RESEARCH ARTICLE Erich BAUER, Zhongzhi FU, Sihong LIU Hypoplastic constitutive modeling of wetting deformation of weathered rockfill materials © Higher Education Press and Springer-Verlag Berlin Heidelberg 2010 Abstract The wetting deformation of weathered rockfill materials has been attracting growing attention from both engineers and scientists. The importance of realistic predictions of wetting deformations for high earth and rockfill dams is a strong motivation to establish a suitable constitutive model. Recently, the hypoplastic constitutive model by Gudehus and Bauer was extended by introducing solid hardness depending on the state of weathering. The extended model takes into account the influence of the current density, the effective stress state, the rate of deformation, and the time dependent process of degrada- tion of the solid hardness. In the present paper, the performance of this model is evaluated by comparing numerical simulations with experiments obtained from a water sensitive rockfill material. In particular, triaxial compression paths and creep deformation under deviatoric stress states are considered. Finally, the constitutive model proposed is used to study the influence of a degradation of the solid hardness on the long term behavior of a hypothetical fill dam. Keywords rockfill dams, weathered granular materials, hypoplasticity, solid hardness, creep, stress relaxation 1 Introduction Long-term settlements of rockfill dams are usually related to loading cycles caused by changes of the water level in the reservoir and to the stiffness degradation of the stressed granular material as a result of the reduction of suction or the progressive chemical and mechanical weathering of the grain material. The latter depends on the chemical reactions of the material with water, i.e., the dissolution of minerals, the plastification of grain contacts, and grain crushing [1–3]. It is experimentally evident that the mechanical behavior of weathered broken rock can be different for the dry state of the grains and for the state after wetting. As shown for instance in Fig. 1, the incremental stiffness under deviatoric loading and the peak friction angle after wetting are significantly lower and both also depend on the mean stress. The degree of geological disintegration has a significant influence on the grain stiffness, the evolution of grain abrasion, grain breakage, and change of the grain size distribution. Depending on the state of weathering, the propagation of microcracks due to water-induced stress corrosion can be accelerated by a change of the moisture content of the grains [4–6]. The degradation of the strength of the grains leads to a reduction of the resistance to compaction and shearing [7]. Progressive weathering will subsequently lead to deforma- tions in the granular body even if the stress is kept constant. This time-dependent process is usually termed as creep deformation or wetting deformation [8–10]. Significant creep deformation after the wetting of an initially dry specimen in experiments was reported by several authors [11–13]. If creep deformation is restricted by the boundary conditions, there will be stress relaxation in the material whose magnitude is related to the potential of creeping. Various constitutive models for wetting deformations have been developed in the past. Most of them relate the wetting deformation to the degree of saturation and suction and fall into the scope of unsaturated soil mechanics. These models can capture particular properties of wet and fine- grained materials but rarely reflect the time-dependent reduction of the stiffness of weathered materials. Bauer et al. [15] were the first to model the wetting deformation of stressed and moisture-sensitive weathered rockfill materi- als in a simplified manner by a degradation of the solid hardness. In this context, it is worth noting that the so- called solid hardness [16] is related to the isotropic Received July 17, 2009; accepted November 4, 2009 Erich BAUER (✉), Zhongzhi FU Institute of Applied Mechanics, Graz University of Technology, Graz, Austria E-mail: erich.bauer@tugraz.at Zhongzhi FU, Sihong LIU Institute of Hydraulic Structures, Hohai University, Nanjing 210098, China Front. Archit. Civ. Eng. China 2010, 4(1): 78–91 DOI 10.1007/s11709-010-0011-8