What is the condition of the Alto Tunnel today?
The northern 170-feet was inspected in June, 2001 and determined to be in good structural condition. The exact condition of the middle 1600-feet of the tunnel is unknown at this time; however, further investigations are being proposed to better determine the condition in this zone. The existing redwood timbers may be in various stages of decay, depending on the amount of moisture in the sealed portion of the tunnel. Regardless of the existing conditions, the existing redwood timber support system will have to be replaced and augmented with a new concrete liner for both structural safety, aesthetics, and to eliminate any fire danger. The Southern 350-feet was backfilled with gravel in 1982 and will have to be excavated and supported with a new concrete structure. MCBC is requesting that the County of Marin do further goetechnical study on the tunnel to learn more about the condition of the tunnel so more accurate cost estimates can be determined.
