Emmanouil Rovithis

Subsoil Interventions Effect on Structural Seismic Response. Part I: Validation of Numerical Simulations

Subsoil interventions to enhance the static soil resistance and reduce deformations may alter significantly the seismic response of the complex soil-foundation-structure system. The aim of this article is to have an insight in the physics of the problems encountered and to validate an adequate numerical modeling procedure to study these effects of the intervention in the global response of the system. Validation concerns wave propagation, site effects, and dynamic soil-structure-interaction issues including the intervention beneath the foundation. Theoretical models-expressions and experimental results from centrifuge tests have been used. The proposed numerical model is proven very efficient to describe the complex dynamic phenomenon and anticipate the seismic response after the employment of subsoil interventions.

Dynamic stiffness and kinematic response of single piles in inhomogeneous soil

The effect of soil inhomogeneity on dynamic stiffness and kinematic response of single flexural elastic piles to vertically-propagating seismic SH waves is explored. A generalized parabolic function is employed to describe the variable shear wave propagation velocity in the inhomogeneous stratum. A layered soil with piece-wise homogeneous properties is introduced to approximate the continuous inhomogeneity in the realm of a Beam-on-Dynamic-Winkler-Foundation model. The problem is treated numerically by means of a layer transfer-matrix (Haskell–Thompson) formulation, and validated using available theoretical solutions and finite-element analyses. The role of salient model parameters such as pile-head fixity conditions, pile-to-soil stiffness ratio, surface-to-base shear wave velocity ratio and rate of inhomogeneity is elucidated. A new normalization scheme for inertial and kinematic response of such systems is presented based on an average Winkler wavenumber. With reference to long piles in moderately inhomogeneous soils, results indicate that: (a) kinematic pile response is essentially governed by a single dimensionless frequency parameter accounting for pile-to-soil stiffness ratio, pile slenderness and soil inhomogeneity and (b) definition of a characteristic pile wavelength allows an approximate estimation of pile elastodynamic response for preliminary design or analysis. Issues related to domain discretization and Winkler moduli are discussed.

A LiDAR-aided urban-scale assessment of soil-structure interaction effects: the case of Kalochori residential area (N. Greece)

We present a methodology for a large-scale assessment of soil-structure interaction (SSI) effects on the vibrational characteristics and the seismic loading of structures in a real urban fabric by combining airborne monitoring techniques, field surveys and simple calculations in the realm of structural and geotechnical dynamics. The proposed procedure combines geometrical features of a building stock acquired from a LiDAR-based 3D city model, material and typology data of structures from in situ inspections in representative building blocks, geotechnical data from field surveys and strong ground motion data from seismic hazard and site effects analyses. The integrated data are employed to compute at a first stage the fundamental period of each building in the urban area by means of code-defined formulas for both fixed- and flexible-base foundation conditions, thus treating soil-structure interaction effects in a simplified manner. The seismic action in terms of spectral acceleration at the fundamental period of structures is then computed, following a series of 1D site-specific analyses of soil response under different seismic scenarios. Spatially distributed ratios of structural fundamental periods and spectral accelerations, corresponding to flexible- over fixed-base foundation conditions, are mapped in GIS environment as an index of SSI effects at large-scale. The methodology is implemented in the urban area of Kalochori located west of Thessaloniki in Greece. The examined case study showed that SSI may be significant even for low-amplitude motions and may lead to higher seismic forces compared to the fixed-base case, depending on the dynamic characteristics of the structures, the soil conditions and the shape of the response spectrum. The above may be of importance in microzonation and seismic vulnerability studies at urban-scale when a building-by-building assessment is not feasible and SSI effects are too important to be ignored.

Dynamic Response of Shallow Rectangular Tunnels in Sand by Centrifuge Testing

A series of dynamic centrifuge tests on rectangular tunnel models embedded in dry and saturated sands is presented. The tests were carried out at the geotechnical centrifuge facility of IFSTTAR in Nantes, France, within the Transnational Access action DRESBUS II funded by the SERIES research project. The experimental program focused on salient parameters controlling the dynamic response of the soil-tunnel system such as soil-to-tunnel relative flexibility, soil-tunnel interface characteristics, soil saturation and characteristics of the input motion. Among the innovative features of the experimental set up were sand pluvation, models saturation and tunnels waterproofing techniques. A dense monitoring scheme was implemented, including accelerometers, displacement sensors, pore pressure sensors and specially designed extensometers for measuring side-wall deformations and diagonal distortion of the tunnel. A preliminary interpretation of the experimental data revealed the effect of the above parameters on the racking deformation modes of the tunnel sections.

