i have clay soil with bad drainage, will digging trenched 8" wide filled with gravel and top soil on top be of any benefit for drainage?

The best way of dealing with this soil is to dig in lots of organic material, plus horticultural grit. It would have been a good time to do it before the winter frosts, which would have broken up the clods for you. Now, though, it's dig, dig, dig. Whenever you plant anything having started the process of improvement, make the hole large and back-fill with compost. In early spring, too, mulch as much as you can and let the worms do some work for you.

In the late autumn, dig over as much as you can and start the process again - leave over winter, then more organic material and grit in spring each year. It will pay dividends.