Experimental Determination and Constitutive Modeling of Time Effects in Sand

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Time effects in soils such as creep and relaxation cause long-term deformation and pressures on buildings, bridge abutments, earth retaining structures, and in slopes. Excessive deformation with time may cause structures to fail. While creep and stress relaxation are caused by the same phenomenon, namely grain crushing followed by grain rearrangement, the prediction of one phenomenon can apparently not be accomplished on the basis of the other. Several series of high pressure triaxial compression tests will be performed to study strain rate effects, creep, and stress relaxation on Virginia Beach sand. Thus, the time to fracture of these particles depends on the proximity of the stress state to the short-term fracture strength. Once fracture takes place, rearrangement of the grain structure will occur due to sliding and rolling of grains. Each of these types of mechanical behavior are time dependent, and together they constitute the phenomena of time effects in sand. This is quite different from the viscous effects seen in clays.The proposed research will contribute to establishment of more realistic modeling of sand behavior as it occurs in-situ. Real sands show time effects as they are deposited or loaded to higher stresses or if they are disturbed, thus creating fresh contacts between the particles. This real time behavior is most often assumed to be nonexistent, and a number of observed behavior patterns of geotechnical structures are therefore not predicted correctly. The benefits of the proposed research to society will be improved predictions of the behavior of geotechnical engineering structures and soil-structure interaction as well as consequent greater safety and economy of construction.