediction concerning the stability of the gully

slopes.

Ilori [15] and Ilori et al. [16] reports on a deep ravine,

with no active erosion. In the report, the geotechnical

investigation on a proposed extension to the existing run-

way of Calabar International Airport, located in Calabar;

which is about 100 km North of Uyo was the focus. The

extension is to pass through a deep ravine that is about

25 m deep and spans about 410 m. The focus was

geotechnical investigation of the ravine. This was carried

out using the German light weight penetrometer, the LRS

10, shallow hand auger boring, for soil sample collection,

and geophysical seismic refraction survey. This ravine

though deep does not have any erosion site. The ravine was

subdivided into Northern and Southern flanks, and valley

floor. The slope on the Southern flank is the steepest at

about 30�. This was observed to be stable, since no bulge,

crack or slide of any soil mass are visible on the slope. The

ground water was not encountered in the shallow boring

made on the valley floor. The orientation of the ravine is in

the South West–North East (SW/NE) direction similar in

orientation to the ravine in this study.

Some reported works on slope stability are mostly on

regional scale. They involve predicting landslide from

previous landslides inventory in the same area, integrated

with rainfall data, some satellite imagery and so on.

Ogbonnaya [26] studied landslides on both sedimentary

and metamorphic terrains using topographic maps, aerial

photographs and geologic field surveys. He concluded that

on the sedimentary terrains landslides consist of mainly

shallow, low-volume movements, material slumps and

short runout slides; whereas on the metamorphic terrains,

complex translational and rotational movements and

mudslides on steep slopes which sometimes involving a

TREET

RE

TAININ

G W

ALL

BH3, CPT 3

BH6, CPT 6

0 10m

5m

BOUNDARY OF GULLY EROSION

BOUNDARY OF GULLY EROSION

Fig. 1 General layout of the erosion site

8 of 19 Innov. Infrastruct. Solut. (2017) 2:8

123

combination of slide and flow with curved head scarps and

slicken sided shear surfaces. His conclusion was that

‘looseness’ of slope materials and their relatively low

strength parameters account for the dominance of land-

slides on the sedimentary zone.

Luo et al. [22] considered the effect of the overland

water flow as one major mechanism that initiates rainfall-

triggered shallow landslide. A model was developed for the

effect of overland flow on slope stability, which was

applied and used to predict stable slope or otherwise in

Dujiangyang area that is on the west side of Chengdu city

in Sichuan Province, China. The area regularly experiences

landslides triggered by rainfall. Using geological data and

satellite imagery, a map showing distribution of unsta-

ble slopes taking into account overland flow was developed

for the area.

Further use of satellite imagery in studying and mapping

geological features in three dimensions was reported by

Marghany [23], where imagery from LANDSAT TM

(Landsat Thematic) and the fuzzy B-splines algorithm were

used to identify 3-D lineaments on section of United Arab

Emirates terrain.

Marghany [24] also simulates 3-D visualization of

coastal erosion, using ENVISAT (Environmental Satellite)

synthetic aperture radii (SAR) imagery. The imageries

were used to estimate coastal erosion rate which was put at

3.5 m per year.

While these methods are suitable for studying and

mapping geological processes and features and can also

forecast such, their use as a sole tool to quantitatively

evaluate stability of slopes and compare same with tradi-

tional tools of slope stability analysis has not been rea-

sonably explored.

Pardesh et al. [30] did a classification of different

techniques used in predicting landslides on regional scale,

which are listed as heuristic, semi-quantitative, quantita-

tive, probabilistic and multi-criteria decision-making

process.

Other reported works on slope stability involve mostly

back evaluation of already failed slopes giving the reasons

why such slope fails, (Omar et al, [28]). The present case is

site specific (not regional) and unique in that it evaluates

and attempts to predict stability of slopes bounding a deep

gully erosion site using geotechnical methods, and dis-

cusses the civil engineering facilities that need to be put in

place to prevent potential slope failure.

Site description, climate and geology

Akwa-Ibom State lies approximately between latitudes

4�2904300N and 5�2904300N and longitudes 7�290700E and

8�170700E. The geographical coordinate 5�20600N,7�5202200E locates a central point on the erosion gully

valley floor. The gully erosion geographical orientation

is in the Southwest/Northeast (SW/NE) direction. The

gully erosion site is bounded in the south west by a

major highway and system of drainages, the North and

South by buildings, and the North eastern end is the

gully erosion outfall. The gully site stretches for about

500 m from South to North. The Eastern side is more

eroded than the western side and this side also has sig-

nificantly built-up areas, which are being threatened by

the erosion. At the near end (Southern), the depth of

gully stands at about 16.0 m which progressively reduces

to about 8.0 m at far end or outfall (Northern end). Both

the western and eastern ends of the gully have extensive

property development, but the development at the east-

ern end is closer to gully than that of the western end

and are therefore at risk should there be a failure of the

slope. Figure 1 shows the general layout of the site,

while Fig. 2 shows the elevation contour of the site with

some other details.

The area is characterized by distinct dry (November–

March) and wet (April–October) seasons. The mean annual

rainfall of the area varies from 1000 to 2560 mm with

mean annual temperature in the range of 22.4–30.1 �C[14].

The site is characterized by Coastal Plain Sands or

Benin Formation, one of the formations that constitutes the

tertiary—recent sediments of the Niger Delta [31]. The

lithology of the Coastal Plain Sands according to Allen [3]

consists of fine-grained sands, pebbly, moderately sorted

with local lenses of fine grained poorly cemented sands and

gravels with clay and shale intercalations. The sands are

sub angular to well rounded.

Study objectives

The study objective includes;

• Geotechnical characterization of the erosion site, valley

floor and adjoining slopes.

• Identify the presence or otherwise of ancient or past

failure surface.

• Setting up geotechnical and geometrical model of the

slope.

• Slope stability analysis of the model setup.

Geotechnical characterization

Site investigation and relevant laboratory work were car-

ried out on the site and on samples to obtain the geotech-

nical properties of the soil at the site, their sequence of

occurrence and their lateral extent. The geotechnical

investigation involved six standard penetrations tests (SPT)

Innov. Infrastruct. Solut. (2017) 2:8 of 19 8

123

borings, and six cone penetration tests (CPTs) soundings.

