The goal of the testing at FAST was to evaluate the effects of repeated heavy loads on the Perforated GEOWEB System installed over a soft subgrade. The soft subgrade was constructed from highly expansive Vicksburg Buckshot Clay, an ASTM “Reference Soil” imported from Mississippi for this purpose. The clay was placed at 30% moisture content in a five foot deep, 700 foot long trench along a section of the High Tonnage Loop, known as the Low Track Modulus (LTM) test zone. The test train operations are designed to accumulate approximately 1 million gross tons (MGT) per day, running on a continuous basis at no more than 40 mph speed. Numerous geosythetic materials had previously been tested under the track in the LTM zone, but ballast tamping was still required on an average every 15 MGT of loading. This time, however, the Perforated GEOWEB System was placed in the sub-ballast layer in readiness for the repeated application of 39 ton axle loads. To ensure that the testing would capture the “worst case” condition for clay subgrades, fire hoses were used to simulate two 500 year rainfall events, saturating the highly expansive clay subgrade during a dry mid-winter period between snow storms and snow melt conditions. Researchers found that the support offered by the Perforated GEOWEB System was not affected by the added water.
Geoweb system stability test over soft subgrades, Assn of American Railroads, Transportation Technology Center, Pueblo, Colorado
The Association of American Railroads (AAR) maintains a very special research facility near Pueblo, Colorado, the Transportation Technology Center (TTC). Within the TTC is the Facility for Accelerated Service Testing (with the apt acronym “FAST”) where years of extreme stress conditions can be applied in a few months. On FAST’s High Tonnage Loop, a 2.7 mile “loop of track” dedicated to High Axle Loads (called HALs, denoting 35-tons or more), some remarkable testing has been performed on the GEOWEB Cellular Confinement System.