LIQUEFACTION-INDUCED EXCESS PORE PRESSURE GENERATION UNDER FOUNDATION AND FREE-FIELD WITH DYNAMIC CENTRIFUGE MODELLING

Historically, earthquakes have been perceived as one of the most damaging natural hazard. Seismic Liquefaction of saturated sand is one of the major sources of damage and reason of the foundation’s failure. Settlement of shallow foundations resting on saturated sand has been repeatedly observed throughout the world as a consequence of liquefaction. The main factor of occurrence of this phenomenon, is the tendency for cohesionless soil to contraction. This contraction occurs when the soil is exposed to dynamic loading. As a result, the tendency to contraction causes an increase excess pore pressure (EPP) and subsequently an effective stress reduction. In liquefy situation soil tend to drown in soil and this causes a dense soil compaction in the lower parts and loosening in the upper parts. If there is foundation on the soil, the effective confiningstress acting on the soil has been observed to influence its potential for EPP generation. In liquefied soil a reduction of the measured excess pore pressure ratio () with depth (i.e. with increasing ) was observed. According to the available result, the probability of liquefaction occurrence in the presence of foundation is more less than free field, as the pore fluid moving up and trap under foundation and create a weak area in the upper part of the soil. (Dashti, 2009). A reduction of the measured excess pore pressure ratio () is believed to depend on two main factors: first, the additional confinement stress caused by the presence of the foundation results in a reduction of the pore pressure ratio (), as the EPP required to cause initial liquefaction is higher. Second, the additional shear stresses due to the foundation may cause dilation in the soil, with the consequent generation of negative excess pore pressures, acting toward further reducing the (), under the foundation. The non-uniform stress distribution underneath the foundation results in non-uniform excess pore pressure generation and subsequent flow of pore fluid driven by the resulting pressure gradients generating between different zones of the foundation soil. Depending on their direction, these pore fluid flows may lead to reduction in the effective stress beneath parts of foundation, resulting in further soil softening. The effect of static shear stress on the cyclic resistance to liquefaction has been investigated by several authors. Most of the experimental work on this topic is based on soil element testing, in particular cyclic triaxial testing. In this article a series of centrifuge test, with presence of foundation on saturated sandy soil for investigating different between excess pore pressure generation in different zone of soil such as under the foundation and free field. Dynamic centrifuge shaking table that produce for generating dynamic shake with PGA about 0.3 g. With dynamic centrifuge shaking table, scaled version of earthquake motions were reproduced in the models tested, enhancing the reliability of experimental results. The soil models tested in this study were constituted of Firoozkooh sand.

Keywords:

Liquefaction , Pore pressure , Dynamic centrifuge analysis , Shallow foundation