Cantilever Walls Design: Advantages - Disadvantages - Analysis Methods
Cantilever walls are walls that have a free unsupported excavation. This section examines vertical cantilever walls, the advantages and disadvantages of cantilever excavations and the basic design methods used for cantilever wall analysis. Cantilever walls restrain retained earth by the passive resistance provided by the soil below the excavation. Many engineers use the cantilever wall term to actually describe gravity walls. In reality both gravity and vertical embedded walls types can be categorized as cantilever if no lateral bracing support is provided by means of tiebacks, struts, etc.
Figure: Cantilever excavation in DeepEX software.
In general, the maximum excavation for a cantilever wall can reach up to 6m or 18ft. Deeper cantilever walls have been constructed but these required more expensive wall systems such as T-type diaphragm walls. Cantilever walls offer some advantages:
Advantages of cantilever walls:
Cantilever walls offer an unbstructed open excavation
Cantilever walls do not require installation of tiebacks below adjacent properties
Cantilever walls offer a simpler construction procedure as the construction staging is much simpler.
Disadvantages of cantilever walls:
Maximum excavation for cantilever walls is rather limited, typically to 18 ft (6m) maximum.
It is generally not recommended to use cantilever walls next to adjacent buildings.
Control of lateral wall displacements depends on the mobilization of passive earth resistance.
For deeper cantilever excavations the wall stiffness may have to be considerably increase. This can limit the available space within the excavation.
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Analysis methods for cantilever walls:
Cantilever walls are very often designed with limit equilibrium methods. In this approach, the designer calculates active and passive earth pressures. Water and other surcharges are also applied if applicable. Two methods in Limit Equilibrium analysis are available:
a) Free earth method for cantilever walls:
This is the simplest approach. It requires that the designer only balances the overturning and resisting moments. Active earth pressures are assumed on the retained side and passive earth pressures are typically assumed on the excavation side. The required embedment for a safety factor of 1 is then obtained at the point of zero moment. The limitation of the method is that shear forces are not balanced. As a result, it is recommended that the obtained embedment for FS=1 is multiplied by approximately 1.2 (to obtain the wall embedment for a safety factor of 1).
b) Fixed earth method for cantilever walls:
The fixed earth method offers a more robust approach as it balances both overturning moments and shears. However, analytical solutions for the fixed earth method are limited to very special conditions. The fixed earth method works by finding the critical pivot point about which rotational moments and shear forces balance. This is done by assuming active earth pressures on the excavation side and passive earth pressures on the retained side below the pivot point. Since closed form solutions with the fixed earth method are limited it is recommended that a Winkler spring analysis will typically offer the best results instead of the classical fixed earth solution.
c) Practical recommendations for cantilever walls
To obtain an adequate safety the typical required wall embedment in cantilever walls is in the order of 1.5 times the excavation height. While cantilever walls offer the advantage of an easy excavation the typical maximum depths are in the order of 15 ft or (4.5m). Beyond that depth, stiffer less economical wall sections will be required to limit horizontal wall displacements to tolerable limits. Whie deeper cantilever walls have been successfully used with small sections, their success was often the result of higher observed soil strengths in the field when compared to the typical conservative design values. Horizontal wall movements should be monitored when constructing deeper cantilever walls.
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