Standalone 20x24 workshop foundation questions

[geshka [OP]]

Hello there.

I am planing to build standalone workshop 20x24 size. Soil has a lot of clay and water is high, plenty of movement at Winter/Spring season, so lots of issues around there. I was talking to several specialists, including geo-tech engineers and got quite confusing and oppositely directing recommendations. So far I am inclined on concrete pad on grade foundation.
Asking this community - from experience what someone may had in this situation - what are my option(s)?
Also, I am in Ottawa area - is there any recommendations for foundation contractor: not too expensive, not too big , willing to have small job.

Thanks
Gene.

P.S. I have workshop already on concrete posts with wooden floor on joists and its not holding my heavy machines (lathe, mill, surface grinder), but I've got what I payed for - flow is uneven and sinking . So I will take it down !!! and rebuild on better foundation/floor

[dave_k]

I built a shop with a Slab on Grade foundation in London around 1990. Clay soil, high water table and the whole neighbourhood has seasonal ponds. I drew up the foundation and my structural design teacher (I was just finishing college at the time) stamped the drawing. It was a 6" slab reinforced with 6 x 6 mesh and 12" ? (coulda been 16" I forget) drop footings around the perimeter. I used 3 - 10m reinforcing bars around the perimeter.

I stripped the topsoil to undisturbed soil and replaced it with compacted A gravel to grade. I built the center up with compacted A gravel leaving the perimeter full depth to form the drop. I used 3/4" form ply ripped to width to form the sides. I forget exactly how I formed it except that I used snap ties and walers simply because I had them and I then used the snap ties to tie the steel and wire mesh to.

It turned out beautiful. I checked the grade regularly with a builders level and then a laser when I finally owned one and it may have moved but it always stayed 100% level. The key to success is in designing the subgrade. Water has to be able to flow freely under the slab. To achieve this I extended the excavation beyond the back of the shop then filled that with 6" stone and topsoil over that.

That's my experience ... yours may vary.

[geshka [OP]]

Thanks a lot Dave. This is very helpful.
Did you take any measures to prevent frost/heave ? I believe in 1990 styrofoam was not popular as insulation, anyway ...did you use anything ?

[dave_k]

Frost heave? No I did focus on quality though. I made sure sub base was well compacted, consistently thick and properly formed .... as I recall I spent a lot of time with a rake and a plate packer. And Styrofoam was very popular back then but I didn't insulate the slab. I did insulate the walls and the ceiling but I just wanted to keep the foundation simple and the building cheap .... I had just spent 3 years in school.

[pmtottawa]

my recommendation is a thickened edge slab on compacted non frost susceptible gravel. 6x6x6 WWF mesh reinforcement. (2) 15m cont bars at bottom of thickened edge footing.
weeping tile around perimeter and a sump pump should your water be high.

[iamtooler]

I second Dave's recommendation but I would bring the floor a good 8'' above grade. As time goes by the surrounding terrain always builds up and you do not want the floor to be the low point! I also highly recommend using a 5000psi concrete, a minimal extra expense on such a small slab and far superior floor. Look at installing heating lines in floor, not too expensive if you lay them yourself.
Rob

[guitarchitect]

I hope this isn't overkill but here's my thoughts - based on what you describe you want to maximize two things... drainage and insulation.

Drainage

if you have a lot of water, you may consider excavating a lot more than you need in order to put in a soakaway / french drain... basically you dig a big hole, fill it with gravel, and cover it with earth - you slope your excavation or weeping tile towards it and it gives the water a place to go if it wants to hang around, and then it "soaks away" true to the name. that's what dave_k is describing. your geotech guy can tell you if there's too much clay in your soil for this to be effective, but it is a good/cheap approach if your circumstances permit because you need the equipment and materials on site to excavate for your pad anyway.

when I did my shop, I didn't build on undisturbed soil so I was able to slope the excavation 2% to one side so that any water that doesn't soak into the ground will find its way out from under the garage. I extended my soakaway up to grade at the back with all of my leftover gravel, and now I have a nice dry pad for building materials / lumber / toys / etc.

