Marin County, California Bridges: Marin County Tunnels

2016 Marin County Tunnels

Are tunnels a kind of bridge? Because of the hilly terrain there are quite a few tunnels in Marin County. The photo above shows the 2350 ft long Cronkhite (Bunker Road) Tunnel (27 0039). It was built in 1917 and carries traffic under US 101 to and from the Marin Headlands (the southern tip of Marin County). It's just wide enough for a single traffic lane so cars (but not bikes) have to wait 5 minutes for the light to turn green on each end of the tunnel.

The nearby Waldo Tunnels (27 0040L/R) are 1000 ft long and were built in 1937 and 1954.  Since they carry busy US 101, they consist of two parallel 4 lane tunnels to and from the Golden Gate Bridge (shown above).

Marin County is big on bicycles and so there are several bicycle routes for commuters.  I rode on the Cal Park Hill Pathway which shares a 1000 ft long tunnel with the Sonoma-Marin Area Rail Transit (SMART) Line. It includes a gate that closes the bike path at midnight (shown above). There are several similar 19th century railroad tunnels that are being renovated for new passenger rail lines and bike paths.

Marin County, California Bridges: Marin County Tunnels by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Alameda County, California Bridges: Posey and Webster Street Tubes across the Oakland Estuary

June 2014 (37.79124 Degrees, -122.27578 Degrees) Posey and Webster Street Tubes

Before we leave Alameda Island I wanted to mention the last two crossings from the mainland. However, these structures are tubes that go under the Oakland Estuary. From 1871 until 1928 the Webster Street Crossing (a swing bridge) connected Oakland to Alameda Island at this site.

In 1923 the Alaska Packers Association promised to build wharves and warehouses on Alameda Island if a tube was built. $4.5 million was allocated and the Posey Tube (after County Surveyor George Posey) was constructed from 1925 to 1928. Precast concrete tubes were built in San Francisco, floated to the estuary, sunk into a trench, sealed together, and then covered in Bay Mud. In 1963, a second tube (the Webster Street Tube) was built.

The tubes performed pretty well until the 1989 Loma Prieta Earthquake shook the estuary. The tubes were sitting in unconsolidated mud and tried to rise to the surface, allowing water to leak into the joints. A long, costly retrofit was performed in the 1990s, which should protect the public but it may not prevent the tubes from being damaged and closed after the earthquake.

Alameda County, California Bridges: Posey and Webster Street Tubes across the Oakland Estuary by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.