Wednesday, July 22, 2015

Phasing the Grand Junction viaduct

In a previous post, this page explored how the Allston Turnpike relocation could be built without a large road viaduct. Thanks to input from several sources, here is a very rough idea of how it could be phased, with no interruption to Turnpike traffic or Worcester Commuter Rail line (aside from quick weekend-long changeovers) and only minimal closures (in the months range, not years) to the Grand Junction railroad. Note that while these are largely to scale, they are by no means engineering drawings, and that (especially laterally) there may be changes, as I am estimating this off of measuring aerial photographs on Google Maps, not actually surveying the highway.

I am first going to show the elevations for each phase, crossing at a point in the middle of the current viaduct. There will obviously be differences at either end, where roads and railroads will have to ramp up or down to reach the construction phase. However, those will be shown in the aerial views, further down. In each of these, the Charles River is on the left, and Boston University is on the right, in other words, they are looking east. The scale in feet, with a grid showing every 20 feet horizontally and every 10 feet vertically.

Current conditions have the Turnpike on a viaduct over the railroad, Soldier's Field Road (SFR) and the bike/ped facility (Paul Dudley White Path, or PDWP).

In Phase 1, several steps would occur:
  1. Soldier's Field Road is relocated towards the river and potentially narrowed slightly (Memorial Drive is 10 feet narrower than SFR yet still has four lanes of traffic). The PDWP is moved slightly towards the river.
  2. The DCR "parkland" between SFR and the Turnpike—currently weeds and gravel—is excavated approximately four feet and three westbound Turnpike lanes are build there. Three lanes are viable during the project, as the current three westbound lanes (owing to the Commonwealth Avenue bridge project) are suitable for traffic volumes. This grade is four feet lower than the current Soldiers Field Road, level with the lowest point where the Turnpike crosses under Commonwealth Avenue.
  3. The Grand Junction is retained under the westbound Turnpike lanes. A new, permanent bridge is built for the railroad over SFR, with extra width to accommodate a bicycle/pedestrian path looping up to the BU Bridge (and eventually the "People's Pike" along the Grand Junction to BU and Allston). The new bridge is also built for the Grand Junction under the existing Turnpike viaduct, while the Grand Junction's current connection is kept to the north. A ramp down to grade is also built below the current viaduct.
  4. Once at grade, the Grand Junction alignment is moved slightly south to allow excavation and construction for foundations for supports for the new Grand Junction viaduct. These can be built during or after the Turnpike viaduct is removed.
  5. The PDWP is relocated under the new, wider Grand Junction bridge along SFR, eliminating the dangerous (narrow, bumpy, slippery when wet, poor sight lines) boardwalk under the BU Bridge. There is plenty of room under the BU Bridge for both a level path and a ramp up to parallel the Grand Junction (24 feet, enough for each path to be 12 feet wide).
  6. The Turnpike is moved under the new Grand Junction bridge and on to the temporary alignment. At this point the Grand Junction's current alignment is removed and service is suspended until the viaduct can be removed above the easternmost section of the Grand Junction. Once this is done, the rest of the removal can proceed with the line in service, with care to be taken to allow for daily and as-necessary traffic on the Grand Junction. 
  7. The westbound Turnpike ramps back up to the viaduct near where the current exit ramp leaves the mainline. A spur track is built under this westernmost area to serve Houghton Chemical. 

Phase 1A (above) shows how temporary supports would be built on the south side of the highway (near BU) and the right of way slightly widened to allow for the Worcester Line to be relocated from underneath the viaduct structure.

Once Phase 1 is completed, half of the Turnpike has been removed, and the supports for the Grand Junction viaduct have been put in to place. In this phase, second half of the Turnpike comes down. For this to occur, the following has to happen:
  1. The three westbound Turnpike lanes are kept as-is, and four eastbound lanes are built between the current eastbound structure, above the location of the Grand Junction viaduct.
  2. The Worcester Line is moved to the outside of the Turnpike structure. This may require temporary supports and/or the temporary taking of some land from Buick Street behind Boston University. This will need to be explored further. Another ramp is built under the to-be-demolished Turnpike, again for the Grand Junction, and a temporary bridge across the location of the eastbound lanes. The Grand Junction will be placed out of service while the viaduct is demolished above this grade; again, this would be the first priority to minimize disruption.

At this point, the main highway work is mostly complete, with the ability to have four 12-foot lanes for each direction of highway in the main cross-section, as the eastbound Turnpike lanes are built at the same level as the westbound lanes where the viaduct previously stood. The Grand Junction is again put on a temporary ramp (with a potential short shutdown), this time alongside the Worcester Line railroad. Some extra room is left in the median of the Turnpike to allow for construction of the Grand Junction viaduct. This could be pre-cast off-site (perhaps in the Allston yards area) and assembled at off-peak times with temporary lane closures.

Once the Grand Junction viaduct is built, several punchlist items are completed:

  1. The bike path and Storrow Drive are moved four feet to the south, as the westbound Turnpike is narrowed slightly taking up space previously used for construction of the viaduct. 
  2. Both sides of the turnpike now reach their final alignment with four twelve-foot lanes with three-foot shoulders (wider than most of the Turnpike extension today; so perhaps a few feet of this leeway which could be better used for the PDWP), and with far better grades and sightlines than the current highway; important since with open road tolling vehicles will no longer be slowing down or speeding up at the toll plaza.
  3. The Worcester Line is double-tracked lowered to the same grade as the Turnpike (or perhaps a couple of feet lower) to allow future overbuild.
  4. Supports for a wide bicycle/pedestrian/park facility are built over the eastbound Turnpike and railroad tracks, connecting Allston to the BU Bridge and PDWP, and eventually along the Grand Junction bridge to Cambridge and beyond.

This is the final elevation of the project. Here are the corresponding aerial views:

The approximate location of the cross section is shown. A is on the right side of the cross sections, B on the left. Colors on the aerial views match those on the cross sections.

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