Checking sheet pile for quick condition
Checking sheet pile for quick condition
(OP)
I am just completing a sheet pile design for shoring an excavation. I am quite satisfied with the design, except for analyzing the potential for a quick condition at the toe. In practical terms I have dealt with the issue by requiring dewatering to a depth that I am comfortable will prevent failure at the toe. (Meaning dewatering depth outside the excavation. Inside the excavation will be dewatered to the dredge line). Still it bothers me not to be able to analyze it numerically. I have reviewed the literature, and most authors seem to take me 90% there and leave me hanging. Most derive the equation for critical hydraulic gradient ic=(SG-1)/(1+e). Now what do I do with the result? The USS manual has a simple formula (reduction in submerged unit weight)=20*Hu/D. Now this I can use in my wall analysis to find the max Hu (unbalance head) that will still leave an acceptable FS. But after giving this simple formula, USS goes on to confuse me with a bunch of criptic nomographs. Can anybody shed some light on this subject? The basics of my design problem: braced wall, 18' dredge depth, 11' penetration, med.-dense fine sand, existing water table 2.5' below grade. Thanks!





RE: Checking sheet pile for quick condition
f-d
¡papá gordo ain’t no madre flaca!
RE: Checking sheet pile for quick condition
RE: Checking sheet pile for quick condition
f-d
¡papá gordo ain’t no madre flaca!
RE: Checking sheet pile for quick condition
RE: Checking sheet pile for quick condition
From a flow net (or a numerical model, which is MUCH more complicated). A suitable flow net (especially for confined flow) should not take more than a half-hour or an hour to complete. Once done, you can then take your ruler (knowing the scale) and measure the flow length at the exit square and divide it by the head drop between the equipotential lines. It's really not that hard.
f-d
¡papá gordo ain’t no madre flaca!
RE: Checking sheet pile for quick condition
Based on the width of the cofferdam and the unbalanced hydrostastic head, the chart gives you the embedment required for safety factors of either 1.0, 1.5, or 2.0 in both loose sand and dense sand. The chart also considers that there is all soil below the sheets or an impervious layer at some distance below the tip of the sheets.
Do an internet search for "Pile Buck Steel Sheet Piling Design Manual." It is a must reference for those designing sheet pile walls.
RE: Checking sheet pile for quick condition
Assume all the soil is the same. Then the difference in elevation between the outside water table and the bottom of the excavation is your head differential.
The flow path is the length of flow from that outside top of water table, down around the tips snd back up to the inside of the excavation.
It really is very simple I wonder why the question.
Divide that head differential, by the flow path to get the gradient. The inverse of that is your safety factor.
Thus the tip embedment below the bottom of excavation should be one half of the head difference.
RE: Checking sheet pile for quick condition
"It really is very simple I wonder why the question."
Its simple if you understand the words! I just needed someone to 'splain it to me, and you've done that; thanks!
RE: Checking sheet pile for quick condition
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