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Apparent Lateral Earth Pressures in Retaining
Walls
Apparent Lateral Earth Pressures what is it and why?
While active and passive earth theory are applicable in simple
cases, multileve braced excavations tend to experience more complex
earth pressures. Since cantilever walls can typically reach only
down to 15ft depth, deep excavations require bracing if an economical
design is to be achieved.
Peck (1969) in a monumental for geotechnical engineering publication
compiled a series of case studies where bracing loads where measured
in struted excavations for the Chicago metro. Peck noticed that
measured maximum bracing reactions did not behave as active or
at-rest earth pressure theory would predict. From back calculations,
Peck noticed that upper struts were more heavily loaded than active
theory would predict, and lower level struts where less loaded
compared to active pressures. Peck summarized his findings in
"Apparent" earth pressures that were purposed as means
to determine maximum strut reactions.
In a mishap, Peck noted the maximum pressures as factors times
gamma x h where gamma = soil density, and h = excavation depth.
Many engineers take this gamma as the total soil unit weight
and do not apply any water pressures. In a personal communication,
Mr. Peck confirmed that the effective unit weight should be taken
and water pressures should be added separetely.
The Federal Highway Administration has also fallen in the same
trap. In its design manual, it recommends using a total unit weight.
Again, as Mr. Peck has quoted, "the apparent earth pressures
are essentially effective active pressures mutliplied by a factor
and redestributed as a rectangle or a trapezoid".
Unfortunately many engineers have misused the original concept
and they typycally apply zero driving soil pressures below subgrade.
This practice can lead to unsafe results and is strongly not recommended.
So if one thing is left in your mind from this webpage, it must
be to use apparent earth pressures with great caution and judgement.
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