Frank Wuttke

Seismic response of lined tunnels in the half-plane with surface topography

In this work, we examine the seismic response of multiple tunnels reinforced with liners and buried within the elastic homogeneous half-plane in the presence of surface relief. The seismic waves are upward propagating, time-harmonic, horizontally polarized shear (SH) waves. More specifically, we examine: (a) the scattered wave fields along the free surface and inside the half-plane with the embedded tunnels; (b) the dynamic stress concentration factors that develop at the soil-liner interfaces; (c) the stresses and displacements that develop inside the tunnel liners. We use a sub-structuring technique that is based on the direct boundary element method to model each constituent part of the problem separately. Then, assembly of the full problem is accomplished through the imposition of compatibility and equilibrium conditions at all interfaces. Next, a detailed verification study is carried out based on comparisons against available analytical and/or numerical results for a series of test examples. Subsequently, detailed numerical simulations are conducted and the results of these parametric studies reveal the influence of the following key parameters on the soil-tunnel system response: (a) the shape of the free-surface relief; (b) the depth of placement of the tunnels and their separation distance; (c) the SH-wavelength to tunnel diameter ratio; (d) the elastic properties of the tunnel lining rings and (e) the dynamic interaction effects between the free-surface relief and the tunnels.

Coda Wave Analysis to Monitor Processes in Soils

AbstractSmall-strain elastic wave propagation is a constant-fabric phenomenon ideally suited to monitor processes in soils. However, the determination of very small changes in travel time limits our ability to resolve changes in soil stiffness caused by internal processes or changes in boundary conditions. The first-arrival reflects the fastest path between the source and receiver of the propagating wave field; later arrivals in the coda correspond to longer paths after multiple boundary reflections and internal scattering. Therefore, time shifts between the codas of two consecutive signals are longer and easier to detect than between the signals’ first arrivals. Slight changes in coda waves can be determined by cross-correlating time windows, time-stretched signals, or frequency-stretched spectra. Basic coda analysis assumes a homogeneous velocity change throughout the medium, propagation modes (P, S) that are equally affected by the process and the preservation of VP/VS ratio during the process. The res...

Advanced Meso-Scale Modelling to Study the Effective Thermo-Mechanical Parameter in Solid Geomaterial

The effects of coupled thermo-mechanical processes under consideration of micro-fracturing of the solid geomaterial on mechanical and thermal properties of geomaterials are investigated and subsequently simulated using advance Lattice Element Method (LEM). As a result of that extension, the alteration of effective parameter due to structural changes become numerically understandable. Hence, the simulation of the coupled processes on the meso-scale helps to develop and validate reliable identification method for real cases. The obtained results make it obvious that LEM has a large potential for fracture problems in geomaterials.

Wave Velocity Change and Small-Strain Stiffness in Unsaturated Soils: Experimental Investigation

Dynamic properties for unsaturated soil are of significant importance for analysing unsaturated soil behaviour for variety of geotechnical design applications. This paper presents an experimental investigation of the small-strain stiffness of unsaturated soil. Bender elements were used to excite and receive shear waves in controlled unsaturated conditions to determine shear wave velocity and corresponding small-strain stiffness. A special cell was designed and produced to control two stress state variables, the net confining pressure and the matric suction and with installed bender elements. The specimen starts at full saturation then it dries gradually while matric suction increases. In each test, the net stress was kept constant whereas the matric suction increases. In the meanwhile, time histories of shear waves were saved for each matric suction step. The objective of all measurements is the identification of the velocity/small-strain-stiffness dependency of the matric suction and saturation level.

Seismic Soil-Tunnels Interaction Via Bem Part I. Mechanical Model

Abstract Two-dimensional elastodynamic problem for seismic response of unlined and lined tunnels located in a layered half-plane with free surface relief is solved. The computation tool uses the idea of the global matrix propagator method which allows derivation of a relation between the wave field quantities along different interfaces in the layered half- plane. The numerical realization of this idea is performed with the help of the sub-structured boundary element method (BEM) well suited when objects with arbitrary geometry are considered. A relation between dis- placements and tractions along the free surface and arbitrary interface of the soil stratum is derived. It works for arbitrary geometry of the interfaces between soil layers. Finally, in the companion paper, numerical results are presented which show both a validation study of the pro- posed computational methodology and extensive numerical simulations demonstrating the influence of some important factors as type and characteristics of the incident wave, dynamic tunnels interaction, soil-tunnel interaction, free surface relief, type of the tunnel construction and mechanical properties of the layered half-plane on the complex seismic field near and far-away from the underground structures.

