Will H5 treated timber piles last 100 years?

The piles are expected to be permanently below ground level after construction. Groundwater level is expected to be not far below the tops of the piles.

Wooden piles in this context should give a 100+ year service life. Untreated wood that is permanently below groundwater level will not decay so treated wood certainly won’t either.

The large size of the piles means that there is much more preservative chemical present than in the 50x50mm stakes on which decay resistance is normally evaluated.  This means that the above-groundwater portion will have a longer life than is assigned to H5 timber which will be longer than H4 treated timber in the same situation.