Grigorios Tsinidis

Performance and Seismic Design of Underground Structures

Underground structures, tunnels, subways, metro stations and parking lots, are crucial components of the build environment and transportation networks. Considering their importance for life save and economy, appropriate seismic design is of prior significance. Their seismic performance during past earthquakes is generally better than aboveground structures. However several cases of severe damage to total collapse have been reported in the literature, with that of the Daikai metro station in Kobe during the Hyogoken-Nambu earthquake (1995) being one of the most characteristic. These recent damages revealed some important weaknesses in the current seismic design practices. The aim of this chapter is not to make another general presentation of the methods used for the seismic design of underground structures, but rather to discuss and highlight the most important needs for an improved seismic performance and design. In that respect it is important to consider that the specific geometric and conceptual features of underground structures make their seismic behavior and performance very distinct from the behavior of aboveground structures, as they are subjected to strong seismic ground deformations and distortions, rather than inertial loads. Several methods are available, from simplified analytical elastic solutions, to sophisticated and in principal more accurate, full dynamic numerical models. Most of them have noticeable weaknesses on the description of the physical phenomenon, the design assumptions and principles and the evaluation of the parameters they need. The chapter presents a short but comprehensive review of the available design methods, denoting the crucial issues and the problems that an engineer could face during the seismic analysis. The main issues discussed herein cover the following topics: (i) force based design against displacement based design, (ii) deformation modes of rectangular underground structures under seismic excitation, (iii) seismic earth pressures on underground structures, (iv) seismic shear stresses distribution on the perimeter of the structure, (v) appropriateness of the presently used impedance functions to model the inertial and the kinematic soil-structure interaction effects, (vi) design seismic input motion, accounting of the incoherence effects and the spatial variation of the motion and (vii) effect of the build environment (i.e. city-effects) on the seismic response of underground structures. The discussion is based on detailed numerical analysis of specific cases and recent experimental results in centrifuge tests. Other important issues like the design of submerged tunnels to liquefaction risk, or the complexity to evaluate the response of the joints of submerged tunnels are also shortly addressed. Finally we present the most recent developments on the evaluation of adequate fragility curves for shallow tunnels.

Physical modeling for the evaluation of the seismic behavior of square tunnels

The Chapter summarizes results from dynamic centrifuge tests performed on a rectangular tunnel model embedded in dry sand. The tests were carried out at the geotechnical centrifuge facility of the University of Cambridge, within the Transnational Access Task of the SERIES Research Project (Project: TUNNELSEIS). The experimental data is presented in terms of acceleration and displacement-time histories in the soil and on the tunnel, soil surface settlements, earth pressures on the side walls of the tunnel and internal forces of the tunnel lining. The goal of the experiment is twofold: to better understand the seismic behavior of these types of structures, and to use the high quality and perfectly constrained data to validate the numerical models which are commonly used for the design of rectangular embedded structures. The interpretation of the results reveals (i) rocking response of the tunnel model, (ii) existence of residual values on the earth pressures on the side walls and on the internal forces and (iii) important influence of the tunnel on the shear wave field. These issues are not well understood and are usually not taken into account in the simplified seismic analysis methods.

Circular tunnels in sand: dynamic response and efficiency of seismic analysis methods at extreme lining flexibilities

The paper discusses the seismic response of circular tunnels in dry sand and investigates the efficiency of current seismic analysis methods at extreme lining flexibilities. Initially, a dynamic centrifuge test on a flexible circular model tunnel, embedded in dry sand, is analyzed by means of rigorous full dynamic analysis of the coupled soil–tunnel system, applying various non-linear soil and soil–tunnel interface models. The numerical results are compared to the experimental ones, aiming to better understand the recorded response and calibrate the numerical models. Then a series of numerical analyses are conducted using the validated numerical model, in order to investigate the effect of the tunnel lining rigidity on the dynamic response of the soil–tunnel system. In parallel, the accuracy of currently used simplified analysis methods is evaluated, by comparing their predictions with the results of the a priori more accurate and well validated numerical models. The comparative analyses allow us to highlight and discuss several crucial aspects of the soil-tunnel system seismic response, including (1) the post-earthquake residual values of the lining forces, which are amplified with the increase of the flexibility of the tunnel and (2) the importance of the soil-tunnel interface conditions. It is finally concluded that simplified analysis methods may provide a reasonable framework for the analysis at a preliminary stage, under certain conditions.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE SEISMIC BEHAVIOR OF RECTANGULAR TUNNELS IN SOFT SOILS

