Your thoughts on my basement plans

[Streetcar]

Eddie
How much does the liner cost?
If you have the three Gs covered,gutters grading and groundwater, you can have a dry basement. My parents have a stone foundation house at the top of a hill. Their basement is dry.

Problems with your design.
A one sided wall will be harder to form.
Root, rock or other debris could puncture your lining.
Water takes the easiest path , with no gravel for drainage it may not exit at the lowest point, your sump
When you excavate , you are disturbing the remaining soil, this can be path for water.
The landfill liners I have seen are on sand bed to help prevent punctures

Friction along wall will counteract the buoyant force of the water to some degree.
A bell shaped foundation will also help due to weight of soil pushing down on bell
Buoyant force of water is based on depth of water not total amount of water.

Good luck with your project

 

[Streetcar]

Duplicate post

 

[BitChin1]

I like the idea of a backup in case of failure. It would be pretty simple to dig a hole into the floor area off to an out of the way corner big enough for a basin for a sump pump. That will take a little more effort to get the liner to fit into there, but like anything, I'll just stick with it until I get it done. I will fill the basin with sand and form it out so concrete does not cover it up, then install a cover over it. If I find there to be a leak into the liner, this will be the lowest spot of the liner and where the water will accumulate. If that happens, then all I will have to do is buy a sump pump. I will have power in the room, but might also look into a 12volt backup system in case the power fails during an extreme storm. This could easily be located under the stairs.[/QUOTE]

Eddie I think in this proposal the liner is your "waterproof" layer and as such I think it would be better if your basin was "outside" of the liner so that any water that reached the liner would drain away vs "pushing" into it (and any potential leak area).
I think that any water that gets through the liner may eventually drain to this basin (if on the inside) but will also wick through the concrete (I'm picturing a tiled shower with edpm liner and concrete floor) - small events will be handled but a larger event will get above the concrete floor I would expect? My real concern is the ability to get it done and done perfect - as is you have 4 corners to seam/seal - adding this basin area interior would increase this complexity, but give you the backup.

Brings up the question that you will need to address is how to detail the top of the liner so that rain hitting/running down the wall does not continue down but "inside" the liner. Most waterproofing approaches are glued/painted on and or have a drip edge to prevent this - I don't believe the liner will be connected to your concrete once it cures.

Any thoughts on how "long" it will take to get done? Probably don't want any rain once it is excavated and liner starts going in?
How are you going to tie the vertical rebar and keep the interior form in place? My quick work to solve this problem leads to some fairly extensive (expensive?) bracing - not impossible though.
You have solved the issue of damaging the waterproof layer during backfill (as you have none) but will also need to protect it during concrete pour - thinking the concern is movement against rebar or any tools/vibrators etc for concrete consolidation, or against the "dirt form", rocks, clods, etc.

as I think about my post here I am most concerned about the question: are you adding complexity when I was under the impression you started this thread thinking this was a simplification?

 

[caver]

If you do go with your idea, how do you plan to brace the inside plywood from the hydraulic pressure created by wet concrete? My dad had Contech cast aluminum forms that looked like brick. Contech/Western If someone forgot a tie we would have a blow-out and a busted form. Not a fun day when pouring concrete. I recall one time dad was putting pieces of smaller forms to get the retaining wall stub out just where he wanted it. There was just enough area missing ties that it blew out after we had over 4' of concrete down inside an 8' wall.

Story time I'm a traditional foundation type of guy because they work. My father owned a set of concrete forms when I was in my early teens and I helped pour many a basement during my summers. This was in the mid 70's up until about 80-81 when he sold the business. I worked a brief period for the GC that bought his forms. Back then we would spray the walls with a non-fibrous tar. Put corrugated drain tile for sump or gravity drain to outside. That was covered with a few inches of gravel and then backfilled at a later date. I had a damp spot when I bought my split-foyer home. It has a concrete basement that is roughly four feet out of the ground. Years ago I dug that area up and discovered a honeycomb at the bottom where the wall meets the footing. This was also the area they had put a tube to allow the city water line to go through the footing. Also, the wall had NO sealant on it and we get a lot of rain here. I did reroute two downspouts to get the water farther away from the house. I used what I learned as a kid to solve that problem. I put tar in the honeycomb and the tube that went through the footing. I sealed that wall with non-fibrous tar diluted. I put a short piece of drain tile to direct water away into a gravel void in my front yard. Dry basement ever since and my basement has always been finished.

 

[MossRoad]

Caver brings up a good question about the forms. Traditional concrete wall forms are tied together inside-to-outside. The ties keep the forms from spreading apart when they are filled, and also as a place to anchor rebar inside the walls. As I recall, I formed my footing with 2x12s spaced 18" apart. I put in my rebar and as we poured the concrete in the footing, I put a 2x4 parallel down the center, that was removed after the concrete set, to create a 2" deep by 4" wide keyway down the center of the footing to help keep the wall from ever moving off the footings. Then we formed up the walls and staked them down so they wouldn't fall over, but also so they wouldn't float up.

How are you going to do a single sided wall form? That's going to require lots of bracing inside. If you build a 16x16 wooden box with lots of bracing inside, you still might not be able to pour one complete wall at a time, as the weight will push the other side out. What's going to keep the box from sliding with no bracing on the outside? You'll probably have to pour just a bit up each wall and constantly move the cement truck around and around the perimeter of the pour, or get a concrete pumper. Just thoughts. Still interesting discussion. :thumbsup:

 

[MossRoad]

Duplicate post removed....

