Pavilion footings

[psmart]

I'm helping a local organization with the erection of a 40x50 foot timber-frame pavilion to be used for outdoor education and events. Basically a timber-frame roof resting on 8 timber posts with open walls. The question is what to use for the foundation under each of the 8 posts. The posts will be in two rows about 30 feet apart, and braced to the roof structure to keep them vertical, so the foundation only needs to provide vertical support for the posts.

The site is very rocky, with rocks and boulders up to 5' diameter (over 10,000 pounds). I've removed half a dozen of the beasts to clear the site so I know there is no easy digging anywhere. The minimal topsoil was removed before digging out the boulders, and the site was been leveled with 1.5" crushed gravel over 1.5" stone. The depth of the stone and gravel ranges from 3" at one end of the pad to 18" at the other. I suspect the water table is near the original surface in the spring, but I encountered no water in 6 foot deep holes in the fall. The surface is extremely firm and well compacted (it was previously a log landing) and there is never any standing water on the surface.

The initial plan was to install sono-tube foundations, but my time on the site tells we that every tube is going to run into rocks we don't really want to move, since each would result in an enormous hole. What are the other foundation options? Would pre-cast concrete bases be appropriate? A shallow foundation would avoid the excavation issue, but might not provide enough frost protection in New Hampshire. (On the other hand, we're not using drywall or other building materials that require an especially rigid foundation.) We could add a certain amount of fill to build up around precast bases, but not too much, since the goal is to maintain full accessibility around most or all of the perimeter. I've even thought about a well driller, that that would probably be too expensive!

Excuse the long question, but I wanted to provide all the background that might be pertinent to your recommendations. Thanks.

 

[EireChch]

Precast concrete foundations would be fine, just as long as they are sized appropriately. I would think that uplift could be your critical case so make sure you have enough mass to resist.

In reality it is a very light structure so it just comes down to whats most practical. Whether you use a precast pad or cast in place, you still need to embed the foundation sufficiently so it looks like you will need a trial hole beneath each of your posts to conform ground conditions.

If you encounter a large boulder then your only option is to remove it. If its impractical to remove then you need to think about redesigning the foundations to span above it or relocate the foundation.

 

[psmart]

Thanks EireChch. Any other comments?

 

[EdStainless]

My grandfathers house was built sitting on 12 very large boulders.
If you have boulders that go 5' into the ground my not build on them?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[le99]

If no uplift concerns, I would consider pad foundation, and won't worry about frost-heave for this rocky site.

 

[HTURKAK]

Probably there will be floor tiling on a SOG . If there are boulders and rocks , the frost depth would not be a concern. Just provide sand and gravel subbase to level and slab on grade concrete foundation .

 

[psmart]

FYI, the finished floor will be crushed gravel - no slab. The only question is how we support the posts.

 

[EireChch]

I dont think any approving agency would accept you bearing on boulders. In all honesty it would likely by ok, however its just not the done thing in my experience.

Just put the posts on concrete pads. A 1 x 1 x 0.5m thk pad is probably fine if your loads are light. Revove a boulder if it is located in a post foundation. You have already removed 6, if you have to remove a few more just do it to give you piece of mind.....

 

[psmart]

The problem is that excavating for 1x1x0.5m pads would encounter large rocks and boulders for almost every footing, so we would probably end up digging out the entire footprint of the building. Once we set aside the boulders there wouldn't be much material left to refill the excavation, and that would be hard to compact. This is a REALLY rocky site, with almost continuous rock-to-rock contact and a minimum of workable fill.

BTW, setting on boulders would also be difficult, since most of the posts would probably land on a steeply sloped face.

 

[HTURKAK]

Still i would prefer depressed slab foundation ( say 200 mm ) with pillars ( for each post ) and planted to raft and provide crushed gravel on it.

My opinion ..

 

[EireChch]

you are only digging out the 0.5m3 of material at ever timber post. I dont see the need to remove everything within the footprint?

Your gravel base (or slab if you go with one) will act independtly of the post foundations. As such it can rest directly on boulders and rocks etc.

There may be a risk of hard spots (should you pour a slab) due to the rocks but i will be very lightly loaded, likely wont be an issue.

 

[le99]

Why not provide above-ground pad/block foundations, then fill gravel in between. The foundation pad/block should have adequate weight to resist the uplift.

 

[psmart]

you are only digging out the 0.5m3 of material at every timber post. I don't see the need to remove everything within the footprint?

Because of the rocks and boulders, each 1x1x0.5m pad would require the excavation of at least 10 times that volume. So the holes for 8 footings would end up disturbing most of the footprint.

 

[psmart]

Why not provide above-ground pad/block foundations, then fill gravel in between.

That's seems an attractive option. The challenge is that we need to maintain ADA accessibility around most of the perimeter, so we cannot add much height to the existing grade, or we will have to build up the entire site to match.

 

[le99]

Without slab, I think there are ways to get around. You should consult someone who's familiar with ADA concerns.

 

[psmart]

You should consult someone who's familiar with ADA concerns.

The entire facility is on an accessible trail system, being built by a contractor with ADA experience. Our goal with the pavilion is to keep the finished floor as low as possible so it will match the surrounding grades with a minimum of additional fill. Hence the desire for in-ground footings that don't raise the finished floor. (At least not too much!)

 

[le99]

Not to argue with you or the expert. But for the site condition (rocky and dense ground) and the rather light structure (except for uplift concern), you can even place the floor below the foundation blocks as there is no need to maintain continuity/structural tie between the two structures.

 

[psmart]

you can even place the floor below the foundation blocks as there is no need to maintain continuity/structural tie between the two structures.

In order to maximize floor space and prevent a trip hazard, we would like keep any foundation pads/blocks at or below the floor elevation. I suspect we will dig in as much as feasible (without blowing out the hole) and then build up as required to burry the pad.

 

[le99]

How thick of the pad is anticipated?

 

[EireChch]

psmart- I see now re the boulder size / footprint issue. Thanks.

To be honest there are not many more alternatives that what I or le99 has proposed. Maybe back to a bored pile as you suggested origionally but again that costs money.

You could consider just cutting out the section of the boulder that is within the volume of the concrete pad. But again, this is not ideal as you may have the corner of a pad sitting on a boulder with the remaining portion of pad on soil.

I do see that it is a lightly loaded structure and will probably be fine. But it is not a common geotechnical recommendation in my experience and I would not be willing to put my liability on it. The liability being that there could be differential settlement which may need correcting at some point.

I accept that you are trying to help a local organization and keep costs low but at some point you have to spend money. Investing in a robust foundation is not wasted money...