I have just seen this and am not an expert at all on this area, however;
spoil with timbers protruding can in my experience indicate either of 2 things
1 They are the remaining uprights of trees put in to support either deadwork or bad ground, and as such could indicate an adit. Example I managed to locate and reopen the deep adit of Great Wheal Charlotte near Porthtowan in Cornwall a few years ago, I saw this and correctly interpreted its meaning.
2 They can be the remaining uprights of the support timbers of an elevated tramway used for the dumping of the spoil. It is not always indicative of an adit at that level. It can emanate from the site of dressing operations, or sometimes be spoil raised to that height artificially by a hoisting tower or similar. Example, Talybont mine near Machynlleth
I hope that this is of some use.
My avatar is a poor likeness.