Temporary Cofferdam design - What FOS should I use? 1

[cyphos168]

Hi all. I am working on a cofferdam design. Since this is a temporary structure, I am wondering if I can use FOS=1 for soil analysis as well as structural design? Thanks.

 

[BigH]

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.

 

[SlideRuleEra]

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.

 

[PEinc]

http://www.PeirceEngineering.com