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Lateral Earth Pressures in Retaining Walls

Lateral earth pressures are the primary driving factor in the design of retaining walls. Soils by the nature of gravity exert both vertical and lateral earth pressures. The initial lateral earth pressure of soisl in nature have an in-situ state of stress commonly refered to as "At-rest" conditions (typically refered as Ko). The design of a retaining wall requires that lateral earth pressures are properly calculated. In this effort various engineers and researchers have proposed a number of lateral earth pressure diagrams. However, actual lateral earth pressures depend on the stress-displacement history. Hence, recommended lateral earth pressure diagrams are in reality only first order estimates. More realistic lateral earth pressure distributions are obtained if an engineer accounts for full soil-structure-interaction (such as in Winkler spring soils model or finite element stability analysis). Non-the-less, our current state of practice currently ignores the effects of wall installation on lateral earth pressures prior to any excavation. The following picture presents some typical lateral earth pressures used in deep excavation analysis.

Lateral earth pressures

On the retained side during excavation the lateral earth pressures gradually descrease from At-rest towards active with increasing wall deflection. On the excavation side, the vertical earth pressures descrease while increasing passive movement tends to mobilize more and more passive earth pressures. The terms "active" and "passive"

An important aspect of "At-Rest" lateral earth pressures is that they typically take place at zero lateral wall displacement. This means that a wall will experience full "At-Rest" lateral pressures only if it does not yield. Such a case could take place if a stiff gravity wall fully bears on bedrock, in such a condition a retaining wall will essentially feel the full "At-rest" driving soil pressures.

Given that "At-rest" pressures are considerably greater than active earth pressures, one might conclude that all braced excavations should be designed with at-rest pressures. Doing so, might actually do greater damage than good. While having a greater capacity might be beneficial, if a series of supports are prestressed to the "full" theoretical "at-rest" load then "in-practice" the wall might actually move back into the retained soil causing a series of unpredicted problems. The author is aware of a diaphragm wall designed this way that moved as much as 12inches (30 cm) back into the retained soil causing severe wall and pavement cracking in the process. Part of the reason for such observations is that engineers tend to be on the safe side when providing "at-rest" pressure coeffiecients and other geotechnical strength parameters. Thus, the actual "at-rest" coefficient might be smaller than originally predicted.

Once you start getting wall movement you are moving into active pressure territory.



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