Stability of Slopes and Earth Pressure Civil Engineering GATE Notes

Stability of Slopes and Earth Pressure is one of the most important topics in the civil engineering curriculum, which is also a vast concern for GATE aspirants. This explains the behaviour of soil when supporting slopes and retaining walls. The knowledge of this topic helps the students understand the design of safe structures.

In short, Stability of Slopes and Earth Pressure discusses forces in the interior of soil and the lateral pressure that soil exerts on walls. These concepts are applied to roads, bridges, basements, and hill-side structures.

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Stability of Slopes and Earth Pressure Video Explanation

It is much easier to understand the Stability of Slopes and Earth Pressure if someone visually explains it. Watching video tutorials is so much easier than trying to visualize from a book the movement in the soil, the pressure direction, and the failure planes.

In this video, a simple explanation is given, in an exam-oriented approach, on the concepts of earth pressure, Rankine and Coulomb theory, and slope behaviour, along with numerical examples. This can be referred to by GATE aspirants for fast revision and clarity of concepts.

Check: GATE Civil Engineering Notes

What Is Earth Pressure in Soil Mechanics

Earth pressure is defined as the lateral force exerted by soil on a structure, such as a retaining wall. Stability of Slopes and Earth Pressure presents reasons why the design for walls should be strong.

When the soil is placed behind a wall, it exerts pressure on the wall sideways. This pressure exerted sideways by the soil is called lateral earth pressure. Earth pressure is dependent on soil type, wall movement, and water presence.

Types of Earth Pressure

There are three main types of earth pressure used in the analysis of the Stability of Slopes and Earth Pressure.

Before looking at the table, remember that the kind of earth pressure is decided by wall movement.

These three conditions help engineers select safe designs.

Earth Pressure at Rest (K₀ Condition)

In Stability of Slopes and Earth Pressure, earth pressure at rest is when the wall does not move at all. Particles of soil remain in their natural position.

For cohesionless soil, the coefficient of earth pressure at rest is given by a simple formula:

K₀ = 1 − sinϕ

Here, ϕ is the angle of friction of soil. This formula is easy to remember and very important for GATE exams.

Earth pressure at rest is common in basement walls and rigid structures.

Active and Passive Earth Pressure Explained

Active earth pressure happens when the wall moves slightly away from the soil. Because of this movement, soil expands and pressure reduces.

Passive earth pressure occurs when the wall pushes into the soil. Soil resists strongly, creating very high pressure. In Stability of Slopes and Earth Pressure, passive pressure is mainly used for stability checks.

Before comparing them, note that wall movement is the key difference.

Rankine Theory in Stability of Slopes and Earth Pressure

Rankine Theory is widely used in Stability of Slopes and Earth Pressure because it is simple and easy to apply. It assumes soil is dry, uniform, and cohesionless.

The active earth pressure coefficient is: Ka = (1 − sinϕ) / (1 + sinϕ)

The passive earth pressure coefficient is: Kp = (1 + sinϕ) / (1 − sinϕ)

Rankine theory also assumes the back of the wall is smooth and vertical. These assumptions make calculations easy for exams.

Earth Pressure Thrust and Pressure Diagram

Earth pressure thrust refers to the total force on the wall acting because of soil. In the stability of slopes and earth pressure, thrust is determined through an area calculation underneath the diagram of pressure.

Before delving into understanding thrust calculation, note that the deeper one goes, the greater the pressure.

The thrust always acts at one-third height from the base for triangular pressure distribution.

Effect of Water Table on Earth Pressure

Water plays a very important role in Stability of Slopes and Earth Pressure. When soil is submerged, water pressure adds extra force on the wall.

In submerged soil:

• Effective unit weight reduces

• Water pressure acts separately

• Total pressure increases sharply

Students commonly forget to add the water pressure in exam problems. This results in wrong answers.

Inclined Backfill and Sloping Ground

The earth pressure is greater when backfill is inclined. Stability of Slopes and Earth Pressure analysis include slope angle β in the calculations.

Inclined backfill is more forceful since the weight of the soil acts along the slope. This variation in the earth pressure coefficients is what engineers need to consider when undertaking safe design.

This condition is common in hill roads and embankments.

Coulomb Theory 

Coulomb Theory is another method used in Stability of Slopes and Earth Pressure. It is based on trial wedges and force balance.

This theory:

• Considers wall friction

• Works for inclined walls

• Uses trial and error method

Though slightly complex, it is useful for real-life designs.

Stability of Slopes and Its Relation to Earth Pressure

Stability of slopes means checking whether a soil slope will stand safely or fail. In Stability of Slopes and Earth Pressure, both topics are linked.

When slope soil pushes laterally, earth pressure develops. If pressure becomes too high, slope failure may occur.

Slope stability depends on:

• Soil strength

• Slope angle

• Water content

• External load

Common Mistakes Students Should Avoid

The small mistakes made by many GATE aspirants in the topic Stability of Slopes and Earth Pressure result in the loss of a number of marks.

Common mistakes include:

• Forgetting about water pressure

• Using an inappropriate formula for Ka or Kp

• Ignoring tension cracks in cohesive soil

• Incorrect failure plane angle

The same sorts of errors may be avoided by reading the questions carefully.

Importance for GATE Aspirants

Stability of slopes and earth pressure is a scoring topic in GATE Civil Engineering. The questions will come totally on formulas, concept-based.

Scoring full marks from this topic is quite easy with crystal clear basics and regular practice.