I don't think wooden wedges are going to up to the task of splitting a 14 -18 inch hickory log!
Nah, those are just to hold the split open while he retrieves his real wedge.
I want to know who carried an 18" diameter green hickory log into a garage for you. I want to hire the guy for some other stuff.
On BIG wood like that, I just started kerfing it deeply with a chainsaw, cutting a channel on one side, preferably one section you don't want, several inches deep into the log. Then I hammer in wedges. Spreading open a kerf takes a lot less energy than driving the wedge in the first few inches, anyway. Keep the saw or an axe handy to cut through hangers-on. Then you'll see that HALF a hickory log is MUCH easier to split than a whole one.
I hope you get some good staves after all the trouble.