Braced cofferdam - Embeddment depth of Piling Question 1

[Bridgegirl67]

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

 

[ishvaaag]

This first sheet gives the safety when no surcharge at the bottom of the cut (Peck Hanson Thornburn)

 

[ishvaaag]

This other is not for water influx in the excavation, but safety on imparted vertical load at a depth of the bottom of the mat, design, from the same text (scarcely relevant to the issue at hand).

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.

 

[Bridgegirl67]

Ishvaaag,
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).

 

[PEinc]

Bridgegirl67, check out Figure 28, Page 7.2-104 of the NAVFAC's DM-7.2, Foundations and Earth Structures. I believe that you can download DM-7.1 and 7.2 over the web. See attached file.

www.PeirceEngineering.com

 

[oldestguy]

I suspect the groundwater table is within the depth of the job. One of the considerations should be potential for heaving of the base due to water upward pressure and flow (possible quick sand condition). Quicksand condition can occur when the head loss per foot of upward flow is one foot (gradient of 1.0).

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.

 

[PEinc]

She said that there is a fairly deep layer of clay till at subgrade. Therefore, I would not expect piping or quicksand.

http://www.PeirceEngineering.com

 

[studio13]

I had actually been looking for this same answer, or at least a better answer then do a flow net. (I seem to recall doing one of those back in college). DM 7.2 is very vague in what to do with minimum sheet pile embedment when excavating in soft to medium stiff clays. The reference above to 7.2-104 is good for calculating the FS, which is a valuable step, however, it states if your FS < 1.5, drive the sheeting deeper than the excavation. But no guidance on what that depth should be.

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?

 

[ishvaaag]

I have made a zip with 5 workshhet related to excavation and mat issues as per my port of what in the Peck, Hanson, Thonburn. One for excavations in sands where the depth of the sheetpile walls is included.

 

[Bridgegirl67]

Ishvaaag,
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...

 

[PEinc]

The different equations you see for base stability in cohesive soils are for sheeting walls that do not extend below the excavation subgrade. In Pile Buck's 1987 Steel Sheet Piling Design Manual (which replaced the USS Steel Sheet Piling Design Manual), Page 130, Equation 32c is shown. The Manual says, "Equations 32(a) and 32(b) apply when sheeting extends only to the base of the excavation. The factor of safety can be increased if the sheeting extends deeper and is adequately designed. In this case, the soil inside the excavation above the base of the piling acts as a surcharge and developes adhesion along the sheet piles, so that the factor of safety becomes:

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.

http://www.PeirceEngineering.com

 

[ishvaaag]

To complement PEinc I may add some pages of

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.

 

[ishvaaag]

Not on the calculation side but very much on the construction viewpoint it is also worth to read

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.

 

[Qshake]

bridgegirl67 - Who is going to check your computations to make sure everything is correct? This thread reads as though your level of experience on this matter is well below that of someone required to undertake such an important task.

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,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[ishvaaag]

Another reference worth to use:

A short course on Deep Excavations in Clay
Kai S. Wong
Nanyang Technological University, Singapore

 

[wmta]

I have read this site and the issues are critical and need a high level of experience

YOU need a seasoned professional

william