Embedded Wall Design Codes
Braced excavation design requires both skill and careful evaluation of many factors that can affect performance. Traditionally in the US, braced excavations are designed with a serviceability approach where soil parameters are conservatively estimated and the performed analysis yields the service displacements, moments, and forces. Design forces are then calculated by applying a global safety factor on the service design results, while the wall embedment is determined by calculating limit equilibrium safety factors against wall rotation and passive resistance that range from 1.2 to 1.5.
In Europe, in contrast to the US, an ultimate limit state design approach has been adopted in geotechnical design including the design of braced excavations. In this design philosophy both wall and supports are designed based on an ultimate limit condition. The ultimate design forces are typically determined by reducing the characteristic soil strength parameters or by multiplying the effects of actions and dividing the effects of resistances by various safety factors. At the end, a safety factor of one or greater is required for all structures and other types of safety factors.
In the US, there is an increasing trend of promoting ultimate limit state design in geotechnical design, including braced excavations. In the author’s experience the ultimate limit state method works reasonably well for most limit equilibrium methods but can produce very inconsistent results in many cases when numerical analyses are employed. Hence, the advantages and limitations of the ultimate limit state design should be carefully weighted by practitioners and academia in the US before, and if, the ultimate limit state philosophy is incorporated in a legally binding building code.
Fig. Excavation Braced with Rakers - AASHTO LRFD Comb. Strength IB - DeepEX
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