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Why Not Cut-and-Cover in Chevy
Chase?
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ACT believes
it is very feasible to construct the Purple Line adjacent
to an improved Capital Crescent Trail at grade along what
was the Georgetown Branch railroad right-of-way between
Bethesda and Silver Spring. It can be done in a manner
that “ensures a safe and accessible pedestrian-bicycle
trail” as specified in recent federal legislation. There
are over 60 trails in the US adjacent to active rail lines
according to the Rails-to-Trails Conservancy.
ACT is opposed to undergrounding
in this segment as a huge sum of money would be required
to put this project underground and would likely put the
project at risk of being declared cost-ineffective by the
Federal Transit Administration. Attractive
landscaping and retaining walls, frequent trail
crossovers, grassy rail beds and other techniques exist
that will ensure that the Georgetown Branch segment
continues to feel like a 66’ to 100’ linear park. On
East-West Highway today -- which is much wider than the
future two-track Purple Line -- there is a tree canopy
that provides good shade over the entire road at
mid-day.
In recent months, some political
candidates have advocated "cut-and-cover"
construction for the Chevy Chase portion of the Purple
Line. This is a method of underground construction in
which a trench is excavated from the surface, the rail
line is built in the trench, and a roof is built over the
trench. This method of underground construction, while
still much more expensive than building at grade, is
usually considered less expensive than deep tunnel boring.
However, site-specific conditions in the Chevy Chase
section of the Purple Line may create additional problems
with cut-and-cover construction above and beyond the cost:
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- The trench must be wider than the tracks because
side walls are required. Tree roots can extend under
at-grade railroad tracks but are blocked by
cut-and-cover tunnels. For these reasons,
cut-and-cover construction in a wooded area like the
Georgetown Branch right of way will cut down more
trees than surface construction.
- For structural support, there will be a six inch
layer of cobbles under the light rail tracks. This
layer is very permeable and acts as a drain if it is
below the water table. With cut-and-cover, this layer
will be below the elevation of Coquelin Run and,
depending on soil conditions and the distance between
the tracks and the watercourse, could drain water out
of the stream. The usual engineering fix for such
problems is a grout curtain, but this is expensive and
could block ground-water recharge into the creek from
the north side of the tracks. The risk that these
problems will cause Coquelin Run (especially its
upstream portion which is closest to the tracks) to
dry up in less rainy seasons can be assessed by
geotechnical and hydrogeologic studies, but such
investigations have not yet been performed.
- East of Connecticut Avenue, drainages cross beneath
the existing rail grade that would be blocked by a
cut-and-cover trench.
- A trench excavated beneath the Air Rights Building
or the East-West Highway bridge would undermine these
structures; so light rail must be built at grade at
these two locations. The preferred grade for light
rail tracks is no more than 3%, although grades up to
6% are possible where geometric constraints require
it. At a 3% grade, there is barely enough room between
the two structures for a track to descend 30 feet and
rise back to grade. Consequently (unless the East-West
Highway bridge is demolished and rebuilt at great
expense and great inconvenience to commuters) a light
rail line built by cut-and-cover would still be above
ground along at least half, and very possibly all, of
the portion of the right of way that adjoins the Town
of Chevy Chase.
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| Finally, there is an
important issue of equity about whether the Purple Line is
above or below ground in Chevy Chase. This right of way,
with its very attractive view, was purchased with
everyone's tax money and belongs to all of our citizens.
The future riders on the Purple Line are much more
numerous than the future trail users (who are in turn more
numerous than the current users of the unpaved interim
trail). Surely, the many Purple Line riders have the same
right to enjoy the beautiful Chevy Chase scenery on their
way to work as the somewhat-fewer trail users and the very
few neighbors.
Some undergrounding of
the Purple Line will probably be required to ensure good
transit service for a section of East Silver Spring and
for the core of the University of Maryland campus. In
these areas, there is no pre-existing rail right of way,
and at some locations at-grade construction will cause
trains to be delayed in traffic. The additional expense of
underground construction buys better transportation. For
more on this subject, see ACT's
position paper on Purple Line alignments in East
Silver Spring.
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