I would suggest that you dig to at least two feet deep if you want to go with a barrier layer over the existing clay. I dug out two feet deep below my raised bed. I then used about one fourth of the native soil. I amended it with bagged soil, and amendments. It seems to work well so far. Last year I did not dig it deep enough. Tomatoes started great till the roots hit the clay. Then the problems began.

It can be done in just the native soil, just not easily, and yields will not be as good.

If you are planing on tomatoes I would advise using transplants. Otherwise you will not get any fruit till the heat wave of summer breaks. Then it is a crap shoot on if they get to full size before the frost hits.

We have two short seasons with maters in this state. The first is if you protect plants that you get in the ground early enough then you will get some fruit to set before the heat of June starts up. They may take a while to ripen, just be patient. The second season is the fruit set that happens when the heat breaks. If they hit full size before the first frost you can wrap them in newspaper. Put them in a box, and let them ripen. My grampa always called them winter tomatoes as you were eating ripe tomatoes in the winter. The ones that are not large enough to ripen can be batter fried, and the super small cherry sized ones can be pickled.

Thank you for your advice. The trench is still empty, so it would be relatively easy (!) for me to dig another 6 inches deeper. I will say that I have had great results growing abe lincoln tomatoes in 18-gallon containers.

I have about 15 transplants that I'm getting ready for transplanting into the trench. I have a few containers that have tomato plants that I grew from seed, too. I bought a variety of tomatoes, ranging from cherry tomatoes to beefsteak, but most are in the intermediate size. The best producers I've ever grown are the abe lincoln.

Google lasagna garden. It's no dig and produces a very nice garden. It can even turn your crappy clay soil into fertile soil. I should know cuz my house sits on mostly (90%) clay and the garden plot was turned into black rich soil with LOTS of fat worms.

Add a lot of earthworms if you can.
-Terry

Here is a link that might be useful: vermiculture

Imagine, digging out clay to bring in sand!

Today I'm going to haul some buckets of clay back home (for my sandy garden) from a job site where we hit a rare band of clay. In fact, we have to do just what you are proposing: dig trenches and put clean sand in the bottom before pouring concrete footers.

Other than that probably quite unhelpful story I have nothing to add, since I know nothing about gardening in clays, expansive or not.

PN, when I buried chunks of clay in my sandy garden (about 10 years ago, I since moved to a clay place) it stayed in lumps for several years and contributed nothing. The few earthworms in that sandy spot completely avoided it (I guess they do not like to break clumps either). You have to mix and break it as much as possible.

Also adding a 2x6 frame can help to raise it up a little more. This would give it a better chance with draining as well. I would suggest mulching as well. Your plants will thank you in the heat of the summer. As will your wallet when your water bill is lower as well.

Glib, yes, I've learnt that from earlier attempts. I'll dry this clay in the sun and then pulverize it and mix with garden soil.

Boy, it turned out to be one hard vein of blueish clay, let me tell ya. Impenetrable to a shovel, even the excavator had some trouble with it. Pretty much rock-free, even though surrounded by very rocky gravel.

Thanks for the replies!

My plan now is to wait for the rain forecast for this weekend to wet my dirt, which usually makes it crumbly, then mix it with the garden soil I've bought.

Oddly enough, I've encountered a lot of earthworms while digging this trench. I wonder if they are related to the batch of nightcrawlers that I dumped in my compost heap the first year? They are on the other side of the yard, most of which is covered by plastic sheets and dirt on top of the sheets.