Although retaining wall was the subject of the last post, their design was not covered. This post will focus on the design of reinforced concrete cantilever walls. There are three different forms of this type of wall, all of which are designed to resist overturning and sliding failure. The primary difference between them is their height. The taller the retaining wall, the more likely that counterforts and beams spanning between them will be necessary. This note describes how all of these forms of retaining wall can be designed.
Reinforced concrete retaining walls have to be able to support the forces that originate from the lateral earth and water pressures that exist within soils. They have to be sufficiently stiff, so not to let the retained material shift to the extent that it will cause the resultant force to move to a location that would cause the retaining wall to fail. It is because of this, as well as the magnitudes of the forces that are being exerted onto the retaining walls, that stiffening elements become necessary, in order to reduce the lateral movement of the wall at higher elevations.
For the shortest form of retaining wall, only two elements need to be designed; the wall and the base it is supported from. Both are subjected to high bending moments and shear forces and care must be taken to ensure that these are addressed. These are referred to as cantilever walls due to the monolithic connection they have with their base
When designing cantilever retaining walls, two separate checks are required. The first is the stability of the wall and the second is the design of the wall’s elements. Stability is dependent on its dimensions and how the various forces that are exerted onto the wall affect it. As with pad foundation design, it is preferable to ensure the resultant force from the soil and any surcharge placed upon it, lies within the middle-third of the base to the retaining wall. In instances where such a condition is not possible, there is likely to be an increase in the bearing stress applied to the soil due to the reduced area of contact between the base and the bearing strata. This is a result of the uplift that occurs in the base of the retaining wall, causing the area bearing onto the soil to be reduced
The combination of forces (when checking for stability) has been explained in the previous post on Analysis of retaining walls. All of the analysis so far described assumes that the consistency of the founding soil material is relatively well understood and uniform. In cases where there is significant doubt over the nature of the soil (due to a lack of site investigation survey data), further geotechnical analysis of the retaining wall’s impact on the soil profile is recommended. A good example of this would be a slip-circle mode of failure analysis.
It is also important to note that the design of retaining walls is very much an iterative process, with the various geometric elements altered until the entire structure complies with all of the design criteria. It is not unusual therefore, for engineers to design multiple wall configurations before an optimum solution is found.
Design of Elements
Cantilever retaining walls have the base designed as a slab with a concentrated bending moment where it interfaces with the retaining wall. It must also be designed to resist the effects of soil pressures, from both the retaining wall and the soil on which it is founded. The wall itself is a cantilever beam/ slab that is considered per metre length
Taller gravity retaining walls typically adopt the counterfort form of construction and their design can be broken down into simple components. The counterfort acts like a cantilevering beam and the retaining wall is a continuous slab supported by the counterforts. Where additional beams are included for taller walls, they are designed as continuous flanged beams that span between counterforts.
Steps in Designing a Cantilevered Retaining Wall
Establish all the design criteria based upon the building codes and compute all applied loads, soil pressures, seismic, wind, axial, surcharge, Impact and others.
Proportion the footing as described in the next section
Check stability of the wall for overturning and sliding: A key or adjusting the footing width may be required.
Calculate the eccentricity of the footing or take moment about the centerline of the stem. Verify if it is within or outside the
Calculate the soil pressure at the toe and heel (must be less than the allowable bearing pressure)
Design the stem and the base for flexure and select reinforcing
The width of the footing for most conditions will approximately be 2/3 of the retained height.
It is usually most advantageous to have more of the footing width on the heel side of the stem. This will put more soil weight on the heel to improve sliding and overturning resistance.
If there is a property line on the toe side try to keep at least some width for additional soil weight, otherwise you’ll have a sliding problem
If you need a key for sliding resistance, try to keep its depth less than one-fourth of the retained height
If there is a property line on the toe side, the footing may need to be wider because soil pressures are usually greater at the toe
A worked example
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