Foundations in sub-base material

nowImonboards

Hi all,

I'm digging some foundations for a veranda to be attached to the house. I have been advised to dig a 300 x 300 trench around the perimeter as the foundation for the main supports. I was initially advised that the foundation has to be dug into undisturbed ground. However, the site is only 3 years old and when I dug down into the topsoil I found it was laid on top of a bed of loose gravel sub-base similar to 804 but without the fines, which I don't think could be classified as undisturbed. I have dug down further and I'm still seeing this sub-base at the 600mm mark with no sign of undisturbed ground. It has since been suggested to me that I have dug down too far and pouring a foundation this deep can cause a weight imbalance that could do more harm than good since the foundation will be poured along with the floor surface all in one go (does this qualify as a raft foundation?).

The advice is to fill it back in with the loose stone and run it over with the compactor in layers to bring it back up to the 300mm mark. However, the fact that this wouldn't be undisturbed ground makes me a bit worried. I would have thought that the deeper trench would be better not matter what. I'm hoping I can get a few opinions here to help me decide how to proceed.

The structure that will be sitting on the foundation will weigh about 400kg per linear metre but I'd like to future proof it to be able to take something heavier down the line if possible. However, this is a DIY dig by hand so if that is just a pipe-dream the I'll be happy as long as I do it properly for the weight specs above.

Thanks in advance for any advice.

Calahonda52

Advice not given on the forum.

.

whats missing here is how far down you went to meet what you call the sub base

the other point is that the 300 by 300 is the size of the foundation, it does not start at ground level

The traditional ROT is that the strip should be 3 times the width of the wall so 100 mm wall = 300mm strip

100 put 50 cavity plus 100 = 750

and so on

A142 or A252 mesh will also help

can be got in 600 and 800 wide

https://www.designingbuildings.co.uk/wiki/Strip_foundation

nowImonboards

Thanks Calahonda52,

I appreciate the input. I initially went down 300 but could see the start of the sub-base layer at that point. However, it wasn't consistent along the trench and went deeper in parts. What was obvious was that the soil layer was put down on top of this sub-base and that it wasn't the original undisturbed ground. With that in mind I though it wasn't going to provide much support underneath the foundation and removed it. However, I'm not sure how far down the sub-base ends.

I thought that ROT applied when you hit undisturbed ground. Am I mistaken there?

Also, you seem surprised I am calling it sub-base. What is the correct term? Here's a pic for reference:

10-10-20

That's not 804, you're right about that. It's more like unbound gravel fill. I think it's far too rounded to compact in the manner required.

I'd refer to AJ's advice here around sub-base and sub-grades as he's the master.

https://www.pavingexpert.com/subbase

I'd like to think that you could backfill those existing trenches with 804 and use a whacker on it to achieve a solid base (as you mentioned), and I think I would take around 30cm's off the sub-base for the floor and also backfill that with 804 compacted in two layers.

All unprofessional advice here of course, but sure it's only a veranda. 😁

nowImonboards

Hi 10-10-20,

Thanks for the input. Funny, I wouldn't have described it as rounded. It's not like a pebble. It's more similar to the rocky parts of 804 but without any fines. More like crushed stone. I would have thought there was some standard material that builders use under the grounds of gardens in new estates before bringing back in the soil that would be easy to identify. I was thinking it would be more equivalent to T2 under SR21 Annex E as described on many of the main quarries as e.g. "4/40mm Gas Permeable Layer"

I was planning to add layers of proper 804 (SR21) on top and in the trenches but I was still concerned about whether this would be suitable for holding a foundation as it wouldn't officially be undisturbed ground. I was hoping that if I could identify this material I could find out whether it is suitable. I have previously read a lot of AJ's stuff on the site you provided but most of the materials he covers in detail are more UK centric with mentions of the equivalent Irish standards. It's a brilliant site. I just can't find the exact answer I'm looking for in this particular scenario there. I have googled around but the more I read the less I know for sure. I have read a few articles from other sites where I found the undisturbed ground rule, e.g. https://resi.co.uk/advice/single-storey-extension/depth-foundations-single-storey-extension

I know this isn't an extension but don't want to under-engineer it either. After all the digging and research it would be a shame to under do it, yet most onlookers think I'm overdoing it.

Calahonda52

You have two choices here once you meet the frost line requirements.

a: Dig out all the stone and go down to undisturbed ground.

b: Go wider and put in a wider foundation with steel mesh, and well vibrated concrete, as described earlier to spread the load on the stone.

