Digging for install of sewage basin

I got my permit this week for basement bath/laundry rough-in. Yesterday I cut and removed floor where I want the basin to go and started thinking about the size of the hole (18x30 basin) I need to dig for the basin.

I had planned on placing the basin near an outside wall but now i'm thinking about and the possibility of cave-in or damage to the footing. The soil under the slab is mostly clay. Is there a general rule to follow as to digging near the footing to ensure that I don't cause a disaster?

I found this when I ended up having to pipe around a footing.

The 45 degree rule is if you are digging a trench parallel to the footing. There is NO restriction when you are just digging a hole for a basin.

hj said:

The 45 degree rule is if you are digging a trench parallel to the footing. There is NO restriction when you are just digging a hole for a basin.

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That is good, because the basin already installed in my basement is right beside a footing.

We also have to dig there when we run pipes under the footings.

Thank you both for your replies- this was exactly the kind of info I was looking for.