Two each of the SPT boring were deployed on the eastern

and western half; one each at the early and far reaches of

the bounding slopes. The remaining four were spaced out

along the valley floor at possible location of potential

erosion control structures. The CPT soundings were loca-

ted within close proximity of the SPT borings; Figs. 1 and

2 show some of these positions. Although the CPT results

were not reported upon here; this is because the refusal

depths are less than five meters in most of the soundings,

and are not useful for adequate interpretations. Disturbed

and undisturbed soil samples obtained from SPT borings

were analyzed and tested in the laboratory. Test carried out

on them includes sieve analysis, Atterberg limits, uncon-

solidated undrained triaxial compression tests, direct shear

box tests, consolidation tests. Sieve analysis results for

some samples obtained at SPT locations 3 and 6 are pre-

sented in Fig. 3. All tests were carried out in accordance

with relevant ASTM standards.

The SPT borings were carried out up to 20.0 m, and a.

2.5 ton Guada cone penetration equipment was deployed

for the CPT. The relevant geotechnical test results required

for slope stability analysis will be presented with the

analysis; however, the following is a summary of the soil

profile as indicated by the SPT borings, and Figs. 4 and 5

show the plot of the SPT values for locations 3 and 6, and

the other four locations on the valley floor.

Soil profile, correlations, and SPT values plot

The soil profile and its geological make-up are essential to

the search for potential failure surface within a slope and

places adjacent to it. In the light of this, it is essential to

map the different soil types and layers that are present on

the site. From the SPT test boring log carried out, the

lithological make-up of the Eastern flank of the slope and

the valley floor can be detailed as follows:

Layer 1 Dark brownish clayey sand, classified as (SC)

using the Unified Soil Classification System, of ‘loose’

consistency with SPT ‘N’ value in the range of 7–9,

occupying a depth range of 0.0–2.0 m.

Layer 2 This layer consists of brown lateritic clayey and

silty sand (SC-SM) from 2.0 m depth to 9.0 m; medium

consistency with SPT N value in the range of 10–15.

Layer 3 This is made up of brownish clayey gravels

(GC); the gravels are predominantly fine. This is in the

depth range of 9.0–11.0 m in boring 3 and is not correlated

across the boreholes 1 and 4 that are drilled on the valley

floor. SPT ‘N’ values for this layer are in the range of

15–17, indicating medium consistency.

Fig. 2 Contour map of site showing location of SPT borings Nos. 3 and 6


123

Layer 4 Consists of well-graded sand, and poorly graded

gravels (GP–SW), occupying a depth range between

11.0 and15.0 m from boring 3. SPT ‘N’ value ranged from

17 to 27, indicating medium consistency. This layer is also

not correlated across the valley floor from boreholes 1 and

4 logs which have three layers of similar lithology and

depth of occurrence, which are light brownish gravely

clayey sand layer (GC–SC), well-graded sand layer (SW),

and well-graded sands and gravels (GP–SW).

Layer 5 this layer is made up of well-graded fine sand

(SW) occurring in the 15.0–20.0 m depth range. The layer

has SPT ‘N’ values in the range of 30–36 indicating dense

consistency. This layer represents the second layer on the

valley floor and is the only stratum that has the same

lithology on the slope side as well as the valley floor.

There is reasonable correlation of soil strata among all

the SPT borings on the valley floor; Fig. 6 presents this

correlation. The plot of SPT ‘N’ values from the valley

floor (borings 1, 2, 4, and 5) presented in Fig. 5 shows

similar log pattern except the log of SPT 4 which shows an

increase in soil consistency with depth and is similar to that

of borings 3 and 6. The valley floor is made of materials

whose consistency ranges from mid-medium to dense as

shown in Fig. 5 and the soil profile consists of alternating

medium dense to dense soil layers. This trend is discern-

able in borings 1, 2, and 5, with exception at boring 4, in

which the SPT log signature shows progressively dense soil

from top to bottom beginning from mid-medium dense to

very dense range.

Whereas there is almost a perfect correlation between

boreholes at SPT3 and SPT6 locations, there is only one cor-

relation between the boreholes on the eastern flank of the slo-

pe and boreholeNo5 on the valley floor (Fig. 7).Also boring 4

on the valley floor has at least two layers that can be matched

with the two bore holes on the slope, as presented in Fig. 8.

From the preceding, there is a general increase in the SPT

‘N’ values with depth within the Eastern flank of the slope

and the lithologies of the soil strata are also different from

those of the valley floor except layer 5 which cuts across

both sides. This suggests that the soil on the valley floor

could have been deposited at about the same time with those

on the Eastern flank but under different deposition energy.

Soil strength parameters and reliability

The undisturbed samples obtained from SPT sampler were

tested to determine the soil strength parameters among other

parameters. Two situations are considered for stability

0

20

40

60

80

100

120

0111.010.0Grain sizes(mm)

Per

cent

age

pass

ing(

%) R50 line

R15

BH6 1.0 mdepth

BH6, 7.0m depth

BH3, 1.0m depth

BH3, 3.0m depth

Fig. 3 Sieve analysis results from SPT borings 3 and 4


123

analysis. The first being the analysis of the slope that is

termed ‘construction period’; and the second after the slope

or adjoining valley has been worked upon termed post-

‘construction period’. For the construction period, uncon-

solidated undrained triaxial tests were performed on the

samples, while for the post-construction period consolidated

drained direct shear tests were performed on samples to

determine their strength parameters. This was carried out for

soil from BH3 which has a slope with threatening height.

These parameters are presented in Tables 1, 4 and 5.

The soil strength parameter values are affected by sample

size, sample disturbance, which depends on the method of

how the sample is obtained. According to a review of shear

test results by Dirgeliene [8], to obtain reliable values of

shear parameters, the ratio of specimen height to diameter of

sample should be reduced from the usual 2–1 to eliminate

friction between the sample ends and the plates. He also

noted that triaxial test results give lager values for sandy soil

than the direct shear test. From the results of the test from the

two boreholes BH3 and BH6, and correlation, the same soil

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40D

epth

(m)

SPT 'N' Values

SPT3

SPT6

Series3

Series4

'N'=10

N=30

Fig. 4 Standard penetration test ‘N’ values plot for location 3 and 6


123

layers are manifest at the two locations. The shear strength

parameters determined with triaxial tests at the two locations

are not widely different for each of the soil layers as pre-

sented in Tables 1 and 4. The strength values obtained from

direct shear test for the same layers for BH3 are smaller than

the ones from triaxial tests, which is in consonance with

statements made above with respect to results for sandy soil.

