Tree-root systems can prevent shallow landslides. In layers permeated by roots, the soil shows greater stability as roots are able to absorb forces. When protective forests die off extensively as a consequence of a bark beetle outbreak or of another disturbance (e.g. storms or fires), their protective power on the slope stability decreases with the decomposition of the roots of the dead trees. By determining the relation between the tensile strength of roots and the tree's time of death, the decrease in stability as a consequence of root decomposition can be estimated. To this end, we measured the tensile strength of roots from: i) freshly felled, living spruces (Picea abies), ii) spruces felled eight years previously, and iii) spruces that had died 10 and 12 years previously in a bark beetle outbreak. Tensile strength decreased continuously with the number of years after death. The results of this study show that within 15 to 20 years of tree death, the root system of protection forests loses most of its soil-stabilising function. It can be assumed that, particularly at high altitudes, this period of time is not long enough for new generations of trees to have grown enough to have the same stabilising effect on the soil.

Among others, the protective effect of the forest against natural hazards is mostly co-determined through the mechanical stability of the trees. Despite this, there is a lack of quantifiable knowledge regarding the mechanical interaction between the trees, the forest and natural hazards. With experiments conducted on spruce trees,and mechanical modelling of trees and tree trunks, the mechanical properties and the energy absorption capacity for a single tree during a rock impact were determined. It was shown that the anchorage properties correlated well with the volume of the tree stem. The four-point bending tests and the dynamic impact tests enabled us to determine the strength of the tree stems. Furthermore, the obtained values for the strength of the tree stems were strongly affected by the wood quality and sometimes deviated substantially from the established material properties of dried wood used for buildings. The behavior of trees when impacted by rocks could be simulated with full-scale impact tests and the energy absorption capacity quantified. These experiments gave important knowledge of how to analyze the interaction between natural hazards, trees and forests,with the focus on Norway spruce. However, to quantify the protection effect of the forest against natural hazards,more experiments on different tree species are required.

Doppelt ist besser: Eine doppelte Metalltemplatstrategie wurde genutzt, um ein „π-konjugiertes Catenan“ aus zwei ineinandergreifenden Makrocyclen mit Oligothienyl- und Phenanthrolineinheiten aufzubauen (siehe berechnete Struktur). Die optischen und Redoxeigenschaften belegen ebenso wie die Struktur- und Konformationsanalysen eindeutig, dass sich die beiden Makrocyclen im Catenan durch Donor-Acceptor-Wechselwirkungen durch den Raum beeinflussen.

Ein „π‐konjugiertes Catenan“ oder, besser gesagt, ein aus zwei ineinander verschlungenen konjugierten Makrocyclen gebildetes Catenan wurde durch eine doppelte Metalltemplatstrategie synthetisiert. Auffälliges Merkmal dieses Systems ist eine durch den Raum wirkende elektronische Wechselwirkung der Donoreinheiten (Oligothiophene, metallische Farbe) mit der Acceptorkomponente des anderen Rings (Phenanthrolin, blau). Einzelheiten finden Sie in der Zuschrift von P. Bäuerle et al. auf S. 367ff.

The interlocked oligothiophene macrocycle 1 was synthesized via a heteroleptic template approach, with the help of a platina-macrocycle intermediate; the corresponding homoleptic approach to the synthesis of 1 resulted in relatively poor yields of the desired product. The structure of the isolated copper(I) catenate was confirmed by ESI-FT-ICR and tandem mass spectrometry.

A method for the synthesis of interlocked pi-conjugated macrocycles is described, Starting from 2,9-bis(oligothienyl)[1,10]phenanthrolines, (trimethylsilyl) acetylene groups were introduced at the terminal thiophene rings by selective iodination and subsequent Sonogashira-Hagihara coupling. Subsequently, we applied our recently developed metal-template approach to macrocyclization reactions by treating the deprotected acetylenes with cis-[Pt(dppp)Cl-2] to yield a platina-macrocycle. Based on this synthetic knowledge, by a

