This document discusses slope stability analysis. It begins with introductions and objectives, then describes types of slope failures such as plane, wedge, toppling and rotational. Factors affecting slope stability and variables to consider in design are outlined. Methods of slope stability analysis including limit equilibrium methods and factor of safety calculations are explained. A case study of a landslide in Nigeria is presented, with soil testing and modeling using Slope/W software yielding a factor of safety near 1, indicating incipient failure. The document concludes with recommendations for further slope stability assessments.

2.  Introduction.
 Aim And Objectives.
 Types Of Slope Failure.
 Factors Affecting Slope Failure.
 Variable To Consider In Slope Design.
 Rock Slope Stability Analysis.
 The Factor Of Safety.
 Case Study; Landslide Investigation of Ikwette, Obudu Local
Government Area of Cross River State, Nigeria
 Conclusion
 Recommendation

3.  Slope stability may be defined as the resistance of any
inclined surface, wall of an open pit or cut, to failure by
sliding or collapsing
Slope failure occurs when the downward movement of
materials due to gravity and shear stresses exceeds the shear
strength.
 Slope stability analysis is an important and delicate problem
in geotechnical engineering, particularly for large projects
such as dams, mining, highways and tunnels.
 The main focus of slope stability analysis is typically to
determine a factor of safety value (Fs) against slope failure.

4. The aim of this write up is based on understanding of
development of natural and man-made slopes over the
life of a mine/engineering projects.
This aim could be achieved under the following
objectives;
 To analyze slope stability and to understand failure
mechanisms and the influence of environmental
factors.
 To assess the possibility of slope failure and stability
involving natural or existing engineered slopes
 Having knowledge of how to analyse these failure
mechanisms for stability.

5. SLOPE FAILURE
PLANE WEDGE TOPPLING ROTATIONAL
NATURAL MAN MADE

6. Plane failure occur when rock block slides on a single face
of bedding plane striking parallel to the slope face
Figure 1: Shows the plane failure mode
Source: Adapted from Goodman, R.E., and Kieffer, D.S., J. Geotech. Geoenviron. Eng.,
126, 675–684, 2000

7. In wedge failure mode, striking obliquely across the slope face,
along their line of interaction dayligthing into the slope face.
Figure 2: Shows A Wedge Failure
,
Source : Adapted from Goodman, R.E., and Kieffer, D.S., J. Geotech. Geoenviron. Eng.,
126, 675–684, 2000

8. In circular failure mode, the heavily jointed and weathered
rock mass, similar to a waste dump rock, slides on a single
cylindrical face into free space/excavation
Figure 3: Shows the circular failure mode
Source: Adapted from Goodman, R.E., and Kieffer, D.S., J. Geotech. Geoenviron. Eng.,
126, 675–684, 2000

9. In toppling failure mode, the multiple rock columns/layers
caused by a steeply dipping joint set rotate about their
bases into the free space/excavation
Figure 4: Shows the toppling failure mode
Source: Adapted from Goodman, R.E., and Kieffer, D.S., J. Geotech. Geoenviron.

10. FAILURE MODE
VERY SLOW
Lateral Spread
SLOW
Rotational Slide
FAST
Debris Flow
VERY FAST
Rock Fall

11. Figure 5: Shows failure mode
Source: Adapted from Goodman, R.E., and Kieffer, D.S., J. Geotech. Geoenviron. Eng., 126,
675–684, 2000

12. Table 1: Shows the summary of factors contributing to slope failure
Source: Adapted from Goodman, R.E., and Kieffer, D.S., J. Geotech. Geoenviron. Eng.,
126, 675–684, 2000

13.  Site topography
 Site stratigraphy and variability
 Geologic origins and characteristics of subsurface
materials
 Groundwater level.
 In- situ soil and /or rock characteristics
 Engineering properties
 Soil & rock behavior

14. SLOPE STABILITY
ANALYSIS
Finite element
Static equilibrium
methods
Limit Equilibrium
Method
Probabilistic
methods
Infinite element

15.  Limit equilibrium methods use representative geometry,
material and/or joint shear strength, material unit weights,
groundwater and external loading/support conditions to
determine slope safety factors based on a set of simplifying
mechanical assumptions.
 Limit equilibrium methods consist in cutting the slope
into fine slices so that their base can be comparable with a
straight line then to write the equation

16. Factor of safety is defined as the ratio of the total force to
resist sliding to the total force tending to induce sliding.
The most basic purpose of slope stability analysis is to
determine a factor of safety against a potential failure, or
landslide. If this factor of safety is determined to be large
enough, the slope is judged to be stable (safe). If it is 1.0 or
less, it is unsafe.

