Braced cofferdam - Embeddment depth of Piling Question
Braced cofferdam - Embeddment depth of Piling Question
(OP)
I am working on a braced cofferdam.
It retains a stratified soil that turns into a clay till at the base (this layer is fairly deep).
My ultimate goal is to determine how deep I need to go with the sheet piling for stability during construction and of course to avoid heave at the bottom of the piling.
I can figure the required embedment requirement to install my first (top) ring. (there are a total of 2 rings)
My question revolves around the bottom heaving.
If (based upon the values) my factor of safety is not large enough to prevent against bottom heave, how do I determine how much deeper I need to go?
Then, how do I "re-check" against bottom heave.
I am reviewing the USS Sheet Pile Manual and Principles of Foundation Eng (4th Ed) by Das and I must be missing it.
I understand that I need to consider the passive forces acting on the embedded portion of the wall (should I need to go deeper) but am just missing how to incorporate that "back" into the "check".
Please advise?!
Thank you
It retains a stratified soil that turns into a clay till at the base (this layer is fairly deep).
My ultimate goal is to determine how deep I need to go with the sheet piling for stability during construction and of course to avoid heave at the bottom of the piling.
I can figure the required embedment requirement to install my first (top) ring. (there are a total of 2 rings)
My question revolves around the bottom heaving.
If (based upon the values) my factor of safety is not large enough to prevent against bottom heave, how do I determine how much deeper I need to go?
Then, how do I "re-check" against bottom heave.
I am reviewing the USS Sheet Pile Manual and Principles of Foundation Eng (4th Ed) by Das and I must be missing it.
I understand that I need to consider the passive forces acting on the embedded portion of the wall (should I need to go deeper) but am just missing how to incorporate that "back" into the "check".
Please advise?!
Thank you





RE: Braced cofferdam - Embeddment depth of Piling Question
RE: Braced cofferdam - Embeddment depth of Piling Question
I may post these two Mathcad 2000 worksheets in a zip file if interested.
May examine more your question looking in some texts. Anyway the 2D and 3D FEM geotechnical programs should give good insight on what expected. Even a gross just elastic analysis will provide you with standing shears to compare with an allowable one; but better from specialized software that can do with more parameters.
RE: Braced cofferdam - Embeddment depth of Piling Question
Thank you, however I am not sure if I clearly understand.
I would love to see the mathcad worksheets, however we do not have mathcad so I am unsure if I would be able to view them properly (but I am willing to give it a try)!
I am newer to cofferdam design (essentially my second one), there are no co-workers that have geotechnical experience and we do not have any software to assist.
Essentially I am working one handed (so-to-speak).
RE: Braced cofferdam - Embeddment depth of Piling Question
www.PeirceEngineering.com
RE: Braced cofferdam - Embeddment depth of Piling Question
Under those possible conditions the bottom will loosen so much that your subsequent structure can settle greatly (like matters of feet).
I usually spec that the flow distance for water from the water table elevation to the inside excavation elevation (along the sheet piles outside and then inside) of the cofferdam is twice the head difference. Then, the tip of sheet piles elevation required is well below the bottom of the excavation and very likely your bottom heave question is no longer critical. Of course with some dense glacial tills, even with this unity gradient, you may get away with it due to minimum flow rate.
RE: Braced cofferdam - Embeddment depth of Piling Question
www.PeirceEngineering.com
RE: Braced cofferdam - Embeddment depth of Piling Question
Later in the evaluating the stability of cofferdams section, it provides minimum guidance for Dense sand to prevent piping. It also states in the section for soft to medium stiff clay to follow guidance on dense sand, which is:
D1=D2=2*H/3, where D1 & D2 are the depths of the sheeting below the bottom of the excavation and H is the depth of excavation.
Anyone know of a better reference to calculate the minimum embedment depth, particularly in soft clays? Or is this pretty much it and I'm over-thinking it?
RE: Braced cofferdam - Embeddment depth of Piling Question
RE: Braced cofferdam - Embeddment depth of Piling Question
I truly appreciate your effort... unfortunately without MathCAD I cannot view the files. =(
On the embedment of the piling
The only reference I have seen in regards to the embedment depth (for clays) has been stated to use:
B/2 - in the USS Sheet piling manual (USSSPM)
and
Up to B/2 - in Foundation Design by Braja M. Das
where B = width of excavation
Is this "it" and I am just overthinking it? (thanks to studio13 for that statment)
On my original question I am facing a potential dry excavation but run the chance of water coming up to above the top ring.
(see attached pdf)..I am just not sure how to (or if I need to) include the water pressure (or the differential head) in the FS calculations regarding heave? According to Pile Buck, "water pressure is not added since it is already included in teh undrained strength considerations" however USS SPM states that it does.....
I am just not sure which way to evaluate this condition and then make sure that I do not add in the water pressure more than once...
RE: Braced cofferdam - Embeddment depth of Piling Question
F.S = [c/(gamma x H)] x (Nc + (2d)/B) (Eq. 32c)
where d = depth of piling below dredge level."
c = cohesion of soil
Nc = bearing capacity factor
H = height of excavation
B = width of sheeted excavation
gamma = average unit weight of soil within the depth of excavation
q = surface surcharge loading
Because the other equations (for sheeting with no embedment below subgrade) use (gamma x H) + q, I assume you should also add q to Eq. 32c.
Ref: Terzaghi's Theoretical Soil Mechanics, 1943 and Pile Buck's Design Example Problem No. 9 on Page 144.
If you have ground water, use the triangular water pressure and use buoyant soil unit weight below the water level.
www.PeirceEngineering.com
RE: Braced cofferdam - Embeddment depth of Piling Question
Deep Excavations: a practical manual, 2nd Edition
Malcolm Puller
Thomas Telford ed.
a book worth to have if one is to deal with deep excavations.
In the first quoted pages there are equations for the cases.
RE: Braced cofferdam - Embeddment depth of Piling Question
Standard Handbook of Heavy Construction, 3d edition.
O'Brien, Havers, Stubbs Jr.
Mc Graw Hill
Part D, Heavy Construction Types
Cofferdams and Caissons
Gerwick Jr.
RE: Braced cofferdam - Embeddment depth of Piling Question
You need more than just a website of helpful engineers, you need someone with experience overseeing what you're doing. This is an important aspect of construction and collapse can lead to injured or dead workers not to mention millions spent re-constructing.
There are companies who've spent the majority of their work on these structures, Hartman Engineering for example. This is not a trivial matter.
Regards,
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RE: Braced cofferdam - Embeddment depth of Piling Question
A short course on Deep Excavations in Clay
Kai S. Wong
Nanyang Technological University, Singapore
RE: Braced cofferdam - Embeddment depth of Piling Question
YOU need a seasoned professional
william