Authoritative Advice on how Thick to Make Garden Retaining Wall Foundation


Postby laurencewithau » Sat Nov 14, 2015 11:51 am

Hi everyone.
I need to decide how thick to make my garden retaining wall foundation. My difficulty is that when I look on the web, ask questions in web forums, ask people I know, or read books I find that there is no consensus on foundation thickness other than that 6" is a minimum.
One person says that for a wall over 1 metre high the foundation should be as thick as the width of the wall. Another will say that the foundation should be 2 feet thick; and so on. By 'thick', by the way, I mean the vertical thickness of the foundation, not its width, or its depth in the sense of the depth of the top surface of the foundation relative to ground level. In other words the depth at which the foundation is buried. Very often it is not clear what is meant.
What I also find is that the advice given takes account of the type of subsoil, for instance whether it is soft clay, but not of some other relevant factors; namely, whether the soil retained will slope down to the wall rather than be level with the top of it; and whether the wall will be buttressed, for instance by side walls at each end and/or by steps from ground level to the level of the top of the wall. The other thing is that very often no reason is given for recommending a particular foundation thickness.
Given all this, I would be very grateful if some knowledgeable person could specify a minimum depth for the foundation I wish to build, if I describe in detail all the factors which seem to me to be relevant.
The wall will be a double- skin hollow-concrete block wall, therefore about 20" wide, with upright rebars through the holes in the blocks, which will then be filled with concrete. The two skins will be tied together with wall ties. The wall will be 30 feet long and 5 feet high. There will be a side wall buttressing each end and a buttress at the mid-point in the form of a flight of steps 4 feet wide from the patio to the top of the wall.
There will be a PVC land drain behind the main wall with gravel above and below it and non-woven fabric used to keep the gravel clean. The retained clay soil will be level at the top of the wall, not sloping down to it. The soil at the base of the wall, which the foundation will sit on, is very hard, clayey, gritty subsoil. Because it is so hard, and because the clay cliff to be retained is unstable, I've decided to make the foundation trench only about 10" deep relative to finished patio level. The foundation will itself have rebars in it.
If the recommended foundation thickness is more than 10" it will be above patio level, but that should be OK because I'll build a raised bed against the wall along its entire length apart from where the steps will be. It should protect the foundation from frost. The bed will be at sitting height and about 2 feet wide, and it will have its own retaining wall.
I would be very grateful indeed if someone who can speak with authority can tell me, taking all relevant factors into account, what the thickness of the foundations should be.
Note: the photos I've attached show the clay cliff and foundation trench. The rough wall sections are there to stop the cliff from collapsing any further.
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laurencewithau
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Postby welsh brickie » Sun Nov 15, 2015 6:20 pm

your foundation concrete thickness should be 225mm thick, any thicker and reinforcing will have to be installed.
If your wall is 200mm wide then the foundation will need to be 500mm wide.
The depth of the foundation needs to be below ground level by at least 600mm
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Postby laurencewithau » Tue Nov 17, 2015 6:23 pm

Hi welsh brickie. Thanks for your reply. The wall will be two single skins of hollow block, therefore about 20 inches ( 500mm) wide. I'll be using rebars and I'm wondering whether the foundation should be about a foot ( 300mm) thick. As for foundation depth, I was wondering whether a raised bed in front of the wall and at sitting height would count as ground level. I assume that it would protect the foundation from frost, so that it could be laid at patio level or above. Thanks again.
laurencewithau
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