Stavros A. Savidis

Soil-structure interaction in the time domain using halfspace Green's functions

Abstract A time-domain formulation is proposed for the transient response analysis of general, three-dimensional structures resting on a homogeneous, elastic halfspace subjected to either external loads or seismic motions. The formulation consists of two parts: (a) the time domain formulation of the soil behaviour and (b) the coupling of the corresponding soil algorithms to the Finite Element Code ANSYS. As far as the structure is concerned, this coupling opens the way for the analysis of non-linear soil–structure interaction. The approach is based on halfspace Greens functions for displacements elicited by Heaviside time-dependent surface point loads. Hence, the spatial discretisation can be confined to the contact area between the foundation and the soil, i.e. no auxiliary grid beyond the foundation as for conventional boundary element formulations is required. The method is applied to analyse the dynamic response of a railway track due to a moving wheel set by demonstrating the influence of ‘through-the-soil coupling’.

Vibro-Injection Pile Installation in Sand: Part II—Numerical and Experimental Investigation

In Part 1 of this series of papers a macroscopic two-equation (two-field) reduced model for the mechanics of the multi-material flow associated with vibro-injection pile installation in saturated sand was derived. Here we employ this model to develop a so-called multi-material arbitrary Lagrangian-Eulerian (MMALE) method. MMALE avoids the disadvantages of the classical approaches in computational continuum mechanics concerning large deformations and evolving material interfaces. The numerical implementation of this method will be outlined, and then the experimental investigations will be presented that have been carried out in order to validate the computational model. Among these investigations, small-scale model tests in chambers with observing window have been designed step-by-step to reveal penetration and vibro-injection pile installation phenomena.

BEM and FEM results of displacements in a poroelastic column

The dynamical investigation of two-component poroelastic media is important for practical applications. Analytic solution methods are often not available since they are too complicated for the complex governing sets of equations. For this reason, often some existing numerical methods are used. In this work results obtained with the finite element method are opposed to those obtained by Schanz using the boundary element method. Not only the influence of the number of elements and time steps on the simple example of a poroelastic column but also the impact of different values of the permeability coefficient is investigated.

3D-Simulation of Dynamic Interaction Between Track and Layered Subground

Three-dimensional numerical models to analyze dynamic soil-structure interaction (SSI) of rail tracks on layered soil have been developed. For track structures on the surface we implemented a frequency domain as well as a time domain approach. To incorporate soil irregularities we also implemented a frequency domain approach for embedded structures. A common feature of all models is the application of the substructure method. The track or any other structure is analyzed be means of finite elements, whereas the unbounded soil is analyzed by means of boundary elements with layered halfspace fundamental solutions. To provide the user with all the capabilities of a powerful commercial finite element software package the SSI-models have been implemented in ANSYS. Therefore nonlinear properties of the track and of the soil up to some extent can be taken into account using standard ANSYS features.

Vibro-Injection Pile Installation in Sand: Part I—Interpretation as Multi-material Flow

The installation of vibro-injection piles into saturated sand has a significant impact on the surrounding soil and neighboring buildings. It is generally characterized by a multi-material flow with large material deformations, non-stationary and new material interfaces, and by the interaction of the grain skeleton and the pore water. Part 1 in this series of papers is concerned with the mathematical and physical modeling of the multi-material flow associated with vibro-injection pile installation. This model is the backbone of a new multi-material arbitrary Lagrangian-Eulerian (MMALE) numerical method presented in Part 2.

Contribution to the Non-Lagrangian Formulation of Geotechnical and Geomechanical Processes

Numerical simulations of geomechanical and geotechnical processes, such as vibro-injection pile installation, require suitable algorithms and sufficiently realistic models. These models have to account for large deformations, the evolution of material interfaces including free surfaces and contact interfaces, for granular material behavior in different flow regimes as well as for the interaction of the different materials and phases. Although the traditional Lagrangian formulation is well-suited to handling complex material behavior and maintaining material interfaces, it generally cannot represent large deformation, shear and vorticity. This is because in Lagrangian numerical methods the storage points (nodes resp. material points) move with the local material velocity, which may cause mesh tangling resp. clustering of points. The present contribution addresses the development of models for geotechnical and geomechanical processes by utilizing Eulerian and Arbitrary Lagrangian-Eulerian (ALE) formulations. Such non-Lagrangian viewpoints introduce additional difficulties which are discussed in detail. In particular, we investigate how to track interfaces and to model interaction of different materials with respect to an arbitrarily moving control volume, and how to validate non-Lagrangian numerical models by small-scale experimental tests.