Underground structures constitute crucial components of the transportation networks. Considering their significance for modern societies, their proper seismic design is of great importance. However, this design may become very tricky, accounting of the lack of knowledge regarding their seismic behavior. Several issues that are significantly affecting this behavior (i.e. earth pressures on the structure, seismic shear stresses around the structure, complex deformation modes for rectangular structures during shaking etc.) are still open. The problem is wider for the non-circular (i.e. rectangular) structures, were the soilstructure interaction effects are expected to be maximized. The paper presents representative experimental results from a test case of a series of dynamic centrifuge tests that were performed on rectangular tunnels embedded in dry sand. The tests were carried out at the centrifuge facility of the University of Cambridge, within the Transnational Task of the SERIES EU research program. The presented test case is also numerically simulated and studied. Preliminary full dynamic time history analyses of the coupled soil-tunnel system are performed, using ABAQUS. Soil non-linearity and soil-structure interaction are modeled, following relevant specifications for underground structures and tunnels. Numerical predictions are compared to experimental results and discussed. Based on this comprehensive experimental and numerical study, the seismic behavior of rectangular embedded structures is better understood and modeled, consisting an important step in the development of appropriate specifications for the seismic design of rectangular shallow tunnels.

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.

Calibration of strain gauged square tunnels for centrifuge testing

Abstract A series of dynamic centrifuge tests were conducted on square aluminum model tunnels embedded in dry sand. The tests were carried out at the Schofield Centre of the Cambridge University Engineering Department, aiming to investigate the dynamic response of these types of structures. An extensive instrumentation scheme was employed to record the soil-tunnel system response, which comprised of miniature accelerometers, total earth pressures cells and position sensors. To record the lining forces, the model tunnels were strain gauged. The calibration of the strain gauges, the data from which was crucial to furthering our understanding on the seismic performance of box-type tunnels, was performed combining physical testing and numerical modelling. This technical note summarizes this calibration procedure, highlighting the importance of advanced numerical simulation in the calibration of complex construction models.

Centrifuge Modelling of the Dynamic Behavior of Square Tunnels in Sand

The Chapter summarizes representative experimental results from dynamic centrifuge tests that were performed on square model-tunnels embedded in dry sand. Two model-tunnels were used, a rigid and a flexible one, with the latter model collapsing during the test. The tests were carried out at the geotechnical centrifuge facility of the University of Cambridge, within the Transnational Access action of the SERIES Research Project (TA Project: TUNNELSEIS). The experimental data is presented in terms of acceleration in the soil and on the tunnel, earth pressures on the side walls of the tunnel and internal forces of the tunnel lining. The collapse mechanism of the flexible tunnel is presented and discussed based on the recorded response. The goal of this program is two fold: to better understand the seismic behaviour of this type of structures, and to use the high quality data to validate the numerical models, which should be used for the design of rectangular embedded structures. The interpretation of the results reveals (i) “rocking” response of the tunnels in addition to racking, (ii) existence of residual values on the earth pressures on the side walls and on the internal forces after shaking, affected significantly by the flexibility of the tunnels, and (iii) modification of the induced shear wave field from the presence of the shallow tunnel, which in turn is affecting its seismic response. These issues are not well understood and often are not considered by simplified seismic analysis methods.

Analysis of the seismic behavior of classical multi-drum and monolithic columns

Greek classical temples constitute monuments of great historical and architectural value. The majority of them remain nowadays as free-standing multi-drum or monolithic columns or portals. Their present typology, in addition to their slenderness, render their response to seismic ground shaking quite distinct compared to that of modern structures. Rocking and/or sliding of the drums along their interfaces are the main modes of response under seismic loading. In this study, the seismic response of classical multi-drum columns is investigated by means of rigorous numerical analysis. An Ionic multi-drum column that has recently been restored at the Akropolis of Lindos in Rhodes, Greece, is used as a case study. The numerical analyses are performed using the finite element code ABAQUS. Numerical modelling is validated with a series of shaking table tests that were performed on a scaled multi-drum model column. Additional analyses are conducted by bonding perfectly the structural elements of the column, to investigate the seismic response of free-standing monolithic columns and compare with multi-drum columns of the same geometry. Through the presentation of representative results, critical parameters that affect the seismic response of this type of structures, including the amplitude and frequency content of the applied base excitation, the effect of uniaxial or multiaxial excitation and the interface characteristics, are highlighted and discussed. Particular emphasis is given on the stresses developed along the interfaces during shaking. The structural performance and overall stability of both the multi-drum and monolithic columns are investigated in terms of the maximum displacement of the capital against various intensity measures of the base excitation. The study contributes towards the improvement of the knowledge of the seismic performance of free standing rocking systems, such as the classical columns.