Apparent Pressures in Deep Excavations
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.
Fig. Anchored Wall - Peck Apparent Pressures Diagram - DeepEX Software
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