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Pecks apparent pressure envelop

Pecks apparent pressure envelop

Pecks apparent pressure envelop

(OP)
Hi all
I have a question regarding pressure envelop for strutted sheet piling .
I have always been calculating strutted forces and sheet pile moments using the triangular Rankine envelop. For strut design, I simply perform structural analysis based on the triangular envelop and design the struts from all layer to as if the heaviest loaded strut.
Now my question is, is this method acceptable structurally? From my understanding, my method is only more conservative than the pecks envelop because my method doesn't consider pressure redistribution among the struts and thus my calculated strut load is deemed to be more overestomated. Is my understanding correct? Or is there a deeper reason to use the pecks envelop?

Thank you

RE: Pecks apparent pressure envelop

(OP)
Just to be clear I am talking about multi level strutted sheet piles

RE: Pecks apparent pressure envelop

If you take the area of Peck's stress envelope you'll realize that it's 30 percent greater than the area of the Rankine triangle. That extra 30 percent is (almost) like reconfiguring the triangular at-rest earth pressures into the rectangular or trapezoidal earth pressures.

Beyond that? You get to decide.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Pecks apparent pressure envelop

Peck derived his apparent earth pressure model (excluding any hydrostatic pressure from groundwater) since the upper supports are more heavily loaded than Rankine theory predicts.
Because you:

Quote (Isaaclau)

...design the struts from all layer to as if the heaviest loaded strut.

I assume the most heavily loaded struts are at the lowest elevation. If so, IMHO, that is conservative.

For what it is worth, I'll pass on a detail I learned from my father. He worked in bridge construction. From the 1930's to 70's he designed/constructed/used numerous deep temporary cofferdams and braced excavations.

Which ever pressure distribution is used, always include wales and/or struts at the top of the sheet pile. Do not cantilever loaded sheet pile above the top wale/struts. Loads at this top elevation are light, but by including support at the top, loading on the second and lower levels are more stable.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Pecks apparent pressure envelop

If you use a triangular load pattern for a multi-level strut wall (instead of rectangular), it seems like the calculated load on the upper strut will be unconservative.
The Terzaghi-Peck-Mersi book terms this distribution "apparent earth pressure" (we all just say "Peck" method). The Tschebotarioff method is similar.
PEinc should comment here.

RE: Pecks apparent pressure envelop

The Terzaghi-Peck envelopes were determined using measurements from their actual projects (the observational method). T-P's empirical pressure envelopes were intended for estimating the maximum anticipated brace loads, not the real, average, or theoretical brace loads. Therefore, brace loads computed by empirical pressure distributions are generally a bit conservative and other structural components of the wall (like soldier beams, SSP, and lagging) can be designed for lower loads, often only 80%. AASHTO, FHWA, and State DOT's seem to have forgotten this. To them, the heavier the wall design, the better. Today, most design manuals show a trapezoidal pressure distribution (symmetrical or non-symmetrical) for both single and multi-tier braced and tiedback walls. I will not use a triangular distribution unless the soil is VERY soft or the wall has a single level of bracing that is very high on the wall, like a typical waterfront bulkhead. While there are still many (older?) engineers that always use a triangular earth pressure distribution, using a trapezoidal or even a rectangular distribution will usually give you a higher, upper brace load. Since the tiebacks or braces are what support the wall, this higher load better supports the wall. Using triangular distributions often results in more load applied on the embedded portion of the wall below excavation subgrade. However, enough walls have been built with many different types of non-triangular, empirical distributions, and research has shown, that very large toe embedments are usually not required, unless there is a lot of hydrostatic pressure or the soil below subgrade is very weak. fattdad is correct about the extra 30% or so increase over the theoretical triangular pressure. While most walls are not built with a support level at the top of the wall, as discussed by SlideRuleEra, it is good practice to keep the upper level of bracing as close to the top of wall as reasonably possible. This helps reduce wall deflection and resulting ground movement as the excavation proceeds to the next lower support level. If the upper support level is too low, the wall will move a lot as a cantilevered wall until the upper support is installed.
While empirical pressure distributions were developed using an assumed support hinge at subgrade (no computers back in those days!), for a braced or tiedback SSP wall, the envelopes do not usually show how to address and apply the active earth pressures below subgrade. Treat the soils above subgrade and behind the wall as an overburden surcharge to get the driving pressures behind the embedded SSP toe.

www.PeirceEngineering.com

RE: Pecks apparent pressure envelop

Rankine is fine for calculating the pressures however I don't know why you would design all struts to the worst case loading (the way you are designing is overly conservative). Calculate the pressures envelope then calculate frame loads based on that. How you calculate the loads depends on the embedment of the sheet pile, but for a basic analysis take the pressure as a variable load and put into a beam analysis program with the supports as reactions to get frame loads. This would be for final stage only though. Each installatiom stage would be different and should be considered separately. There are programs (some of which are free) that will do this for you.

As a side note, the previous example showing an additional prop at ground level is in my opinion unnecessary, but that would be based on allowable deflection and bending capacity of sheet. If you have a stiff enough sheet there is no reason to add a prop at ground level 'just because' as this is not standard practice (anymore). I wouldn't say the top levels are significantly more loaded than rankine suggests either. Sorry to jump in on something which was pretty well answered, just my two pence worth.

Oh and technically if you have a multiple frame design you should ideally be using sophisticated software (finite element)

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