عنوان مقاله [English]
Loose saturated sand and alluvial strata are susceptible to liquefaction during earthquake. Simple and empirical method for evaluating their resistance against liquefaction is based on the results of standard penetration tests (SPT). One of the most important parameter which affects liquefaction resistance is initial stress; Standard procedures for considering this effect require correcting penetration resistances to an equivalent overburden stress of one atmosphere and adjusting the cyclic resistance ratio for the effects of overburden.
Liquefaction potential in alluvium will change after construction of heavy structures on them, which is due to changes in earthquake loads, overburden pressures and soil densification. In this paper, a new methodology is explained to calculate changes in compaction of alluvial foundation after dam construction, which influence both shear strength and dynamic shear stresses. With this new formula, one can determine shear strength of alluvium against liquefaction and compare its values with induced dynamic shear stresses to evaluate factor of safety .
Effect of dam construction on liquefaction potential of alluvium is investigated through dynamic analysis based on equivalent linear approach. Liquefaction potential of alluvial foundation without dam construction is compared to that potential after construction in two ways; in the first method, effect of induced stresses on liquefaction potential is considered in the numerical analysis based on new stress states and related functions, and in the
second method, in addition to these changes, relative density of the alluvium is corrected due to construction; for instance a method is described to update alluvium relative densities and update material parameters. To clarify
effectiveness of the dam body on the liquefaction potential of foundation, three different compressibility conditions, i.e. weak, medium and stiff states, are considered in the alluvium to discover changes in factor of safety after dam construction. Results of analyses show that liquefaction potential of alluvial foundation decreases significantly after construction in various alluvial strata. Considering results derived from this study, if selected site
for dam construction is prone to liquefaction, engineers might have economic project with keeping alluvium beneath the dam instead of totally excavation.