That clay appears to be decently friable and easy to break up when dryish? If it's not really tooo bad of stuff, what I do is remove a good portion of it and amend with regular garden soil and peat and compost. Clay contains and holds onto nutrients and isn't a bad thing if it's not the nasty thick sticky stuff. Otherwise, start a pottery outlet :)

i'm not sure about other quadrants but I'm dealing with these suckers in the NW. Last year I tried to plant some peonies damn near broke my back trying to dig a decent sized hole. Sick and tired of it but I use coconut coir dust with essential oil to amend the soil though. I picked a up a bag from Plantation gardens. Hope this helps.

Good heavens...! I’ve never dug a planting hole that deep! Our soil here is heavy clay - but the red stuff that looks like you could make terra cotta pots with it! When we first started gardening here (18 years ago) I used a lot of the mycorrhizal fungus planting supplement in the first 10 years or so (Myke is the product name we used) in every plant hole. The holes were just deep enough to plant things more or less at the same depth they were in the pots.... We don’t mulch but do plant densely and let a lot of the dead foliage and fallen tree leaves cover the beds and rot down to ‘compost in place’. In most of the garden beds now the soil is good but it’s still heavy clay in areas where there aren’t garden beds. Good luck with your soil!

Bring it to me! I will take enough, to mix with my sand, for a decent sized lot! :-)

Seriously though, What's the top soil like? Does it drain well?

What problems are you having with clay that is that deep underground?

I rarely dig a planting hole that deep. Even for a 6' tall tree, 12" is usually enough depth for a decent sized root ball.

If the clay layer is 12" to 18" below the top level, I would think that would be a good thing for retaining moisture in the upper layer of top soil but have no experience in clay soils so idk.