MS7.14 - Dynamic Soil-Structure Interaction and Wave Propagation

The Wave Based Method (WBM) is an indirect Trefftz Method, which uses weighted wave functions to approximate the field response of a boundary value problem. These wave functions weakly fulfill the underlying differential equations and pass through a weighted residual approach by applying boundary conditions. This method has firstly been introduced for vibroacoustic problems to simulate excitations in the midfrequency range. The accuracy of the WBM strongly depends on the relation between the geometrical size of a boundary value problem and the applied excitation frequency. This permits to transfer the WBM from vibroacoustics to soil halfspaces without significantly increasing the number of wave functions, and hence the number of unknown weighting values. As WBM domains are based on analytical solutions of their differential equations, these can directly be coupled to other adjoining continuous systems. In the proposed contribution, a Timoshenko beam is implemented to reduce the structure of an elastodynamic trench within a halfspace. The Timoshenko beam permits to simulate the mitigation effect of a trench against incoming waves, by allowing shear deformation and reducing the number of unknowns of the numerical model. The performance and accuracy of the Timoshenko beam is compared with the results of the full order model for an elastodynamic trench.

The reinforcement technique with rigid inclusions is a practical, economical, and time-saving foundation solution. This technique has been already utilised with success to support structures undergoing high seismic demands, e.g., the Rion-Antirion bridge in Greece. In a performance-based design approach, the dynamic soil-structure interaction is an important phenomenon that should be considered. Direct approaches simulating the whole soil-structure system within the same FE or FD model are still computationally expensive and the classic modal response spectrum analyses, based on the superposition principle, are not capable to take into account most of nonlinear mechanisms. Therefore, the macro-element approach, that allows to model non-linear SSI mechanisms at the base of the structure without increasing the numerical cost, is a promising method showing especially good performance in the dynamic analysis of shallow and piled foundations. In a macro-element model, the behaviour of a foundation is entirely described at the foundation centre at a macro-scale by means of a set of generalised variables: force (moment) and displacement (rotation). The non-linearities of the soil and at the soil-foundation interface are generally reproduced within the framework of the plasticity theory. This paper aims to extend the application of the macro-element approach to inclusion reinforced foundations. The dynamic soil-structure interaction effects on several typical structures are investigated using a 3D non-linear model with a macro-element. The work is a part of the ASIRI+ French National Project.

In most civil engineering applications, the seismic Soil Structure Interaction (SSI) is assessed by performing blind numerical analyses with no detailed verification of the accordance between numerical results and in situ observations of SSI. The most widespread way to account for dynamic SSI is to associate a set of Impedance Functions (IF) to the structure’s foundation. The implementation of this method is simple and widely used in practice since it allows to describe the relationship between efforts and displacements at the soil-foundation interface for harmonic stresses depending on the frequency of interest. However, this method is based on many simplifying assumptions, such as rigid foundation and homogeneous and horizontal soil layers amongst others. The purpose of this paper is to present a measuring methodology which aims at acquiring in situ data to characterize the IF and therefore, allowing a prior validation of numerical analyses of IF within the following framework: • a linear behaviour of the soil, of the foundation and of their interface; • a relatively rigid and superficial foundation (compared to a considerably softer soil). The main difficulty of such a task is the measurement of the evolution of the forces at the soil-foundation interface. To overcome this difficulty, in this work, we propose to induce the ground-foundation motion with applied forces. At this point, it is sufficient to measure the displacements of the foundation with traditional devices (velocimeters and/or accelerometers) to get the knowledge of the in situ IF. Indeed, within this framework, one can derive an explicit function relating the measured displacements to the FIs terms based on the mechanical equilibrium of the soil-foundation interface using the principles of rigid body dynamics. For the foreseen device, the forces are generated by mechanical vibrations applying unidirectional sinusoidal forces which frequency is to be monitored. Eventually, this paper presents a reduced scale experimental mock-up of the proposed set-up and the set of equations leading to the measurement of in situ SSI in the absence of material and interface non linearities. This is a first step towards the validation of SSI calculations in numerical models. Forthcoming work will cover the evidence of feasibility at a full structural scale before aiming at the extension of the framework to include material and interface non linearities.