Theory and Numerical Modeling of Geomechanical Multi-material Flow

Multi-material flow describes a situation where several distinct materials separated by sharp material interfaces undergo large deformations. The research presented in this paper addresses a particular class of multi-material flow situations encountered in geomechanics and geotechnical engineering which is characterized by a complex coupled behavior of saturated granular material as well as by a hierarchy of distinct spatial scales. Examples include geotechnical installation processes, liquefaction-induced soil failure, and debris flow. The most attractive numerical approaches to solve such problems use variants of arbitrary Lagrangian–Eulerian descriptions allowing interfaces and free surfaces to flow through the computational mesh. Mesh elements cut by interfaces (multi-material elements) necessarily arise which contain a heterogeneous mixture of two or more materials. The heterogeneous mixture is represented as an effective single-phase material using mixture theory. The paper outlines the specific three-scale mixture theory developed by the authors and the MMALE numerical method to model and simulate geomechanical multi-material flow. In contrast to traditional flow models which consider the motion of multiple single-phase materials or single multi-phase mixture, the present research succeeds in incorporating both the coupled behavior of saturated granular material and its interaction with other (pure) materials.

Web-based Data and Monitoring Platform for Complex Geotechnical Engineering Projects

Data management is the key of geotechnical risk management and disaster prevention providing right information at right time and right place. It supports regular construction cycles as well as handling of exceptional situations occurring probably during execution stages where the detailed knowledge of the actual state of construction is especially important. The web-based client–server software platform DoMaMoS was developed to cover all aspects in a new fashion. Main parts of the software are a graphical user interface, a SQL database and a controller application. Software development concerned user-friendly handling, geotechnical monitoring, security aspects, rapid access and adaptivity during a running project. Basic ideas and main features of the developed software are described and the practical application is shown.

Investigation on the sensitive and insensitive zones of the rail support stiffness for the dynamic response of a vehicle system under low excitation frequencies

ABSTRACT The variation of the rail support stiffness is an inherent issue of railway tracks. There is still no consensus on the influence of the rail support stiffness variation on the dynamic response of the vehicle–track system. One view indicates that changes of the support stiffness do not have considerable influence on the vehicle dynamic response. The main influence factor is the rail deflection. However, the opposite view presents that the influence of the support stiffness on the system dynamic response is obvious. Reasons that lead to the dispute of previous studies are the neglect of the influence of the excitation frequencies and a lack of understanding of stiffness sensitive zones. In this study a vehicle–track coupling model with equivalent overall support stiffness is employed to investigate the response of the vehicle to changes of the track stiffness and excitation frequencies. Results show that for each of frequencies (1–40 Hz) the dynamic response of the vehicle is only sensitive to a certain range of the support stiffness. A stiffness sensitive zone for each excitation frequency can be observed. In order to further study the influence of the subgrade on the vehicle system dynamic response a vehicle–track-subgrade model is utilised. The subgrade stiffness belonging to the stiffness sensitive zone has a significant influence on vehicle vibrations. For overall support stiffness of the rail higher than 20 kN/mm, the stiffness sensitive zones of low excitation frequencies can be avoided.

New Design Approach for Large Diameter Offshore Monopiles Based on Physical and Numerical Modelling

Monopiles with diameters of more than 4 m are suitable foundations for offshore wind farms in water depths of about 30 m. They are subjected to high numbers of cyclic horizontal loads due to wind and water waves. The paper presents a new practical design concept for cyclic horizontally loaded monopile foundations in water saturated sands and considering pile displacement and pore pressure accumulation effects. The design approach is based on physical and numerical modelling. In the design concept, the pile displacements due to cyclic horizontal loads are predicted by means of an Extended-Strain-Wedge-Model (ESWM) approach. The extension of the known SWM (Norris, 1986) mainly concerns a more realistic representation of the cyclic soil-monopile behaviour in water saturated sands. A possible pore water pressure accumulation around the pile is evaluated on the basis of the three-dimensional fully coupled two-phase finite element analysis. The design approach has been validated using the results from the physical 1g pile model tests.