Soil-bridge system stiffness identification through field and laboratory measurements

Despite the major advances in finite-element (FE) modeling and system identification (SI) of extended infrastructures, soil compliance and damping at the soil-foundation interface are not often accurately accounted for due to the associated computational demand and the inherent uncertainty in defining the dynamic stiffness. This paper aims to scrutinize the effect of soil conditions in the SI process and to investigate the efficiency of advanced FE modeling in representing the superstructure-soil-foundation stiffness. For this purpose, measured, computed, and experimentally identified natural frequencies of a real bridge were used. Field measurements obtained during construction were reproduced both in the laboratory and by refined FE modeling. In addition, to understand the physical problem more thoroughly, three alternative soil conditions were examined: rock, stabilized soil, and Hostun sand. Discrepancies on the order of 3-13% were observed between the identified and the numerically predicted natural frequencies. These discrepancies highlight the importance of reliable estimation of soil properties and compliance with the SI framework for extended bridges under ambient and low-amplitude vibrations

Elastic Wavefield Evaluation in Discontinuous Poroelastic Media by Bem: Sh-Waves

Elastic Wavefield Evaluation in Discontinuous Poroelastic Media by Bem: Sh-Waves This work examines the anti-plane strain elastodynamic problem for poroelastic geological media containing discontinuities in the form of cavities and cracks. More specifically, we solve for: (i) a mode III crack; (ii) a circular cylindrical cavity, both embedded in an infinite poroelastic plane; and (iii) a mode III crack in a finite-sized poroelastic block. The source of excitation in all cases are time-harmonic, horizontally polarized shear (SH) waves. These three cases depict a situation whereby propagating elastic waves are diffracted and scattered by the presence of discontinuities in poroelastic soil, and this necessitates the computation of stress concentration factors (SCF) and stress intensity factors (SIF). Thus, the sensitivity of the aforementioned factors to variations in the material parameters of the surrounding poroelastic continuum must be investigated. Bardets model is introduced by assuming saturated soils as the computationally efficient viscoelastic isomorphism to Biots equations of dynamic poroelasticity, and stress fields are then evaluated for an equivalent one-phase viscoelastic medium. The computational itool employed is an efficient boundary element method (BEM) defined in terms of the non-hypersingular, traction-based formulation. Finally, the results obtained herein demonstrate a marked dependence of the SIF and the SCF on the mechanical properties of the poroelastic continuum, while the advantages of the proposed method as compared to alternative analytical and/or numerical approaches are also discussed.

Time-Lapse Monitoring of Fabric Changes in Granular Materials by Coda Wave Interferometry

The determination of wave velocities in soils captures an important role for small-strain parameter in constitutive modeling as well as in monitoring processes of the state. The wave velocity differs substantially with the porosity, surrounding pressure, degree of saturation and other parameters in soils. In particular when the environmental conditions are non-stationary. For studying the change of state parameters in granular materials, the identification of the velocity evolution can be a challenge in particular if the perturbations are small. We discuss the application of a new seismic method for geotechnical experiments—the Coda wave interferometry. The method is used to detect the wave velocities change related to small soil perturbations. Whereas the perturbations are caused by small changes in stress and void ratio. Different material, well-known sands and artificial glass beads are analysed in this experimental study by conventional volume measurements and seismic methods to detect the stress and porosity changes. The wave excitation and recording was done by piezoceramic bender elements. In result of the tests, the coda wave interferometry emphasized its large potential for the time-lapse monitoring of soils.

Toolbox for Applied Seismic Tomography (TOAST)

TOAST (Toolbox for Applied Seismic Tomography) makes methods of full-waveform inversion of elastic waves available for the practitioner. The inversion of complete seismograms is an utmost ambitious and powerful technology. One of its strengths is the enormously increased imaging-resolution since it is able to resolve structures smaller than the seismic wave length. Further it is sensitive to material properties like density and dissipation which are hardly accessible through conventional techniques. Within the TOAST project algorithms available in academia were collected, improved, and prepared for application to field recordings. Different inversion strategies were implemented (global search, conjugate gradient, waveform sensitivity kernels) and computer programs for imaging the subsurface in 1D, 2D, and 3D were developed. The underlying algorithms for the correct numerical simulation of physical wave propagation have thoroughly been tested for artefacts. In parallel these techniques were tested in application to waveform data. They proved their potential in application to synthetic data, shallow-seismic surface waves from field recordings, and microseismic and ultrasonic data from material testing. This provided valuable insight to the demands on seismic observation equipment (repeatability, waveform reproduction, survey layout) and inversion strategies (initial models, regularization, alternative misfit definitions, etc.). The developed software programs, results of benchmark tests, and field-cases are published online by the OpenTOAST.de initiative.

Seismic Soil-Tunnels Interaction Via Bem Part II. Numerical Results

Abstract. The mechanical model for seismic response of unlined and lined tunnels located in a layered half-plane with free surface relief was described in the first part of this work. The computational methodology developed in the first part is based on a combination of both the idea of the global matrix propagator method which allows derivation of a relation between the wave field quantities along different interfaces in the layered half-plane and boundary element method (BEM) using the elastodynamic fundamental solution in frequency domain. The solution of the problem for transient waves is solved by the usage of direct and inverse Fast Fourier Transform (FFT). The aim of this second part is to demonstrate the accuracy and the convergence of the proposed computational tool. Furthermore, subsequent extensive numerical simulations help in