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Numerical simulation of the seismic distress of harbor quay walls

Ntomopoulou Thomai

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Summary

Quay walls are retaining structures that constitute important components of port infrastructure. The seismic design of these walls is of paramount importance, since ports are the centers of transportations and economy in many regions, and especially islands. The present diploma thesis focuses on the numerical simulation of the seismic distress of quay walls. The parametric investigation is carried out via 2-D dynamic analyses using PLAXIS 2D software. This software is based on the finite element method and it is widely used to simulate static and dynamic geotechnical problems.More specifically, the main topic of this study is a system of two rigid port walls, which are located opposite to each other at a relatively close distance that is often constructed in harbors. The analysis of this system is based on the analytical model of a double retaining wall system, which has been presented by Wood (1973) based on an elastic method of dynamic analysis. The two walls retain an isotropic-homogeneous and elastic soil material, while their base foundation is considered to be rigid. The analytical solution, developed by Wood under dry conditions, was used to verify the adopted numerical simulation approach. Subsequently, this geometry was modified by defining a soft soil layer at the foundation of the retaining system, thus, enabling deformation of the walls. The purpose of this modification was to create realistic soil conditions occurring at a port. The calculation of soil thrusts for flexible quay walls can be achived using the method of Mononobe – Okabe (Okabe 1926, Mononobe & Matsuo 1929), which is based on Coulomb’s method and can calculate the inertial forces which are exerted on the walls under pseudostatic conditions.Initially the numerical models were validated with the aforementioned analytical methodologies. In particular, the model of the two rigid walls was verified by Wood's method, while the verification of the flexible quay wall system was performed via Mononobe – Okabe method. The numerical analyses were conducted considering two different geometries with reference to the distance among the two walls, which was defined based on the ratio of length to height. Initially, a small distance between the two walls was examined, i.e., for length to height ratio L/H=3. In the sequence, the same analyses were performed for a much greater distance between the two walls, i.e., L/H = 10.In order to realistically examine the seismic response of a port quay wall system, three different cases were examined, in which the main variable was the angle of friction. The first case refers to a model with typical values at the friction angles of the retaining soil and the foundation soil that are often observed in ports. Then, in order to simulate liquefaction conditions of the soils, a reduction of their friction angles was imposed in order to drastically reduce soil shear strength, which is caused due to liquefaction. Accordingly, the second case comprises of a model with a reduced friction angle at the foundation soil layer. Moreover, the third case represents the most unfavorable soil conditions by reducing the friction angle, not only at the foundation soil layer, but also for the backfill soil material among the two walls.According to the literature, during a severe earthquake, the two quay walls tend to turn and/or slide towards the sea at both sides. In the present research, in order to strengthen the system of the double quay walls against earthquakes, diagonal and horizontal tie-rods were installed to connect the two walls. The proposed schemes of tie-rods were tested for the three aforementioned cases using Ricker pulses with frequencies 6Hz and 2Hz as base excitations. Lastly, an important parameter that affects the response of the system is the foundation soil, thus, two types of soil with shear wave velocities equal to Vs=100m/s and Vs=300m/s were examined, the second is a soft surface soil layer that commonly occurs in coastal areas. As verified by the results, the proposed tie-rods assist to maintain the stability of the double quay walls system and avoid substantial seismic damages.

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