Slope analysis procedure

Ancient or previous landslide

In assessing a slope for stability, one of the errors (called

seven deadly sins) listed by Conforth [6] is the non-

recognition of past landslide and its failure surface. He

indicated that the presence often indicates high probabil-

ity of another occurrence. The Inclinometer is often the

primary tool for recognizing such failure surface when

used in drilled borehole for site investigation. A sec-

ondary tool is the SPT. Inclinometer was not available in

this case so the SPT was examined to detect such failure

surface. The presence of a previous landslide surface will

be indicated generally by soil of low shear strength when

Bore Hole Shear Test (BST) is carried out. Synonymous

with that is a soil with ‘very loose’ to ‘loose’ soil con-

sistency at some depth. A check of the entire SPT plot

does not present such situation; hence, there is no previ-

ous landslide surface.

0

5

10

15

20

25

0 10 20 30 40 50 60

Dep

th(m

)

SPT 'N' Values

SPT 1

SPT 2

SPT 4

SPT5

N =10

N= 30

Fig. 5 Standard penetration test ‘N’ values plot for location 1, 2, 4, and 5


123

The two locations indicated by the geotechnical inves-

tigation as Boring No 3 and Boring No. 6 also designated

as BH3 and BH6, respectively, were locations chosen to

setup models for the slope stability analysis. These two

locations are said to be ‘unaffected’ by the erosion yet.

BH3 is located in the early reach of the gully erosion while

BH6 is towards the lower reach of the gully. The two points

are on the eastern side of the gully, going into the gully

from the main road, (that is from south to north).

The different soil layers and their thicknesses at the

boring locations as indicated by the geotechnical investi-

gation were used to set up the stability analysis model.

30

40

50

60

70

80

Gravel Clayey Sand

GC - SC

GC - SC

GW - SW11.0 m

10.0 m

GW

GW - SWSC - CH 12.0 m

GC - SC

SC Dark Brown Sand

SW Well Graded Sand with Fines

GW Well Graded Sand with Gravels

GC Clayey Gravel

GW - SW Well Graded Sands & Gravel

SC - SM Light Greyish Sandy Silt/Clay

BORE HOLE

ELEV

ATI

ON

(m) SW

GW -SW

SW

GW -SW

SWGC

SW

SW

GW -SW

SW

GW -SW

SW

BH1 BH2 5HB4HB

ELV. 69 mELV. 65 m

ELV. 61 m

ELV. 47 m

1.0 m

12.0 m 6.0 m

12.0 m

18.0 m

1.0 m

9.0 m

14.0 m2.0 m

13.0 m

18.0 m

Fig. 6 Correlation of soil layers within the valley floor as indicated by boreholes 1, 2, 4 and 5

30

40

50

60

70

80

2.0 m

10.0 m12.0 m14.0 m

GW

GW - SWSC - SM

15.0 m

10.0 m

SC

GC

GW - SW

SC - SM

2.0 m

12.5 m

SW

BH 5 BH 6

SC Dark Brown Sand

GC



Clayey Gravel

GW - SW Well Graded Sands & Gravel

SC - SM Light Greyish Sandy Silt/Clay

SC - SM Brownish Silty Sand & Clayey Sand

BORE HOLE

ELEV

ATI

ON

(m)

ELV. 47 m

ELV. 76 m

SW

GW -SW

Fig. 7 Correlation of single soil layer of borehole 5 on the valley floor with borehole 6 on the eastern slope


123

From the topographical survey of the area, the slopes cross

sections at the two boring locations were extracted and

were superimposed on the lithological drawings of the

location. These drawings are designated Figs. 9 and 10,

representing BH3 and BH6, respectively. The drawings

were set up using Autodesk Autocad 14 Educational soft-

ware [4]. This software has the capability of extracting

areas, angles and distances very accurately. From the the-

ory of slope stability, potential failure circular slip surfaces

were drawn on both BH3 and BH6 with different radius,

the centers of which are different and not collinear either

vertically or horizontally.

Stability analysis

The stability analysis is carried out for both construction

and post-construction period. A number of methods of

analysis have been developed over years which are detailed

in literature. These include: Fellenius method of slices [9],

Bishop simplified method [5], Janbu generalized method

[18], Janbu simplified method [19], Morgenstern and Price

method [25], and Spencer method [32], 1967). Each of

these methods starts with the basic method of slices anal-

ysis, in which particular assumption on inter-slice forces is

made in each case. For example, the simplified Bishop

method of slices assumes that the resultant of the inter-slice

forces is horizontal and has no inter-slice shear forces;

simplified Janbu method assumes the same about the

resultant inter-slice forces, but a correction factor is used to

account for inter-slice shear forces). Most of these methods

assume non-strata, homogenous soil.

From the geotechnical investigation, the slope being

analyzed consists of different soil types occurring in layers.

The ground water table was not encountered during the

SPT borings.

Before analyzing the slope for stability, the lithologies

and apparent stiffness of the slope materials were first

examined from SPT borehole data. These were carried out

40

50

60

70

802.0 m

9.0 m

11.0 m

1.0 m

9.5 m

15.0 m

11.0 m

SC

GW - SW

2.0 m

13.0 m

GW - SW

11.0 m

15.0 m

GC

SC Dark Brown Sand



Clayey Gravel GC

GW - SW

SC - SM

Well Graded Sands & Gravel

Light Greyish Sandy Silt/Clay

GC - SC Gravel Clayey Sand

BORE HOLE

ELEV

ATI

ON

(m)

ELV. 80.0 m

SC- SM

SW

GW - SW

SC

SW

SW

GC - SCELV. 61.0 m GC- SC

SW

SC- SM

ELV. 76.0 m

BH 3 BH4 BH6

Fig. 8 Correlation of borehole 4 on the valley floor with boreholes 3 and 6 on the eastern slope

Table 1 Soil properties utilized in stability computation for slope at BH3

Layer Depth of

occurrence (m)

Soil

type

Unit weight,

c (kN/m3)

Angle of internal

friction (undrained)

Øa (�)

Soil cohesion

value (undrained),

C, (kN/m2)a

SPT ‘N’

values

I 0–2.0 SC 18.1 10 28 7–9

II 2.0–9.0 SC–SM 18.2 12 35.2 10–15

III 9.0–11.0 GC 18.9 17 30.4 15–17

IV 11.0–15.0 GW–SW 21.7 32 0.0 17–27

V 15.0–20.0 SW 22.1 32 0.0 30–36

a Values obtained from undrained-triaxial test. Angle of slope 35�


123

to determine the existence or otherwise of the potential

failure surfaces. Classical potential failure surfaces include:

• Soft soil occurring within the stronger soils that makes

up the slope,

• Hard soil stratum occurring within softer soil strata.

