Laying slab for porch, undermining existing slab

[TheKeymeister]

Hi, i've got a load of spare concrete on a job next week so have started to dig out for a porch (2m x 1.2m) to save it going to waste. My house is built on a raft foundation. I have read up on how deep to dig out for a slab for the porch, there were various answers but as the concrete isn't costing me anything I thought the deeper the better, was aiming around 450-500mm as I will be putting ducts in the slab for water, BT, etc. so will have to dig down to at least that depth anyway.

I have got about 300mm deep at the moment, the ground has been topsoil, sand, and type 2 stone. The edge of the raft/slab for the house only seems to be 140mm thick, with a membrane and then type 2 stone. Am I going overkill being as the house seems to be sat on such a thin slab, and will I do more harm than good by digging deeper and undermining the existing slab?

When I've worked out what depth to go to, should I put new stone in the bottom and compact it before putting in the shuttering in and the membrane? PS also got some steel reinforcing to go in as well.

As you can probably tell I haven't done this before lol so any help is much appreciated

 

[r896neo]

perhaps someone more knowledgable will be able to confirm but if your house was built on a raft it may have been because the ground is poor.

If this is the case then a massive lump of deep concrete which will be hugely heavy and have a small footprint may not be a great idea. It may begin to gradually lean away from the house.

It may be better to try and dowel it into the existing footing use use a 150mm thick reinforced raft.

an engineer would know better but Just something to consider/ try and get more advice on

 

[TheKeymeister]

Ground isn't great, I think it's been built on made up ground and there was a mine not too far from here, house was built in the early 70s so hopefully any settling will have occurred already. So far I'm thinking compact a load of type 2 up to around the existing level and pour a slab the same thickness as the house. Fair point about attaching to the existing slab, I've got some concrete anchor bolts so could use them and some threaded rod to keep it all together...

Pics to follow soon anyway