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Temporary Cofferdam design - What FOS should I use? 1
- Thread starter cyphos168
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geotechguy1
Civil/Environmental
Sounds like you want to design a one hoss shay
You may find something of use in the old USACE manual:
You may find something of use in the old USACE manual:
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 )
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1
- #7
SlideRuleEra
Structural
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."
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.
- Thread starter
- #9
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.
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.
- Thread starter
- #11
@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 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.
- Thread starter
- #14
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 have another alternative. Would doing the sinking method using sheet pile cofferdam as a guide be a better solution?
JedClampett
Structural
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.
They can tell you if they've encountered your issue before and likely give you some preliminary guidance.
- Thread starter
- #17
@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.
- Thread starter
- #18
@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.
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.
SlideRuleEra
Structural
cyphos168 said:
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.
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