Investigation of liquefaction and post liquefaction behavior of layered sands during earthquakes by means of model tests

Liquefaction and consequent lateral spreading is one of the major topics of investigation in geotechnical engineering following their sometimes drastic effects on structural damages caused by earthquakes. In this study, in order to be able to investigate the liquefaction phenomenon in uniform and layered sands, five series of shaking table model tests were conducted by means of an experimental set-up developed in the laboratory. The experiments were carried out on uniform sand columns, silt inter layered sand cloumns and two layered sand columns deposited at various relative densities and subjected to different input accelerations. During the experiments, excess pore water pressures generated during dynamic loading were measured by pore pressure transducers installed at three different depths and, the surface settlements and water film thicknesses were measured from the images recorded by a digital camera. The results of the model tests have shown that the excess pore water pressures and volumetric strains are largely influenced by the shaking intensity and relative density. The results from the silt interlayered sands demonstrated that a water film is developed underneath the less permeable silt layer. The formation of the water film is seen to be influenced by the relative density of the sand layer underneath the silt seam, shaking intensity and the developed excess pore water pressure ratios. The observations regarding water film formation point out that the adverse effects of water film is expected to be more significant in loose sands rather than the presence of water film increasing the level of excess pore water pressures. In the field, the water film may not dissipate for days after an earthquake shaking and may lead to significant engineering problems such as flow failures in slopes. The model tests are also numerically modelled to analyse the dynamic response of uniform and layered sand columns with finite element analysis using the program called DIANA, using the Towhata-Iai liquefaction model. As a result of the numerical analysis, excess pore water pressures were computed and compared with the experimental results. Generally a good agreement between the results of computed and measured excess pore water pressures is observed. The results of the numerical analyses have demonstrated that the build-up of excess pore water pressures is highly influenced by the shaking intensity and relative density of the sand deposit. The numerical analysis results also revealed that the presence of less permeable silt interlayer within the sand deposit can cause an abrupt increase in pore water pressures underneath the silt layer and reduce the excess pore water pressures developed in the upper sand layer. Key Words: Liquefaction, numerical modelling, water film, model tests, silt interlayer