While a full-blown tunnel was never really on the table for Mountain View, the city had been considering a partially covered trench to provide a walking path between the downtown area and Rengstroff Park. It appears that the city would be on the hook for paying for that partial trench covering.
Mountain View was the only Peninsula city to have the aerial viaduct dropped from the option list. The aerial viaduct option was widely unpopular in recent public hearings on high-speed rail here.
"I could not imagine how an aerial structure could possibly be fit into Mountain View without having a huge impact on us," said Mayor Ronit Bryant, who attended the board meeting.
In Mountain View, a deep tunnel option has not been a studied alternative, although a partially covered trench has been supported by the City Council. The city had recently asked the rail authority to study a deep tunnel in Mountain View was well, which is now unlikely.
A new staff report lists just two design options for the Peninsula segment: one relies on street level and aerial structures and another includes tunnels at several portions of the segment. Tunneling is still an option for a stretch in San Francisco, in the Burlingame/Millbrae area and in Santa Clara.
In Option A, Mountain View would have tracks run entirely at grade, presumably with Castro Street and Rengstorff running underneath. Option B is much the same, except an open trench runs from Stevens Creek and under Castro Street and Rengstorff Avenue before returning to grade level.
East of Highway 237, both options call for an aerial viaduct.
A "sub-option" called B1, would run an open trench all the way from the town of Atherton to Fair Oaks Avenue in Sunnyvale.
Issues with tunnel option
A deep tunnel would "result in critical risks due to ground conditions, have major constructability issues, lengthy construction schedule and substantial cost features," the report said.
But there are exceptions. "Partially or completely covered trench or short-tunnel sections may be constructed to ameliorate either narrow right of way or environmental concerns" on the Peninsula segment, the report states.
That includes creeks.
"The San Francisquito Creek in Palo Alto could be a location where a short tunnel underneath the creek would be necessary in order to not interfere with the creek's water flow."
Trench covering at city's expense
It says that in other sections, trenching would be designed "to not preclude future decking or coverage" which "would allow cities to cover sections of the trench if they found it desirable and if it were acceptable by Caltrain and the Authority."
Covered sections of less than 600 feet could be added later "without requiring sophisticated fire/life safety systems," the report said.
Bryant said she believed the cost of covering the trench would have to be carried by the city.
This story contains 542 words.
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