Fill for detached garage
Fill for detached garage
(OP)
Hi, I'm planning on building a detached garage. I started with a floor plan from an internet site and drew up a rough set of plans for a basic 30' wide X 44' deep building. Facing the front of the garage, my lot slopes down to the right and down to the back. The front left corner will be closest to grade. So, in order to maintain footings below frost depth, I was planning on a stepped footing arrangement, following the grade. The stepped footers would have a stem wall poured on top. The back right corner would be almost 8' unbalanced and I've learned that I can't just fill this big box with fill, compact it and pour a slab on top. My calculations show I need something like 375 cubic yards of fill. I've attached a picture, viewed from the right, rear corner.
So, my problem is that I don't think this will work. Compacting the fill inside the garage will bulge the walls. Not compacting the fill will allow my slab to settle. Using something else for fill (sand, aggregate, etc.) is going to cost a fortune. Although honestly, I don't see this arrangement as much different than my house foundation except the open space/soil is reversed. Maybe the difference is I'm not compacting the back fill against my foundation.
My background is mechanical engineering, so I'm not a civil or geotech. But I know enough that I know I need some help. My current plans are to talk to my geotechs/civils and let them see my plans and come up with an affordable solution. Prior to our meeting, I'd like to get some ideas from you folks.
My building site and proximity to future house don't really allow me to compact and build-up a pad site and have balanced fill on both sides of my foundation walls. So, I' really like to keep my current thought process and come up with a solution that meets my needs and budget. My thoughts for "solutions":
1. Shrink the overall size of the building and put in a full basement with steel I-beams supporting a concrete slab. Concrete slab above would need to support vehicles, max load of 5,000 lbs etc. This would be complicated as I want to put a car lift in and that would be difficult on a suspended slab I think. Also, I know steel prices and they're not bad, but the engineering for such an arrangement and the decking to pour the slab over the steel don't have a good feel for cost.
2. Keep the current arrangement and fill with something that doesn't exert horizontal force on the walls. Sand? Flowable fill? No idea of this cost or if this would agree with the building officials (not strict here, but not stupid).
3. Engineered slab with some grade beams that doesn't require compacted fill underneath. No idea of cost. Maybe pour two piers down to virgin soil with independent footings to support each side of my car lift.
4. Compact up a pad with slopes on each side and then cut it down to size to make an "earth plug." Pour my footings on virgin material and fill the gap between the compacted plug and the poured walls with sand or aggregate. I still think I need to compact these, so this probably won't work either. I have my own backhoe, so some of the dirt work is low cost and basically just diesel.
5. Engineering foundation walls that would withstand the fill would get really expensive I think and be much thicker than the 8" wall I'm planning.
So, for a homeowner (i.e. residential dollars, not commercial), what do you guys/gals think for a best course of action? There may be something I haven't though of. Thanks a lot. I enjoy this forum--lot's of smart folks on here.
-Matt
(located in central VA)
So, my problem is that I don't think this will work. Compacting the fill inside the garage will bulge the walls. Not compacting the fill will allow my slab to settle. Using something else for fill (sand, aggregate, etc.) is going to cost a fortune. Although honestly, I don't see this arrangement as much different than my house foundation except the open space/soil is reversed. Maybe the difference is I'm not compacting the back fill against my foundation.
My background is mechanical engineering, so I'm not a civil or geotech. But I know enough that I know I need some help. My current plans are to talk to my geotechs/civils and let them see my plans and come up with an affordable solution. Prior to our meeting, I'd like to get some ideas from you folks.
My building site and proximity to future house don't really allow me to compact and build-up a pad site and have balanced fill on both sides of my foundation walls. So, I' really like to keep my current thought process and come up with a solution that meets my needs and budget. My thoughts for "solutions":
1. Shrink the overall size of the building and put in a full basement with steel I-beams supporting a concrete slab. Concrete slab above would need to support vehicles, max load of 5,000 lbs etc. This would be complicated as I want to put a car lift in and that would be difficult on a suspended slab I think. Also, I know steel prices and they're not bad, but the engineering for such an arrangement and the decking to pour the slab over the steel don't have a good feel for cost.
2. Keep the current arrangement and fill with something that doesn't exert horizontal force on the walls. Sand? Flowable fill? No idea of this cost or if this would agree with the building officials (not strict here, but not stupid).
