[Jack E]

I'm planning to build a detached garage next to my house. The location of the garage has a 3.5 - 4 foot slope down from the driveway of the main house - I want the garage to be on the same level as the existing driveway. Question - can I have a concrete foundation that is 3-4 foot high at the low point of the slope and gradually gets shorter as it approaches the existing driveway? I was thinking I'd fill the lowest areas of the pad with rip-rap/3/4- until level. I'll be pouring a concrete floor for the garage. The other option is to build a retaining wall and then the garage foundation on that. I'll be checking, but I'm hoping I don't have to have an engineered wall. Thanks

 

[Nealtw]

What ever we build here requires a geo tech engineers report.
They check the soil for strength and content after you have it dug out. He decides the width of the footing and where and how much rebar may be needed. He and the city inspect before concrete is poured. We also have to have it surveyed for where to dig and then they come out and stake the corners of the foundation and then comeback and and check the top corners of the foundation before we pour concrete in the walls,

The bottom of the footing wants to be below frost depth and the top of the foundation wants to be 8" above the ground level at the highest point, (driveway).


You can do the footing in steps up the hill but we always charged a lot more for the work. It is a pain in the but for building forms and if you are using rental form you have to get the steps very close to what works for them.

 

[mark sr]

We don't see many poured concrete foundation walls around here but it isn't uncommon to see block foundation walls go many feet up to match the grade on the other end.

 

[3onthetree]

No problem with a high foundation, two critical things:

- compaction, compaction, compaction (not just with a vibratory plate)
"Rip rap" material around here would be too big, may be defined as something else where you live

- interior drain tile - you can get the subsurface water in there and be held against the back wall

 

[mark sr]

I painted for one builder that would back fill slabs like that in stages, compacting as he went, he might take months to get it back filled and the concrete poured. Painted one house for him where I used an extension ladder to cut in the interior garage walls [8'high] because I got tired of waiting on concrete [and getting paid]


But then I painted for another builder that didn't think twice about backfilling and pouring concrete in the same day :surprise:

 

[Jack E]

Thanks for the comments and ideas. I'm thinking a poured foundation then add back fill with good compaction is probably the way to go. Otherwise build a retaining wall - back fill/compact - then pour a foundation; might as well go directly to the foundation. Since the tallest point of the foundation would be around 4 feet, does that require any special reinforcement above and beyond a usual foundation? In other words would it need an engineer to draw a foundation plan? $$$$$:surprise:

 

[Nealtw]

Thanks for the comments and ideas. I'm thinking a poured foundation then add back fill with good compaction is probably the way to go. Otherwise build a retaining wall - back fill/compact - then pour a foundation; might as well go directly to the foundation. Since the tallest point of the foundation would be around 4 feet, does that require any special reinforcement above and beyond a usual foundation? In other words would it need an engineer to draw a foundation plan? $$$$$:surprise:

Just draw up your plans submit them to the city and they will tell you if you need an engineer.

I would bild the footing out of 2x6 and left the 1 -2 inches off the ground to level and call it 8" and 16" wide with an 8" foundation centered on that.

Add some rebar dowels maybe ever 4 ft at the deep end and a length of rebar around the top. 5/8 rebar is the standard here.

 

[Photobug]

Thanks for the comments and ideas. I'm thinking a poured foundation then add back fill with good compaction is probably the way to go. Otherwise build a retaining wall - back fill/compact - then pour a foundation; might as well go directly to the foundation. Since the tallest point of the foundation would be around 4 feet, does that require any special reinforcement above and beyond a usual foundation? In other words would it need an engineer to draw a foundation plan? $$$$$:surprise:

Not really.

I designed my own garage build and drew it myself and had someone online finish my architectural drawings for cheap. Then hired an engineer to finalize the plans. The engineer was the most expensive part of the process short of supplies and labor. I can't even remember what it cost but it was a fraction of the overall costs. It might cost a bit more for you with a sloped or hilly wall but it is not something you want to get wrong.

One thing is the engineers may not need to do a soil test but if so it would be a different engineer than the one who would design the foundation. You would need a geologist to do soil tests then an structural engineer to do the designs. It can start to add up.

The other option is they do not need to reinvent the wheel each time. An engineer can have an idea of the soil quality in your area. They can assume the worst case scenario in your area and design the footer and foundation based on this. You also have the option to have the soil being tested and if the soil is not worse case scenario, you can design the footer/foundation to be less substantial. I looked into this and the geologist just told me to use a bigger foundation profile and save the money.

