Planting apple trees in a restrictive soil

I am planting a row of semi-dwarf fruit trees. There is about a foot of decent soil, but beyond that there is a very heavy clay subsoil with very slow water infiltration. The trees are semi-dwarf, so they tend to have smaller roots anyway. The land is sloped, so there won't be standing water. The climate is semi-arid. Given all those factors, I think I should be able to make this work? My biggest concern is a restricted rooting depth leading to water stress in the summer, but irrigation will be necessary for these no matter what.

Any thoughts or suggestions? I'd rather not plant on berms or mounds, due to site, time, and money constraints, and because it would make establishment watering more difficult.

Gilbert,

Do you know what is the thickness of the clay layer? if it's reasonably thin, for example 20" then it would make sense to dig the hole deep enough to remove the clay and replace it with clay/compost mix. When I encounter spots of clay, I dig it down as much as I can and make clay + coarse sand + compost mix. Also, some rootstock will handle heavy soil better than the other.

The restrictive layer is at least three feet thick, probably thicker. I don't want to amend the planting hole; Edible Forest Gardens and other resources suggest that his will paradoxically lead to pooling of water in the hole.

There are different approaches to tree planting. At the beginning I was planting "naturally" and everything was dying, also "naturally" in bad soil, with manual watering, no amendments and gophers. Later I have noticed that the spots that got deep ripped and disturbed produced extremely thick vegetation.
It's possible that mature tree will handle bad soil, but before it grows - it will be tormented (in my case) by extreme heat, late frosts, sun, winds, hard-as-brick clay (in some spots), gophers. Since I was losing a lot - due to simplified planting and vast array of species and cultivars I started improving things.

My current approach is:
-plant more moisture demanding trees at the bottom of the slope: all prunus, pears, quinces, chestnuts, higher I plant apples, pistachios, mulberries, persimmons and at the top olives, some figs, Italian pines, pomegranates
-avoid cultivars that were developed in oceanic climate of central Europe/Pacific North/humid Asia or from east of the Rockies
-if I encounter heavy clay I'm trying to put there prunus on Myrobalam rootstock
-I plant in January and February
-all plants (except older, large transplants) go into wire cage (1/2" openings), fi 12", height 16-24" to protect from gophers
-I dig a hole at least twice the cage diameter at 1.5 times the height
-the soil from the hole is mixed 1:1 with compost (eucalyptus chips + sheep manure)
-if the soil is heavy clay I use 1:1:1 soil, compost, coarse sand
-I mulch with wood chips
-if the tree is very thin or sensitive I put a protective sleeve
-they are drip irrigated with 2 emitters (40 l/h each) and irrigated for half an hour at a time (two irrigations in summer per week, 3 if temperatures are over 40 C)

After I tested multiple cultivars and implemented all improvements the trees are dying at much lower rate and they finally produce fruits - more every year.