the seward wall of the western engine shaft at Trewavas has been gradually entered by the sea, there was a request by the National Trust to do a detailed survey but this was deferred by choughs. Even if the whole seaward side went, it is doubtful if it would effect the engine house however.
At Wheal Rose Porthleven there were fragments of a possible engine house surviving at the bottom of the cliff which may now have gone.
There are a lot of adits between there Marazion which were choked by cliff falls &/or storm debris, which could have either become more choked or opened up, all need a look at again. In some areas the cliffs have retreated a fair distance over the last 100 years, eg the cliffs of head where Trebarvah adit comes out & around Wheal Charlotte (St Agnes).