The effect of isolated damping layer system on earth dam under earthquake loading

Abstract

The structural behavior during an earthquake is one of the major concerns for earth dam of a medium size about 30 meter height and 90 meter width. The body crack is created by relative vertical displacement at both edges of the crest.The failure is recorded with the crack development in dam body by interaction between dam and reservoir. To reinforce dams, some methods were used with respect to literature such as perpendicular drain, prefabricated vertical drain, geotextile layers, pile group, micro pile injection and cutoff wall system. This research included three objectives; (i) Identifying damage location in earth dam with respect to case study (Bakun dam), (ii) Studying the effect of Isolated Damping Layer (IDL) system in blanket layer using physical modeling on top of the vibrator table, and (iii) Evaluating slope stability based on seismic motion. In terms of methodology, Finite-Element method using ANSYS13program and equilibrium method using Geostudio 2007 (Slope/W) were used. Series of soil mechanic test to design IDL and small-scale model (1/100) using IDL were carried out. Displacements, shear stresses and shear strains of dam were evaluated using nonlinear analysis under strong earthquake intensity of 0.6g. The major effect on the displacement of dam was due to different foundation properties (soft, medium and stiff soil) in comparison to different core configuration in terms of geometry. The best elastic modulus ratio between unsaturated part of dam and foundation, ß was 0.66 for saturated part and foundation, ? was 0.13 in order to reduce response of the earth dam. Time-history and response spectra analysis of Bakun dam showed the minimum relative vertical displacement between both edges of crest by peak ground acceleration less than 0.24g. For all site classes, the displacement ratio (?=2) for return earthquake period from 2500 to 500 years was recorded. Based on modal analysis, the rigid behavior of foundation was achieved by modulus ratio more than three. Effect of modulus ratio on dominant frequency was greater than depth ratio. The minimum relative vertical displacement was attained when modulus elasticity ratio between shell and core clay was less than five. The optimal behavior was obtained by using clay in blanket layer when a modulus elasticity ratio was equal to 2.50, between this layer and weak foundation.The blanket layer was designed based on mixed product of laterite soil with shredded tire and micro silica. The main role of silica was to control seepage. The qualified combination by comparison of thirteen samples was distinguished. Subsequently, nine physical models were vibrated using dominant frequency. Most of damage occurred at upstream of one third near to the crest.The best absorption of energy without any destruction was observed when the layer thickness of reinforced blanket was one fourth of dam height.The safety factor was increased using blanket reinforced layer. Finally, IDL system showed the best performance in order to reinforce dam under resonance seismic motion.