This article focuses on evaluating the dynamic behavior of ballast mixed with tire derived aggregate as an economic and environmental preserving method to obviate the rigidity problems in railway tracks. For this purpose, the ballast was mixed with 0, 11, 22, and 33 volume percentages of tire derived aggregate in a loading chamber and was examined under different types of dynamic loads such as Sweep sine imposed by a shaker on a rigid square plate. Considering the plate as rigid foundation resting on ballast mixed with tire derived aggregate as subgrade, frequency response functions of both foundation and subgrade were calculated by recorded accelerations and force using the installed accelerometers on loading plate and the wall of loading chamber and load transducer in shaker tip. The obtained results for pure ballast first proved that Sweep sine shaker test leads to more smooth spectra in comparison to other shaking methods. The obtained values for subgrade stiffness and damping ratio showed considerable decrease and increase with respect to increase in tire derived aggregate percentage in ballast mixture. As an optimum percentage of tire derived aggregate, values around 10 can be proposed for practical uses.

The phenomenon of track rigidity in desert areas due to sand dune movement and ballast contamination is one of the most important causes of ballasted railway track degradation and consequently increase in train-induced vibrations. In addition to looking at the existing methods for reducing the track rigidity such as using ballast mat and installing the under sleeper pad, this paper focuses on evaluating the dynamic behavior of railway track including ballast mixed with tire-derived aggregate in a field study. Based on prior laboratory tests done by the authors and latest researches, 10 volume percentages of tire-derived aggregate were adopted as maximum suitable amount to be mixed with ballast for decreasing the track rigidity and increasing its damping property maintaining the basic ballasted track capabilities. In this regard, in a test track located in central desert of Iran in Bafgh-Jandagh railway route, the ballast was mixed with 0, 5 and 10% of tire-derived aggregate by tamping machine in three different sections with 20 m lengths. Moreover, another test section in the same route with fully fouled ballast was considered as a critical bench mark. Many tests were carried out on the mentioned sections using impact test method. In the next stage, the operational modal analysis method was utilized to achieve the tested sections' mode shapes, natural frequencies and damping ratios as well as geometrical attenuation from the recorded responses. By knowing the modal properties, numerical models of tracks were developed, and the effect of tire-derived aggregate as well as fouling on track stiffness was evaluated.