Evaluation of liquefaction potential of sands under existing buildings

The paramount rule of damages imposed during earthquake due to liquefaction in soil layers have been pointed out by numerous researchers. By and large, liquefaction occurs when effective stress of sand drops immediately and excess pore pressure rises. Under structures bearing capacity of the sand during liquefaction decreases radically. Furthermore, structures built on liquefiable soil as a result of liquefaction can have stability problems, rocking or can be collapsed. Nowadays, the simplified procedure has commonly been utilized to estime the liquefaction potential. In this analysis, cyclic stress ratio (CSR) due to soil's movement which is caused by an earthquake is compared by cyclic resistance ratio (CRR). Cyclic resistance ratio is defined as soil resistance against liquefaction. To determine the potential of liquefaction, the simplified procedure solely considers the free-field condition when there is not any structure or slopes. The effective stress of the soil increases as a structure is built on the soil. In another word, when structure added on the top of a soil layer the foundation load of the structure increases the effective stress of soil. Moreover, for observing the structure effect on the soil, it is recommended that K and K correction factors should be multiplied with the CRR equation and static shear stress in the soil should be considered as well. Due to the fact that under K factor is difficult to determine under the foundation, its contribution is ignored and K is considered as one. The scope of this thesis is analyzing the impact of the structure on the potential of liquefaction of sand under the structure and providing a modification for simplified procedure. The distributed load was converted to the strip load, then with the help of Boussinesq method, in the static condition, vertical effective stress increment and static shear stresses were calculated. As a result, the effect of structures on the Kα (shear stress correction factor) and Kσ (overburden correction factor) were estimated and then multiplied with CRR values. Besides, during the calculation of the values of CSR, the effect of structure on the effective stress and total stress were considered. Afterwards, the results obtained after these modifications were used to compare with the analysis results of 1-D and 2-D numerical analysis. In 1-D analyses on the soil profile, the ground response analyses of the soil were conducted in free-field conditions. The equivalent linear and non-linear methods were employed to acquire cyclic stress ratio values. Additionally, peak accelerations (amax) values which were used for calculating CSR values with the simplified method under foundation, were obtained from these analyses. HS-Small and UBC-Sand soil models which use the finite element methods in 2-D analyses were utilized in dynamic analyses. In both soil models, free-field and under-foundation cases were modeled to compare the effect of foundation during liquefaction. As a result of free-field analyses ground response results were obtained. In addition, two different foundation loads were used to observe the effect of various stresses on the liquefiable soil. In the existence of foundation, the effects of foundation on excess pore pressure formation and cyclic stress ratios were examined. The result of 1-D and 2-D ground response analyses for free-field were compared. amax and spectral acceleration values on the top of rock were relatively close together when it is compared in 1-D and 2-D analyses. These results indicate that both models operate properly and give the desired results. Also, the amax values on the surface were the close in both 1-D and 2-D analyses. UBC-Sand and HS-Small models gave closer results to non-linear 1-D analyses and maximum acceleration values were almost identical on these analyses on the surface. CSR values with depth were approximately the same in the analyses. As a result of these comprehensive analyses, it is well understood that the behavior of the soil under foundation was altered during the earthquake and the maximum accelerations of soil are higher under the edge of foundation than free-field on the surface. The maximum acceleration under the center of 100 kPa foundation was lower than that of free-field. As a result of this situation, although the CSR values under the center of the foundation was lower than free-field, under the edge of foundation the CSR values are much higher than free-field. Under the edge of foundation soil exposed to much higher stresses. In 2D analyzes based on the UBC-Sand model, the effect of foundation on the excess pore water pressure and potential liquefaction on the sand were investigated. As a result of the analyzes, it is evident enough that foundation changes liquefaction potential when it is compared to free-field conditions. It is obvious from analyses, albeit liquefaction potential increases under the edge of the foundation, it decreases under the center of the foundation. Increase in the surcharge pressure under the edge of foundation increases the potential of liquefaction. In liquefiable soil, the points that at the free-field were liquefied, in presence of foundation and under the center of foundation at the same points liquefaction did not occur. During the earthquake liquefaction potential differs under center of foundation from free-field. The risk of liquefaction was raised under edge of foundation when it is compared to the free-field. It has been determined that the main cause of the critical increase in liquefaction at the corner line is the high shear stresses in the corner. In these studies, analyses carried out within the scope of the thesis, factor of safety against liquefaction was calculated in the center line and corner of the foundation by using the simplified method. In these calculations, the cyclic shear stress ratio is given from both the simplified approach and the analyses. In the calculation of the cyclic shear resistance ratio, the approach based on the standard penetration test was used. It has been understood that the simplified procedure is modified by considering the stress increase and the shear stresses originating from the basic analysis can be used to determine the liquefaction potential due to the earthquake if the cyclic shear stress ratio values are obtained by the finite element method. However, it has been determined that under the center of foundation the liquefaction characteristics have changed and that the liquefying potential with depth under the corner lines is much higher than in free-field.