Evaluation of Earth Dam Construction on Mitigation of Liquefaction Potential within Alluvial Foundation

The evaluation of the liquefaction risk of soil during earthquake and its consequences on the structures as well as the mitigation methods are major tasks for the safety of populations. In construction of earth dams، the alluvium deposit usually removed so that the dam lays on the rocky foundation. This solution in the case of thick deposits is expensive. On the other hand، it is possible to construct the moderate height dams on mitigated alluvium foundations، if the seismic behavior of system is guaranteed. In this study، the construction effect of earth dam on liquefaction potential of its alluvial foundation and reciprocally the effect of liquefied foundation on the stability of dam have been investigated for two different geotechnical conditions. The analyses are nonlinear and the constitutive law of the foundation material was assumed to follow Finn model incorporated into “Flac 2D” finite difference analysis program. The factors such as initial shear modulus، variation of shear modulus versus shear strain، generation and dissipation of pore pressure and hysteretic damping are considered in this study. The results of these analyses then compared with the results of dynamic analysis of earth dam on a rocky foundation. The evaluations show that the dam construction increases the relative density، effective stress، shear modulus، and thus decreases the shear strain and water pore pressure within alluvial foundation under the crest of dam. Depending on the relative density، depth of layer and level of applied acceleration، this may lead to mitigation of liquefaction potential. This effect decreased toward the upstream and downstream of the dam. It was observed that the liquefaction could be mitigated in the region close to dam crest when the relative density of soil is 65% to 85% and subjected to a maximum acceleration of 0. 3g. However، the construction of dam finds little decreasing effect on liquefaction when the relative density of alluvial foundation is less than 65%. The main settlement and maximum horizontal displacement in foundation is occurred under the core and downstream of the dam، respectively and reached up to the depth of 10-15 m of foundation. In spite of the good performance of dam weight on lowering the liquefaction potential during earthquake loading، a large deformation and even instability condition can be achieved within the alluvial foundations. However، deformation of dam and its foundation are strongly dependent on the geotechnical specification of alluvial foundation، density، thickness and depth of liquefiable layer and the level of applied acceleration. The results present that in very good quality alluvial foundation where the liquefiable layer has small thickness and is located at a deep position، and in the case of acceleration lower than 0. 2g، the effect of liquefaction in deformation of dam will be insignificant.