Can you get a small digger in?

then dig out the stone

nowImonboards

Thanks Calahonda52,

Can you point me to some info that can tell me how to determine the frost line requirements for my area. I have done a bit of searching and some sites suggest there is a way to look this up by location (although I think they are mostly US). Is there an equivalent lookup by area for Ireland?

No chance of getting a digger in at this stage but since all the soil has been removed I can easily make the trench parts of the foundation wider.

Since you mentioned about going wider to spread the load, I should clarify that I'm planning to fill the trench part and the floor area all in one pour, so effectively it would be one big slab with deeper edges for foundations (does that qualify as a raft foundation?). I would have thought that would spread the load throughout the entire slab. Does that make a difference?

10-10-20

https://www.boards.ie/discussion/comment/122414701#Comment_122414701

Yes, seems like the best plan.

Funny, I wouldn't have described it as rounded. It's not like a pebble.
It's more similar to the rocky parts of 804 but without any fines. More
like crushed stone. I would have thought there was some standard
material that builders use under the grounds of gardens in new estates
before bringing back in the soil that would be easy to identify.

That's exactly what I was thinking - general infill stone. It just looks rounded from here as maybe the photo isn't picking up the oblique angles of true crushed stone.

Calahonda52

Frost line is between 450 and 600.

After the 2010 big freeze Dof E reminded us that their requirement is 600mm deep for cold water pipe

https://www.boards.ie/discussion/2056128547/depth-of-burial-to-stop-mains-water-supply-freezing

Are you building walls along the perimeter or what, your first post refers to future load

google raft foundations

nowImonboards

Hi all,

I had googled raft foundations but I was getting thrown off by mentions of supporting the raft with piles or beams. Since my structure will be supported mainly on corner posts I'm wondering whether I need some additional support there.

The closest I can find to what I was planning was similar to one of the examples on AJ's site mentioned earlier.

Source: https://www.pavingexpert.com/concrete

(I presume it is ok to include the image here since I have attributed the original source)

The above is referenced for a garage with a brick wall and only goes down 300mm. I am not planning walls at this point but I would like to future proof it within reason in case I change my mind down the line. I hate the thoughts of having to pull it all up and do it again later for the sake of a few extra hours digging up front.

Here's the full plan I have:

Finished floor:

• 10mm porcelain tiles (suitable for indoor/outdoor use)
• 10mm tile adhesive (without gaps to protect from frost damage)

Slab:

• 130mm concrete in main slab area (30n20 with extra sand)
• Single layer of A142 6mm mesh within the slab
• DPM (1200 gauge)
• 20mm quarry dust for a blinding layer
• 150mm SR21 compliant hardcore
• Stone infill to unknown depth and whatever is underneath that

On that note, here's a (hopefully) clearer pic of the stone in-fill:

I'll initially just be using aluminum posts at the corners of the veranda instead of the brick walls pictured in the example above. My walls and roof are glass though so there sill be some weight (around 500kg per linear metre). Here's an image for reference:

I was given the following pieces of advice which I'm having doubts about. I was told that:

1. I could bring the posts right to the edge and so didn't nee the 100mm margin
   This makes me nervous that the concrete could crumble at the edges of the post
2. I wouldn't need the T20 bars and could use a single layer of A142 in the slab section
   I initially wasn't so worried given there are no walls planned but since it isn't dug into solid ground I'm second-guessing this
3. I only needed to dig 300mm into the stone infill for the trenches of the foundation at each edge
   This is the part I'm having most trouble with
4. Going too deep with the foundation trench at the edge could actually cause it to sink due to the weight
   I can't see how this makes sense since under normal circumstance we would be planning to build walls over the foundation.

The people who have given me this advice have infinitely more experience that I do and they have given me solid advice before. It's just these points in particular that don't seem logical to me.

I appreciate everyone's input so far. I need to start compacting and doing the form work today to make the deadline so any more opinions (I realise no official advice is given) would be greatly appreciated.

muffler

I think it's time you engaged a structural design engineer for your project.

nowImonboards

I'm trying not to over complicate it. It was supposed to be a simple footing for a veranda. There are about 10 verandas in my estate and they are using simple post foundations made by digging a small hole and pouring a cement mix in. One veranda installer told me he just puts down a concrete block under each post. I'm just trying to understand how things should be done.

The only real difference with mine is I wanted a tiled floor (as opposed to paved) to create a more indoor/outdoor experience. To do that I needed a concrete slab. I'm just trying to make sure that the concrete slab is a little bit future proofed.

