Pressure Measurements and Structural Performance of Hot Mixed Asphalt Railway Trackbeds

In order to assess pressure distributions within the track and support structure, earth pressure cells have been installed in selected hot mix asphalt (HMA) trackbeds on a CSX Transportation heavy tonnage revenue freight line and at the TTCI heavy tonnage test track at Pueblo. The measured values of pressure distribution within each layer were used to establish a pressure index. This pressure index is the ratio of pressure at the interface of a layer, to the predicted bearing capacity of the layer. A pressure index value of one implies failure of the layer. The pressure index and other measures of structural performance such as track modulus and subgrade stiffness were assessed. From these assessments, relationships were established whereby the pressure distribution and deflection characteristics of the trackbed were quantified in terms of material type and layer thickness. Thus, allowing for a rational performance assessment of the overall trackbed system. and Lees, 2008; Anderson and Rose, 2008). The most common HMA trackbed, termed asphalt underlayment as shown in Figure 1b, incorporates a layer of HMA in lieu of the subballast. Ballast is used above the HMA layer in a similar manner as the conventional all-granular trackbed. The ballast provides a protective cover for the HMA by blocking the sunlight, protecting the surface from air and water, and maintaining a relatively constant temperature and environment. (a)