When the heavy clay excludes oxygen, which it can, the fungi can't do their magic on decomposing the wood.
I mix in sand with my heavy clay so that there will be some pore space in the soil on top of the wood and it's not sitting under a piece of terra cotta baking in the sun. Even then, sometimes I don't put in enough and then after a couple of rains, the clay on top makes a crust that can get pretty hard.
If you stack the logs and keep them covered with a tarp, then that will keep them moist and they can rot faster than if they dry out in the sun. The manure is not necessary, it may add a little nitrogen for the fungi to use, but they are adept at finding their own nitrogen. What keeps the wood from rotting in your Mediterranean climate is that you don't get much rain in the hot summer months. Once those logs dry out, the fungi figure it's time to call it a season, so they sporulate and go dormant until the rains return in the fall. You have to convince them they are in an all-year rainy season, like we have here in Georgia. Get out once a week, pull the tarp back, soak it down good and cover it back up. The more miserable the level of humidity is under that tarp, the better the fungi will grow and the faster your wood will decompose.
Manure/compost seems to help keep the wood wet, in my experience anyway. However, getting enough material to cover the logs thoroughly is the biggest hurdle to hugelkultur, and also one of the reasons I'm not so fond of it anymore. Wood chips are easier to cover, rot faster, attract more worms, and if you spread them out instead of making a solid layer then plant roots can grow between them without trouble (logs not so much).
I would not add sand to clay. Sand doesn't add anything of value to soil, and if you add it to the sort of clay you've got it makes a nice concrete. Greensand is a whole different story, but I'd put that under a sheet mulch rather than on top or mixing it in. Mixing and digging are worm jobs. On the other hand, a tall hugelbed covered with soil is so far from the underlying dirt that it's more or less like a giant pot, so things mixed under it may not do anything for plants for years. I've still got some small-medium branches that haven't rotted away after 2-3 years or so of being covered, and it's very warm and humid here.
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