In both cases, the strong and the weak strata of soil must

daylight into the slopes for them to be the slip surface. To

determine whether a soil stratum is soft or hard and is on

another hard or soft stratum, recourse is made to the SPT

‘N’ values. The first situation can be the case in practice

where:

1. A soil stratum which has ‘N’ value is in the ‘‘very

loose’’ or ‘‘loose’’ range and is lying between two

strata which has SPT ‘N’ value in the extreme medium

(‘N’ is equal to or between 25 to 30); or the two outer

bounding strata have dense or very dense consistency

or;

2. The stratum has the ‘N’ values in the loose to medium

dense and is between strata with extreme values in the

dense and very dense (‘N’ is equal to or is between 45

and 50 or greater).

The second situation is a reverse of the (1) and (2)

above, which is a stronger stratum lying between two softer

soil layers. The lithologies of the strata and their consis-

tencies as indicated by the SPT ‘N’ values for the slope

material do not present any of the above classical situa-

tions. However, layer 1 loose consistency and layer 2 and 3

early to mid-medium dense consistency present a loose

situation, on extreme medium dense layers 4 and dense

layer 5 at the two borehole locations. But this is more

pronounced at location 6 as evident from the SPT log

signature in Fig. 4 in which layers 1–3 have a kick towards

the negative side and layers 4 and 5 have a positive bell-

shaped kick. The apparent loose consistencies of layers 1–3

soil make it easily erodible if the slope and hydraulic sit-

uations are favorable. Layer 4 surface is, therefore, a

potential slip surface, although a potential circular slip

surface is assumed for analysis as against plane failure

surface since a circular arc slip surface gives a lower factor

of safety than a planar one. Layer 1 exists only in the upper

reaches of BH3 and not on the slope. It also covers only

small area around BH6.

For non-homogenous soil like the one under consideration,

the Simple (Swedish) method of slices and simplified Bishop

Layer 1, C1, Ø1, γ1 2.00

7.00

4.701 2 3 4 5

6 78

910

11 12

15.70

2.00

37°

w1

w735°

w9Layer 2, C2, Ø2, γ2

Layer 3, C3, Ø3, γ3



A2

A3

A4

w5

Fig. 9 Model to illustrate simple method of slices equations


123

method of slices were the methods employed in the analysis.

Total stress analysis was considered for the construction

period. Using these methods of slope analysis, a circular

potential failure surface which passes through the toe of the

slope is assumed. There are infinite numbers of such potential

failure arcs, having different radii. The arc that gives the least

factor of safety is assumed to be critical one; locating this arc

is one of the main objectives of slope analysis.

The simple method of slices involves straight compu-

tation if relevant parameters are known, while the simpli-

fied Bishop method of slices involves trial and error

solution starting with an assumed factor of safety, ‘F’,

which is used in computation to obtain an ‘F’. When the

‘F’ obtained is the same as the one assumed that ‘F’ rep-

resents the true ‘F’.

The simple method of slices involves:

• Assuming a potential circular of failure surface.

• Dividing the slope into a series of slides of equal width,

with their end on the sliding surface.

• Estimating the weights of individual slide and summing

their resolved magnitude along the failure surface as the

driving force.

• Determining the total shear resistance offered by the

failure surface to the driving forces by the weights of

the slides.

The factor of safety, F, is given for a homogenous soil by:

F ¼ CLþ tanuPi¼n

i¼1 NiPi¼n

i¼1 WiSin hi; ð1Þ

where C is the effective soil cohesion value which is

modified for a layered soil (ref Fig. 9) as:

C ¼ C1 þ C2 þ � � � þ C5 and

L ¼ L1 þ L2 þ L3 þ � � � þ L5;

whereC1,C2,…,C5 are cohesion values for layers 1,2,3,…,

5 and L1, L2, …, L5 are total length of the failure surface

passing through each layer of soil, respectively (which from

Fig. 9 is teal blue for layer 1, pink for layer 2, red for layer 3,

and yellow for layer 4). L is the total length of the failure

surface, hi is the angle between the tangent of the bottom of a

slice and the horizontal, N is the normal reaction due to the

weight of each slide, porewater pressure, and frictional shear

forces between the slice and the failure surface, and

N = WiCoshi - uiDli, Wi = weight of each slice for

homogenous soil, and for layered soil,Wi is the unit weight

of each layer multiplied by the area of the slide within that

layer. For example, with reference to Fig. 9,

W5 = c2A2 þ c3A3 þ c4A4, ui ¼ effective angle of internal

friction of the soil which for layered soil is taken as the angle

of friction for that layer applied to the sum of net forces,

(WiCoshi - uiDli) cutting the failure surface in that layer.

While in simplified method of slides, moments of the

weight of individual slices are taken about the toe of the

slope and factor of safety is calculated based on the ratio

summation of moments of resisting forces to that of driving

forces and is given by

F ¼

PCD xi þðwi � ui DxiÞ tan �u� �

1=MiðhÞ½ �Pi¼n

i¼1 Wi Sin hi; ð2Þ

where Mi(h) is the moment of individual slices about the

center of the failure arc and all other parameters being the

same as defined in the simple method of slices

The first method is reported to give a conservative value

that is values less than the true result. The second method

(simplified Bishop method of slices) is reported to give

Geotechnical and GeologicalCharacterization site

Lithology and Soil Parameters

Correlation of Lithology and

Analysis of SPT Values

Presence or otherwiseof ancient landslide

Preliminary Slope Stability Analysis Using;1. Angle of repose method2. Unsupported height method

Precise Analysis1. Simple method of slice2. Simplified Bishop method

Yes

No

Determine shear parameteron ancient slide surface

Fig. 10 Flow chart showing procedure for slope stability


123

more reliable result. The two methods were used for

computations in this analysis.

