Creating a safe support system for sequential excavation through soft ground conditions

The challenge

Phase I construction included twin tunnels 1,700ft (518m) long and 26ft (8m) in diameter near Tysons Corner, Virginia, a busy suburb.

Working two 12-hour shifts per day, crews constructed the tunnels using the new Austrian tunneling method (NATM) through soft ground geology comprising gravel, clay and sand.

To protect adjacent buildings and utilities sensitive to ground movement, as well as to accommodate the alignment’s low cover of 15–38ft (4.6–12m), tunneling took place under a grouted pipe arch umbrella.

The solution

Testing demonstrated that Dramix® RC65/35BN steel fibers could meet the project’s specified toughness requirements for the Metrorail twin tunnels through Tysons Corner.

The project’s initial ground support used 10 inches (250mm) of steel fiber reinforced shotcrete – a wet mix design that is self-compacting.