Fondation on bedrock
thierry19
| Posted in General Questions on
Hello
I’m looking for a good resource on building a fondation on bedrock. All I can find is a few articles here and there but nothing that really satisfies me.
I’m building in a cold climate (Quebec) so it would be helpful if the effect of climate on exposed bedrock/fondation was covered.
Thank you kindly
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Hi Thierry,
Just to clarify so you may get more pertinent answers: As you building on bedrock that will stay ex[posed around the house, or will there be fill around the foundation walls once the project is complete?
Sorry, I didn't want to get into all the specifics of the site. The fairly steep slope will allow me to bury the back half of the fondation (road and driveway side). The front of the house (looking over a lake) will have a walk-out basement and exposed fondation.
I found some articles about the challenges of building on sloping bedrock but I'm still looking for a better one.
Thanks
My experience with bedrock and foundations is limited to my current homestead. On our property everything is sloped (we live in the front range of the Rockies in MT). On every building and outbuilding I either have hit rock immediately or within a few feet. Some of the rocks I have been able to move and others are impossible to move without either bringing in (really) big equipment or blasting. In talking with three of the concrete/foundation contractors in the area, the consensus is a) they never blast, b) two of them pin/epoxy into the footings, and c) one pours 3/4 inch clean gravel 3 to 6 inches deep over them assuming they are underground.
I don’t have any background in engineering nor did I consult a soil or structural engineer for my house and garage and shop but opted to use the “gravel” approach with either a frost wall and continuous footings or basement on top of the same. For our barn (48x36) I poured 20 inch concrete piers - when I hit a rock that I could not move I stopped...some are at 5 feet and some are at 2. Take that for whatever it’s worth.
Thank you Peter. What you hear from contractors is also what I understand to be the best approach. I will not blast and will pin the fondation to the bedrock. I have the solid Canadian shield to work with.
Thierry,
Vancouver island is all rock. We pour on sloped bedrock all the time. No footings, just pin the stem-walls. Much easier when there is fill on the up-hill side, as then you can bury the perimeter drains. On sites like yours where the lower part will be exposed, I prefer to delineate the point where the fill ends with a retaining wall. It makes the transition (and end of the perimeter drains) cleaner.
Thank you Malcolm. I'm starting to learn about the challenges and things to consider and watch for when building on bedrock.
The no-footing part is interesting and makes sense when you think about it. You don't need a wider base to spread the weight of the house when you're sitting on solid granit.
I would still like to find a good reference book or manual about building on bedrock...like "Everything you ever wanted to know about fondation on bedrock" or something like that :) I still have so many questions. A good resource would likely answer most of my question without having to bug the experts on this forum.
Thanks
Thierry,
I don't think such a book exists. The differences between building on bedrock and conventional foundations are probably seen as not being significant enough to warrant one.
As an additional note about pinning foundation walls to bedrock, ICF forms are great for this. You can cut the foam to follow the shape of the rock very easily.
Thanks Yupster.
Yes I read about ICF. Unfortunately my fondation contractor is not working with them.
Thanks
Hello Thierry
Since you are building in Quebec like I am, have you checked to use helical piles? I think they can pin them to the rock and that would solve your sloping problem. From what I understand they can put them underground as long as you put enough insulation to prevent the soil from freezing (same detail as a frost procted shallow foundation).
Helical piles on areas with anything larger than softball sized rocks are problematic. Once they hit rock and can’t go further they spin and undermine the soil above them thereby loosing nearly 100 percent of their uplift strength. We sunk 20 of them to support a solar panel array, they had challenges driving all but a few of them and none of them were in the right spot (due to constantly hitting rocks and not installer incompetence). Before even putting panels up on the frame the wind was able to pull a few of them several inches out of the ground...we ended up pulling them from the ground after rethinking the strategy and many of them pulled free with surprisingly little effort. On certain soil types I’m sure they are brilliant but my experience is that for us that build among enormous rocks, helical piles are not a viable option.
Are you speaking of hand driven piles? These are not worth a dime. I'm talking about real helical pilces driven in by machinery. There is no way you can pull these by hand or just by the wind blowing. If that's the case, there is something really wrong with your installer frankly. Skycrapers and factories are built on these types of piles so as long as you choose the right pile and drive them at the correct depth, they can support whatever load you want.
Anyway, they should be able to guarantee the bearing capacity by monitoring the torque required to drive the pile in the soil. Then if they hit a rock, the pile will usually push it aside. If the rock is too big they can always excavate it or drill through it or move the pile if it's feasible. When the bearing capacity is high enough, they just pin the pile to the rock hence my suggestion since he's building on bedrock.
Helical piles are really popular here. The modern technique of monitoring the torque was invented in Quebec so there are at least 10 installers that I know of and why I made that suggestion. I have seen house additions, sheds, decks built on these kind of piles in various conditions with no problem. It's a really clean option and remember that the frost line here is at least 4 feet deep so it can save you lot of excavation work.
Yes real helical piles driven by a skid steer attachment by a crew that has been doing them for years. Once you start hitting big rocks with them all bets are off as far as uplift and placement. On several of them they bent the piles hitting large enough rocks before they even got a foot down. If you have to drill and tap or dynamite the stubborn rocks I don’t see the advantage, especially when invariably the rocks could throw the pile off course. Again I’m sure in the permafrost in Alaska and other areas and soil types they solve a lot of problems and are brilliant.
Alex,
(Tu parles français j'imagine)
I'm on exposed bedrock, no soil. Surface has been cleaned already and loose rocks have been removed. I'm on a smooth but very uneven granit bedrock. Helical piles is not an option.
Have you already found this JLC article? Seems like it might be relevant.
