ECE 2201:BASIC GEOLOGY AND SOIL MECHANICS

GROUP 7
1. Cynthia Ongachi - ENC221-0107/2022
2. Nerhius Ogutu – ENC221- 0115/2022
3. David Siapai – ENC221- 0099/2022
4. Kelly Rotich – ENC221- 0096/2022
5. Yvonne Moraa- ENC221- 0097/2022
CYNTHIA ONGACHI -ENC221-0107/2022
SLOPE STABILITY
Slope- A surface on which one side is at a higher level than another. OR
-An inclined ground surface which can either be natural or man-made
Slope stability refers to the condition of the inclined soil or rocks to withstand movement.

TYPES OF SLOPES
Slopes can be classified based on;
1. Method of construction - natural and man-made slopes
2. The extent - infinite and finite slopes
3. The type of soil
- Cohesive soil slope; Slopes having purely cohesive soils as its content.
-Frictional soil slope; Slopes having frictional soil as its contents
-Cohesive frictional slope
4. The stability of the slope
- Stable slope- Slopes that are unlikely to undergo failure
- Unstable slope- Slopes that are prone to collapse
5. Slope angle
- Steep slopes; Slopes with a high angle of inclination
- Moderate slopes; Slopes with a medium angle of inclination
- Gentle slopes; Slopes with a small angle of inclination
There are two main types of earth slopes namely;
1. Natural slopes
2. Man-made slopes

Natural slopes
Such slopes exist in hilly areas
Examples of natural slopes include:
- Hillslopes: These are natural landforms characterized by gradual to steep
inclinations formed by geological processes such as weathering and erosion.
- Mountainsides: These are the slopes of mountains, characterized by steep inclines
and rugged terrain.

Artificial slopes
 These slopes are formed by unnatural processes. They are formed by humans
Embarkments: Theses are man-made slopes constructed for various purposes
Cut slopes: These are slopes created by cutting into natural terrain

The slopes whether natural or artificial may be; infinite or infinite

• An Infinite slope is used to describe a slope extending up to an extent whose
boundaries are not well defined. For this type of slope the soil properties for all
identical depths below the surface are the same. The long slope of the face of a
mountain is an example of this slope
• The finite slopes are limited in extent. The length of the slope depends on the
height of the embarkment or dam. For examples slopes of embankments and earth
dams

DAVID SIAPAI- ENC221-0099/2022

Terms used in slope description
a) Toe: The bottom part or lowest point of the slope where it meets the ground or
another surface.
b) Crest: The topmost part of the slope, usually referring to a ridge or highest point.
c) Backslope: The inclined surface that extends upward from the toe to the crest of the
slope. It's the main slope face.
d) Face: Refers to the vertical or near-vertical surface of a slope.
e) Shoulder: The transition zone between the backslope and the crest or between the
backslope and the surrounding terrain.
f) Bench: A relatively flat step-like surface partway up a slope, often created
intentionally in engineering projects or formed naturally.
g) Scarp: A steep slope or cliff created by erosion, faulting, or other geological
processes.
h) Ridge: A long, narrow elevation of land that is often the highest point along a slope
or a mountain.
i) Gully: A narrow, deep channel or ravine formed by erosion, often found on steep
slopes.
j) Terrace: A flat or gently sloping segment of land built into a slope, typically formed
by human activity for agricultural purposes.
 TYPES OF SLOPE FAILURE
A Slope may have any one of the following types of failures.
1) Rotational failure:- This type of failure occurs by rotation along a slip surface by
downward and outward movement of the soil mass. The slip surface is generally
circular for homogeneous soil conditions and non-circular in case of non-
homogeneous conditions.

Rotational slips are further divided into 3 types.

 i) Toe failure, in which the failure occurs along the surface that passes through the
toe.
ii) Slope failure, in which the failure occurs along a surface that intersects the slope
above the toe.
iii) Base failure, in which the failure surface passes below the toe.
2) Translational failure:- A constant slope of unlimited extent and having uniform soil
properties at the same depth below the free surface is known as an infinite slope. It
occurs in an infinite slope along a long failure surface parallel to the slope.
Translational failures may occur along slopes of layered materials.

3) Compound failure:- A compound failure is a combination of the rotational slips and

the translational slip. A compound failure surface is curved at the two ends and plane
in the middle portion. A compound failure generally occurs when a hard stratum
exists at considerable depth below the toe.
4) Wedge failure:- A failure along an inclined plane is known as plane failure or wedge
failure or block failure. It occurs when distinct blocks and wedges of the soil mass
become separated. It can occur in both infinite and finite slopes.
5) Miscellaneous failure:- In addition to above four types of failures, some complex
types of failures in the form of spreads and flows may also occur.

