Dig it man;)

So you intend to have the digger in the hole? 50mm of soil each side won't support it... You can dig from one side but neither wall will be pretty, I guess that doesn't really matter though.

Is the ground such that they need to be 600mm wide? Generally 400/450mm would be plenty for an extension. 600mm was the traditional width when you poured a 225mm footing in the bottom of the trench and so needed enough working room to raise the brickwork off it, but IM (now not so recent) E most builders prefer 'trench fill', pouring concrete to within about 150mm of ground level so minimum width isn't an issue.

Usually the 600mm wide bucket on the 700 - 900 kg class machines is designed more for levelling and spreading rather than hard digging (you can tell just by looking at it that its not built to anything like the standard of the digging buckets). So you will most likely need to use the 300 or 450mm bucket with nice big teeth on it do do the brunt of the excavation.

The machine probably won't have the power to dig with the 600 bucket, unless the ground is very soft, too soft to take the foundations. Start by digging the bits that abut the existing walls, dig them full depth and width but don't dig them all the way. leave enough solid ground to take the digger whilst you dig the footing that's parallel to the existing building. Then go back and finish the abutting footings at the corners. You will probably be able to dig one abutting foundation full length before doing the parallel one, but certainly not both, Its best not to in case you get a problem and have to dig from the other direction.

I hope that makes sense!

Mike

Umm ... 700 don't go into 600 Dave, and its not going to happen here;-!...

Well plans drawer upper and building inspector sez 600 so I can't argue;!..

Also it seems walls are wider nowadays cavity ids to be 100 mm ..

In article , MuddyMike scribeth thus

Thanks for that seems the way to go..

Seems we're on what's described as river bed gravel's here so quite good in that respect..

Noted...

will

I very much doubt that 50mm of soil each side of soil will support the weight of the digger. It will go into the hole possibly only on one side...

Well theres the digger on firm land and the trench being cut in front of it as it works backwards over the as yet uncut earth.

I somehow doubt that you'd be able to dig underneath the machine.....