1D Harmonic Response of Layered Inhomogeneous Soil: Exact and Approximate Analytical Solutions

The seismic response of inhomogeneous soils is explored analytically by means of one-dimensional viscoelastic wave propagation theory. The system under investigation comprises of a continuously inhomogeneous layer over a homogeneous one of higher stiffness. The excitation is specified at the bottom of the base layer in the form of vertically propagating harmonic S waves. Shear wave propagation velocity in the inhomogeneous layer is described by a generalized parabolic function, which allows modeling of soil having vanishing shear modulus at the ground surface. The problem is treated analytically leading to an exact solution of the Bessel type for the natural frequencies, mode shapes and base-to-surface response transfer function. The model is validated using available theoretical solutions and finite-element analyses. The exact analytical solution is compared with energy-based Rayleigh techniques and equivalent homogeneous soil approximations. The latter are defined by means of alternative definitions for the representative shear wave velocity in the inhomogeneous layer. Results are presented in the form of normalized graphs demonstrating the effect of salient model parameters such as layer thickness, impedance contrast between surface and base layer, surface-to-base shear wave velocity ratio in the inhomogeneous layer, rate of inhomogeneity and hysteretic damping ratio. Harmonic response of inhomogeneous soils with vanishing shear wave velocity near soil surface is explored by asymptotic analyses.

Assessment of seismic loading on structures based on airborne LiDAR data from the Kalochori urban area (N. Greece)

Airborne LiDAR monitoring integrated with field data is employed to assess the fundamental period and the seismic loading of structures composing an urban area under prescribed earthquake scenarios. Α piecewise work-flow is adopted by combining geometrical data of the building stock derived from a LiDAR-based 3D city model, structural data from in-situ inspections on representative city blocks and results of soil response analyses. The procedure is implemented in the residential area of Kalochori, (west of Thessaloniki in Northern Greece). Special attention is paid to the in-situ inspection of the building stock in order to discriminate recordings between actual buildings and man-made constructions that do not conform to seismic design codes and to acquire additional building stock data on structural materials, typologies and number of stories which is not feasible by the LiDAR process. The processed LiDAR and field data are employed to compute the fundamental period of each building by means of code-defined formulas. Knowledge of soil conditions in the Kalochoti area allows for soil response analyses to obtain free-field at ground surface under earthquake scenarios with varying return period. Upon combining the computed vibrational characteristics of the structures with the free-field response spectra, the seismic loading imposed on the structures of the urban area under investigation is derived for each one of the prescribed seismic motions. Results are presented in GIS environment in the form of spatially distributed spectral accelerations with direct implications in seismic vulnerability studies of an urban area.

Field monitoring of strong ground motion in urban areas: the Kalochori Accelerometric Network (KAN), database and Web-GIS portal

The Kalochori Accelerometric Network (KAN) has been operating since 2014 in the urban area of Kalochori, 7 km west of Thessaloniki in Northern Greece. KAN is composed of seven accelerometric stations: three ground stations installed in distinct urban zones (i.e., residential, industrial, and tanks zone); three stations on top of a selected structure within each urban zone; and one free-field station away from the built environment. The stations are documented with installation and operating features, available characteristics of the housing structures, and geotechnical data. A set of 78 earthquakes that have been recorded by KAN between 16 January 2014 and 31 December 2016 are reported, allowing investigation of local site effects on seismic motion, variation of ground surface motion within different urban environments, and evaluation of dynamic response features of the instrumented structures. KAN stations monographs, processed acceleration recordings, and metadata of the recorded earthquakes are available online through a Web-GIS platform. Indicative example applications are discussed for possible data use in the field of soil and structural dynamics. The DOI linked to the complete set of KAN data is 10.6084/m9.figshare.5044804.

EFFECT OF SOIL-STRUCTURE INTERACTION ON THE SEISMIC RESPONSE OF AN INSTRUMENTED BUILDING DURING THE CEPHALONIA, GREECE EARTHQUAKE OF 26-1-2014

An investigation of SSI effects on the seismic response of the Prefecture building in the island of Lefkas is conducted, based on recordings of an earthquake excitation at its base and at a nearby free-field station. Detailed F.E. models of the building are developed and used as a basis for the development of simpler, yet reliable models for the numerical investigation of SSI effects. Both kinematic and inertial interaction are taken into account, using the substructure approach. The comparison of the predicted response with the actual one recorded at the basement of the building is presented and discussed, as well as the various repercussions due to the potential underestimation of the seismic hazard from recording stations housed at the basement of buildings. 1375 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 1375-1387