Analysis of slope at BH3

This was done in two stages;

Stage 1: angle of repose method—approximate analysis

Though the site is an active erosion site, before construc-

tion activities began, slope failure of any reasonable

magnitude was not observed on the site. It was thought,

therefore, that the natural geotechnical properties of the

soil could be such that the soil can stand for some height

without failing. Hence, the approximate analysis was

employed first to assess the slope before exact analysis.

For purely cohesionless soil, if the slope angle is more

than the angle of repose of the soil, the soil will not be

stable and it will flow downhill [21].

The angle the slope makes with the horizontal shown in

the Fig. 11 (BH3) by points ABC is 35�. Basic mechanics

states that cohesionless soil mass will flow down a slope if

the angle of slope is greater than the angle of internal friction

of the soil which is true in the present situation (angle of

internal friction are 10�, 12�, and 17�, respectively, for layers1, 2, 3.) and 32� for layers 4 and 5. Layer 1 only exists at theupper reaches of the slope at each of this borehole location.

The soils in these layers, however, possess some cohesion,

and for such a soil the free and vertical standing height of the

soil without lateral support is given from Rankine theory of

earth pressure for a C - [ soil by

z0 ¼2c

cffiffiffiffiffiffiKa

p ð3Þ

where C is the soil cohesion value, Ka is the coefficient of

earth pressure at rest, and, c is the unit weight of the soil.

Using equation 3, this vertical standing height for each

of the layer are 3.70, 4.77, 4.3 m that is if each soil layer is

existing alone. While when in layers, equivalent analysis

using average values obtained as weighted mean of each of

the parameters for layers 2 and 3 gives values of ‘c’ as

32.9 kN/m2, Ø, as 12.6�, and unit weight as 18.3 kN/m3.

The analysis assumes that there is no slip between the

surfaces of these two layers; this ensure that the layer acts

like a homogenous unit. This is a reasonable assumption

for two reasons:

� = 18.1kN/m³, Cu = 28.1, angle of internal friction = 10°

� = 18.2kN/m³ Cu = 35.2angle of internal friction = 12°

� = 18.9kN/m³, Cu = 30.4 angle of internal friction = 17°


SC

SC-SM

GC

GP - SW

2.00

7.00

4.701 2 3 4

56

78

9

10

11

R=18.91m

12

15.70

35°2.00

� = 22.5kN/m³, Cu = 0angle of internal friction = 32° SW

R=26.00m

37°

R=37.50mCritical Radius

Fig. 11 Slope analysis model for slope at SPT boring 3 (BH3) showing different potential failure surfaces and approximate line of locus of

radius potential failure surfaces


123

• The shear strength parameters of each of the layers are

not widely different and hence their replacement with

equivalent one.

• The assumption is consistent with the one made earlier

that eliminates any of the layers contact surfaces as a

potential slip surface.

The analysis gives an unsupported vertical height of

4.48 m and the slope length of 7.81 m at 35� slope angle.

This represents the maximum unsupported height that the

combined layers will stand, above which the soil mass will

fail. The natural slope stands at vertical height of about

16.0 m here and hence will be unstable, even though the

soils cohesion will contribute to counteract possible slope

failure, though with a slight margin within this height. A

summary of procedure of stability is presented in flow

chart given in Fig. 10.

Stage 2: precise analysis—method of slices

A potential failure surface for the slope is defined with a

circle having a radius of 19 m. The potential failure surface

passes through the crest and the toe of the slope. This

failure surface (the one that passes through the toe and

crest) represents the optimum failure pattern that could

exist in an eventual failure. The failure mass is divided into

11 slices of 2.0 m width each. The slices cut across five

layers of soil with different properties. From the top layer,

one contribution to the analysis given by slice ‘11’ is very

small. Table 1 presents the soil properties for the five

layers. Table 2 presents the full analysis using simple

method of slices while Table 3 presents the analysis using

simplified Bishop method of slices. The analysis was car-

ried out with the aid of excel worksheet. Another trial

circle was carried out with radius of 21.0 m. In choosing

these radii, a line of locus of potential critical circles was

constructed based on Fellenius [9] principle, which is for

homogenous C - [ soil and not in layers. The circles

considered were around these lines and not exactly on it.

These are presented in Fig. 11, in which the line is repre-

sented by ‘LS’.

Analysis of slope at BH6

The slope is oriented at 33� to the horizontal. The slope

here stands at an average height of 12.0 m. At a thickness

of about 7.1 m, the soil of layer 2 dominates the slope in

this location. For the slope here, four potential slip surfaces

were considered. The first is with a radius of 32.0 m. This

was chosen because the slope at this location has an

extensive flat top, a potential failure circle that cuts through

the slope toe and this flat top was considered. Examination

of the geometry of this failure surface suggests that factor

of safety for this failure surface will reasonably be higher

than 2. Three other circular failure surfaces that cut through

the toe and the crest of the slopes each with a radius of

17.0, 16.74, and 13.0 m were considered. Figure 12 shows

the failure surfaces. The analysis was carried out using both

methods of slices. The parameters used are presented in

Table 4, while Table 5 presents the computations details

for method of slices with radius (R) equal to 16.74 m

Table 2 Slope stability analysis at BH3 using method of slices R = 19.0 m

(1) Slice Wi Dxi (m) h Sinhi WSinhi Coshi WiCoshi Ui (kN/m) Dli (m) Ui Ni

1 18.900 2.000 40 0.643 12.15 0.766 14.48 0 2 0 14.48

2 213.490 2.000 0 0.000 0.00 1.000 213.49 0 2 0 213.49

3 246.980 2.000 10 0.174 42.89 0.985 243.23 0 2.24 0 243.23

4 224.066 2.000 10 0.174 38.91 0.985 220.66 0 2 0 220.66

5 215.8554 2.000 16 0.276 59.50 0.961 207.49 0 2.24 0 207.49

6 295.485 2.000 21 0.358 105.89 0.934 275.86 0 2.24 0 275.86

7 298.227 2.000 37 0.602 179.48 0.799 238.17 0 2.24 0 238.17

8 294.406 2.000 40 0.643 189.24 0.766 225.53 0 3 0 225.53

9 270.976 2.000 51 0.777 210.59 0.629 170.53 0 3 0 170.53

10 193.045 2.000 60 0.866 167.18 0.500 96.52 0 4 0 96.52

11 71.392 2.000 68 0.927 66.19 0.375 26.74 0 5 0 26.74

1072.02 1932.71

C1 (layer 1) C2 (layer 2) C3 (layer 3)

71.71 280.13 51.8

Summation of resisting forces = 71.71 ? 280.13 ? 51.8 ? 1098.0 = 1501.64 kN. Summation of driving forces = 1072.0 kN. Factor of safety

against slope failure = 1501:641072:0 = 1.400


123

Results and discussion

The reliability of the results of stability analysis depends to

what extent the model used represents the condition in the

field.

