Focus On: Mission Bay Grade Separations

The San Francisco downtown extension, as environmentally cleared and approved, ends at the existing grade crossing at 7th and Mission Bay Drive (formerly Common Street).  Somebody apparently forgot that this will leave grade crossings at Mission Bay Drive and busy 16th Street, where it still isn't exactly clear how to cross a trolley bus line with a 25 kV electric railroad at grade.  With train traffic expected to double in the next 15 years, conditions at those intersections are expected to become unacceptable, a problem which is now being considered along with redevelopment ideas for the Mission Bay neighborhood.

A number of radical solutions were proposed by outside groups (including the city!) coloring outside the lines of the environmentally cleared and approved DTX project, thus causing significant unease at TJPA headquarters.  To regain control of this issue, the TJPA commissioned a white paper study by engineering consultants Parsons Transportation Group and Jacobs Associates (organizations with considerable tunneling expertise), entitled Proposed Alternative Tunnel Connections to the San Francisco Downtown Rail Extension.

Grade Separation Tunnels Proposed

The billion dollar PowerPoint slide,
from the city's presentation,
used as pretext for dismissing a trench

Curiously, on page 1 of the white paper, the most logical option is summarily dismissed: 

• City SPUR Presentation Option 1 – This option consists of a trench trackway in between the I-280 freeway pilings. The presentation noted that this option may not be feasible, and therefore it is not discussed further in this white paper.
  

This seems like a rather weak rationale for not further discussing or studying a particular option: somebody else, presumably far less-qualified than the Parsons / Jacobs engineering team, happened to cast doubt on it.

One could cast a similarly dismissive eye on the entire study.  It boils down to considering various complex and over-wrought tunneling options (considering arcane subjects such as boring machine types, ventilation zones, and even tsunami run-up) in order to grade-separate these two streets for an estimated cost of well over a billion extra, compared to the current DTX plans.  That's right, BILLION.  Two grade crossings.  If you do the math, at least $500 million a pop works out to about ten times what a typical grade separation should cost.

The study's conclusion, not surprisingly, is to proceed with the TJPA's environmentally cleared and approved DTX plan so as not to upset or delay anything, with the understanding that nothing in the current plan precludes grade-separating in the future, if someone finds more than a billion under a rock.

Grade-separating a couple of streets doesn't need to be so ridiculously expensive or complicated, of course.  It requires an open trench built under the I-280 freeway, along the current alignment of the tracks and between the large pilings that support the freeway.  This trench would pass under Mission Bay Drive and 16th St, then climbing steeply (2.5% grade) to meet the existing portal of Caltrain Tunnel One.  No new tunnels would be built, and the freeway could later be torn down if the city so desires.

This modest 1/2 mile grade-separation trench extension would not require complex phasing; it would be built as part of the DTX tunnel, at an incremental cost far smaller than the proposed grade separation tunnels, with approval of a minor supplemental EIR.  There would be no need for fancy U-walls or tunnel stubs or temporary structures that would have to be modified at some future date with great disruption to train service.  The vertical profile would look very much like this:

Grade separation trench profile view (from west side of tracks)

There are a number of arguments raised against a trench at this location.

Constraint #1: Freeway Pilings

1987 grade separation trench concept

To see what a trench built between the I-280 freeway pilings might look like, we can set our way-back machine to the year 1987 -- more than a quarter century ago -- when Morrison-Knudsen Engineers performed a study entitled Peninsula Commute Service Interim Upgrade Study that included the I-280 diagram at right.  In 1987, the idea of building a rail trench under an elevated freeway wasn't simply dismissed without further consideration.

I-280 straddle bents at 16th Street

And suppose the trench did come too close to some I-280 piles?  The 1989 Loma Prieta earthquake triggered a massive seismic retrofit of the I-280 freeway, during which Caltrans demonstrated a surprising ability to add or move bents under an active freeway.  Even temporary solutions would be acceptable, since the city plans to tear down this section of I-280 in the short to medium term.  The biggest shame would be to spend a billion-plus dollars to avoid freeway pilings that are about to be dismantled.