A Foundation on Ledge
Rebar and epoxy provide the grip on a sloping coastal site
https://www.jlconline.com/how-to/foundations/a-foundation-on-ledge_o
Thanks Kohta,
I had not seen the one you sent me but I found the following one which is closer to the situation I'm facing. Ready-mix trucks can access the site so I'll have a full concrete foundation.
https://www.jlconline.com/how-to/foundations/pinning-to-ledge_o
I just wish I could find lots of these articles and also construction-detail drawings for foundations on bedrock. It is not rare yet finding documention is difficult, I find.
Thank you
Hi Thierry
I just did this on my project on the other side of the country. There's no manual and very little information on best practices etc but here's what I did after reading a ton of GBA and some sage advice from Malcolm Taylor. I also had a great geo tech to work with.
I was initially considering a slab but we switched to a 4ft crawlspace-the cost of fill and access was prohibitive and a crawl space is so much easier to run mechanicals etc. I had to blast down on the uphill side to get my entrance close to grade and on the down hill side it's just over an 8ft wall up which is a few inches above my crawlspace slab. The grade is about a 5ft change in elevation over 30ft. The walls step down in between. We're in an area with an earth quake risk so my geo tech wanted everything pinned where we could hit bedrock, which ended up being the entire perimeter. Our excavation costs were significantly higher than I expected because you just don't know what you're going to find, so budget more than you might think. The blast rock was crushed on site and later used as structural fill which ended up saving money, despitee the cost of crushing. We drilled 12 inch deep holes 16" OC and used Microsill anchor grout at the geo tech's recommendation to anchor 15m bar. After drilling the holes were blown out and the depth was checked and then again each time before bar went in. Good drill bits are a must, the guys had some Hilti bits which while pricey, cruised through without issue. Microsill was easy enough to get and straightforward to use. My forming guys assured me they'd used it before but mixed the first batch too thin. It didn't harden and didn't hold the bar so I had to pull it out and redo it. Whatever you use, make sure it's rated for the application-there's a popular product out here guys use all the time to anchor deck hardware in concrete etc but if you read the website it's actually not rated for exterior use. At the geo tech's recommendation, weep holes were placed anywhere the contour of the rock inside the perimeter sloped towards the concrete at 4ft intervals, so the entire front wall and half way along the side walls. I cut some squares of non woven geotextile fabric and used 3m hi strength adhesive to stick it to the concrete and stop the weep holes plugging during backfill, it worked really well. In hind site I might have included a couple in the uphill wall where I found a little water pooled at rain at low points in the rock once the concrete had been poured. It's wall below the level of the crawlspace slap though so I'm not too concerned.
Before closing in the forms, the rock surface was power washed clean as best we could to help adhesion of the concrete.
Layout must have been a bit tricky for the forming crew and I discovered when it got to framing the concrete was a little out of square and parallel. I have since seen there are more sophisticated laser systems which can help with layout accuracy on sloping sites like this.
Waterproofing-my priority was preventing hydrostatic pressure build up at the height of the crawlspace slab and above. Because I wasn't sure exactly where my final grade would end up and different slope profiles on opposite sides of the house, I didn't want to use a black damp proofing product which could end up being visible and would just look a bit shoddy. Add in the rough looking finish to the concrete in the most visible areas and I choose to plug the snap tie holes with hydraulic cement and use grey Masterseal 581 over the entire wall surface. It's not a perfect finish but it's an improvement over the poured concrete finish na doesn't draw the eye like exposed portions of your typical damp proofing would. Although Masterseal should be waterproof at the correct application, being I first time user I didn't want to solely rely on this. I used Delta MS dimple mat on the upslope side so that any surface water would be directed down the face to perimeter drainage below the level of the crawlspace slab. I got an 8ft high roll which covered the wall and overlapped on to the undulating rock profile. I cut slits to shape it down in to the low spots and undulations as best I could. It's possible some water can make it's way to the base of the wall but it's well below slab level. Installing the Delta was a bit of a pain in the ass, nails fairly frequently just blew out chunks in the concrete, which got frustrating, despite using the recommend nails and charge strength for the gun.
For my perimeter drainage, the high point in the rock on the upslope wall was in the middle. I ran two independent lengths of pipe in opposite directions to follow the slope down each side of the high point, overlapping them in the middle to provide an effectively continuous drain which meets on the downslope side. I could also build in a small swale or edpm lined shallow drain upslope a little but I don't think it's necessary at this point.
I don't think an IFC would have been possible, the rock profile undulated so much. Further more, the area is prone to carpenter ants and some termites. Add in the cost of ICF and parging the large exposed sections on the downslope side and it would have been cost prohibitive, not to mention the fact in my rural island location ICFs and experience with them is not common. I ended up using 4 inch EPS being sold off cheap from a commercial jobsite to line the interior of my crawlspace walls and under the slab. I couldn't see a way to insulate the exterior of the concrete within my budget-how would one detail 8ft of concrete covered in rockwool with 6ft above grade on the down hill side and make it look good? I couldn't come up with a reasonable solution.
Alex,
Great exhaustive description!
Hi Alex
Awesome summary, thank you so much. I'll bookmark it for later. Many details you mentioned are similar to my situation.
I'm just still disappointed that there are no better resources to refer to for foundations on bedrock. A nice manual with pictures and details drawings, etc. would be so useful.
Thanks again Alex.
Thierry
Details, drawings, etc...sounds like you should hire an architect/designer/engineer ;)
Agreed, I couldn't fathom doing this without a geotech
I sure will Yupster, but I'm also someone who likes to learn and prepare so I know what's coming. And if there's something I learned on this site and elsewhere it's that someone, somewhere has found a better way to build and we should be ready to adapt and possibly challenge the local building code if needed.