hi, dig a min of 600mm down and you need to hit clay most of the time its not more than a meter, the width of the concrete needs to be 150mm ither side of your bricks, so if your cavity width is 275 mm it should be 575mm wide, i make them 600mm its just easier, your concrete mix should be 3 to 1 and thats the same for any bricks laid below dpc, above dpc 6 to 1 is what you will need!
Please read our project on foundations. There is no situation where a solid wall should be built on foundations less than 150mm thick. This is the minimum depth required to stop concrete cracking with slight ground movement. Our project on mixing concrete explains a little more.
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