Table 7 presents the summary of results for both loca-

tions BH3 and BH6. Analysis was carried out for stability

during construction and post-construction periods for

location BH3 but only during construction for location

BH6. Effective parameters were determined for BH3 but

not for BH6. These parameters are presented in Table 6.

Without the privilege of a full version 4 of a computer

program for slope stability analysis that could allow rapid

and accurate analysis with possible trial failure circles, the

failure circles employed in the analysis were arrived at

based on the principle that the optimum circle is one that

passes through the toe and crest of the slope. The centers of

the circle were moved around vertically and horizontally;

the circles presented are the ones that give least factor of

safety. A commercial slope stability computer program will

usually use the grid search method by iterative computation

of factors of safety for a given x, y, coordinates that define

a radius of a slip surface or could use optimization methods

such as steepest descent, Davidson–Fletcher–Powell

method [7, 10].

Other algorithm exists in carrying out global search of

minimum factors of safety. This includes: the RST-2

algorithm implemented by Jade and Shanker [17]. This

algorithm is said to be easily implemented, robust, and

efficient. The starting point is x, y, coordinates of the

Table 3 Slope stability analysis at BH3 using simplified Bishop method of slices R = 19.0 m

1 2 3 4 5 6 7 8 9 10 11 12 13

(1) Slice Wi Dxi (m) h Coshi �CDxi ui uiDxi Wi - uiDxi ; tan ; (6) tan ; (3) ? (9)

1 18.900 2.000 40 0.766 0.000 0.000 0.000 18.900 32.000 0.625 11.810 11.810

2 213.490 2.000 0 1.000 0.000 0.000 0.000 213.490 32.000 0.625 133.403 133.403

3 246.980 2.000 10 0.985 0.000 0.000 0.000 246.980 32.000 0.625 154.330 154.330

4 224.066 2.000 10 0.985 0.000 0.000 0.000 224.066 32.000 0.625 140.012 140.012

5 215.855 2.000 16 0.961 0.000 0.000 0.000 215.855 32.000 0.625 134.881 134.881

6 295.485 2.000 21 0.934 0.000 0.000 0.000 295.485 32.000 0.625 184.640 184.640

7 298.227 2.000 37 0.799 0.000 0.000 0.000 298.227 32.000 0.625 186.353 186.353

8 294.406 2.000 40 0.766 60.800 0.000 0.000 294.406 17.000 0.306 90.009 150.809

9 270.976 2.000 51 0.629 60.800 0.000 0.000 270.976 12.000 0.213 57.598 118.398

10 193.045 2.000 60 0.500 70.400 0.000 0.000 193.045 12.000 0.213 41.033 111.433

11 71.392 2.000 68 0.375 70.400 0.000 0.000 71.392 10.000 0.176 12.588 82.988

14 15 16 17 18 19 20 21 22 23 24 25tan hi tan ;i

FF = 1.46 F = 1.46

(13)/(15)

F = 1.45 F = 1.45 F = 1.45

(13)/(17)

F = 1.4 F = 1.4 F = 1.4

(13)/(21)

F = 1.20 F = 1.20 F = 1.20

(13)/(24)

0.359 1.041 11.343 0.362 1.043 11.323 0.524 1.168 10.114 0.441 1.104 10.702

0.000 1.000 133.403 0.000 1.000 133.403 0.000 1.000 133.403 0.000 1.000 133.403

0.075 1.059 145.715 0.076 1.060 145.644 0.110 1.093 141.158 0.093 1.076 143.431

0.075 1.059 132.196 0.076 1.060 132.132 0.110 1.093 128.062 0.093 1.076 130.124

0.123 1.079 124.979 0.124 1.080 124.885 0.179 1.133 118.996 0.151 1.106 121.954

0.164 1.087 169.868 0.165 1.088 169.703 0.240 1.158 159.514 0.202 1.122 164.598

0.323 1.056 176.436 0.325 1.058 176.140 0.471 1.175 158.640 0.396 1.115 167.185

0.176 0.901 167.445 0.177 0.902 167.273 0.257 0.963 156.674 0.216 0.931 161.953

0.180 0.742 159.466 0.181 0.743 159.299 0.262 0.795 149.020 0.221 0.768 154.137

0.252 0.626 177.985 0.254 0.627 177.738 0.368 0.684 162.895 0.309 0.655 170.208

0.299 0.487 170.553 0.301 0.487 170.282 0.436 0.538 154.226 0.367 0.512 162.089

1569.389 1567.821 1472.702 1519.785

Assumed F F = 1.46 F = 1.45 F = 1.4 F = 1.19

Computed F 1.46 1.46 1.45 1.42

Computed Fs =1569:3891072:02 ¼ 1:46, 1567:821

1072:02 ¼ 1:46, 1472:7021072:02 ¼ 1:45, 1519:785

1072:02 ¼ 1:42


123

failure surface. Another one is the Monte Carlo, proposed

by Greco [11]. Kim and Lee [20] proposed a technique that

uses finite element method. The above references were

reported along with numerous other optimization tech-

niques by Taha et al. [34].

