Experimental Study on Earthquake Resistance of Underwater Granular Structures

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A new experimental technique to visualize the dynamic behavior of underwater particle assemblage (LAT: Laser-Aided Tomography) was developed. According to the proposed method, a granular structure model, made of particles of crushed glass and immersed in liquid with the same refractive index, consequently becomes invisible. An intense laser-light "sheet" (LLS) is then passed through it, illuminating the contour lines of all the particles within a cross-section. Scanning the model with LLS allows a 3D image of every discrete particle's shape and motion to be obtained as well as the whole-field deformation of the model. Utilization of this method with a smaller particle size will expand its application to analyses of structures made up of finer grains. In this case, however, it is not easy to have a clear look at each particle on the LLS. Some techniques were developed to overcome the problem, and we could reduce the grain size of the employed glass material to lmm or smaller.Dynamic failure test of embankment-shaped models were conducted using the proposed technique. The models made of screened particles ( 2mm < grain < 5mm ) were shaken sinusoidally in the liquid. The surface of the model began to slide when the amplitude of base acceleration exceeded a threshold. The threshold acceleration increased with excitement frequency, while variation of threshold velocity was rather small, and was scattered in the range of 0.5 to 2.0 kine.The failure process and its frequency dependence observed in LAT experiments was then discussed using the Discrete Elliptic Element Method (DEEM), considering contributions of overall slip and roll to the failure process. The DEEM simulation gave us a general idea of the dilatation effect on overall behavior and its frequency dependence.