Experimental Determination and Constitutive Modeling of Time Effects in Sand

土壤中的时间效应，如蠕变和松弛，会对建筑物、桥台、挡土结构和斜坡造成长期变形和压力。随着时间的推移过度变形可能导致结构失效。虽然蠕变和应力松弛是由相同的现象引起的，即颗粒破碎后颗粒重排，但一种现象的预测显然不能根据另一种现象来完成。将对弗吉尼亚海滩砂进行几个系列的高压三轴压缩试验，以研究应变率效应、蠕变和应力松弛。因此，这些颗粒的断裂时间取决于应力状态与短期断裂强度的接近程度。一旦发生断裂，由于晶粒的滑动和滚动，晶粒结构将发生重排。这些类型的力学行为都是时间依赖性的，它们共同构成了砂中的时间效应现象。这与粘土中的粘性效应是完全不同的，所提出的研究将有助于建立更真实的模拟现场发生的砂土行为。当真实的砂被沉积或加载到更高的应力时，或者如果它们被扰动，则显示时间效应，从而在颗粒之间产生新的接触。这种真实的时间特性通常被认为是不存在的，因此不能正确预测岩土结构的许多观察到的行为模式。所提出的研究对社会的好处将是改善岩土工程结构和土-结构相互作用的行为的预测，以及随之而来的更大的安全性和经济性的建设。

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.