Civil Engineering ► Soil Mechanics I ► Earth Pressure and Earth retaining Structures
Types of Earth retaining Structures
Retaining walls:
A retaining wall is a structure designed to sustain the material pressure of earth or other materials as grains, ores, etc
Gravity Retaining wall structure:
It is the simplest of all and is made up of:
- Concrete
- Brick Masonry
- Stone Masonry
Gravity wall retains the backfill due to its weight. It is also much thicker in section.
Cantilever Retaining Wall: 
It resists pressure due to its bending action. Cantilever wall is usually made up of R.C.C. It is more conveneint and economical to use. Rankine's and Coulomb's theories can be used to find active earth pressure on the wall.
Counter Fort Retaining Wall:
It is just like a cantilever wall but is much longer as compared to cantilever wall. If it is to be used for even longer distances, some supports are provided to it at required intervals.
Sheet Pile Wall structure:
In sheet pile wall thin sheets are used which are driven through the soil upto the required depth. It cannot resist very high pressure. Sheet pile walls are light in weight as compared to the other typed of retaining walls and is used where the soil strata is weak, especially when the upper strata is weak. Some supports are provided to it if it is to be used for high pressure.
Stability of earth retaining structures
In case of stone/brick masonary:
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It is strong in compression and weak in tension so no tension should be allowed in any part of the retaining wall, especially the base of the supporting wall.
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The maximum base presuure applied should not be greater than the safe bearing capacity of the wall.
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The wall should be safe from overtuning.
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Factor of safety should be considered. Factor of safety shoud not be less than 2 for lateral forces and 1.5 for overall net forces.
Stability of Slopes
Slope is defined as:
The inclined boundary layer/surface of materials between earth and the air.
Purpose of slope:
As the soil cannot stand vertical, thus the phenomena of slopes is used to keep the soil intact and stable.
Uses:
Earth embankments/slopes are commonly required on railays, roads, earth dams etc. The stability of these slopes must therefore be thoroughly analysed as their failure may lead to loss of human life as well as economic loss.
Types of slopes
On the basis of method of construction:
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Natural slope: The slopes formed due to natural process and exist naturally are called natural slopes.
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Artificial slope: The slopes formed by unnatural process. Artificial slopes are formed by humans as per requirements.
On the basis of type of soil:
- Cohesive soil slope: Having purely cohesive soil as its content
- Frictional soil slope: Slopes having frictional soil as its contents
- Cohesive frictional soil: Slopes made up of soil which has both frictional as well as cohesive properties.
According to extent:
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Infinite Slope: The type of slope extending infinitely, or upto an extent whos boundaries are not well defined. For this type of slope the soil properties for all identical depths below the surface are same. In the making of natural slopes, thier is no contribution from our side.
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Finite Slope: The slope that is of limited extent. We the engineers deal with this type of slopes.
Failure of slopes
The failure of slope occurs when the shear stress of the soil exceeds its shear strength OR when the pushing (disturbing) forces becomes greater than the resisting forces. The slope fails when it is not properly designed. The forces causing the failure are
- Gravitational Forces
- Seepage Forces
Types of failure of slopes
There are two types of failures:
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Parallel to the slope: In this case the particles move down parallel to the slope. Parralle type of slope failuree occurs in the case of cohesionless soil.
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Curved Shape failure: In this case the slope failure surface is of curved shape, like an arc of a circle. This occurs in case of cohesive soil.