From Table 7, the factor of safety (‘F’) against failure

for the 19.0 m radius at BH3 location is smaller than at

radius 21.14 m during construction period. For post-con-

struction period, the values of factor of safety are smaller

by method of slices which for the 19.0 m radius is 1.10,

while for the 21.14 m radius, ‘F’’ is 1.32. The analysis

results with simplified Bishop method follows the expected

trend. For both construction periods and by the two

methods, the dominant value of ‘F’ is less than 1.50, which

is the normally accepted as the lowest minimum for a safe

slope [21], US Army engineering manual (EM 1110-2-

1902, 2003) [33]. The factors of safety computed in slope

analysis occurred in grid form at centers of potential failure

radii, from which the least value is obtained. The radius of

this least values is called the critical radius. With respect to

the slope being investigated, values computed forms part of

this minimum search grid. They can therefore be used to

produce contours or generate other values using some of

the optimization methods mentioned above. Thus, two or

more close values can have radius that is reasonably dif-

ferent from each other. Commercial slope stability com-

puter software will have some or near some of this factor of

safety values in its results or in the extreme case values less


� = 18.4kN/m³,Cu = 34.2angle of internal friction = 12°

� =18.9kN/m³, Cu = 30, angle of internal friction = 18°� = 22.4kN/m³, Cu = 0, angle of internal friction = 30°

SC

SC-SM

GC1

2 3 4 5

6 7 8 9

R16.74

10

R32.17

2.00

R13.45

51°

R17.04

14°

27°32°

49° 33°

GW - SW

2.00

9.11

2.002.00

5.00� = 22.4kN/m³, Cu = 0,angle of internal friction = 30° SW

Critical Circle R = 27.5m

Fig. 12 Slope analysis model for slope at SPT boring 6 (BH6)

Table 4 Soil properties utilized in stability computation for slope at BH6

Layer Depth of

occurrence (m)

Soil type Unit weight,

c (kN/m3)

Angle of internal

friction (undrained)

[a (�)

Soil cohesion

value (undrained),

C, (kN/m2)a

SPT

‘N’ values

I 0–2.0 SC 18.1 12 26.7 6–8

II 2.0–11.0 SC–SM 18.4 12 34.2 10–16

III 11.0–13.0 GC 18.9 18 30.1 16–21

IV 13.0–15.0 GW–SW 22.0 32 0.0 29–30

V 15.0–20.0 SW 23 30 0.0 28–34

a Values obtained from undrained-triaxial test. Angle of slope 33�


123

than those presented in Table 7. Proprietary software

developed in-house named Ibeslope [29], was used to

analyze the slope. The software is limited to circular failure

analysis only. For BH3 location, this software obtained a

factor of safety of 1.32 at a critical radius of 33.798 m.

A demo version of a commercial software, Geo 5 (fine

software), which has all the capability of a full version

except that it is run online and its results cannot be printed

or exported was employed to carry out the slope stability

analysis and gave a critical radius of 37.5 m and a factor of

safety of 1.30. The closest value to this from Table 7 is

1.32 at a radius of 21.14 m. This will form a near lower

band of the contour for the 1.30 value. All the three

approaches produce similar results. Hence, valid deduc-

tions can made based on the results presented in Table 7.

The implication of the result is that the slope is marginally

stable. It could not, however, be left in this natural state as

the site is an active gully erosion site, which is hydro-

dynamically active, and can lead to situations where the

slope becomes very unstable. A retaining wall was adopted

to be constructed at a location after the point BH3. The

wall is to allow back up of sediments within the reach

behind it up to the full depth of its height and sediments

that will provide support for the adjoining slope, repre-

sented in the analysis by BH3.

For BH6, the result of the analysis is also presented in

Table 7. The analysis shows that massive slope is stable;

with a failure surface of radius, R = 13.5 m gives a factor

of safety of 1.83, failure surface with R = 16.74 m gives a

factor of safety ‘F’ of 1.38, while R = 17.0 m gives a

Table 5 Computation of factor of safety at BH6 using method of slices (R = 16.74 m)

(1) Slice Wi (kN) Dxi (cm) h Sinhi WSinhi Coshi Wicoshi Ui (kN/m) Dli (m) Ui (kN) Ni (kN)

1 12.096 2.000 0 0.000 0.000 1.000 12.096 0.000 2.000 0.000 12.096

2 32.367 2.000 0 0.000 0.000 1.000 32.367 0.000 2.000 0.000 32.367

3 76.90543 2.000 0 0.000 0.000 1.000 76.905 0.000 2.240 0.000 76.905

4 147.200 2.000 13 0.225 33.113 0.974 143.427 0.000 2.000 0.000 143.427

5 202.400 2.000 13 0.225 45.530 0.974 197.213 0.000 2.240 0.000 197.213

6 239.200 2.000 26 0.438 104.858 0.899 214.992 0.000 2.240 0.000 214.992

7 239.200 2.000 28 0.469 112.298 0.883 211.201 0.000 2.240 0.000 211.201

8 220.800 2.000 33 0.545 120.256 0.839 185.178 0.000 3.000 0.000 185.178

9 184.000 2.000 44 0.695 127.817 0.719 132.359 0.000 3.000 0.000 132.359

10 73.600 2.000 50 0.766 56.381 0.643 47.309 0.000 4.000 0.000 47.309

Sum 600.253 1253.047 24.960 0.000 1253.047

Summation of resisting forces = C1Dl1 þ C2Dl2 þ tan 12P

Wi cos h = 143.85 ? 387.99 ? 266.34 kN = 798.19 kN. Summation of driving

forces = 1253.057 kN. Factor of safety against slope failure = 798:19600:25 ¼ 1:38

Table 6 Effective stress values of soil parameters at BH3 from direct

shear box test

Layer Soil

type

Unit weight,

c (kN/m3)

Angle of internal

friction [ (�)Soil cohesion

value C,

(kN/m2)

I SC 18.1 5 12.6

II SC–SM 18.2 3.2 18.4

III GC 18.9 4.4 14.2

IV GW–SW 21.2 24.6 1.2

V SW 24.0 28.8 0.8

Table 7 Slope stability results

summaryArea of slope Construction period Radius of potential slip

surface ‘R’

Factors of safety

Method of slices Simplified Bishop

method of slices

BH3 During construction 19 1.40 1.46

21.14 1.68 1.78

Post-construction 19 1.10 1.12

21.14 1.32 1.40

BH6 During construction 32 ? ?

17 1.70 1.89

13.5 1.83 1.86

16.74 1.38 1.67


123

Table

8Comparisonofstabilityanalysisresultswithsomepreviousworks

S/N

12

34

5

Slopestabilitystudylocation

Highway

Hwy34Mp175.5’

MonroeCounty,Iowa

Hwy169Winterset,

MadisonCounty,Iowa

Uyo,Southeastern

Nigeria

BH3

Uyo,Southeastern

Nigeria

BH6

Slope15,(embankmentfillslopes)

Hwy63Sugar

Creek

Wapello

County,Iowa

Slopecondition

Stable

Failed

Stable

Stable

Failed?