Insulation

If the ground can't freeze, it won't heave. you can accomplish this by implementing the principles of a Frost-Protected Shallow Foundation to your eventual slab approach. Think of it like this - if your frost line is around 5' down, and you put an insulated wall 5' down, the ground under the wall won't heave and neither will the foundation. This is why foundations are made the way they are. But concrete pony walls that deep are expensive. So you can accomplish the same thing by going horizontally. 5' out is the same as 5' down if you use the right thickness of insulation for your climate. I didn't have the space to do it horizontally (tight urban site) so I extended my perimeter insulation 2' downwards (see my detail below, although i didn't draw the extended insulation). If I had the room I would have extended it 4' horizontally. Here's some more information about it.

Beyond
Assuming you take care of the drainage and you properly insulate it against freezing, you've put on your belt and your suspenders. The last thing left to do is over-engineer it! The first 4" of slab are the most expensive... every inch you add is a marginal increase. I did 5" with a wider-than-typical footing at the perimeter, with added rebar and wire mesh in the slab. I used 36mpa concrete just like iamtooler suggests (typical for the heavier loads in a garage, a regular basement slab would be 20mpa). Altogether it makes the whole thing very strong and rigid for the heavy loads of a garage... so putting shop tools in it is no big deal, and it should be pretty resistant to movement. If it does move, it's moving as a single unit.

Attached is a detail of my own foundation. In order to accomplish good drainage you might consider something similar - establish a 6" compacted bed and build your foundation in insulation foam on top of it. I was originally going to use a Legalett system (which should be considered by anyone hoping to build a garage, it's a great system) but the way I built it ended up being a lot cheaper with many of the benefits. The form of the monolithic slab was achieved with sand infill rather than gravel or foam. The only reason the perimeter foam isn't thicker is because it would have been proud of the wood siding, forming a little "shelf" that would require thick metal (and a tricky forming job) to protect it. Too finnicky for a shop! But as described above the whole strategy basically allows for full drainage under and away from the foundation. It's not as frost-protected as I would like, but such is life - that's why I thickened the slab and the perimeter. I'm lucky to be in a very sandy area so heaving isn't as much of a concern or problem - there's no sign of it anywhere, even on the parking pad out front or the 80' of driveway back to the shop. I was definitely over-doing it with my approach but I didn't want any surprises in 5, 10 years!

That's a lot of info so let me know if you have any questions!

[Egon]

Will you have to meet any local codes?

[geshka [OP]]

WOW, thanks for the responses, folks.

@Egon . I do want to meet local building code, though I don't have to. I am in rural area with no neighbors close to me or other constructions, so I am a bit relaxed in this regard.

@guitarchitect . Thanks a ton for the detailed answer. Few more questions if you don't mind: Shed that I am planning to build will have no heating at winter (it is actually three season shop) . Will it change anything (from frost heave perspective) in your proposed method of building foundation? Water right now is 2 feet deep from surface level - very high... and it is like that every spring and fall Should I use sump pump or there are other solutions?

Thanks.
Gene.

[guitarchitect]

WOW, thanks for the responses, folks.

@Egon . I do want to meet local building code, though I don't have to. I am rural area with no neighbors close to me or other constructions, so I am a bit relaxed in this regard.

@guitarchitect . Thanks a ton for the detailed answer. Few more questions if you don't mind: Shed that I am planning to build will have no heating at winter (it is actually three season shop) . Will it change anything (from frost heave perspective) in your proposed method of building foundation? Water right now is 2 feet deep from surface level - very high... and it is like that every spring and fall Should I use sump pump or there are other solutions?

Thanks.
Gene.

I designed mine as an unheated building - that's why it has the insulation fully under the slab and it sits on a drainage layer. My shop isn't "heated" per se, but i try to keep it slightly above freezing and around 10C during project time. If you're going the horizontal FPSF route (which I would be tempted to do - i think you get into dodgy territory putting frost/pony walls in when the water is that high), you would want to put your horizontal insulation on a minimum of 6" of compacted gravel (maybe 12"), and the gravel under the monolithic slab in my scheme I would say should be more like 12". Again though it's worth discussing with your geotechnical guy to figure out if the soil has too much clay to allow something like a soakaway to work. In some clay-heavy soils digging a hole is like putting in a pool - if water gets in it stays there. In that case you have to pump it out + away.