NERHIUS OGUTU-ENC221-0115/2022

FACTORS AFFECTING SLOPE STABILITY

1.Hydrological Conditions:

a) Groundwater Seepage:
Changes in groundwater levels and seepage patterns may adversely impact slope stability
by increasing pore water pressure and reducing soil strength.

b) Rainfall and Surface Water Flow:

In particularly in poorly drained or highly permeable soils, heavy rainfall or surface water
flow can saturate slopes, raising pore water pressure and causing instability.

2.Slope geometry

a) Slope angle and height

Steeper slopes are prone to instability than gentle ones
Taller slopes experience greater gravitational forces than shorter ones hence causes more
stress on the slope materials.

3. Geological and Geotechnical Properties:

a) Type and Composition of Soil:

The stability of a slope is greatly influenced by the type and characteristics of the rock or soil
that makes up the slope. In general, soils with stronger cohesion and internal friction angles
are more stable.

b) Changes in the amount of moisture in the soil:

Whether brought on by precipitation, groundwater levels, or other circumstances, can have a
big impact on the stability of a slope. Elevated water content weakens the soil and raises the
possibility of slope collapse due to the high level of saturation.

c) Soil Structure:

Soil structure, which includes features such as bedding planes, joints, and faults, can have an
impact on slope stability because it provides potential failure surfaces.

d)Shear Strength:

One important factor influencing slope stability is the soil or rock mass's shear strength. It is
influenced by elements like internal cohesiveness.

4.Human activities

a) Excavation and construction

This alters the slopes’ stability by changing the slope geometry, adding loads or even
changing the drainage patterns.
b) Deforestation and land use
This alters natural drainage patterns, increase erosion, and destabilize slopes.

5.Climate and Environmental factors

a) Temperature variations
Freeze-thaw cycles and temperature fluctuations causes expansion and contraction of slope
materials which leads to stress and potential instability.
PRINCIPLES OF SOIL STABILITY ANALYSIS

Slope stability is analyzed by considering the factor of safety against failure.

Generally, the factor of safety is defined as
 𝑇𝑓
Fs =
𝑇𝑑

Where; Fs= factor of safety

Td= average shear of the soil
Tf = average shear strength developed along failure surface

Shear strength of the soil can be written as;

Tf = c’ + Q’ tan ∅’

Where c’= cohesion of the soil

Q’= normal stress
∅ ’= angle of friction of the soil

Shear strength developed in the failure surface

Td =Cd’ + Q’ tan ∅′

Cd and Xd are respective cohesion that develop along potential failure surface.

𝑇𝑑 𝐶 ′ + 𝑄′ tan ∅′
Fs = =
𝑇𝑓 𝐶𝑑′ + 𝑄′ tan ∅𝑑′

The factor of safety against cohesion

𝐶′
Fc’ =
𝐶𝑑′

Factors of safety against angle of internal friction

tan ∅
F∅’=
tan ∅𝑑′

Stability analysis of infinite slope (no seepage of water)

Infinite slope is one that extends along distance.
Consider a slope in which;
H= height of the slope
𝛽 = slope angle of inclination to the vertical
Y= unit weight of the soil.
Fs = factor of safety
 𝑐′ 𝑡𝑎𝑛∅′
Fs = +
𝑦𝐻 cos2 𝛽 tan 𝛽 𝑡𝑎𝑛𝛽

If a soil possesses cohesion and friction, the depth of the plane along which critical
equilibrium occurs may be determined by substituting Fs = 1 and H = Hcr into the equation

Thus ;
𝑐′ 1
Hcr =
𝑦 cos ^2𝛽(𝑡𝑎𝑛𝛽−𝑡𝑎𝑛∅′ )

VYONNE MORAA – ENC221-0097/2022

FACTORS OF SAFETY CONSIDERED IN SLOPE STABILITY.

The factor of safety is the load-carrying capacity of a system beyond which the system
actually supports. The minimum safety factor for permanent embarkments, cuts, fills
and landslide repairs is used at 1.25.
There are three types of factors of safety:
✓ Factor of safety with respect to strength
✓ Factor of safety with respect to cohesion/height.
✓ Factor of safety with respect to friction.

