The old Doyle Drive had to be torn down because it seismically unsafe, but like the new Bay Bridge, designers wanted something not just functional, but also beautiful. Now, one of the key features is well on its way.
Engineers are concentrating on the twin main post tunnels right now, hoping to get as much work done as possible before the rainy season. Tunnel construction happens in distinct phases -- first, excavation, then comes stabilizing the marshland soils.
Base rock is laid down. Concrete goes on top of that, followed by water-proofing. Steel rebar and concrete is used to build the road bed, followed by rebar rising up as the first step in building tunnel walls. When done, the tunnels will be covered up again with native soils and landscaped, providing an uninterrupted view and pedestrian access to the bay.
A Caltrans animation shows the transformation. As surface roads are replaced with tunnels, cars disappear underground. At the other end, closer to the Golden Gate Bridge, the piles and columns are going in for what will be another viaduct or bridge, side-by-side to the new one already in use.
Some nighttime lane closures and alignment are coming in a few weeks as the project gets ready to build and connect the new viaduct to existing roadway.