Retaining walls are as the name suggests any wall that is designed to retain any material. The material could be earth, water, anything else that needs to be retained. A common example of retaining wall in everyday life is Basement walls, Swimming pool walls, Landscape walls.
Before we discuss how to design retaining walls, I want you to watch a simple but excellent video of how the soil fails behind the retaining wall. Video courtesy of British Geological Survey. This video perfectly shows the failure plane that forms at an angle behind the wall. In this video the soil behind the wall is granular soil. This video kind of emphasizes that what we do on paper is not just a math problem, it is actually a structure that will be built and care should be given as to how best to design it to prevent failure.
Types of Retaining walls:
* Gravity walls
* Cantilever walls
* Counterfort walls
* Tieback walls
* Drilled Pier walls
* Soldier Pile walls
Of the above, Cantilever retaining wall, Tieback walls, Driller Pier walls and Solider pile walls are the most commonly engineered walls. Gravity walls are mainly used for shorter landscaping type of walls as it becomes less efficient for taller walls. The main difference between cantilever retaining walls and the other walls mentioned is the way the foundation is designed. Tie back walls are completely different retaining walls and rely on pre-stress in ties that hold back the wall there by retaining the soil.
Before one can design retaining walls, a little understanding in soil mechanics is essential.
Soil mechanics and Assumptions: In order to design a retaining wall, understanding the soil behavior is critical. The design engineer needs to know some basic soil parameters.
Soil Parameters needed:
– Soil type (Granular or Cohesive)
– Unit weight
– Angle of Friction
What is cohesion? It is the binding ability of soil. According to OSHA.gov, cohesive soils is a soil with high clay content. It is plastic when moist but becomes hard to break when dry. When dry, cohesive soils can be excavated with almost vertical slopes. Good example of cohesive soil is clay.
Granular soils are opposite to cohesive soils. The angle of internal friction plays a major role in granular soils since their cohesion value is zero. Example of granular soil is sand.
The Geotech usually takes samples of the soil over which a structure is to be built and gives the results of the type of soil that is present at the site. One of the tests that Geotech uses to find out the angle of internal friction of the soil is a Direct shear test. Please watch the following video of direct shear test courtesy of Carleton University.
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