3. Engineered slab with some grade beams that doesn't require compacted fill underneath. No idea of cost. Maybe pour two piers down to virgin soil with independent footings to support each side of my car lift.
4. Compact up a pad with slopes on each side and then cut it down to size to make an "earth plug." Pour my footings on virgin material and fill the gap between the compacted plug and the poured walls with sand or aggregate. I still think I need to compact these, so this probably won't work either. I have my own backhoe, so some of the dirt work is low cost and basically just diesel.
5. Engineering foundation walls that would withstand the fill would get really expensive I think and be much thicker than the 8" wall I'm planning.
So, for a homeowner (i.e. residential dollars, not commercial), what do you guys/gals think for a best course of action? There may be something I haven't though of. Thanks a lot. I enjoy this forum--lot's of smart folks on here.
-Matt
(located in central VA)





RE: Fill for detached garage
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Fill for detached garage
I understand your response. However, I was under the impression that I cannot just take fill from my basement and spread it out for a garage pad--maybe I'm wrong. Everything I've read (a lot from this site) says that I can move fill around as much as I like, but even if I properly compact the fill in 12" lifts, I would have to dig THROUGH the fill to virgin soil to place my footings. Also, I have no idea if the fill they will take out of my basement is "good fill." I'm building in a virgin forest, so I'm not concerned with "hidden" things like stumps and landfills--just soil quality. I'm having a soil test done later in the process once I get the location of my house fully defined.
RE: Fill for detached garage
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Fill for detached garage
One method may be to run vertical re-bar in the stem wall and bend into slab. But all that needs to be calculated.
RE: Fill for detached garage
Anyway, I'm planning on talking to these guys. I was just hoping to come with some good ideas and work through some of the obvious "bad solutions" to limit the size of the wheelbarrow of cash I need to bring. Actually, a bigger part is that my "design" I've got is flexible. I don't want to show my design and say "make me a foundation for this" as I'm willing to make changes to keep the cost under control--maybe substantial changes. We might want to shrink the size of the building and come up with some novel ways to keep my square footage. -Matt
RE: Fill for detached garage
RE: Fill for detached garage
You didn't mentioned any special compaction or wetness of the fill or even what kind of soil material. A little money with a good geotech/structural will save you big headaches later.
RE: Fill for detached garage
The foundation walls can be designed to handle the added lateral load of compacted fill. No big deal...but it has to be designed...see Mike and Mike above.
I see no issue with building a foundation wall, filling inside using light compaction equipment and thin lifts, and then placing your slab on the fill. If you go to an elevated slab, you will incur lot more expense and effort.
If placed properly, fill is no more susceptible to settlement than natural soil, often less so. Make sure the soils below the fill are competent and won't settle excessively from the weight of the fill.
RE: Fill for detached garage
RE: Fill for detached garage
RE: Fill for detached garage
How shallow is groundwater in your area?
Looking are your concept drawing, it seem like you might need a ramp to get to the garage? If that is the case, why not just lower the short side of the wall and put the garage at a lower elevation?
RE: Fill for detached garage
I have to disagree here too with regard to the fill. It can be done in two ways here...
1. Design the exterior foundation walls as yielding retaining walls, allowing for the induced pressure of a tamped backfill, and restricting the equipment weight and time of execution so that the green concrete of the wall is not cracked. I have seen this happen too many times.
2. Design the exterior walls as basement walls with the resteel at the top of the wall tied to a partlially poured interior slab with a 4 to 6 foot pour strip, then backfilling in the pour strip and pouring the strip later. This serves as a tension tie for the top of the slab. Done this many times and it works well for garages and other structures with this scenario.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Fill for detached garage
Let's assume a granular fill material with a unit weight of 110 pcf after compaction. Further, lets assume that compaction is done with a plate compactor in 12" lifts (thus not exerting a great deal of lateral pressure on the wall during compaction). Now if we dump all the soil in between the walls and don't compact it, we will likely have between 85 and 90% compaction. If we compact it, we'll push that up to 95%. The difference is 5 to 10 pcf, which in the scheme of design for a reinforced masonry or reinforced concrete wall of limited height, is very small. When tied at the top as you suggested, negligible.
RE: Fill for detached garage