Factor of safety is calculated as follows:

1. Identify the critical failure surface- This involves identifying the surface
where the soil or rock mass is most likely to fail under given conditions.
2. Estimating the driving forces- These forces primarily include the weight
of the soil or rock mass and any additional loads such as water pressure
or seismic forces.
3. Determining resisting forces-These are forces that are primarily governed
by the shear strength of the soil or rock mass.
4. Apply the equilibrium conditions-The sum of the resisting forces should
be equal or exceed the sum of driving forces that is,
Resisting forces≥ Driving forces

Then we calculate the factor of safety

Resting forces
Driving forces
 S=Maximum shearing strength
FoS=S/ =average value of mobilized shearing
But S=c’ + tan ’ strength.
c’=mobilized cohesion
So c’+ tan ’/ =effective stress
=mobilized angle of friction.

The mobilized shear strength at each point on a failure surface is given by;
=c’/FoS + tan /FoS

In reality, shearing resistance does not develop in the same way at all point on
a failing surface and the shearing stress isn’t always consistent.
If the safety factors for cohesion and friction are different;
=c’/Fc + tan ’/F
Fc= Factor of safety with respect to cohesion
F =Factor of safety with respect to friction.

N/B: Fc is dependent on the slope’s height.

KELLY ROTICH- ENC221-0096/2022

TENSION CRACK EFFECTS ON SOIL STABILITY
• Tension cracks are vertical cracks that form in soil due to shrinkage caused by loss of
soil moisture.
• They can also form due to expansion of soil when it absorbs water.
• Clay soils are more prone to tension cracks as compared to sandy soils as it is
considered an expansive type of soil.

EFFECTS

1. Tension cracks create pathways for water infiltration, leading to increased porosity
within the soil. This can result in reduced soil cohesion and stability, particularly in fine-
grained soils like clay.
2. As tension cracks form, they weaken the soil structure by breaking up cohesive bonds
between soil particles. This reduction in shear strength makes the soil more susceptible
to erosion, landslides, and other forms of instability.
3. Tension cracks provide channels for water to flow, which can lead to surface erosion as
water removes soil particles and carries them away. This erosion further compromises the
stability of the soil.
4. Tension cracks can extend deep into the soil profile, affecting subsurface stability. This
can result in subsidence or collapse of overlying structures.
 5. Tension cracks can disrupt the root systems of plants, leading to vegetation loss and
further exacerbating soil erosion and instability.
6. Tension cracks near buildings or infrastructure can lead to structural damage or
foundation failure due to soil subsidence.
7. Tension cracks can disrupt agricultural activities by reducing soil fertility and water
retention capacity.
8. Tension cracks pose safety hazards to people and vehicles, especially when they occur
near roads, trails, or residential areas.
9. Tension cracks expose finer soil particles to wind erosion, leading to increased dust
emissions and air pollution.
10. Tension cracks contribute to sediment transport by funneling eroded soil particles,
affecting downstream ecosystems.

REMEDIES TO SLOPE FAILURE

There are several methods for preventing or mitigating the effects of slope failure. These
include:

• Planting vegetation on slopes helps stabilize soil by reinforcing it with roots,

reducing erosion, and increasing slope cohesion.

• Constructing terraces or retaining walls on steep slopes helps to redistribute the

weight of the soil and reduces the likelihood of mass movement.

• Proper surface drainage systems, such as channels, ditches, and surface drains,
divert water away from slopes, preventing saturation and erosion.

• Installing subsurface drainage systems, such as French drains or perforated pipes,

helps to lower groundwater levels and reduce pore water pressure within the
slope.

• Using geotextiles or geogrids as reinforcement within the soil can improve slope
stability by increasing soil strength and preventing erosion.

• Installing rock bolts or soil nails into the slope helps to stabilize loose or
fractured rock masses by providing additional support and reducing the risk of
rockfall.
• Employing bioengineering techniques, such as using live woody vegetation in
combination with geotextiles or soil bioengineering methods, can enhance slope
stability and reduce erosion.

• Constructing terraces or retaining walls on steep slopes helps to redistribute the

weight of the soil and reduces the likelihood of mass movement.

• Hydroseeding involves spraying a mixture of seed, mulch, fertilizer, and water onto
slopes to establish vegetation quickly, reducing erosion and enhancing slope
stability.
• Modifying the slope gradient through grading or reshaping helps to reduce the angle
of repose and minimize the risk of slope failure.

• Replanting native vegetation or using erosion-control species helps to stabilize

slopes, improve soil cohesion, and reduce surface erosion.

• Removing loose debris, such as rocks, soil, or vegetation, from the slope reduces the
potential weight and instability of the slope.

• Installing mechanical reinforcements, such as soil anchors, gabions, or crib walls,

can provide additional support and stability to slopes

• Implementing water management practices, such as reducing irrigation or controlling

stormwater runoff, helps to minimize the effects of water on slope stability.

• Regular monitoring of slope conditions and maintenance of stabilization measures

are essential to identify early signs of instability and prevent slope failure.