Transportation professionals on BART extension: Let's actually examine costs of single vs. twin-bore tunnels
René Magritte: Time Transfixed, 1938. Image by Vilseskogen on Flickr
Next Wednesday, a new ad hoc committee overseeing the BART Downtown San Jose extension will meet for the first time. In preparation, Bay Area Transportation Working Group (BATWG) members have compiled a list of key questions for the steering committee to address. BATWG's letter, in its entirety, follows.
Dear Chair Burt and other members of the BART Phase II Steering Committee:
It is now clear that the new Steering Committee will be examining all aspects of the BART Phase II Project. This comes as welcome news. A fresh look at the situation is needed.
There are important questions that the Staff and its consultants should be called upon to answer. Here are a few that we think may qualify:
Comparison of Tunneling Alternatives
Width of Center Loading Platform:
The alternative set forth in the EIR consisted of two tunnel bores, each 20 feet in diameter between stations, with the three subway stations to be constructed by cut and cover methods. This twin-bore option included 28-foot-wide center loading platforms. This arrangement was later changed to provide a 54-foot single tunnel bore arrangement with everything, including the stations, placed inside. This option includes 20-foot wide center loading platforms.
BART’s other downtown platforms range from 30 feet to 40 feet in width. Have any passenger throughput analyses been run? What assurance is there that 20 feet of platform adequate for access, circulation and safety?
It has been strongly suggested that to ensure passenger safety, partitions with sliding doors that automatically open only in front of stopped trains be added. What would this additional feature cost *?
Cost of 54-foot diameter tunnel alternative compared to the cost of the twin bore alternative?
How many additional cubic yards of tunnel excavation along the five-mile subway including portals would there be with the 54-foot tunnel? At what cost?
How many additional cubic yards of concrete tunnel lining? At what cost?
How much bigger would the portals have to be? At what cost?
How many cubic yards of cut-and-cover excavation would there be with the Twin Bore alternative? At what cost?
How many cubic yards of cut-and-cover excavation would there be with the Single Bore alternative? At what cost?
Deeper Subway compared to Twin Bore Alternative:
How many feet of additional descent would it take to get from sidewalk level to train level?
How many feet of additional emergency egress facilities? At what total cost?
How many feet of additional escalators? At what total cost?
How many feet of additional elevators? At what total cost?
How many feet of additional vent structure? At what total cost?
What is the total cost of deepening the subway by roughly 35 feet?
Life Cycle Cost Comparison
Assuming an economic life of 100 years, what is the Present Worth of the total operating, maintenance and security cost of deepening the subway by roughly 35 feet? Please provide a life-cycle cost comparison between the two construction approaches?
* All costs should be in 2036 U.S. Dollars
The Crossover: A crossover is shown in the original twin bore layout as located just east of the Downtown Station. Why there? Why couldn’t the crossover be located directly under the Diridon Station or at some other, less disruptive, location?
The Cut and Cover Stations: This method of building stations has been used the world over for well over a century. Through the use of temporary timber decking, most of the work takes place underground and out of sight after decking is put in place. By opening only half the street at a time to install the decking, and at the end of the job remove it and rebuild the street, the street is never entirely closed to traffic flow. It appears that switching from this tried-and-true way of building subways to a much larger and deeper tunnel has added significantly to the capital and O&M costs of the project.
Gerald Cauthen
Gerald Cauthen P.E.
Co-Founder and President,
Bay Area Transportation Working Group (BATWG)
510 708 7880
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