17. A CASE STUDY:
Landslide Investigation of Ikwette, Obudu Local
Government Area of Cross River, Nigeria
 The landslide of Ikwette occurred in November, 2013 and
the affected area is in OB1 community.
 The methodology employed for the analysis of the landslide
were carried out systematically in four phases, which includes:
(i) Site investigation carried out in the field, (ii) Geotechnical
analysis carried out in the laboratory, (iii) Application of the
results from some geotechnical parameters to a Slope/W
Geostudio 2012 software program, to generate a factor of
safety value on the other hand.

18.  Some obtained parameter were then applied in a slope/W
Geostudio 2012 software program which uses the
conventional limit equilibrium methods to simulate the
dominant factors of safety of the instability of the area.
 Results from geotechnical investigation of the samples
shows an average maximum dry density value of 1.63kg/m,
moisture content valve of 18%, Analysis from particle size
distribution showed that the particle sizes where silty sand,
with a coefficient uniformly and curvature value of 1.8 and
0.968 respectively.
 Triaxial compression test showed angle of internal friction
and cohesion values averaging around 12.65 and 43kpa
respectively.

19. Fig. 6 Front view of the google earth satellite image of the landslide

20. Fig. 7 Side view (from the right) of the google earth satellite image of the landslid

21. Fig. 8 Picture of slope OB1 showing the scarp of the slide, as well as the
rocks and debris moved.

22. Fig. 9 Picture of slope OB1, showing the threat posed by this slope instability on roa

23. Soil type Property Average values of Soil
geotechnical properties
Grain Sizes (%)
2.0 – 4.76 (mm) 11
0.42 – 2.0 (mm) 81
0.074 – 0.42 (mm) 5.5
0.002 - 0.074 (mm) 2.5
Coefficient of Uniformity (Cu) 1.8
Coefficient of curvature (Cc) 0.968
Optimum Moisture Content (%) 18
Maximum Dry Density (Kg/m3) 1.63
Wet Density (Kg/m3) 1.92
Angle of Internal Friction (°) 12.65
Cohesion (kPa) 43
Permeability (cm/s) 3.48×10-3
Table 2. Summary of the Soil Geotechnical Properties.

24.  The factor of safety value for the slope in OB1, where the
landslide occurred was 1.114, which is close to the value for
an incipient failure.

25. Fig. 9 Geostudio Model diagram of the slide at OB1.

26.  Assessing the stability of slopes required a proper
investigation in order to avoid any risk. In which the following
conclusions is based on;
 Rock slopes can be natural or engineered (excavated). Plane,
wedge, circular and toppling failures are the four basic modes of
their failure.
 The objective of a slope stability analysis of a rock slope is
to identify the most likely mode/mechanism of slope failure and
to determine the associated minimum factor of safety.
 The factor of safety of a rock slope is defined as a ratio of
total force available to resist sliding of the rock block to the total
force tending to induce sliding. In practice, rock slopes with FS
= 1.3 to 1.5 are considered to be stable less than this is unstable.

27.  There are new technologies, with possibilities to take
automatically readings and send them via wireless links to an
interpretation center; there are also interesting progresses in our
understanding of some aspects of soil and slope behaviors; there
has been development of very powerful coupled hydro-mechanical
numerical models that can consider saturated and unsaturated soils,
progressive failure, etc.
And all these new technologies been used for the analysis include
the following;
 Artificial Neural Network
 Fuzzy Inference System
 Rock pack III
 Dips And Slope/W Mine Software.