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At-Rest Lateral Earth Pressures in Retaining
Walls
Soils in nature have an in-situ state of stress. This "in-situ"
state of stress is commonly refered to as "At-rest"
conditions. If a natural surface is level and all stratigraphy
is also level, then the "At-rest" state of stress can
be described by two main stresses:
a) The vertical stress (effective and total)
b) The horizontal stress
All effective horizontal stresses are typically defined as a
ratio of the effective vertical stress times a coefficient of
lateral pressure. For "At-rest" conditions, this coefficient
is typically defined as:
Ko1 = [1-Sin(friction angle)]
However, as many researchers (Ladd et. al) have reported, the
initial lateral state of stress is linked to the soil stress history.
For example, soils that have experienced a greater vertical state
of stress in the past tend to hold memory of their overloaded
history. As a result, these types of soils tend to "lock-in"
greater lateral stresses in "At-rest" conditions. These
types of soils are typically referred to as overconsolidated.
In such cases, the coeffiecient of at-rest lateral earth pressures
can be defined from an equation relating to the Overconsolidation
Ratio (OCR) such as:
Ko = Ko1 x (OCR )^n
Where OCR is the ratio of maximum past to current effective vertical
stress. The exponent n can be defined by running a series of laboratory
or insitu experiments.
When a sloped ground is included Eurocode 7 recommends multiplying
the above coefficients by (1+ sin (Beta)) where Beta is the surface
inclination angle.
Do you have to include At-Rest Pressures for Retaining Wall
Design?
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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