Numerical Simulation of Liquefaction Phenomenon and Its Effect on the Pile and Quay Wall

The economy of a large number of industrialized countries is on the basis of import and export goods through waterfronts. The waterfront is a marine structure that links the ship to the land and is one of the most important parts of the port. In case of waterfront damage, adjacent structures such as cranes will be damaged and important activities of the port will be stopped. Piles inside the soil are also one of these structures that in case of pile failure, the structures over it will be lost. Because of this, the behavior of offshore structures against damage factors such as an earthquake is very important. Numerous experimental modeling has been done with the pile, but its behavior in the waterfront has rarely been studied.
One of these experimental models was presented by Motamed and Towhata in 2010. This experimental model presents results of 1-g shaking table model tests on a 3×3 pile group behind a sheet-pile quay wall. The main purpose was to understand the mechanisms of liquefaction-induced large ground deformation and the behavior of the pile group subjected to the lateral soil displacement. The sheet-pile quay wall was employed to trigger the liquefaction-induced large deformation in the backfill. Furthermore, distribution of maximum lateral force within the group pile was thoroughly studied.
In this study, the experimental test has been modeled numerically, and the accuracy of the results in the numerical model has been investigated using experimental tests. Numerical simulation has been done using Flac3D software that is a three-dimensional explicit finite-difference program for engineering mechanics computation. The Software is able to simulate the behavior of three-dimensional structures built of soil, rock or other materials that undergo plastic flow when their yield limits are reached. Besides, it simulates the excess pore pressure and liquefaction with the help of Finn constitutive model.
Using this software, the pile group behind the quay wall has been loaded dynamically using shake table and the possibility of liquefaction has been studied in the model. The lateral displacement of the soil flow in the numerical model has been compared with experimental results and these displacements have been analyzed. Moreover, the bending moment and the lateral force of the piles and bending moment and lateral displacement for quay wall obtained from numerical simulation have been compared with experimental results. The results show that liquefaction in the soil has occurred and its time is consistent with the experimental model. The soil liquefaction behind the quay wall and the pile group causes the lateral spreading of the soil. The maximum lateral displacements are formed on the surface and behind the quay wall, which decreases with distance from the quay wall and by moving toward the free surface of the soil. The behavior of the pile in this research is similar to a cantilever beam whose maximum bending moment occurs at the bottom of the pile. The results show that the piles near the quay wall can tolerate bending more than the other piles, the reason of which is the greater displacement of the soil in the place of these piles. Comparison of the bending moment of the piles in the numerical model with the experimental model shows that the matching of the results is very good for the piles near the quay wall and is acceptable for other piles. The lateral force of the liquefied soil flow exerted on the piles was obtained by two different methods including
back-calculated and software outputs. The total force on the piles was compared in different scenarios, including the JRA (Japan Road Association) design code, the model of the experimental, the numerical model using back-calculated and the numerical model using the software output. The JRA instruction proposes non-conservative values, so that experimental results and numerical model provide more values. The sheet-pile quay wall was modeled as a floating type quay wall without any constraint at the bottom, so that quay wall moves completely with the surrounding soil and its lateral displacement is similar to the displacement of the soil. The maximum bending moment on the wall is well suited to the experimental result, which indicates the accuracy of numerical modeling.