Concrete slab on top of dwarf wall or adjacent to...

[Carrerahill]

Hi guys, I have a question regarding my garage slab, I'd just like to see what you guys think and also what you may have experience with. I have attached some sketches to help explain this.

 

My garage site is on a slope, it slopes to the back. I dug out and poured at the rear elevation a C35/40 foundation 400-450mm wide and min 200mm deep. I then built a 2 block high wall, I may add a single row of engineering brick along the top to get the level spot on, so this is going to become the rear wall.

 

It was done like this because of the slope, the amount of shuttering and the strength of said shuttering would have been fairly heavy duty and because of the proximity to the neighbours it would mean needing to remove mature hedges to build the shuttering structure - their only stipulation when I said I wanted to build this garage was to leave the hedges, although they are my hedges I have respectfully done this. The rear found and wall also forms a back shear key of sorts to help support the slab and stop it slipping (there will be another mid-point shear key cast as part of the slab). It's all quite well thought out and a civil engineer gave me the design spec but I have got some questions now.

 

I was thinking about the slab and how it will work in relation to this little wall, the next stage would be to attach 6x2/8x2 to the wall running up to the front to form the shuttering. The concrete will then fill the shuttering up to the rear wall, but my concern is that the slab could push upon this wall and potentially push it out in the future if there was any slippage or settlement. 

 

So I have decided that I think the slab should sit on top of the small block wall, this would then mean that the rear wall would restart on the slab, and if slippage settlement occurs it would be less likely just to push out the rear wall - instead the wall would move with it. So I would build the shuttering to the back of the wall and let the concrete sit on top of the wall. I would need to insure that the infill was very well whacked to ensure the slab didn't settle on the inside of the wall and crack as it cannot move because of the wall. I was tempted to make up a no-fines concrete and back fill on the inside of the wall so that it was a bigger concrete base. 

 

I have sketched out some thoughts and options. I am an engineer so I tend to look at things in every fine detail, but not being a structural/civil engineer I can only start letting my mind run away with ideas and thoughts that I don't have training in.

 

 

[RichS]

Personally I'd go with 1 or 3. I'd probably do 3 but not bother with the additional concrete.

Deffo not 2.

[ProDave]

The sheer key will do nothing imho. It's only keying into hardcore that is laid on a slope, and the whole lot, hardcore included could, no make that probably will slide down the slope.

 

I would level the site in steps so at least the infil is sitting on level ground. Perhaps another footing and dwarf wall at each step?

 

Buttress walls on the inside of your wall which I think is what drawing 3 is trying to show? and buttress walls on the downhill side of any intermediate dwarf wall.

 

To level the site in steps, you will be removing a lot of soil. Use that removed soil to raise the ground around the garage. You will be removing the top layer of organic soil anyway won't you?

[Jeremy Harris]

As Dave rightly says, the shear key won't do much at all if it's in the sub-base.  To work it needs to be in compacted and undisturbed soil, with shear properties that have been used in the sliding stress calculations that were a part of the retaining calcs.

 

I have a spreadsheet to do gravity wall calcs on my main PC, so could run some numbers and see what it looks like later.  To do this I'd need all the base data on loads, soil type and bearing/shear stress, any surcharge from structures or imposed loads higher up the retained area, etc.

[Carrerahill]

46 minutes ago, ProDave said:

The sheer key will do nothing imho. It's only keying into hardcore that is laid on a slope, and the whole lot, hardcore included could, no make that probably will slide down the slope.

 

I would level the site in steps so at least the infil is sitting on level ground. Perhaps another footing and dwarf wall at each step?

 

Buttress walls on the inside of your wall which I think is what drawing 3 is trying to show? and buttress walls on the downhill side of any intermediate dwarf wall.

 

To level the site in steps, you will be removing a lot of soil. Use that removed soil to raise the ground around the garage. You will be removing the top layer of organic soil anyway won't you?

Hi Dave, thanks for your input.

 

The sketch is inaccurate, it's being dug down into undisturbed boulder-clay. But had it in real life been as drawn, I'd agree!

 

Yes, the site is being excavated down to the hard clay, the soil being used to lift the lawn.

 

I like your idea of the stepped site, then level each site onto level ground - the sketch was not ideal, it didn't show it all properly it was just to show the wall issue. Yes the third sketch is the wall with a engineering block buttress then filled with concrete. I am just in this minute from the builders yard with engineering brick so that's the weekend sorted!

[Carrerahill]

3 minutes ago, JSHarris said:

As Dave rightly says, the shear key won't do much at all if it's in the sub-base.  To work it needs to be in compacted and undisturbed soil, with shear properties that have been used in the sliding stress calculations that were a part of the retaining calcs.

 

I have a spreadsheet to do gravity wall calcs on my main PC, so could run some numbers and see what it looks like later.  To do this I'd need all the base data on loads, soil type and bearing/shear stress, any surcharge from structures or imposed loads higher up the retained area, etc.

Hi - we just posted at the same time. Noted the shear key comments but see my reply above - this is OK.

 

I am interested in your calcs spreadsheet. That could be a goer to get me some figures. 

 

Thanks!

 

 

Edited June 16, 2017 by Carrerahill

[Jeremy Harris]

I can stick the calcs spreadsheet up later, either here or as a download from my website.  It was designed to allow me to do some comparisons of retaining wall construction methods, when I was trying to find the best compromise between width and cost, and although it uses the same recognised methods for calculating earth retaining structures that an SE would use, I'd not rely on it as proof that a structure is safe.

 

If the shear key is in undisturbed ground then I'm pretty sure it should be OK.  As a general rule, for soil types that have a reasonable max allowable shear stress, the shear key will probably be a bit of an overkill in terms of resistance to sliding, anyway.  The base sliding load from a structure like this is going to be fairly low, I think.

Edited June 16, 2017 by JSHarris

[Mr Punter]

Would beam and block be better here?

[Carrerahill]

On 2017-6-16 at 17:18, Mr Punter said:

Would beam and block be better here?

 

Been back to the engineer and discussed the wall, he is happy with what has been built and confirmed it was all as planned, the back wall is not going to be doing a hell of a lot apparently. I think I just had an overactive imagination of the slab slipping slightly, but it won't due to the ground prep planned etc. So I am happy, I did however build engineering brick buttresses and finished the wall. Looks good. I am happy with it. It looks a bit German Bunker now actually. 

 

I mentioned the no fines concrete back-fill and he said if I wanted to, it would do no harm and would mean we didn't need to ensure he back-fill was whacked.