Supporting stairs base

[DIYer Damo]

Hi everyone,

I'm in the process of replacing my old suspended timber floor down stairs with a solid, insulated one.
Whilst ripping up the hallway i discovered that the stairs posts don't penetrate into the ground but are nailed straight onto the floor boards themselves.
The stairs are the type that turn 90deg after the first 3 steps so have a large timber post at the very bottom with another where they turn 90 deg. (See pic)
Since i'm pouring in a 100mm slab of concrete i wondered what would be the best way of securing the stairs posts? The 100mm concrete will be beneath 100mm insulation and 60mm screed so won't meet the bottom of the existing posts.

Options i can see so far are:
1) Dig down and pour a mini foundation pad, build up from this using brick and blue brick to base of post. What size / depth would i need?
2) Extend the post some how so it is encased in the 100mm slab. I was thinking i could drill out a large hole in the post centre underneath and then add something like stud bar of a decent diameter?

Any other ideas?
many thanks

 

[blup]

The joists support the floorboards on which the posts sit and which in turn are notched into the staircase.

Would it not be wise to leave this whole structure in situ?

Blup

 

[noseall]

Dig a hole to the right of the newel post and concrete-in a 1.5m long piece of 100mm x 50mm timber so that it finishes along side the newel. Make sure that the conc' goes beneath the prop. When set, screw the prop to the newel. I'd also pop a few screws into the stringer from below to help prevent movement.

You can then pack the stairs on bricks and saw the prop off prior to screeding when the concrete has set.