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Alto Tunnel 1975 lean concrete fill near North Portal - Jacobs Assoc. report indicates no proof exists that this was ever done

Can anyone tell me how to verify that a lean concrete plug was in fact poured near the north portal of the alto tunnel in corte madera (reportedly in 1975), and if so, by whom?
I've seen many references to it, but the most recent geotechnical/engineering report from Jacobs Associates of San Francisco indicates that no proof exists that this was in fact ever done.  Thanks!

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Hello Michelle,

The only reference I found on the web to the this concrete plug, one which you may already have seen, is here:

I also attached a copy of the PDF file, below, if the link above becomes unavailable for some reason. This report was prepared for the Marin County Department of Public Works by Jacobs Associates in 2001 -- perhaps one or both may have more information about the plug in question. Failing that, you might call the NCRA ( and ask if they have additional information. Regards,

Mark D.
P.S. Mention of the plug is at the top of page six of that report.


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