If I'm reading the plans correctly.....
There will be a second carraigeway up and westwards of the existing road. It looks like being at the level of the Drum and Monkey
pub -which will alas be demolished.
If so, the caves in the gorge (Nant Rhin, Capel, etc ) will not be affected. Ogof Craig Y ffynnon is much further up so
also shouldn't be affected. Waterfall cave looks close to the proposed route.
I guess during the work access might be poor and I'm sure it will generate lots of pollution.
....It might just uncover some interesting stuff though......