unreinforced concrete 1

[wildehond]

I have a temporary situation where a concrete plug has to be cast into a caisson below the water table under tremie conditions. The water then gets pumped out resulting in a head of approx 4m between the invert of the caisson and the surrounding water table. (the final situation sees a seond reinforced concrete slab cast on top of the tremied concrete.

The caisson is 4.7meter by 5.5 meter and the tremied plug will be 500mm thick.

Would it be unreasonable to treat the unreinforced tremied concrete as a bending element and to accept that the concrete will respond safely in an elastic manner to the bending loads.

Based on the "slab" operating as a 2-way spanning slab, the tensile bending stress would be approx 6% of the concrete compressibe strength.

Thanks for responses

Alten

 

[SlideRuleEra]

A tremied seal should be mass concrete, where bending and other forces may be ignored. The main reason is because there is no control over what "happens" when the concrete goes underwater. You really don't know how well it is consolidated nor what it's in-place dimensions or compressive strength are.

I had wondered why you were having uplift problems for this caisson, see thread507-163450
With a reasonable tremie seal, say 2 meters thick - or more, design problems are reduced:

1. The extra weight of a thicker concrete seal will help overcome uplift.

2. A thicker seal increases the meaningful contact area between the edges of the seal and the caisson. Thru friction, this allows the seal to act as a "true", heavy bottom to the caisson.

3. Stresses within the seal, which can't be accurately predicted anyway, may be ignored.

There are many places for detailed, elegant design, but this is not one of them - putting in lots of seal concrete is safer, more predictable, and cost effective.

http://www.SlideRuleEra.net

 

[wildehond]

SlideRuleEra, Thanks for your reply.

1. Uplift problem. I'm trying to get away with a tremied seal of 500mm. While making the seal thicker provides extra deadload which will contibute to resist floating, its only the submerged weight of the concrete that benefits us. And this deadweight is costly.
2. We are able to do a bulk excavation down to the current water table which is approx 4m below NGL before beginning to sink the caisson. Current plan is to cast the first 4m lift of the caisson and then to build a 1m wide protruding apron slab at this level. This apron will then have 4m of soil sitting on it in the final situation which provides the stability we require.

Hope this makes our decision making clearer.

Alten