Slopeheight(m

)7

716

12

20

Slopeangle

(�)

22

13

35

33

18.4

(proposed)

Soiltype(lithology)

loess,glacial

till,or

alluvium

Loess,glacial

till,alluvium,

shale,

limestoneand

sandstone

CoastalPlain

Sand

CoastalPlain

Sand

4layers,thin

lean

clay

withsand

andgravel,clayey

sandandsilt

(alluvium);highly,moderately,

andslightly,weathered

shale

Soilclassification(U

SCS)

CL

CH

SC,SM

SC,SM

CL,CH

Shearstrength

param

eters

Angle

ofinternal

friction[

18�

18�–35�

10�–32�

5�–28.8�

12�–23�

Cohesion‘C’

(kPa)

911–45

28–35.2

28.8

29–97

Groundwater

level

Groundwater

table

present

within

theslopeheight

Groundwater

table

present

within

theslopeheight

Water

table

notmet

Water

table

notmet

Water

table

parallelto

slope

Methodofstabilityanalysis

Morgenstern–

Price

method

Bishop

Morgenstern–

Price

method

Bishop

Sim

plemethod

ofSlice

Bishop

Sim

plemethod

ofSlice

Bishop

Morgenstern–

Price

method

Bishop

Criticalfactorofsafety

1.366a,1.001

1.364a,

0.998

4.895a,1.000

4.966a,

0.997

1.10,1.32b

1.12,

1.40b

1.70,1.83b

1.89,

1.86b

0.583–2.610

(min

tomax)

0.583–2.584

(min

tomax)

aThetwovalues

listed

ascritical

factorofsafety

wereanalysisresultsusingshearparam

etersfrom

BSTandbackcalculation,respectively

bValues

arefordifferentradius


123

factor of safety ‘F’ = 1.70. Ibeslope obtained a factor of

safety of 1.69 for a critical radius of 23.79 m. The Demo

version of ‘Geo 5’ for this location gave a critical factor of

safety for the slope of a value of 1.65 at a radius of

27.55 m. Based on the three results, the slope is safe.

However, analysis with radius less than 13.5 m cuts the

mass of soil on the slope surface and gives a factor 1.39.

Hence, the soil mass bordering the slope surface is unsta-

ble; the possibility that the soil here will gradually be

removed sheet wise is very high; if this happens the soil

mass towards the upper extreme reaches of the slope may

become unstable and flow down to a more stable geometry.

A cover system which consists of geosynthetics was

installed on the slope. On top of this grasses, ornamental,

and fruit trees were also planted. This was carried out after

backfilling the second retaining wall located in the outfall

reaches of the gully near BH6 slope.

Comparison with previous works

A number of slope stability analysis cases abound in the

literature; some are presented with the ones in this study in

Table 8

In a Highway Research Board report, IHRB Project TR-

489 [12], 15 cases of slope stability evaluations on a number

of highways in Iowa, USA were reported, of which 13 of the

slopes have failed. The failed slopes include both embank-

ment slopes (comprising compacted fill) and back-slopes

(formed by cutting). The Lithology of the area is diverse, and

includes shale, glacial till, silty clay and weathered shale.

Slope angle ranges from 11� to 23�, and vertical slope heightfrom 6.0 to 23 m. Field investigations included survey of

slope geometry, borehole drilling, soil sampling, in situ

Borehole Shear Testing (BST) and ground water table mea-

surement. Laboratory investigations mainly comprised ring

shear tests, soil basic property tests (grain size analysis and

Atterberg limits test), mineralogy analyses, soil classifica-

tions, and natural water contents and density measurements

on the representative soil samples from each slope. The

factors of safety were evaluated based on shear parameters

determined by BST, ring shear and back calculation.

For the locations quoted from the report, the lithology

essentially are inorganic clay and silt,while theones in this study

are sandy clay and silt. For both, there is no organic soil, even

though shale soil is present in the lithology of the quoted slopes.

Factors of safety are either 1.00 or less in those slopes

that have failed when back calculated; this is with reference

to highway Hwy169, although the failure here is a steep

scarp slide with a maximum height of 1.7 m, and not a

global failure. Highway Hwy34 though stable still has

minimum factor of safety of 1 and 0.998. The highway

‘HWY63’ slope analysis is completely different. Here, the

natural slopes were evaluated and found not to have global

stability, but with a proposed embankment on the native soil

designed with appropriate slope angle, and suitable materi-

als, the stability value increases up to a value of 2.584. The

Morgenstern–Price method of stability analysis generally

gives higher values than the Bishop’s method; behaving like

Simple method of slice and Bishop’s method of slices when

compared. The radius of factors of safety for the stability

analysis performed in the report was never mentioned. The

software employed in the stability analysis report is the

Geoslope.

Conclusions

In carrying out stability analysis of the slopes bounding a

big and active erosion site in Uyo, Southeastern Nigeria,

geotechnical investigation was carried on the site to char-

acterize the slope. This was done with both SPT and CPT

followed by laboratory testing. The soil making up the

slopes was essentially silty sand(SM) and clayey sand

(SC). The shear strength properties of the soil were also

determined. The slopes were then examined for the pres-

ence or otherwise of ancient or previous landslide sur-

face(s), but these were not present. They were analyzed

using Simple Method of slices and simplified Bishop

method of slices. The factor of safety values is consistent

with generally accepted values. The two methods indicates

that the slopes in the early part of the erosion site is

unstable, and will therefore require some constructional

measures to keep it safe. While the slope at the far reaches

of the erosion though stable but could be subjected to sheet

erosion. A covering system consisting of grasses, trees, and

geosynthetics was installed at this location to aid stability.

The major runoff into the ravine gully is taken through a

constructed channel located on the stable western flank and

discharged through a series of spillway into the far reaches

of the gully beyond the location in which the slopes are

threatening.

Acknowledgements The lead author wishes to acknowledge Auto-

desk Inc for providing the Autocad 14, Education software that was

used in producing some of the slope model drawings used in this

article.

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