Organic semiconductors containing metal binding sites within their molecular backbones are of a general interest in organic materials chemistry. In this paper, we describe a straightforward synthetic procedure, which gives access to a series of 2-(oligothienyl)-[1,10]phenanthrolines (nT-phen), 2,9-bis(oligothienyl)-[1,10]phenanthrolines (nT-phen-nT) and 2,2'-(oligothienyl)bis-[1,10]phenanthrolines (phen-nT-phen). By a Negishi-type cross coupling of 2-iodo-[1,10]phenanthroline or 2,9-diiodo-[1,10]phenanthroline with in situ generated alpha-zinc derivatives of different mono-, ter-, and quinquethiophenes we were able to synthesize the corresponding oligothienyl-phenanthrolines in medium to excellent yields. Furthermore, characterization of the optical properties of the new materials indicated that the two subunits, oligothiophene and phenanthroline, are in pi-conjugation. Characterization of the redox properties revealed additional evidence for the role of [1,10]phenanthroline as a pi-bridging unit in the nT-phen-nT series.

AbstractWe present a biomechanical application of our parallel finite element model for coupled problems in solid mechanics. This programming framework provides a very lean and flexible interface, which allows to realize time‐dependent nonlinear simulations. In this context, a special variant of a stabilized local Gauß‐Seidel preconditioner is introduced, which can be successfully employed to large scale computations. Finally, the efficiency of the implemented algorithm is shown by a numerical example considering the axial compression of a L4‐L5 motion segment of the spine. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deformation and localization analysis is a crucial issue and has thus been intensively investigated in the last decades. In particular, geotechnical applications do not only concern a single solid material but they also affect the interaction with the pore-fluids, water and air. As a result, both the deformation and the localization analysis must be applied to a triphasic material consisting of a materially incompressible elasto-plastic or elasto-viscoplastic skeleton saturated by two viscous pore-fluids, a materially incompressible pore-liquid and a materially compressible pore-gas. Based on a continuum mechanical approach, unsaturated soil can be described within the well-founded framework of the Theory of Porous Media (TPM). The numerical computations proceed from weak formulations of the momentum balance of the overall triphasic material together with the mass balance equations of the pore-fluids. The resulting system of strongly coupled differential-algebraic equations (DAE) is solved by use of the finite element tool PANDAS. Furthermore, several initial boundary-value problems are presented demonstrating the efficiency of the overall formulation.

We present a new concept for the realization of finite element computations on parallel machines which is based on a dynamic data structure address by points. All geometric objects (cells, faces, edges) are referenced by its midpoint, and all algebraic data structures (vectors and matrices) are tied to the nodal points of the finite elements. Together, they build Distributed Point Objects (DPO), where the parallel distribution is made transparent by processor lists assigned to the points. All objects are stored in hash tables (where the keys are points) so that pointers can be completely avoided. Then, we consider the application of the parallel programming model to a geomechanical porous media problem. This work complements our previous work [C. Wieners, M. Ammann, S. Diebels, W. Ehlers, Parallel 3-D simulations for porous media models in soil mechanics, Comput. Mech. 29 (2002) 75-87], where the geomechanical model, the interface of the finite element code and the parallel solver is described in detail. Here, we discuss the parallel data structure and the parallel performance for a characteristic application. Together, this demonstrates that demanding 3-D non-linear and time-dependent engineering applications on unstructured meshes can be parallelized very efficiently within a very small overhead for the parallel implementation.

We introduce a general parallel model for solving coupled nonlinear and time-dependent problems in soil mechanics, where we employ general purpose linear solvers with specially adjusted preconditioners. In particular, we present a parallel realization of the GMRES method applied to a triphasic porous media model in soil mechanics, where we compute the deformation of unsaturated soil together with the pore-fluid flow of water and air in the soil. Therefore, we propose a pointwise preconditioner coupling all unknowns at the nodal points. In two large-scale numerical experiments we finally present an extended evaluation of our parallel model for demanding configurations of the triphasic model.

Ru(II)- and Os(II)-polypyridine termini are linked by a quinquethiophene bridge ( the inter-metal separation is ca. 1.9 nm) wherein excitation energy flows into the luminescent Os-based unit by way of a conductive level.

