Fill for detached garage

[20turbo]

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)

 

[dicksewerrat]

I would build the house first. Move the basement material to you garage site . Once the house is completed and you are moved in, reevaluate your garage. A 30 x 44 is a six bay garage. You may be able to move the basement material around to make a 4 bay wide and only one bay deep without needing more fill. You can buy a lift that bolts to the floor. you will have to design part of the floor to accomodate the extra weight for the lift.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[20turbo]

Richard, agreed, it's a big garage. Let's call it what it really is and that is a garage/shop. Most of the space is workshop and it only has two garage door. The 30' presents itself to the road to not look so wide and the 44' deep is for space, workshop, metal lathe, metal mill, welding, sandblasting, etc.

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.

 

[msquared48]

There are numerous ways to handle this structurally, and we can give you some general guidlines here. To get the best answer though, you really need to combine our thoughts with the expertise of a local structural engineer.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

M^2 has it right. Get a good Geo/structural guy involved. This is actually done all the time.

One method may be to run vertical re-bar in the stem wall and bend into slab. But all that needs to be calculated.

 

[20turbo]

I've got a local company that includes Geotechs, Civils and Surveyors all under the same roof--they've already done my site survey and will be doing my soil testing. Not sure if they do Structural or not. I'm also going to talk to an Architect. He might tell me to go talk to an engineer.

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

 

[hokie66]

If it were mine, I would use a suspended slab. Either form the slab or cast it on the fill. You will need a structural engineer to design it, either way you go.

 

[oldrunner]

You are building your garage on a fill?? Well it's going to move and settle and not necessarily evenly. I would have this as either a very stiff floating slab where portions of the slab will span over settled areas or a ribbed slab that's also very stiff. And if the fill isn't the same depth, over time, the slab might tilt.

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.

 

[Ron]

What's the big deal about fill? It's done all the time and yes, you can even put your foundations in fill, provided you are below the frost zone in your area.

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.

 

[hokie66]

I will have to disagree with Ron. Maybe the fill won't settle after construction, maybe it will. But in my opinion, and this would depend on location and cost of fill material, a suspended slab would be a more economical solution. You would have to do the numbers to determine whose advice to take.

 

[Ron]

hokie66...you might be right on the cost issue. My consideration was more of effort with the construction being done by the owner. Skill level has to go up for suspended slab. Dumping soil and compacting doesn't require much brain damage!

 

[Riggly]

Turbo, I think your idea is quite constructable, and depending on the foundation condition and type of fill, I do not see much of a problem. The soil test should reveal whether you will have a settlement issue. Depending on where you are in Virginia your foundation can be founded on soft soils or quite competent soils. One of the biggest contributer of settlement of the slab would be the 8ft surcharge of soil that you will be placing on your foundation, not the cars or even the lift. If you used a good granular backfill material (sand, gravel), settlement would be expected to be insignificant within the fill, and most additional settlement when you apply the slab would be immediate. For gravel, the compaction effect would be minimal. Furthermore, your enclosed region would be good to provide the necessary confinement for compacting the granualar soils. I also see backfilling with gravel and faster than sand - you can use much higher lifts for gravel compared to send, but in terms of cost sand might be a little cheaper. A slab supported by thw walls would be more technical and take longer to construct, and also to design. Depending on the type of soil from the basement, it can be used as backfill material. However, depending on which region you are, you will have a layers of good and bad soils, and that would have to be sorted out during excavation, which could be additional cost, and a challenge for the operator.

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?

 

[msquared48]

Ron:

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

 

[Ron]

Mike...I agree with your premise, I just think that the lateral load difference on the stem wall between compacted and uncompacted is being way over-rated here.

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.

 

[hokie66]

Give a dog a bone...my objection to placing a slab like this on fill is due to being called to too many jobs which have suffered from time dependent consolidation of fill. In the OP's example, his footings and walls will be founded in original ground, then his slab will be partly supported on the wall (I think), and partly on the fill. If time dependent consolidation occurs, the slab will dish between the walls as the fill support is lost. This is the reason we often design slabs adjacent to basements as "semi-suspended" for say 10 ft from the wall. Where I am, the building authorities got sick of this, and require any house slab placed on fill more than a metre thick to be supported on bored piers.