One of TTC’s test tracks, the High Tonnage Loop, has had too much of a good thing: a dense silt-sand subgrade providing an excellent support condition along the entire 2.7-mile loop. But TTC needed to evaluate and test poorer subgrade conditions that exist at many locations in North America.
How did TTC prepare to test these conditions? With Mississippi clay. After importing a soft clay from Mississippi, TTC track crews began placing and compacting it in a five-foot-deep, 12-foot-wide, 700-foot-long excavated trench, which is now a track modulus of 2,000 pounds/inch/inch, “soft” by any standard.
We got what we asked for: a “high-maintenance” track at the Facility for Accelerated Service Testing (FAST) with an average tamping cycle duration of about 15 MGT, although it has ranged from a high of about 30 MGT to a low of 1.0 MGT under 315,000-pound car operation.
While the track was experiencing the 1.0 MGT tamp cycle conditions, a geosynthetic material known as GEOWEB® was tested by placing it in the sub-ballast layer, as shown in Figure 4. The GEOWEB® is a cellular confinement system designed to accommodate subgrade movement without loss of structural integrity (see illustration, above right). In this application, sub-ballast was placed in the cores of the GEOWEB® forming a composite layer to provide a greater stiffness and resistance to track settlement.