VIRGINIA BEACH, Va. (AP) — Construction of an additional tube for the Chesapeake Bay Bridge-Tunnel is now two years behind schedule. The wrap-up date is now expected to be 2024.

The Virginian-Pilot reported Tuesday that the delay is due in part to the granite boulders that were used to build the bridge-tunnel 60 years ago.

The project’s main contractors are using tunnel boring machines. But it’s taking longer than expected because of the boulders. The boulders help to form four artificial islands that anchor the portals of the bridge-tunnel’s original two original tubes.

Expansion plans for the bridge-tunnel will add two additional tunnels. The Chesapeake Bay Bridge-Tunnel connects Virginia Beach to Virginia’s Eastern Shore.