Why Not Cut-and-Cover in Chevy Chase?

     The Purple Line can be built 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.

     The Maryland Transit Administration has prepared drawings and videos showing what the trail will look like alongside the Purple Line.   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 the past, 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:

1. 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.
 
2. 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.
 
3. East of Connecticut Avenue, drainages cross beneath the existing rail grade that would be blocked by a cut-and-cover trench.


4. 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.
   

      Some undergrounding of the Purple Line will be required to ensure good transit service for  East Silver Spring. In these areas, there is no pre-existing rail right of way, and at some locations at-grade construction  is not feasible due to steep slopes or will cause trains to be delayed in traffic. All light rail alternatives under consideration involve a mixture of at-grade and underground construction in East Silver Spring; ACT prefers the at-grade route on Wayne Avenue with a short tunnel section east of Sligo Creek.