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To explore the microscopic responses of granular materials to wetting, the inter-particle lubrication effect and particle breakage in an odometer were simulated using a two-dimensional discrete element method. The lubrication effect was modeled by reduction of the inter-particle friction coefficient and particle breakage was initiated by decreasing the particle strength. Once the strength of the particles decreased to a threshold value, the particles began to break so that new contacts could be established to transfer the external loads. Numerical simulations successfully reproduced the additional compaction of the material and the intensification of the horizontal stress in addition to the microscopic responses of the granular assemblies. The microscopic interpretation of the earth pressure coefficient at rest and the evolution of the grain number distribution during particle breaking were also investigated. granular materials, discrete element method, wetting, lubrication effect, particle breakage Granular materials are extensively used in civil engineering applications such as the foundations of railways and the shells of rockfill dams. These materials are exposed to changes in climate and the environment, which inevitably leads to changes in their long-term mechanical behavior [1]. One typical environmental factor that deserves special attention when designing a granular structure is the interaction between the particles and water from both underground and surface runoff. Water infiltration into the inter-particle pores and the micro-cracks in particles has at least three effects, i.e. the lubrication effect, loss of matric suction and particle breakage [2,3]. These factors often occur in combination and lead to the so-called wetting-induced collapse [3][4][5]. Alonso et al.[3] distinguished the predominant factors according to the sizes of the particles and pointed out that the wetting-induced collapse is primarily caused by particle breakage for coarse granular materials such as rockfill and gravel, while the loss of matric suction is responsible for the collapse of fine materials with grain sizes ranging from 10 to 0.1 mm, such as sands. Both particle breakage and the loss of suction overwhelm the effect of inter-particle lubrication.The wetting-induced collapse may result in unfavorable deformation and stress deterioration in structures; therefore, this phenomenon has attracted a great deal of attention over the past three decades. Many experimental studies [6][7][8][9] have been conducted and great efforts have been devoted to constitutive modeling of this particular behavior of granular materials with the goal of understanding and explaining phenomena observed in the engineering field and numerically solving the boundary value problems in practice [10][11][12]. Among these experimental and theoretical investigations, a phenomenological approach has generally been employed and the macroscopic responses of materials were described within the framework of continuum mechanics [5,[10][11][12]. Nevertheless,...