The ones I've seen in Cornwall (Ding Dong, St Ives, some others in Mounts Bay) have been built up from solid rock and are like circular arches, ie:- the shaft can't do 0>>o and drop in. I imagine Trevoole will be of the usual timber collar (probably gone with a big crater) into hard rock. I don't think there is an epic amount of soil around there.
Sadly, I can't post anything due to the computer not wanting to manipulate pictures, but there are a whole series of shafts on the 1909 6" map (not the 1888) and I reckon it would be prudent to find out what goes where. I think the adit (looking at the map, without going for a poke around) comes out in Pendarves woods....The ponds you can see on GE were not originally there, so it would be unlikely the adit is the source of them. Running in the other direction, there is a shaft (x2?) in the patch of ground across the field to the SW, however due to the lack of air shafts and anything resembling them, this is unlikely to be related, IMO.
Interestingly, I note the W's as in wells on the 1909 map. I wonder if these are shafts on one of the other lodes.....They (OS) sometimes confused the two.
There is a shaft in the field and 2 other possible sites NW from it which look interesting..... I imagine the obvious shafts are bins, but I gather the adit level is over 500m long, so there has to be something to look at.
Anyway, it's weirdy beardies tomorrow night, so I'll ask some people who will have had a rummage up there.