AbstractWhile the theoretical background of various porous media models is well understood, it is still a demanding task to deal with these models numerically. In this contribution, a triphasic model is presented, which is capable of describing partially saturated soils. In quasi‐static conditions, this model results in the primary variables solid displacement, pore‐liquid pressure and pore‐gas pressure. For a stable numerical implementation, Taylor‐Hood elements are required, which need quadratic ansatz functions for the displacement and linear ansatz functions for the pressure terms. Looking at numerical simulations in 2‐d, challenging finite element calculations have already been realized in combination with adaptivity in time and space [1]. Nevertheless, new strategies have to be considered for a realization of applications of the model in 3‐d in order to handle the huge amount of unknowns arising from the discretization with Taylor‐Hood elements. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deformation and localization analysis is a crucial issue and has thus been intensively investigated in the last decades. However, in contrast to solid mechanical problems, geotechnical applications do not only concern a single solid material, the soil, but they also affect the pore-fluids, water and air, and, consequently, the coupling of the solid deformation with the pore-fluid flow. As a result, both the deformation and the localization analysis must be applied to a triphasic material consisting of the soil skeleton, the pore-water and the pore-gas, which, in geotechnical engineering, is known as unsaturated or partially saturated soil. Based on a continuum mechanical approach, unsaturated soil can be described within the well-founded framework of the Theory of Porous Media (TPM), thus including saturated soil (solid matrix and pore-water) as well as empty soil (solid matrix and pore-gas) as special cases. It is the goal of the present contribution to investigate the deformation and the localization behavior of unsaturated soil and to exhibit the influence of the solid–fluid coupling on the localization analysis. In the framework of a triphasic formulation, unsaturated soil is considered as a materially incompressible elasto-plastic or elasto-viscoplastic skeleton saturated by two viscous pore-fluids, a materially incompressible pore-liquid and a materially compressible pore-gas. Assuming quasi-static situations, the numerical computations proceed from weak formulations of the momentum balance of the overall triphasic material together with the mass balance equations of the pore-fluids and Darcy-like relations for the seepage velocities. As a result, a system of strongly coupled differential-algebraic equations (DAE) occurs, which is solved by use of the FE tool PANDAS. In particular, various initial boundary-value problems are treated on the basis of time- and space-adaptive methods, thus demonstrating the efficiency of the overall formulation. Furthermore, the influence of the pore-gas constituent on the material behavior of partially saturated soil is studied with respect to fluid-flow simulations or embankment and slope failure problems.

Contact problems are highly non‐linear because of the non‐penetration inequality in the contact interface. To solve such problems, different constraint methods are used to transform the inequality into an equality problem [4]. Concerning the contact of two isotropic fluid‐saturated porous solids which consist of an immiscible mixture of a solid skeleton and a fluid pore content [3], the non‐penetration constraint can be applied to the mixture approach by accounting additionally for the pore‐fluid interchange between the contacting porous solids. Except for the contact between the solid skeletons of two solids, it is important to determine the interaction forces exerted by the pore‐fluid on the pore‐fluid and the porous skeleton. In this contribution, it is assumed that at the permeable contact boundaries, the fluid effective pressure and the fluid mass flux across the surface are continuous [1].

In this contribution, a new finite element method in the temporal domain is presented, in which the time step size is introduced as an additional variable. Thus, the variation of the time integral of the Lagrangean resulting from Hamilton's principle has to be carried out with respect to the rules of the generalized variational calculus. Apart from the usual time integral of the Euler‐Lagrange differential equations, the so‐called transversality condition is obtained as an additional result representing a time‐boundary term, which is used to obtain an optimal step size in the time domain.

A major problem in using the finite element method for solving numerous engineering problems in the framework of single‐ and multiphase materials is the assessment of discretization errors and the design of suitable meshes. To overcome this problem, adaptive finite element methods have been developed. Based on the error indicator by Zienkiewicz and Zhu, it is the goal of the present paper to present a new error indicator which is especially designed for multiphase problems. Furthermore, efficient h‐adaptive strategies concerning both the generation of new meshes in the framework of independent and hierarchical remeshing strategies and the data transfer between old and new meshes are pointed out. Finally, numerical examples are given to exhibit the efficiency and the quality of the presented h‐adaptive methods and to compare the different strategies to each other. Copyright © 2002 John Wiley & Sons, Ltd.