It's been an allotment since about 1940. There's a layer of stone-filled clay under it, & digging into that is pointless. It keeps on going. Above that (a fork depth down in places, but mostly more) the soil's loose enough for daikon etc. without deep digging.
Previous tenant had put some raised beds in, I presume to deal with that problem. I repaired 'em (skip timber & some old roof timber donated by a builder rather than putting in his skip), raised 'em a little (they'd sunk in a bit), & keep topping 'em up with compost, leaf mould, & any other organic material I can get hold of.