I didn't think it would be that difficult to warrant involving a structural design engineer. I'd be the only person I have ever heard of who had to involve a structural design engineer for a veranda. Where did I go wrong?

DOCARCH

Mod Note: Just reminding posters seeking or giving structural advice on the forum is not permitted (see forum charter).

nowImonboards

Noted, thanks.

Nothing taken as structural advice. All decisions at the end will be my own. Hoping someone can point me to some publicly available information that I can use to make such a decision.

Some examples I would expect that would be allowed, would be along the lines of:

• Whether this sub-base is standard for use in new builds
• When this sub-base should not be used or is not recommended
• Whether it is typically left there and/or even generally preferred as a base for concrete slabs
• Why the 100mm clearance is recommended at the edge of a foundation
• What are the typical weight limits that would warrant the use of T20 mesh in the foundation
• Why digging too deep in the edges of a raft foundation might be a bad idea
• What would be the typical weight limits for a foundation of 300mm in this type of base

I wouldn't imagine the answers to any of the above would be classes as advice. Rather direct answers to direct questions based on common standards.

Opinions are only related to whether my concerns are valid.

Other input could be simple references to situations from others who may have experienced similar scenarios before.

If I have misinterpreted the rules here then please accept my apologies.

bfclancy2

you are overthinking and over-engineering it but i fear you won't be deterred so it is prob best to engage an engineer as no one else will satisfy you at this stage, most people would have lobbed down some concrete on top of some hard core with some mesh

nowImonboards

https://www.boards.ie/discussion/comment/122416596#Comment_122416596

I completely agree with you on all counts :)

Unfortunately I am one of these idiots who wants to understand things to the determent of actual progress.

nowImonboards

On that note I'm hoping people can see I'm not looking to be spoon-fed here and actually trying to do the research. I'm just learning the terminology too slowly to narrow down my searches quick enough.

I guess I'll just have to trust what I have so far and go for it.

I'm just used to having to give reasons for the decisions I make.

nowImonboards

After rereading above again I'm a bit confused about the statement:

"no one else will satisfy you at this stage"

@bfclancy2: Was there a response above in particular that you think answers my concerns or was this statement just intended to fortify your statement that I'm overthinking it? I don't mind that statement. I'm just wondering whether I have missed something.

Lumen

I have no idea why you need a perimeter foundation for point loads.

What's the load through each corner post, roughly? 1 tonne? That's almost nothing. You need stability to stop the frame flexing and cracking the glass, but don't go overboard.

This isn't structural advice, but if I was doing that I'd probably just dig round, straight-sided holes down to 70cm-ish, throw some 804 at the bottom and tamp it, and then pour concrete into 300mm twinwall corripipe as a form (lifting a bit to leak it out around the bottom of the hole), backfill around the pipe and wack around it.

nowImonboards

Hi Lumen,

Thanks very much for the input. Absolutely not taken as structural advice but hearing how others have done it / would do it is very much appreciated.

I'm reason I was opting for the perimeter foundation was because, although most of the load from the glass roof would be transferred to the posts, the walls are glass sliding doors and there are no catches/stops to limit movement on them so at any point in time, all glass doors (6m + 3m if two sides are open to the same corner) can be moved to the same point and that point can be anywhere along the perimeter.

From there, I was thinking, well if I'm going go to the trouble of digging the perimeter then I may as well future proof it for another structure (e.g. a conservatory) down the line as the Veranda may only last 10 years. It would make sense to me to put in an extra day or two digging now than to have to pull it all up later, destroying the rest of the work I have done all summer (and previous years) putting in a new garden and paving.

I can totally understand people reading this without all the context wondering why I'm trying to plan so far ahead but it's more a case of not wanting to lose everything I have put in so far. I have been working on the garden and saving for materials for the last 3 years. The veranda is the last step. I should have done that first but didn't have the means at the time. I just don't want to make a total balls of it at the last hurdle.

Thanks again to all for sharing their thoughts.

10-10-20

You're right to ask as you do want the best result. Hopefully you understand the limitations and risk of us specifying a foundation, but I think you're on the right track - just need to join the dots and decide what you need against the trade-off of going over the top.

Keep us posted!

nowImonboards

Thanks 10-10-20,

I completely understand indeed. I'll do my best at joining the dots and no matter what I have read, I know all decisions are my own, my mileage may vary and all that.

I've had an unrelated set-back so I have a few more days to think about it.

Thanks to everyone for sticking with me.