I was drawing up a plan to create a connection between two ponds using cinder block walls to create a wide chute and weir boards to control the water levels. (Think of an Agri-Drain box control structure that is 4' wide.)
However, the concrete footings must be on clay (instead of the normal gravel-packed base) because I do NOT want subsurface water flows between the two ponds when one is drawn down.
Clay is considered a weak soil and is also problematic due to swelling and shrinking during wet/dry cycles. I do not think the latter will be a problem because this clay should always remain moist from at least one of the ponds. I am worried about the clay "weakness".
This clay visibly looks like it is perfectly sealed since it is very dense and has zero plant roots. However, when I excavate below the groundwater level, it will weep tiny amounts of water - so it is not perfectly compacted.
I will compact some more prior to pouring concrete. I am also expanding the footing from the recommended 16" wide to 24" wide. Finally, I am going to add rigid supports connecting the two walls. Basically, as one wall tries to push "in", it will be opposed by the other wall trying to push "in" in the opposite direction.
Question:
These two cinder block walls will have a dirt/clay berm piled to the "outside" of each. The forces will be essentially the same as those on a 6' high retaining wall.
Can this work with a concrete footing on clay, or will the footings shift enough that the mortar in the cinder block joints will fail?
My vote is fail. Why not use this instead? Block is much more porous than concrete. If you are determined to go with an open topped system, I'd pour a concrete chute with a concrete footing and have a groove for the boards in the walls and the bottom of the chute.
I want the chute to be wide enough to: Lower water level in Pond 1 and collect fish in the chute. Seal access from chute to Pond 1, and then re-fill the chute to equalize with the water level in Pond 2. Open chute to Pond 2 to release the fish with no handling.
The system needs to function for controlling the water level, and being capable of also serving as a fish holding tank.
I am not worried about the minimal weeping of water due to the concrete (cinder) block wall construction. I am worried about the foundation footing being so weak that the walls will tip over.
I guess if I poured a solid concrete structure using forms, then the foundation might not matter if the entire construct was solid with rebar? A little tilting and shifting would not matter on that type of structure?
That height walls you will need a good foundation with rebar coming up from that foundation a good way into the walls, and the horizontal rebar needs to tie to that vertical rebar.
Mom & Dad had 40"' vertical walls that tapered to 18" height, poured, 8" thick to hold back soil. The contractor didn't put rebar up from the 4" thick slab and didn't have a footer under the wall. It lasted for 6 years before it started tipping in at the top due to the soil pressure (clay). EVERYTHING had to be removed, a footing dug out, rebar put in and everything re-poured. This was on a 8' wide, 50' long "chute" that went from 6" height to 40" height at the walk-out basement doors. I can't remember if the 2nd contractor used #6 or #5 rebar for vertical pieces, but they weren't #4's.
Concrete block walls will crak easily, and eventually fail. I have recently experienced it, with a retaining wall. The blocks were cracked many years before they began to collapse. Poured, thick, highly reinforced concrete walls on wide footings is the only way to go in my opinion.
That is what I was afraid of, just not strong enough for the task at hand. Wet soil is really heavy, and I can't use weep holes to lighten the load for obvious reasons.
Not sure if this is useful, but we built a concrete board weir and gate to control input to our pond. We weren't concerned about leaking of the dam and gate so just used boards. These are intentionally leaky but left open the possibility of adding gaskets. As you mentioned, Agri-Drain sells a range of items including stop logs, we are low tech. I included a few pictures of the weir/gate in operation, the structure dry, and the rebar used. The forms were all scrap plywood and dimensional lumber coated with soybean oil and sprayed with PAM. These can bet seen piled up in some pictures. We poured the pad off site then assembled the forms and lateral rebar on site and poured the walls. Worked fairly well.
Don't know if this was of any use. Your structure will be very different
That is pretty similar to the type of system I was considering.
Were you diverting flow from a bar ditch that runs after a decent rain?
I think I need to do poured concrete like you did. Unfortunately, I need to have the water level to be about 6' lower in one pond part of the time. I know how to do the project "right" with a compacted gravel base, but I can't do it the right way.
I was mainly wondering if it would work on compacted clay - that would form a water-tight seal beneath my concrete structure.
Looking at your work, I think it might. Your structure is clearly strong enough to withstand some lateral forces and some ground shifting. I think making the concrete structure with high mechanical integrity might be the key to my project.
FishinRod, glad to support your idea. The drainage ditch collects runoff from about 25 acres and can really get running in a big storm. I took some of my inspiration from "rice gates." The lateral extension in the front make the structure more stable and prevent flow around the sides, akin to the baffles you put on conduit under a dam. I was wondering if adding an extension downward might eliminate any remaining issues with clay and stability.
My theory is the same, that a block wall wont hold up, long term, the poured concrete, with a high psi mixture, would last a lot longer, I am not sure that I would be as worried about wet clay pushing it as much as frost pushing and cracking it, maybe a little aggregate on the dirt side of it would keep either one, frost or shrink swell clay, from pushing too hard on it. I like the looks of the one RO 40 shows in his pictures, If I understand that is similar to what you are wanting, except maybe a little wider and longer with a weir on each end. kinda like a lock and dam on the Mississippi River. Good Luck! keep us posted with pictures.
All the really good ideas I've ever had came to me while I was milking a cow.
Working with clay can be tricky, but it sounds like you're on the right track with compacting and widening the footing. The rigid supports between the walls should help counteract any pushing. I'd say keep an eye on that weeping clay during excavation, make sure it's as compact as possible.
