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.