Excavation

Excavation to any serious depth should not be undertaken without professional advice, especially when weight bearing foundations are going to be set in place.

Calculations should be made prior to excavation taking place with disposal, earth movement and groung conditions being of prime importance.

When you're excavating for garden walls and concreting, a basic rule of thumb for the concrete size would be to add 100mm(4") either side of the width of your wall and the width of the wall equal to the depth of concrete.

So a 100mm(4") wall would require a 300mm(12") X 100mm(4") foundation. A 200mm(8") wall would require a 400mm(8") foundation.

When calculating excavation for trenches deeper than this you're entering the area of quantities having to be calculated for load bearing, tensile and compressive strength.

Trenches for foundations are difficult to level as you work so cut wooden pegs twice the depth of your proposed concrete, hammering them into the soil approximately the length of your level away from each other.

By doing this you won't have to try and level the base of an uneven trench while digging out, leaving all levelling above the messy base of digging, allowing you to transfer a level to each peg in turn.

Bearing the above in mind there must be a starting point and this would be as follows;

Dig down to find a suitably compact bed.
Knowing the depth of concrete, hammer your peg in leaving whatever depth it is you want, showing between the bottom and top of your peg.
Dig along the trench and set another peg in place as before, transfering the level at the top of the pegs.
Your guide is the depth from the top of the pegs down, which should be the same as peg 1.
Whenever the level of the base of the trench changes you will be alerted by measuring down from the top of the pegs as you progress.
In an ideal world, all pegs should be level...and the base should be consistently level.

What happens when you have to create a step due to the ground level changing drastically?

Adopt the same method as above EXCEPT where the step occurs there must be an overlap equal to the width of the concrete by double the depth.

e.g

When the concrete is 300mm(12") wide and 100mm(4") deep it will overlap by 300mm(12") to create a step 200mm(8") deep.

The rise will have to be contained when pouring concrete as the concrete will be parallel to your excavation.

Mike Lyons.....Any questions! Please contact me.

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