Help with my calculations, please. How deep should the foundation be ?

[joe90]

45°.   Wow, every day is a school day 

[saveasteading]

9 hours ago, SteamyTea said:

just to be a pendant,

spelcheque striques agen. 

 

8 hours ago, ToughButterCup said:

one car-door width plus a bit next to the wall.

As all loads know to disperse at exactly 45° (symbol cut and pasted from above)  the wheels will  be far enough not to load the wall at all, and only a problem if the car users are very heavy indeed.

You can scrape a footing and build straight on to of it to hold back 450mm,   for more you might want a wall with some structure, but in your circumstances i think I would simply lean it back towards the cars, or make it thick.

[George]

On 13/09/2023 at 12:04, ToughButterCup said:

I need to prepare our carpark: one edge of it has a low wall which supports part of the parking area. The stone (local stone) wall forms the border to a footpath. 

 

I would like to calculate the minimum depth of the foundation.

 

I need  to take the wall down, dig out the back of the wall to make space for the foundation [ and the shingle drainage and french drain] , then put a block wall up, and face that block wall with the stone I took down. 

 

The wall height varies between 450mm and 700mm. The ground is medium clay, and it's fairly damp most of the year. (I think its glacial till ) Wet Clay repose is (say) 20 degrees or so. 

 

This is the first site I googled that gives me a formula to calculate the minimum depth of the foundation

 

The formula is given as 

where p = the soil bearing capacity

w = density of the soil

and the angle of repose is that funny phi (or is it summat else ?) looking thing

 

That means

p = 250 kN per square meter 

w = 1760 kg per cubic meter

Angle of repose is 20 ish

 

 

1 - sine 20 is 1 - .342   =  0.658

1 + sine 20                    = 1.342

 

Minimum depth is ( 250 / 1760 )  *  ( 0.658 / 1.342)  

 

= 0 .1420 *  0.490

 

= 0.6958

Here's where I'm stumped ---- is that 0.6958 mm or is it 69.58mm or is it something else ? 

 

Thats the trouble with google: a little knowledge = danger. hence this post

 

I 'feel' the foundation needs to be 100mm of C25. But it would be good to know why I'm wrong - or, more rarely, right.

 

 

 

 

 

That formula is for non-cohesive soils so isn't applicable here anyway.

 

 

A formation level a minimum of 450mm below finished ground level is good for most of the UK. For capacity, in the absence of ground information, go 150mm into good virgin ground. For mass concrete, foundation thickness a minimum of half the width (although depends a bit on the wall - part A rules should be ok for a small retaining wall). For retaining wall, top of concrete ideally 200mm below low side ground level.

 

Edited September 15, 2023 by George

[Russell griffiths]

Row of gabion baskets. 

Dig trench, bit of stone, run the wacker up and down, face up with the stone you were going to use. 

Natural looking, flexible so no cracking if you get any movement. 

Fast, and not a lot of skill needed. 

 

Sounds perfect. 

[joe90]

31 minutes ago, Russell griffiths said:

Row of gabion baskets. 

Perfect indeed (if that’s what @ToughButterCup wants). When I worked fir a large company all the acronyms used to pizz me off, however the one I did like and still use to this day is.  “Work smarter not harder”.

[ToughButterCup]

16 hours ago, saveasteading said:

....

As all loads know to disperse at exactly 45° ...  the wheels will  be far enough not to load the wall at all, and only a problem if the car users are very heavy indeed.

...

 

Thanks very much for that. Very useful to know.

 

2 hours ago, George said:

That formula is for non-cohesive soils so isn't applicable here anyway.

 

 

A formation level a minimum of 450mm below finished ground level is good for most of the UK. For capacity, in the absence of ground information, go 150mm into good virgin ground. For mass concrete, foundation thickness a minimum of half the width (although depends a bit on the wall - part A rules should be ok for a small retaining wall). For retaining wall, top of concrete ideally 200mm below low side ground level.

 

 

Thanks be to God for people who know their stuff. I was fishing for a reply from you @George 

 

1 hour ago, Russell griffiths said:

Row of gabion baskets. 

Dig trench, bit of stone, run the wacker up and down, face up with the stone you were going to use. 

Natural looking, flexible so no cracking if you get any movement. 

Fast, and not a lot of skill needed. 

...

 

Pragmatic, sensible, cheap. It's the  '...not a lot of skill...' bit I like best

 

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If anyone ever wonders why BuildHub attracts such a wide audience among self-builders, this thread is more than adequate demonstration.  I've gone from - How the Hell do I do this, to  - why I should do this, but not that AND why I should or shouldn't. Saving money time and effort.

 

For people new to BH

Quick cautionary note 🙂 the responses given above  are not professional opinions - they are merely the advice of interested online  colleagues . It's up to me to evaluate their answers and take the final decision.

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Edited September 15, 2023 by ToughButterCup

[saveasteading]

12 hours ago, ToughButterCup said:

As all loads know to disperse at exactly 45°

I hope everyone realsies that forces dont go at 45°, but it is a reasonable starting point in wondering if downward forces might cause problems.

 

 

12 hours ago, ToughButterCup said:

the responses given above  are not professional opinions

But they may be the opinions of professionals....not the same thing as we don't have all the info, are influenced by the way the question is put, and perhaps can't be bothered in thinking it right through, or its late on Friday evening.