I dont design cofferdams but definitely not FoS of 1. That is very close to the bone!

Sounds like you want to design a one hoss shay

You may find something of use in the old USACE manual: https://www.publications.usace.army.mil/Portals/76...

For temporary structures you still have the same factors of safety you just get to reduce the design earthquake intensity and flood level.

I believe, per USACE, FS of Temporary Structures is 1.3 and FS of Permanent Structures is 1.5. You need to verify using USACE documents however, but I understand USACE may not control what you're attempting to design.

For Sheetpiling cofferdams ( failure mode is shear ) F.S = 1.2 to 1.6 suggested ( Foundation Engineering by Bowles ).

IMO, F.S.=1.2 should be adopted for temporary cofferdam design .

Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

cyphos168 - Are you designing a "cofferdam" (in water or soil with high water table) or is it a "braced excavation" (in soil, no groundwater)?
The following applies for a "cofferdam".

See DRC1's comments on Cofferdam Factor of Safety.

Specifically: "Designing cofferdams is considerably different than typical structural or foundation engineering, and is based on considerable part on experience, contractors construction methods, and local conditions."

Also, my comment: "The shape, size, and connection details of a cofferdam are more important than a large safety factor for sizing the structural members."

If this is a cofferdam with one or more sides out in the water, don't forget to consider stream flow pressures.

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Hi all,

Thanks for your input. After speaking to a local geotech expert and the software designer, I will be doing the following.

FOS 1.2 for materials
FOS 1.2 for earth pressure (active side)
FOS 1.2 for moment, shear and prop forces

I am pretty much covering all angles here when it comes to FOS so I think I am on the safe side unless I have missed something.

The cofferdam is on land. But water table is high. I have taken this into consideration. The soft clay property has much to be desired but calculation showed base heave to be within spec.

Since this is a temporary design I am using total stress analysis. I have been advised to do an effective stress analysis which I have done and the forces look to be less than short term analysis.
So I am using the forces from total stress analysis for my structure design.

I would be reducing your passive resistance by at least 1.2, if not 1.5. I dont think that is an overly conservative move.

I also don't think its in anyone's benefit to skin this to its bones.

@EireChch - I applied a FOS of 1.15 on gross passive pressures . And the contractor in a way is improving the soil on the passive side as well using driven piles to competent stratum. They will drive piles into the ground prior to setting up the cofferdam and excavating to formation level. The piles will be used to support a water intake structure. But I hear you. Will look into higher FOS for passive side as well.

Where did you get 1.15 from, very specific?

I would not rely on any improvement from that. You said there is a layer of soft CLAY at the base, likely no improvement at all in that layer.

If anything, there could be less passive resistance if anything. You will drive piles in CLAY and temporarily increase the pore pressure which will reduce your strength in the short term.

I ended up using 3 as a SF and because failure would have meant serious loss of life.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

After much deliberation, I have decided to forego the cofferdam method of construction due to the poor ground condition.

I have another alternative. Would doing the sinking method using sheet pile cofferdam as a guide be a better solution?

I'd call Nucor/Skyline. I've attended many of their free Seminars and they have hands on design engineers on staff. Once again, as I've said in other threads, these People will help you out for the business lead.
They can tell you if they've encountered your issue before and likely give you some preliminary guidance.

whats the difference in a sheet pile cofferdam and the other cofferdam?

@EireChch, Just thinking out loud. The contractor did think about using sinking method to do but their concern is that if the structure they are sinking into the ground tilts too much then it will be end game for them. So maybe using an extra sheet pile cofferdam to help as a guide? I know this will just add to the cost. And of course other problems may arise potentially. Certainly not an easy job due to site condition. There is an existing intake station around 4m away from the new one which basically adds to the complexity of the task. Its identical design and the way the first intake was constructed, the contractor had opted for partial open cut and coffer dam design (If the info given to me is correct) which isn't helpful at all as the site was a green-field when the first intake was constructed.

@JedClampett, Not in the US and plus the contractor hasn't actually priced the work according to the complexity. Temporary work is always at the back of their mind when tendering. Otherwise they wouldn't get the job. I have yet to agree to proceed unless the contractor has given me free hand to decide how to build the cofferdam. i.e. budget wise. Otherwise as advised by others I would avoid doing this job so I prefer to sleep soundly at night. LOL!

cyphos168, if you try to sink a prefabricated SSP retaining structure down during excavation, as you would a caisson (like for the Brooklyn Bridge), how will you get it deep enough below subgrade to prevent excessive water seepage and an unstable bottom due to unbalanced hydrostatic pressure?
It seems to me like most of the above responders are talking about using a high safety factor(s) but no one except you defines the safety factor(s). Read various books and design manuals on retaining wall and sheeting design. If you are doing allowable stress design (with unfactored service loads and safety factors), The materials' allowable stresses will already have built-in safety factors. Generally, the only safety factor you would need to apply is one for the passive pressure during design or you use full passive resistance and just increase your calculated pile embedment to provide a safety factor on the passive resistance. If you start compounding safety factors (i.e., putting safety factors on materials, loads, and soil properties), you will have a safe but very expensive cofferdam.

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Quote (cyphos168)

The cofferdam is on land. But water table is high.
The piles will be used to support a water intake structure...
There is an existing intake station around 4m...
...contractor had opted for partial open cut and cofferdam design...

Has this contractor constructed a similar project in the past?
You have not mentioned the depth, but below, say, 9 meters, cofferdams can be really tricky.

IMHO, from the info posted, the best option is for a full depth (from top of ground) braced cofferdam.
Not a (potentially unstable) circular cofferdam.

SRE - just thinking - years ago in the Buffalo area they did a deep cofferdam - very large deep circular excavation. As they went down the used inside steel i-beam rings for support - hoop stress and all that. Seemed to work. I had the article at one time . . . circa mid 1970s.

BigH - Construction of circular cofferdams require and even higher level of Contractor competence than typical braced cofferdams. For the questions being asked in this thread... I don't think circular would be wise.

Also circular has what I consider a glaring flaw in some circumstances. To be truly stable the external pressure needs to be uniform all around the circumference. No problem with this issue in water. Not too much problem with this on land. Use circular on the waterfront, like for a water intake structure, with part of the circle resisting only water pressure and the remainder resisting both water and soil pressure... a disaster waiting to happen, even it can be successfully dewatered.

At one of our generating stations we had an experienced Contractor who was "blinded" by the prospect of using less sheeting and having unobstructed interior space for circular compared to rectangular braced. This was for a water intake, half on land, half in the water. I tried to talk him out of it. Anyway, he tried and failed several times to construct circular... finally did the right thing (for this situation) and used braced rectangular, with no problems.

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