Remember that the water level is the water level no matter where your foundation is. If you dig a hole to 1'10" and your water is at it's highest at 2', it's not getting in. If you build frost walls below the water line, you now have to manage and deal with *all* of the water from 2' down to 5', both between the walls and outside of them, and you may need to deal with water movement as well. With a properly drained monolithic slab-on-grade you are doing your best to keep the building above the water, and the drainage layers help ensure that any water that does get in will be encouraged away. The intent of the FPSF insulation under the building is partly to ensure that any water remaining won't actually freeze before it gets away, and the wings help ensure that the freezing is pushed as far away from the building as possible.

I have to issue a caveat though to say that I've never done a FPSF in an area with water that high, so I don't know what the best approach is. To be clear it will never eliminate heaving - the idea is just to keep any frost movement away from the building. You definitely want to try to work with a contractor who has done it before, because the local climate partly dictates the amount of insulation.

[dave_k]

That's a lot of info so let me know if you have any questions!

Great post and I realize that the section detail you included was simply for illustrative purposes however I have a few minor issues with this detail

It calls for "compacted sand" ... sand doesn't compact, unlike gravel which is graded, sand particles are of uniform size .... they won't compact. Run a plate packer over it and it only displaces the sand. I've worked with sand under rigid insulation building ice pads for arenas, getting the sand flat and level so your insulation sits flat is by far harder than placing and finishing the concrete. I guarantee by the time you get your wire mesh in the sand will look nothing like the dwg and have filled in your drop footings. In theory it's a good idea but not in practice.

I would also add a third bar on top along the line of the anchor bolts and extend the gravel base a foot or more beyond the base of the footing for stability. This is academic really because this type of foundation has to be designed by an engineer.

[stotto]

Following original post and not trying to derail thread.

What could you have done to improve on existing shop with concrete piles ( maybe screw piles) and wood floor?

You say it's not holding larger machines and is sinking.

Are your concrete piles actually moving? If so, how deep and how big around?

Are joists 16" on center? 10" or 12" ?

What size/ply beams?



If floor was beefed up and maybe screw piles ( instead of concrete) used it may be a much less expensive venture than a slab. Or not.

Concrete pricing out west has gotten ridiculous but have no idea what it costs out your way.

I'll follow post with curiousity as I was also planning on building a shop this year ( or having a small modular built and set on site) but the property owners we were talking with decided not to sell.

My thought ( and I'll ask the same questions to OP ) was why spend thousands of dollars on concrete?



I'm far from expert in this area so very interested in the many experienced folks on here chiming in.

[guitarchitect]

It calls for "compacted sand" ... sand doesn't compact, unlike gravel which is graded, sand particles are of uniform size .... they won't compact. Run a plate packer over it and it only displaces the sand. I've worked with sand under rigid insulation building ice pads for arenas, getting the sand flat and level so your insulation sits flat is by far harder than placing and finishing the concrete. I guarantee by the time you get your wire mesh in the sand will look nothing like the dwg and have filled in your drop footings. In theory it's a good idea but not in practice.

I get what you're saying but - it is indeed compactable. There's a big difference between sand that has been dumped in place, and sand that has been compacted. Once compacted, construction sand has very high compressive strength (it is coarse), much more than typical sand (like beach or desert sand, which is fine and rounded) which wouldn't hold a footprint.

[geshka [OP]]

What could you have done to improve on existing shop with concrete piles ( maybe screw piles) and wood floor?

I leveled floor every year jacking it and putting shims. One time I had to cut off concrete post in the middle as it was risen after winter. I am not screwing my floor panels to the joists as I need to get down every spring and re-level it. I have my posts on depth 4.5 feet down and about 1 feet above ground. Nevertheless, this spring posts are leaning sideways and making my floor crooked again. Also - I started to do some metalworking and brought heavy lathe and milling machine, which , of course contributed to the problem.

I probably had it enough.

I have several quotes from 7K up to 12K for the excavation , grade and concrete.

BTW -if somebody knows decent concrete foundation contractor in Ottawa area, I will be happy to continue shopping.

[Egon]

If winter water frost heave is an issue build above grade with concrete piles set below the frost level.