While Caltrans is consistent with claims that they don't know enough to recommend a tunnel, this memo offers some insight into what they actually know:
Here are some selected quotes:
"We have completed the update of the Advance Planning Study for the Devil's Slide Bypass Tunnel alternatives."
"Alternative #1 2-lane (46' - 0" roadway) Length 4450' Total Cost $77,500,000 Cost Per Linear Foot $17,400"
"The tunnel costs and cross-section are based on conventional hand tunneling that uses drilling and blasting methods in rock."
"Geology information ... should be considered preliminary."
"The southern approach to the tunnel on the 1974 alignment passes through the McNee Ranch State Park. There appear to be other alignment alternatives that would not affect the Park and could result in a shorter tunnel length such as the 1980 Sierra Club alignment."
"...any new alignment would require extensive geological investigations."
"Caltrans does not have an extensive data base on tunnel costs, in fact the last major tunnel designed by Clatrans was the Caldecott Tunnel built in the 1970s."
"If the material were to be soft to medium rock costs could decrease by as much as 30%."
"If the purpose of this project is to bypass the Slide and not to increase the current highway capacity, then Alternative No. 1 should be selected (2-lane single bore)."Original signed by James E. Roberts, Chief, Division of Structures, Department of Transportation
See a copy of the original memo. Each page is about 40K.