## 1 Introduction  
With the rapid development of transportation in China, highways or high-speed railways would inevitably encounter the situation of soft soil foundations. How to deal with unacceptable settlements of embankments on these foundations has become one of the most challenging tasks for engineers [1, 2]. Traditionally, the composite foundation and plastic drainage are always utilized to reinforce the soft soil foundation to diminish the potential settlement [3, 4]. However, use of embankments with lightweight backfill material, such as expanded polystyrene (EPS) blocks (also called EPS geo-foam) or EPS beads mixed with soil and binder [5, 6], can also reach this goal. Obviously, the latter is much more time saving, cost-effective, and even environmentally friendly. Since [PERSON] [7] firstly reported that the EPS geo-foam was used as an embankment fill in Norway, more and more studies about this artificial material have been conducted and its good performances have been confirmed. For example, replacing the typical embankment fill material in highways [5], diminishing the maximum lateral earth pressure of a reinforced soil platform [8],and reducing the swelling pressure by expansion of soil behind the retaining wall [9], etc. At the same time, a lightweight fill consisting of dredged soil and air foam and cement (i.e., EPS beads) was adopted to reduce the embankment self-weight on soft foundations in Japan [10]. [PERSON] [11] pointed out that the EPS beads could even reduce the weight of the fill to a great extent, even to be 50%, which is of great beneficial for the post-construction settlement control. [PERSON] et al. [12] pointed out that the unconfined compressive strength of the lightweight fill, as well as the shear strength and stiffness, increases considerably if the cement-soil ratio of 10% to 15% is used. Moreover, [PERSON] et al. [13] inspected the mechanical properties of the lightweight fill through a series of road performance tests, verifying that embankments with the lightweight backfill obtain an obviously smaller settlement over embankments with the conventional lime-stabilized fill. As a subgrade filling replacement material, the lightweight soil should have good bearing capacity and deformation properties under static loading, but it also needs good mechanical performance under repeated traffic loading. As early as 2002, [PERSON] et al. [14] has pointed out that the mechanical behaviors of lightweight soil under static and dynamic loads would be quite different from the natural soils. At present, the research on the dynamic characteristics of lightweight soils mainly focuses on the acquisition of basic dynamic characteristics parameters and the effects of various influencing factors. For example, [PERSON] et al. [15] analyzed the characteristics of skeleton curve, dynamic shear modulus, and damping ratio of EPS composite soils based on nineteen combined axial-torsional tests on hollow cylinder specimens. Through resonant column and cyclic triaxial tests on sand-EPS bead mixtures, [PERSON] et al. [16] discovered that the material damping is relatively unaffected at small shear strains but it increases at larger strains, and the decrease in shear stiffness with increasing bead content occurs at all strain levels. Moreover, [PERSON] et al. [17] carried out a series of laboratory tests to evaluate the monotonic, cyclic, and post-cyclic behavior of the contact interface between lightweight soil and reinforced geogrid. Nonetheless, the road performance of the lightweight fill, especially the mechanical behavior under complex loading conditions as well as the difference with different natural soils is still not clear. In this study, a series of triaxial shear tests, under repeated loading-unloading conditions, were conducted to investigate the characteristics of deformation and damping of EPS beads-mixed lightweight soils.