I am building a retaining garden wall that is 1.5 metres high. It is in an 'L' shape, 5m down one side and 3.7m down the other.
I intend to build this on a strip foundation but I have numerous questions that I cannot find answers too. The footings will be 0.6m wide and 0.45m deep.
I am unsure whether to build a double skin cavity wall filled with rebar and concrete or use 9inch hollows rebarred with concrete. I assume that the double cavity wall will have more lateral strength but wonder if it's more important to put less weight on the footings by using a single skin of 9inch hollows - it's difficult to find solid ground around here and I don't want the footings to sink! I am also unsure as to the pros and cons of each wall type and how that will affect the foundations.
I intend to bend the rebar at 90degrees inside the footings but need to know if it will be beneficial to put horizontal rebar in the footings sitting on top the vertical rebar to spread the load and stop them breaking out.
Do I need a DPC?
Will it make much difference putting fibres in the foundation concrete?
Should I use steel mesh in the footings?
Would it be beneficial to add a third foundation strip to the 'L' shape in order to help stop the foundations tipping over?
If I can't find solid ground, can I use a layer of crushed hardcore in the base of the trench and if so, how deep should this be?
The foundations depend on the ground condition. See our foundation project. Foundations on squashy ground are useless regardless of how much hardcore you put down. You must get to a solid base as explained in the project. Once you have established a solid base 450mm is good for concrete and the strength of the wall as descibed above is greater, the greater the thickness. The foundation width should be a minimum of 100mm either side of the wall. With water pressing in the soil both against the back of the wall and ground pressure pushing water upwards, a dpc on the wall is completely ineffectual and you would be advised to leave weep holes, formed by inserting 50mm pipe through the wall, to allow any build up of water to get through the wall. Rebar should not be required in the foundations and fibres will not make a difference for this construction.
In another article on this site you say that a square peg 50mm x 50mm should not be able to be banged into the ground more than 6 inches 'without difficulty'.
'without difficulty' is a rather subjective phrase since it depends on the size of the hammer and how much force you exert which will feel different from person to person. Do you have a clearer description for determining solid ground?
This is how solid ground is determined in the building regulations. Without difficulty is a logical phrase and if you want to use a 25lb hammmer then we would suggest this involves a great deal of difficulty. Common sense is involved in 99% of construction operations and the scale of the work needs to be taken into account. A standard lump hammer and a square peg are available to every builder and if you can get it in easily with this then you have got to go deeper.
I have had 10 builders come and look at this 'simple garden wall'. None of them agree on any of the important aspects, which is why I have decided to do it myself. One guy will say the footings have to be 800mm deep, another 300mm, another says that you need to dig down to solid ground however deep that is but then you fill with hardcore up to the bottom of the footings. I went to the local merchants yesterday to get a price of hardcore and he tells me you can't build footings on crushed hardcore!
So can I or can't I? If I can then do I need to compact this hardcore before laying the footings?