Foundation depth near drain

MrMJB · 3 May 2011

Hi all,

I am after some advice about a free-standing brick wall.

I want to build this in my garden, well clear of boundaries. I have spoken to building control who say I don't need approval for it.

I plan to have a small (pedestrian) gateway at one end of the wall with a brick pillar (concrete reinforced) on each side of the gateway as I want it 6ft high. My problem is that the main drain off my property (not shared) passes under what will be the centre of the gateway but at a depth of about 4 foot. The other end of the wall abutts the house which is victorian and built on pyramidal footings with no foundations.

From reading books and online guides I think I cannot get away with anything but foundations with their top below the level of the drain invert and then lintel over the drain. This then makes my wall 10 foot high!

Am I correct or is there an alternative? Should I get an engineer to design this (rather not fork out for that though as I suspect the answer will make the wall financially non-viable)?

This is making my 'simple' garden wall project turn into a major undertaking!

Thanks in advance.

Matt.

wavetrain · 3 May 2011

Please draw us a colour CAD drawing of your concerns. I am not able to follow your reasoning of a 10' high wall, certainly there will be some footings around the drain so as not to damage it, but you would build up to near ground level in concrete.

BTW what will be the footings width and depth for the 6' wall and what is the width of the wall?

Drawings are always a good idea.

^woody^ · 3 May 2011

Just build your wall on suitable foundations at a suitable depth (450-600mm?) and don't worry about the drain

MrMJB · 3 May 2011

Larger image now uploaded here: View media item 33764
Unfotunately lost clarity in upload process, the measures are 6' for the brick pier heights and 4' for the drain depth. Pier A is one gatepost (the far left of the image), pier B is the second gatepost and pier C is the one abutting the house (far right of image).

I can alter/move the gate opening to be centred over the drain, therefore the gateway width is not fixed yet.

My understanding was that wall height was measured from the top of the foundation and that the foundation needed to be below the invert of the drain, therefore 4 foot below ground and with 6 foot above ground the wall is now 10 foot high?

Matt.

wavetrain · 4 May 2011

Matt I asked you 'BTW what will be the footings width and depth for the 6' wall and what is the width of the wall?'

Please answer my questions and draw the footings you plan for the wall (not the gate area since you don't yet know what to do here) on your drawing and increase the text size so we can read it and then repost.

RedHerring2 · 4 May 2011

As I understand, having recently built a wall alongside the foul drain run, and if you were building a wall in conformity of Building Regs, the bottom of the footings must be below the invert level.

In my case it was only about 50mm below, at the lowest level of the drain run, which was only about 100mm away from the footings. BCO was happy with it.

I'm not sure how close the footings have to be before this is the requirement.

Just done a search and come up with this:
//www.diynot.com/forums/viewtopic.php?t=155754

And I'm certainly comfortable with Noseall's advice.

MrMJB · 4 May 2011

Aha, I have it now, as long as the bottom of the trench for the foundation is at or below the drain invert, the load is transferred to the subsoil below the drain and not onto the drain itself. My plan was for a 450mm depth footing 450mm wide for the wall (one brick width) and expanding out around the piers to 600mm width.

I guess I have a lot of digging to do - just as well I'm on paternity leave for the next week then!

Thanks guys, sorry for the useless drawings!

wavetrain · 4 May 2011

Matt, I am trying to help you, but unambiguous info from you is not forthcoming. You did state "(one brick width)" but I cannot assume you mean the length of the brick.

Is the width of the wall about 225mm and 450mm for the piers, or is the wall about 102mm wide and the piers 225mm?

Your footings are wider than they need to be for the weight of the wall but not deep enough to ensure the wall does not lean in a few years time.

You should dig footings to 1000mm depth and a width of 300mm. If the piers are wider than 300mm then increase the footing width there to the width of the pier. If you have time to get hold of some free used bricks then incorporate these (50% bricks by volume) in with the concrete to reduce your concrete cost.

^woody^ · 4 May 2011

wavetrain said:
Matt, I am trying to help you .....

You should dig footings to 1000mm depth and a width of 300mm. If the piers are wider than 300mm then increase the footing width there to the width of the pier. If you have time to get hold of some free used bricks then incorporate these (50% bricks by volume) in with the concrete to reduce your concrete cost.

I don't think that is any help at all

1 metre deep foundations for a wall? And only 300mm wide? And only 300mm wide for such a deep strip or just a shallow strip?

And adding large unit bricks into concrete wont lead to it cracking will it?

The OP's wall will be horizontal straight after he first slams the gate

wavetrain · 4 May 2011

A trench foundation receives pressure from the ground around it, why should it crack? But if it did crack, it is not going to fall apart due to the ground pressure pressing it together at the sides and pressing top and bottom from the weight of the wall pushing down and the soil pushing up on the footing respectively.

The purpose for having a deep foundation is so the ground around it stops it from tilting.

Around 10 years ago a friend and I dug a footing for a 1.2m high 225mm wide wall with piers as narrow as we were able to and to a depth of perhaps 80cm. I remember it well as while we were digging, a neighbour told us a plane had just flown into one of the twin towers - at the time I thought it must have been a light aircraft...

The wall has not tilted any measurable amount since it was built later that year.

How could the wall fall over after a slam of the gate using my suggested design? SE standard explanations only please Woody.

tkmax · 4 May 2011

Couple suggestions:

- Take a 45 degree angle up from the invert level (bottom) of the drain and make sure any foundations are below that line, this will keep any foundation pressure off the pipe.

- Excavate the drain 4 feet each side of the gateway and encase it in concrete, then do what you want with the foundations.

tkmax · 4 May 2011

MrMJB said:
Aha, I have it now, as long as the bottom of the trench for the foundation is at or below the drain invert, the load is transferred to the subsoil below the drain and not onto the drain itself. My plan was for a 450mm depth footing 450mm wide for the wall (one brick width) and expanding out around the piers to 600mm width!

PS Foundation dimensions sound good to me, depending on the ground conditions.

PPS Don't put bricks in your foundation concrete.

^woody^ · 4 May 2011

MrMJB said:
I guess I have a lot of digging to do - just as well I'm on paternity leave for the next week then!

Well, if you want to waste your time digging instead of spending it with the family, then that's OK

There is no need to excavate down to invert level for this wall. The loads on the drain are not significant.

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Foundation depth near drain

RedHerring2

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