Floating Slab w. unlevel grade

[[memphis]]

I would like to build a detached garage approximately 20x30
Lot grading is different in this old subdivision; street is high, rear of the lots are all low. Previous flooding issues until they put a catch basin at the back of my property with a feed line to the street running down the middle of my driveway.

I am not sure how deep the pipe is but I am doubtful that I can utilize a true foundation as I have to get below the frost line (4ft) which requires building over top of the pipe.

Anyone ran into a similar issue? I own the catch basin as the project was terminated two years ago, no easement issues.

I am not sure of the slope that the rear of the lot tapers off to but I imagine I would have to come up at least three feet. If I built a floating slab, how would I retain the fill?

Red is the drainage pipe
Yellow is a new fence the PO put in that would have to be modified. I'd like to start the garage exactly where the driveway ends as I want to save as much of my back yard as possible

 

[Kaizen]

looks like you can build really close to the lot line. can you move the pipe closer to the fence? If not its going to have to go around to the right.
If it were me i'd put the garage closer to the house like the lot on the right side of the picture. looks like 15 feet closer so tear up the driveway a little and preserve the back yard.
2nd choice would be all the way to the back corner but if you want to park in the garage that wouldn't work and drainage is an issue.
To put it where you want you will need to move it to the right blocking the back yard view from the house. Reason is you will need to excavate all the soil out. The lower level also needs to have it removed and fill will have to be brought in to mound it up level with the driveway. so it can't be right next to the fence. Other more expensive option is to pour a footer and foundation near the fence, fill, and slab. Not easy no matter where you put it.

 

[finn]

Looks like the building density is already too high for the ground to absorb rain rather properly.

Are there storm sewers or is the street the only conduit to get storm water away from the buildings?

 

[TommyK]

You really need to determine the elevation of the pipe and its relationship to your proposed finished floor and bottom of footing elevations to get any type of substantive design suggestions.

 

[kbs2244]

This is more than a home owner project.

They terminated a surface water improvement project and left you holding the bag?
There has to be legal tracks on a deal like that.

I would call a real estate lawyer

 

[ConCretin]

I agree that you are probably going to need some professional help to design this project with all the constraints and variables but from a conceptual standpoint, I don't think a floating slab is a viable option.

There really is no way to 'retain the soil' with a floating slab. The slab would need to be built on an engineered fill that would taper out from finish grade. From the looks of it, you don't have room for this relative the the property line.

The best option would probably be foundation walls that extend above grade far enough to bring your slab to level. If you can't go to frost depth as you suggest, maybe you could do a variation of a frost protected shallow foundation where the footings are only down a foot or so and rigid insulation keeps the frost out. No idea if you have the room for this option or if it's allowable under your local codes.

As I mentioned, probably time for some professional help.

 

[JamesW84]

I'm no expert, but I'm with Kaizen. I'd move the pipe to the right and make sure it daylights at the end.

Put the shop at the end of your driveway so you have access to it. You'll have to do footers & foundation walls and add fill to level the pad to pour your floating slab on. Make sure the clean fill is compacted well...no more than 12" lifts. You'll most likely need an engineer to approve the fill and test the compaction before you get a building permit.

You'll have to get water away from the building on the left side (all sides, really). You'll probably end up doing footer drains for drainage.

 

[Fatboy148]

First thing is not to guess at your grade. This can be found out exactly what you want for a finish grade with a piece of garden hose filled with water and a ruler. Read up on using a water level. Using your scenario of wanting to come up three feet with grade, without using any sort of retention, you would need to project at least three feet outside of your building footprint (a one to one ratio) to be stable. So, if you "would like to build a detached garage approximately 20x30", I would want the top of the fill to be at least 26'x36' so the base of the slope would be 32'x 42'. For a rough estimate, you can figure a yard of gravel at 1.5 tons and the area you want to cover one foot deep to be 32'x42' that would be about 50 cubic yards to raise grade one foot. So 50 cubic yards at 1.5 tons /yard would be about 75 tons of gravel to raise it one foot. To raise it three feet with a 1/1 taper, you would need roughly 141 yards or 212 tons .


I think my first stop would be to talk with the local building department to find out if you can add additional structures to your property and what the building composition needs to be, height and size requirements, etc. I am sure you can read horror stories here of getting a project underway without knowing or disregarding the rules you need to play by.