 

[CurlyDave]

If you do go with your idea, how do you plan to brace the inside plywood from the hydraulic pressure created by wet concrete? ...

Concrete guys call a one-sided form like this a "loose form". I did this once for a foundation/retaining wall that was only 8' high. I poured a floor and embedded 5/8" J-bolts with about 6" sticking out of the floor. I put one 4 x 6 at the base of the form, one 4' away from the base and one 6' away. Then I braced the form walls with 2x4 s angled up from the 4 x 6s to the form. I had the 2 x 4 s on 12" centers. Took a long time to put together, was very expensive, and when all was said and done, didn't work that well. The wall was 12" thick at the bottom, but despite enough bracing that it looked like Sherwood forest in there, the top was 14" thick.

Wet concrete exerts tremendous force. And, you do not want to inside the area when the concrete is poured. If a form gives way, it will hit you like a giant flyswatter.

I don't know how deep your basement will be, but the torque on a retaining wall increases as the cube of the height, so a 9' wall is a lot more force that an 8' wall, not just a little bit, and a 10' wall is almost twice the force.

This was a long time ago, and I am a lot more educated now. If I had to do it again, I would fill the forms much more slowly, about 6" every 20 minutes, so the concrete at the bottom could set up a bit before more was placed on top, which would reduce the hydrostatic pressure and vastly lower the stress on the forms.

My back up plan would be to make the whole basement about 14" deeper than absolutely necessary. I very much doubt that water would come through the walls with the membrane behind them, but it might leak in from the floor. If this happens, add a 10" layer of gravel on top of the old floor, and 4" of concrete on top of that with a sump pump at the right location. I don't think this would happen, but by making the basement deeper, you have the possibility of an easy fix if it does.

Why 14" instead of 12"? 14" is two 7" steps and makes everything come out easier if you have to add the sump pump.

MossRoad says: "If you build a 16x16 wooden box with lots of bracing inside, you still might not be able to pour one complete wall at a time, as the weight will push the other side out..."

I would not pour one wall at a time. Build all of the forms at once, get a pumper and place the concrete all around at the same time. Do this in about 6" lifts going around and around the forms. Let each lift set up for at least 20 minutes before starting the next one.

 

[SandburRanch]

Our finished basement / game room / storm shelter is 14x20 with walls inset from the exterior building line as follows. E wall about 20ft., W wall about 35 ft., N wall 3 ft. and S wall 5 ft. Egress is the stairwell. House is concrete floor with basement top poured when floor was poured.
This was built by a basement contractor that only does basements. Their plywood forms were set by the 2 man crew and held with looped wire snap ties through the plywood with vertical steel rods dropped through each set of loops and bracing only at the corners. I've not been able to find a picture of that kind of snap ties.

A French drain / with sock below footing leading to an exterior 10" PVC sump that extends above grade. With surface drainage correct in sandy soil there hasn't ever been water in the sump in 32 years. Basement floor poured on compacted sand, no plastic, liner material etc.
In the central plains there really isn't such a thing as a gentle rain very often. 3" per hour is common and the record is 3" in 12 minutes and that's not a typo.

Would I do it like this again? You bet.

 

[EddieWalker]

Thank you, that helps clarify my thought process on the pour. My plan for the forms is to build a stud wall with 2x6's and 3/4 plywood on the inside of the wall to create the form. I will drill half inch holes into the concrete slab through the sill plate of the stud wall and pin it into place with half inch lengths or rebar. This will stop the forms from sliding or moving from the weight of the concrete. The studs will be 12 inches apart. I can go closer if you think that will help. Then I plan on building a L beam along the sides of the forms every two feet. Basically screwing in one 2x6x16 onto the studs on the flat side, then screwing another 2x6 onto that one on the edge. The L shape beam along the face of the stud wall will provide a huge amount of stiffness ans strength to the forms. It will also give me a solid mounting position for bracing that will go across the length of the room. The bracing will go from wall to wall, and in my mind, looking down on it, it will look like a checker board.

This is going to require a lot of lumber just for the forms, but when I take down the forms, I will use them to frame up my addition. It will be 24ft by 48ft long with 12 foot walls. So I will cut every one of those 2x6x16's down to 140 5/8 inches for the studs of my walls. Then use the rest for blocking between the studs. I am going to cover the interior walls in 3/4 inch plywood, so that will also be reused

Instead of hiring a pumper, I was thinking that I could have two cement trucks there at the same time, but at either corner. I didn't think about having them go slow and doing it in lifts. That makes sense, so I will probably have to pay a little extra to keep them on sight longer then they want to be just to keep them going slowly.

 

[BitChin1]

How high above ground level will the top of will be? You will need some sort of exterior form for this portion

Interior forming will need angle braces (between floor and wall and also between end and side walls to keep it square. (look at how icf's are braced and poured for ideas on what level of bracing you will need)

I think you can make this work, but still wondering if this complexity and effort "saves" you anything.

Go back to my input on a PWF - if you use in-ground treated plywood your forming/bracing activities would be minimal and you could SKIP the concrete walls entirely (having said this I too am not convinced wood in the ground is a great idea, but your proposal would be protecting it with edpm which is more than > most basements interior framing is protected)