Designing pit / trench concrete thickness 1

[777gne]

This, I'm sure, is basic stuff but I don't deal with a lot of concrete work so I could use a little help. I've got to put some pits into the ground in a processing plant. The pits are roughly 4'x10' x 4' deep and have vertical walls. We will be having these poured. Is there a design reference that I can use to determine how thick the walls of the pit should be, re-bar centers, re-bar size, etc...? Any help or nudge in the right direction would be appreciated.

 

[daddymem]

Check out

http://www.oldcastleprecast.com

for utility vaults similar to you use. You'll find rebar and concrete thickness in the details.

 

[SlideRuleEra]

daddymem's advice for utility vaults is a good way to go.
However, if you do need cast-in-place concrete for pits, such as pump sumps, here are a couple of things to keep in mind:
1. If there is a high ground water table, use enough concrete in the walls and pit bottom to keep the (empty) pit from "floating" out of the ground.

2. Cast-in-place concrete walls less than 6" thick are difficult to construct (space inside the forms is very limited). Eight inch thick walls (minimum) are much more constructable.

http://www.SlideRuleEra.net

 

[777gne]

Thanks guys. I checked out that link, the only real detail I could see was in the CAD drawing(s) and didn't include any rebar detail. I was hoping for more of a general reference or at least a few rules of thumb (thanks SlideRuleEra).

We have poured a few footings before but no in-ground vaults. These are open on top and won't be holding anything other than the occasional worker. I had planned on going w/ 6" walls and #4 rebar 12" O.C., plus pinning the walls to the existing concrete floor w/ #4 rebar 12" O.C. all around the upper perimiter. I think it should be ok, but 'think' is the key word there.

 

[cvg]

you will need the #4's at 12" oc both vertical and horizontal. You will also need bent bars to tie the bottom slab to the walls - maybe #4 at 24" oc

 

[dicksewerrat]

Why not use precast? then all you do is dig drop and backfill outside the walls. No formwork, no steel work, no sweat.

 

[777gne]

CVG, Thanks for your input, that is exactly what I did except everything is on 12" centers except where pinned into the existing floor which are 24" centers.

Dick, this is going into a plant with finished epoxy flooring, I am neat saw cutting the floor to have the cleanest end result that won't require any epoxy work.

 

[dicksewerrat]

the concrete edges will need work no matter how neat you try to be. Do an economic work up. Precast vs. cast in place. Leave the precast 1/4 low and fill with epoxy. I still bet it is cheaper and faster.

 

[DRC1]

I agree with Dicksewerrat, if I had my druthers, I would look more closely at precasting. Small pours are always very expensive. One day to tie brs and pour bottom. Next day form and brace walls and pour. Third day day strip and rub. To set a precast sump, probably an hour, then a little time to repair the epoxy. Note that no matter how you do it, placing concrete will bee messy. You will need time to protect the floor, probably with quite a bit of plywood.

 

[jheidt2543]

Older editions of the "Concrete Reinforcing Steel Institute's Handbook" (CRSI), the 1950's and 1960's versions, have design tables in the back of the book for CIP pits with rebar and constrcution details.