 

[123pugsy]

I'm planning to build a detached garage next to my house. The location of the garage has a 3.5 - 4 foot slope down from the driveway of the main house - I want the garage to be on the same level as the existing driveway. Question - can I have a concrete foundation that is 3-4 foot high at the low point of the slope and gradually gets shorter as it approaches the existing driveway? I was thinking I'd fill the lowest areas of the pad with rip-rap/3/4- until level. I'll be pouring a concrete floor for the garage. The other option is to build a retaining wall and then the garage foundation on that. I'll be checking, but I'm hoping I don't have to have an engineered wall. Thanks

Mine is similar with a 2' slope.
They stepped it wherever it came up closer than 4' frost level.
Mine is blocks, but I now prefer poured with rebar. I knew nothing when I built. I didn't even draw the foundation close to what was built, but everything passed.

 

[Jack E]

Just draw up your plans submit them to the city and they will tell you if you need an engineer.

I would bild the footing out of 2x6 and left the 1 -2 inches off the ground to level and call it 8" and 16" wide with an 8" foundation centered on that.

Add some rebar dowels maybe ever 4 ft at the deep end and a length of rebar around the top. 5/8 rebar is the standard here.

I appreciate the suggestions. So I understand how the foundation will go - Will need to excavate a 20" (approx.) wide footing trench; the lowest part of the slope will have a 12" deep (below undisturbed ground) trench, as the trench moves up the slope, the trench depth will increase until there is a 4' deep trench at the top of the slope - the footing will be level. Build the foundation (stem wall?) on top of the footing? So at the low point I will have approx. 3-4' of stem wall showing? Thanks

 

[Nealtw]

I appreciate the suggestions. So I understand how the foundation will go - Will need to excavate a 20" (approx.) wide footing trench; the lowest part of the slope will have a 12" deep (below undisturbed ground) trench, as the trench moves up the slope, the trench depth will increase until there is a 4' deep trench at the top of the slope - the footing will be level. Build the foundation (stem wall?) on top of the footing? So at the low point I will have approx. 3-4' of stem wall showing? Thanks

The bottom of the footing has to be below frost depth for where you live

If that is 12" then it has to be more than 12" below the ground at the bottom. At the top it can be just the same and you just put in big steps in between.



So start with frost, how deep is it where you live. Often if you google frost depth and the city, it will come up.

 

[Nealtw]

It can be like this and if I was digging it by hand, this is how i would do it.
All the grass and organic material has to be removed from the area too.

 

[Jack E]

So I can step the footing up the slope making sure the footing is below the frost line all the way up the slope? If so, I won't have to dig down so far at the top of my slope; I was thinking I had to have a level footing all the way around. I like the idea of not having to excavate a deep footing trench. I'm getting motivated to get this project going.

 

[Nealtw]

So I can step the footing up the slope making sure the footing is below the frost line all the way up the slope? If so, I won't have to dig down so far at the top of my slope; I was thinking I had to have a level footing all the way around. I like the idea of not having to excavate a deep footing trench. I'm getting motivated to get this project going.

What are you planning to use for forms for the walls, knowing that ahead of time will give you an idea of where to start and stop the steps in the footing.

We rent them and those people really don't like you cutting them to fit. :surprise:

 

[Jack E]

I was thinking I would excavate the trenches - thus the questions about trench depth - and hire out form framing and concrete work. I'll get quotes for the foundation and see what part of the foundation I want to do, if any. One thing I realized, and it has me a bit worried, is the garage slab will be on top of the power line that feeds my electric distribution box - we don't have any overhead electric wires, all underground. Need to check with county and power co., could be a deal breaker. :sad:

 

[Nealtw]

I was thinking I would excavate the trenches - thus the questions about trench depth - and hire out form framing and concrete work. I'll get quotes for the foundation and see what part of the foundation I want to do, if any. One thing I realized, and it has me a bit worried, is the garage slab will be on top of the power line that feeds my electric distribution box - we don't have any overhead electric wires, all underground. Need to check with county and power co., could be a deal breaker. :sad:

If it is in a conduit like ours I don't think that is a problem, just don't damage it.

If you are going to hire some one to do it, it might be cheaper to have it dug out level. Find your crew first and work out the details with them.