Beobachtungen an der Küste von Suriname

Ohne Zusammenfassung     

Optimization of reinforced soil embankments by genetic algorithm

A Genetic Algorithm (GA) is described, which produces solutions to the cost optimization problem of reinforcement layout for reinforced soil slopes. These solutions incorporate different types of reinforcement within a single slope. The GA described is implemented with the aim of optimizing the cost of materials for the preliminary layout of reinforced soil embankments. The slope design method chosen is the U.K. Department of Transport HA 68/94 ‘Design Methods for the Reinforcement of Highway Slopes by Reinforced Soil and Soil Nailing Techniques’. The results confirm that there is a role for the GA in optimization of reinforced soil design. Copyright © 2000 John Wiley & Sons, Ltd.     

Accelerated initial stiffness schemes for elastoplasticity

Iterative methods for the solution of non‐linear finite element equations are generally based on variants of the Newton–Raphson method. When they are stable, full Newton–Raphson schemes usually converge rapidly but may be expensive for some types of problems (for example, when the tangent stiffness matrix is unsymmetric). Initial stiffness schemes, on the other hand, are extremely robust but may require large numbers of iterations for cases where the plastic zone is extensive. In most geomechanics applications it is generally preferable to use a tangent stiffness scheme, but there are situations in which initial stiffness schemes are very useful. These situations include problems where a nonassociated flow rule is used or where the zone of plastic yielding is highly localized. This paper surveys the performance of several single‐parameter techniques for accelerating the convergence of the initial stiffness scheme. Some simple but effective modifications to these procedures are also proposed. In particular, a modified version of Thomas' acceleration scheme is developed which has a good rate of convergence. Previously published results on the performance of various acceleration algorithms for initial stiffness iteration are rare and have been restricted to relatively simple yield criteria and simple problems. In this study, detailed numerical results are presented for the expansion of a thick cylinder, the collapse of a rigid strip footing, and the failure of a vertical cut. These analyses use the Mohr–Coulomb and Tresca yield criteria which are popular in soil mechanics. Copyright © 2000 John Wiley & Sons, Ltd.     

Deglacial palaeoclimate at Puerto del Hambre,subantarctic Patagonia,Chile

The primary objective of this study is to further substantiate multistep climatic forcing of late‐glacial vegetation in southern South America. A secondary objective is to establish the age of deglaciation in Estrecho de Magallanes–Bahía Inútil. Pollen assemblages at 2‐cm intervals in a core of the mire at Puerto del Hambre (53°36′21″S, 70°55′53″W) provide the basis for reconstructing the vegetation and a detailed account of palaeoclimate in subantarctic Patagonia. Chronology over the 262‐cm length of core is regulated by 20 AMS radiocarbon dates between 14 455 and 10 089 ¹⁴C yr BP. Of 13 pollen assemblage zones, the earliest representing the Oldest Dryas chronozone (14 455–13 000 ¹⁴C yr BP) records impoverished steppe with decreasing frequencies and loss of southern beech (Nothofagus). Successive 100‐yr‐long episodes of grass/herbs and of heath (Empetrum/Ericaceae) before 14 000 ¹⁴C yr BP infer deglacial successional communities under a climate of increased continentality prior to the establishment of grass‐dominated steppe. The Bølling–Allerød (13 000–11 000 ¹⁴C yr BP) is characterised by mesic grassland under moderating climate that with abrupt change to heath dominance after 12 000 ¹⁴C yr BP was warmer and not as humid. At the time of the Younger Dryas (11 000–10 000 ¹⁴C yr BP), grass steppe expanded with a return of colder, more humid climate. Later, with gradual warming, communities were invaded by southern beech. The Puerto del Hambre record parallels multistep, deglacial palaeoclimatic sequences reported elsewhere in the Southern Andes and at Taylor Dome in Antarctica. Deglaciation of Estrecho de Magallanes–Bahía Inútil is dated close to 14 455 ¹⁴C yr BP, invalidating earlier dates of between 15 800 and 16 590 ¹⁴C yr BP. Copyright © 2000 John Wiley & Sons, Ltd.