Experimental Study On Liquefaction And Behavior Of Post-liquefaction Deformation And Strength In Saturated Sand-gravel Composites

Damages of structures may be caused by liquefaction and post liequefation deformation of saturated soil due to seismic load. The former researches of liquefaction and post liquefaction mostly aimed at saturated sand or silt sand. Due to biggrain size and high hydraulic conductivity, gravels and gravelly soils were once thought to be unliquefiable. However, the results of field earthquake investigation have shown that saturated sand-gravel composites may occur liquefaction during earthquake, and most liquefied gravelly soils comprise both sand and gravel. However, the study of dynamic properties, especially the liquefaction characteristic of saturated sand-gravel composites is not sufficient partly for the reason of test apparatus and technology. The present research is supported by the National Science Foundation Project of Study on Liquefaction and Relationship Between Parameters of Deformation and Strength of Saturated Sand-gravel Composites (No. 50578029) and the State Key Lab of Coastal and Offshore Engineering Foundation Project of Study on Liquefaction Characteristics of Saturated Sand-gravel Composites (No. LP0505), the liquefaction, post-liquefaction deformation and strength, reconsolidation volumetric, modulus and damping behavior of saturated sand-gravel composites are thoroughly investigated. The main contents of the current research are as follows:The development of axial strain and pore water pressure in sand-gravel composites and substituted sand during liquefaction is studied contrastively, by using of medium-scale and small-sacal DS-2T dynamic triaxial apparatus respectively, and scalping the oversized gravel particles in sand-gravel composites and taking similar gradation method to get substituted sand. The development of shear strain and pore water pressure in sand-gravel composites and substituted sand specimens due to cyclic loading are compared principally. A pore pressure model of sand-gravel composites is suggested based on test results, and the range of parameters of the model is given.A static reloading on liquefied specimens test method is put forward and a series tests are processed. In this study, the effects of relative density, initial consolidated pressure, liquefaction safety and cyclic stress ratio are thoroughly investigated. The results show that liquefied sand-gravel specimens reloading behavior is distinctly different from those without dynamic loading. The reloading stress-strain curve is composed of three parts approximately, which is stiffness-recovering part, stiffness-stabilising part and plastic flow part. The phenomenon of zero stiffness in a relatively large strain range, which often occurs in liquefied sand, is not observed for sand-gravel specimens. A method is given to define the characteristic parameters, which avoids the uncertainty of the method by Yasuda and assures the accuracy of parameters analysis. A three-linear static stress-strain model of liquefied sand-gravel composites is established based on test results, in which initial consolidated pressure, liquefaction safety and cyclic stress ratio can be concerned synthetically. The method of defining model parameters is presented. This model is added into an FEM program and is used to analyze the deformation behavior of a simple sand-gravel dam after earthquake and liquifaction. The numerical results show that this model is competent.A series of tests are performed to illustrate the reconsolidation volumetric behavior of sand-gravel composites after dynamic loading. The results show that, there is a good correlativity between the reconsolidation volumetric strain, the pore water pressure ratio, and the maximal double amplitude shear strain induced by dynamic loading, though the initial effective consolidation pressure has a great effect on the reconsolidation volumetric strain. The reconsolidation volumetric strain is not affected by the characteristic of dynamic loading and reduces with the increase of specimen relative density. Based on the test results of sand-gravel and sand by Ishihara, an experiential relation of reconsolidation volumetric strain of sand-gravel and sand is suggested. The application method of the experiential relation is given for predicting the ground settlement.A number of tests are progressed to investigate the modulus and damping characteristics of saturated sand-gravel composites, and the effect factors such as relative density and initial consolidated pressure are studied. The varying ranges and average curves of the normalized dynamic shear modulus and damping ratio versus dynamic shear strain are proposed, which are compared with those of Seed and Kyle. The parameters range of formula for evaluating the dynamic shear modulus is proposed on the basis of experimental results.Undrained dynamic strength tests are performed on sand-gravel specimens after cyclic preloading under drained condition. The results show that the liquefaction resistance of sand-gravel increases with cyclic preloading. A number of tests are accomplished to investigate the modulus and damping characteristics of liquefied sand-gravel composites. The correlation of dynamic and static parameters of sand-gravel is discussed briefly. An experiential linear relationship between G_dmax/[F(e)σ₀] and q_f/σ₀ is suggested.