Constraint #2: Major Sewers

There is already a large sewer passing under the tracks at Division Street (see profile diagram above).  Furthermore, the San Francisco Public Utilities Commission is in the planning stages of a $1 billion sewer project known as the Central Bayside System Improvements, which will involve tunneling a new 27-foot "Channel Tunnel Connection" underneath the tracks (see profile diagram).  The approved DTX project artfully avoids the sewers by surfacing before the sewer crossings, but a trench or tunnel would inevitably conflict with them.  The TJPA study assumes the new sewer tunnel forms the controlling constraint on the vertical alignment of a grade separation rail tunnel, which sends the rail tunnel so deep underground that it can no longer connect to the existing Caltrain Tunnel One.  That's what makes the proposed rail tunnels so long (all the way to Cesar Chavez!) and so expensive.

This raises an interesting question of underground right-of-way: should the rail tunnel yield to the sewer tunnel, or the other way around?  Without very much expertise in the art of pumping enormous flows of sewage, one might guess that sewage is more flexible than trains when it comes to changes in vertical profile.  The SFPUC and TJPA need to be prodded to come to a more realistic agreement on this issue, rather than flushing well over a billion of scarce rail dollars down that new sewer.

Constraint #3: Track Gradient

There isn't a lot of distance between the portal of Tunnel One and the 16th Street crossing.  If 16th Street is left where it is, the tracks need to dive about 28 feet down in that short distance, resulting in a rather steep grade of 2.5%, after accounting for vertical curves.  The TJPA study points out:

• insufficient length is available to transition from the tunnel below 16th Street to at-grade at Tunnel One while complying with vertical grade and curve requirements of DTX, Caltrain design criteria, or accepted railroad engineering practice.
  

Tunnel One North Portal

2.5% is not an impossibly steep grade for EMU electric rolling stock of the sort contemplated by Caltrain and the high-speed rail authority; portions of the high-speed rail system are being engineered with grades approaching 3.5%.  If there is a problem here, it is clearly with overly conservative DTX vertical grade requirements and Caltrain design criteria.  American "accepted railroad engineering practice" is totally irrelevant here, being firmly rooted in freight train operations.

If anybody tells you a 2.5% grade is impossible, ask for a second opinion, preferably from a European engineering consultant.  And there are ways to reduce the required dive: 25 kV overhead electrification clearances could be reduced by several feet by using rigid overhead conductor rails (instead of wires), or 16th Street could be raised by a few feet.  It sure is tight, but it most definitely can be done.

Constraint #4: Keep Caltrain Operating

Shoo-fly concept

Old wye track viewed from
under Mariposa, since removed

Pair of yard tracks to the east of the
main tracks, looking north from 16th

Keeping Caltrain operating during construction of the DTX connection is obviously a major concern.  It would be necessary to construct a temporary shoo-fly track on the east side of the existing tracks, to enable the trench to be constructed where the current tracks exist.  There are already some siding tracks to the east under I-280, but the challenge is to dodge out of the forest of freeway bents at Mariposa Street without violating side clearances between trains and concrete, along the path of an old wye track that was recently removed.  This can most likely be done as shown in the diagram at right; it is a puzzle that requires all the ingenuity our transportation industrial complex can muster.  If bents must be moved, then move them.  If clearances are truly insufficient, CBOSS can be used to enforce single-occupancy of the narrow section.

The TJPA study describes how the DTX project would be staged to allow tying in to a future grade separation tunnel.  The staging concept described would require disconnecting the Transbay Transit Center for several weeks or months while the new connection is made, with the 4th and King terminal used on an interim basis.  This staging concept is surely more time-consuming and disruptive than the trench tie-in work implied in the shoo-fly diagram above, which could probably be pulled off over a three-day holiday weekend.  Far more complex operations were pulled off on that time scale at the Bay Bridge.

Conclusion

Dismissing the trench grade separation alternative is premature and poorly justified.  Boundaries should be pushed and "requirements" should be questioned.  Coordination between the city, the TJPA, Caltrans and the SFPUC should be improved with the help of the Mayor's Office, and if necessary the Governor.  Most importantly, the small sliver of land between Mariposa and Sixteenth should be temporarily preserved from development until the DTX is completed, so that a shoo-fly can be built there.

Without a doubt, this trench would be a tough grade separation to engineer and construct, but when the only alternative is a billion-plus bored tunnel, there are suddenly a billion-plus reasons to take another look to make it work.  With the extensive means of excavation to be mobilized for the DTX project, the incremental cost may be small enough that Mission Bay and Sixteenth can be grade-separated concurrently with the DTX, without delaying the project.

The TJPA and its consultants should be sent back to the drawing board to study the feasibility and cost of the trench option in much greater detail.