|The Dumbarton water tunnel TBM,|
being assembled for the start of its
five-mile drive under the Bay in 2011.
Boring a new tunnel under the Dumbarton corridor, through muddy soils right under a sensitive national wildlife refuge
, seems like an impossibly difficult, risky and expensive undertaking in this day and age. But here's a little-known fact: it's already been done
From 2011 to 2013, a 15-foot diameter tunnel boring machine (TBM) quietly bored a new five-mile tunnel under the Bay from Menlo Park to Newark. The $288 million project, the first tunnel ever bored under San Francisco Bay, is part of the Hetch Hetchy Water System and was built to contain a 9-foot diameter drinking water supply pipe that feeds San Francisco and the peninsula. The TBM that bored the tunnel was an EPB (Earth Pressure Balance) machine and advanced so quickly that it had to wait underground at the far end of its drive, while an access shaft was prepared so the machinery could be retrieved. There were few geotechnical surprises along the way, of the sort that can sometimes blow out tunneling budgets and schedules. The geological layers of clay, gravel and rock under the Bay along the Dumbarton corridor are now better known than they have ever been, and any "geotechnical risk" is effectively retired after the actual boring of an actual tunnel.
Of course, a rail tunnel would be larger and cost far more than the $288 million water tunnel. To safely carry train traffic at speeds of 125 to 150 mph, two parallel tunnel bores about 30 feet (10 meters) in diameter would be needed, connected by cross-passages about every 1000 feet and with a handful of ventilation and emergency evacuation shafts to the surface.
How Much Would a Dumbarton Rail Tunnel Cost?
The costing of bored rail tunnels is reasonably predictable, with models having been developed for example by the High Speed 2 project
in the United Kingdom. The HS2 tunnel cost model can be applied to estimate the known cost of the Dumbarton water tunnel, as a sanity check. The model uses 2011 British pounds, which we convert to dollars using the exchange rate of $1.57 in 2011. The length of the water tunnel is about 8000 m, and it took about 100 weeks to drive and clear out (100 m/week drive and 400 m/week clear-out). Tunnel construction cost is scaled by bore diameter as indicated by section 4.2 chart G.1; the single-bore water tunnel has 23% of the perimeter of a twin-bore 9.6 m tunnel considered in the HS2 document. Disposal cost is scaled by bore area; the single-bore water tunnel has 11% of the area of a twin-bore 9.6 m tunnel. Note the water tunnel does not require portal or ventilation / evacuation facilities.
|Purchase of TBM||EPB Boring Machine||1||ea.||$28M||28|
|Support Costs||Fixed Costs (EPB Machine)||1||ea.||$55M||55|
|Tunnel Construction||EPB Tunnel (single bore)||8000||m||$8000/m||64|
|Disposal of Material||Off-site disposal||8000||m||$800/m||6.4|
The HS2 model seems to predict the direct construction cost of the existing Dumbarton water tunnel reasonably accurately, landing within ~12% of its actual cost. Most of that difference can be ascribed to the much smaller boring machine, which the HS2 model cannot account for; the Dumbarton TBM cost about $10M.
Scaling It up for Trains
The unit costs from the HS2 model can be used directly to scale up to a Dumbarton twin-bore tunnel ready for high-speed electric trains. This tunnel will be a bit longer than the water tunnel, since unlike water, trains can't just climb vertically into and out of the tunnel. Assuming 2025 dollars, which are worth about 20% less due to inflation, you get the following direct construction costs:
|Purchase of TBM||EPB Boring Machine||2||ea.||$35M||70|
|Support Costs||Fixed Costs (EPB Machine)||1||ea.||$69M||69|
|Tunnel Construction||EPB Tunnel (twin bore)||10000||m||$43000/m||430|
|Disposal of Material||Off-site disposal||10000||m||$9000/m||90|
|Tunnel Shafts||Ventilation / Emergency||3||ea.||$39M||117|
|Systems||Electrical / Mechanical||10000||m||$8000/m||80|
The basic construction bill comes to $1.2 billion in year-of-expenditure dollars for a state-of-the-art twin-bore electric rail tunnel built in the middle of the next decade. This figure is then burdened roughly as follows:
- 3% environmental mitigation
- 25% contingency
- 6% engineering design
- 3% program management
- 4% construction management + 0.5% agency fee + 4% mobilization costs
These overhead rates compound with each other, combining to 53%. The expected all-up cost of a twin-bore Dumbarton tunnel is then about $1.8 billion. Add to that the expense of removing the old bridge, estimated by Samtrans at $150M
, and we reach almost $2 billion.
As we are often reminded on the peninsula, a tunnel puts the trains out of sight and out of mind. In this case, it actually makes sense to build one because it crosses a terrain obstacle, San Francisco Bay. A new tunnel avoids visual and noise impacts, removes the blight of the old bridge, enables higher train speeds without endangering wildlife, and can be made more resilient to sea level rise than a new bridge. A new tunnel is not much more expensive than the options now being contemplated as part of the Samtrans Dumbarton Transportation Corridor Study
, where it was summarily and improperly dismissed as too expensive, risky, burdensome and impactful (see Table 6-4). The tunnel option deserves a second and more serious look.
A Dumbarton tunnel could extend under University Ave and Willow Road in Menlo Park, grade separating both for a marginal cost that our model places at $132k per meter of twin tunnel (in 2025 dollars). The Samtrans study estimates each grade separation to cost about $200M (in 2017 dollars), so the two grade separations are worth about a mile of extra twin tunnel if you've already got TBMs in the ground. That's before the grade separations have to be rebuilt to accommodate sea level rise.
A Dumbarton tunnel would provide more cost certainty than a bridge. The last bridge the region built
overran its cost estimates by several hundred percent, while the Dumbarton water tunnel was on time and on budget. Tunnel boring is a well-developed technology that is highly automated and doesn't use a lot of expensive construction labor. Some people are working on making it even more automated
|San Francisco to Tracy in 35 minutes|
A Dumbarton tunnel could serve as a key component of a new regional rail link
between the Bay Area and the Central Valley, putting San Jose much closer to Sacramento, and San Francisco under an hour from Stockton. It could eventually serve as the entry point of high-speed rail into the Bay Area, making faster trips from anywhere in the Bay Area to Sacramento and southern California. The performance simulation at right shows a high speed train passing through Tracy just 35 minutes after departing San Francisco Transbay, traveling along the Altamont SETEC alignment
. This would vastly simplify the "blending" of Caltrain and high-speed rail since the latter would enter the peninsula rail corridor at its midpoint, sharing slow tracks for only half the distance of the existing plans and requiring fewer overtake maneuvers.
A new Altamont / Dumbarton high speed regional rail link could replace and combine the fragmented hodge-podge of projects and agencies variously pushing Altamont Commuter Express
extensions, Valley Link
, Livermore BART
, a second BART Transbay Tube
, the high-speed rail system, and whatever Cross Bay Transit Partners
might come up with for Dumbarton, each of which nibble at different edges of the same basic problem: our regional mobility is inadequate and relentless traffic jams are crushing the souls of hundreds of thousands of people in the I-580, I-680, I-880, US-101 and CA-92 corridors.
The Dumbarton rail corridor needs to be thought of as so much more than a simple bay crossing that relieves traffic for people who work at Facebook. This is a one hundred year piece of infrastructure that can unclog an entire region, and it needs to be engineered for it. A tunnel for $2 billion (in 2025 dollars) is a sound and future-proof investment.