CO-SEISMIC AND POST-SEISMIC BEHAVIOR OF AN ALTERNATELY LAYERED SAND-CLAY GROUND AND EMBANKMENT SYSTEM ACCOMPANIED BY SOIL DISTURBANCE

In the evaluation of the damage caused by earthquakes, particular attention has been paid until now to the unstable behaviour (the phenomenon of liquefaction) of sandy grounds during tremors. However, studies of post-seismic damage and damage to clayey grounds have also been reported. In this paper, the behaviour of an actual alternately layered sand-clay ground and embankment-coupled system before, during, and after an earthquake is investigated. The investigations were carried out by soil-water coupled finite element analysis (GEOASIA), which is capable of handling inertial forces and utilizes the SYS Cam-clay model as the elasto-plastic constitutive model of the soil skeleton. By accounting for the effects of the soil skeleton structure (structure, overconsolidation, and anisotropy) at work, the above constitutive equation is capable of expressing the reduction of shear modulus, etc. caused by disturbance in loose sands and naturally deposited clays. In the analysis, (1) the effect of the rigidity of the embankment and (2) the effect of the penetration depth of the sheet pile in a strengthened ground with retaining walls and sheet riles joined by tie rods were examined. The main results are as follows: (1) If the embankment is soft, liquefaction of the upper sandy soil layer during the earthquake causes the embankment itself to collapse. In contrast, if the embankment is relatively hard, the shape of the embankment remains stable. However, a decrease in shear rigidity occurs in the lower clayey soil layer due to degradation (disturbance) of the soil structure, resulting in increased settlement both during and after the earthquake. (2) In the case of grounds strengthened by the sheet pile method, if the tips of the sheet piles are located in a soft soil layer with a high degree of structure, large disturbance may occur at such locations during an earthquake and cause increased settlement during and after the tremors.