Test Measurements and Performance Evaluations of In-Service Railway Asphalt Trackbeds

During the past twenty-five years the use of Hot Mix Asphalt (HMA) as a subballast layer, or underlayment, within the track structure has steadily increased until it is becoming standard practice in many areas of the United States. This asphalt-bound impermeable layer, typically 5 to 8 in. (125 to 200 mm) thick, provides a “hardpan ” to protect the underlying roadbed and to support the overlying ballast and track. Long-term performance studies on numerous HMA installations attest to the improved attributes and economic benefits of the asphalt layer, particularly on heavy tonnage lines traversing areas of marginal geotechnical engineering characteristics. Recent studies involve instrumenting HMA trackbeds with earth pressure cells and displacement transducers to measure pressure levels and distributions within the track structure and rail deflections under moving trains. These tests, conducted in real time domain train operations, confirm the positive attributes of the asphalt layer. Results are presented in detail for test installations on a CSX Transportation heavy tonnage mainline and at the Transportation Technology Center (Pueblo) low track modulus heavy tonnage test track. KENTRACK, a finite element, layer-elastic trackbed structural design computer program, is utilized for determining the effects of varying trackbed materials, component thicknesses, and designs on pressure distributions in the track structure. Predictive values from the KENTRACK model are compared with in-track measurements. The predictive values, obtained from KENTRACK, compare well with in-track measurements for similar conditions providing a measure of credibility for the model