Clau soil particles are a mineral and do not "break down", it is the organic matter thaat gets mixed into the clay that gets digested by the soil bacteria. Some types of clay do not seem to allow organic matter to mix, there seems to be some kind of barrier at the soil/mulch interface that prevents mixing so those soils need to have some organic matter tilled into the soil first. Once that is done then mulches add to the OM in the soil with no problem. However, wood chips and shredded bark seem to be the most common types of mulch material that presents this problem.
I would find some good compost and work that into that soil first and them apply the mulch.

We have heavy clay soil here in Madison, Wisconsin. Every spring I am out there digging it up somewhere, and then mixing in compost, about 50/50 by volume, to get soil that can be worked easily. I have also used peat moss, but we have a large compost pile so I no longer need to buy organic material to add to our soil. There is effort involved, but the results are worth it. I suppose that you could put down compost, or shredded leaves, on top of the clay, and wait for the worms to work it in, but I am not that patient.

Unforutnately because of the drip irrigation tilling and otherwise amending the soil is a major project I'm trying to avoid. I, ok with something that might take time. But I wonder how effective adding organic mulch and compost can be and whether worms or maybe microbes, over time will help losses up the soil enough S&P that as the organic material breaks down it works its way into the clay...too much to hope for?

The only way you are going to get a significant amount of organic material worked into the clay in any reasonable amount of time is to do it yourself. Mulch will help but it's not a miracle worker.

Even with drip irrigation you should be able to hand dig compost in between the plants. As well as compost I also use alfalfa pellets which IMO are a really good soil amendment. If you do that as well as mulch you won't have to 'add' earthworms as they will come when there is something for them to feed on. Earthworms provide a lot of castings which are one of the best fertilizers.

You may try using a surfactant (soap) on the soil to break down the surface tension thus enabling it to absorb water.
Also when homes are built the builder will often remove a lot of the top soil and vegetation. Along with it he remove most of the soil life. Much of the microbial system will come back on it's own by the addition of compost and mulch. Some will not. I would recommend the addition of a good reputable organic starter containing a mixture of microbes including mycorrhizal fungi, which by the way will not come in with any compost or through natural process any time soon.

One way you could get a good amount of organic matter in the soil without digging would be to grow a season of cover crop. Chop and drop it so the roots are left to decompose and mulch some more over top.

Let us know where you are located so more specific information could be conveyed. The clay soil found down here in Georgia (for example) responds to certain amendment techniques which may not be appropriate for other areas of the country. Best Wishes--Carl

I was in the same situation 2 years ago. Newer build, unworked clay. Clay is not entirely evil. It's hard to work but has a lot of nutrients. Depending on what you are planting, a top layer of compost, wood chips on top and patience may be your best bet. My roses have thrived in our clay as has the Japanese maple and all of the perennials. Gradually it is improving as the wood chips and other ammendments break down. Other than taking longer to plant things it really hasn't been an issue.

Komehtm

If not already done, you will need a raised surface for the bed, 6 to 12 inches. If you really have a high percentage clay soil (not all soil called clay does), you can fight it forever or fix it once with sand and enjoy carefree gardening thereafter.