A Generalized Backward Euler algorithm for the numerical integration of an isotropic hardening elastoplastic model for mechanical and chemical degradation of bonded geomaterials

An extended version of the classical Generalized Backward Euler (GBE) algorithm is proposed for the numerical integration of a three‐invariant isotropic‐hardening elastoplastic model for cemented soils or weak rocks undergoing mechanical and non‐mechanical degradation processes. The restriction to isotropy allows to formulate the return mapping algorithm in the space of principal elastic strains. In this way, an efficient and robust integration scheme is developed which can be applied to relatively complex yield surface and plastic potential functions. Moreover, the proposed algorithm can be linearized in closed form, thus allowing for quadratic convergence in the global Newton iteration. A series of numerical experiments are performed to illustrate the accuracy and convergence properties of the algorithm. Selected results from a finite element analysis of a circular footing on a soft rock layer undergoing chemical weathering are then presented to illustrate the algorithm performance at the boundary value problem level. Copyright © 2002 John Wiley & Sons, Ltd.     

Modelling mechanical behaviour of limestone under reservoir conditions

High porosity and low permeability limestone has presented pore collapse. As fluid is withdrawn from these reservoirs, the effective stresses acting on the rock increase. If the strength of the rock is overcome, pore collapse may occur, leading to irreversible compaction of porous media with permeability and porosity reduction. It impacts on fluid withdrawal. Most of reservoirs have been discovered in weak formations, which are susceptible to this phenomenon. This work presents a study on the mechanical behaviour of a porous limestone from a reservoir located in Campos Basin, offshore Brazil. An experimental program was undergone in order to define its elastic plastic behaviour. The tests reproduced the loading path conditions expected in a reservoir under production. Parameters of the cap model were fitted to these tests and numerical simulations were run. The numerical simulations presented a good agreement with the experimental tests. Copyright © 2006 John Wiley & Sons, Ltd.     

A cosmological study of the star formation history in the solar neighbourhood

We use a cosmological galactic evolutionary approach to model the Milky Way. A detailed treatment of the mass aggregation and dynamical history of the growing dark halo is included, together with a self-consistent physical treatment for the star formation processes within the growing galactic disc. This allows us to calculate the temporal evolution of star and gas surface densities at all galactic radii, in particular, the star formation history (SFH) at the solar radius. A large range of cosmological mass aggregation histories (MAHs) is capable of producing a galaxy with the present-day properties of the Milky Way. The resulting SFHs for the solar neighbourhood bracket the available observational data for this feature, the most probable MAH yielding the optimal comparison with these observations. We also find that the rotation curve for our Galaxy implies the presence of a constant density core in its dark-matter halo.     

Stress–dilatancy based modelling of granular materials and extensions to soils with crushable grains

Stress–dilatancy relations have played a crucial role in the understanding of the mechanical behaviour of soils and in the development of realistic constitutive models for their response. Recent investigations on the mechanical behaviour of materials with crushable grains have called into question the validity of classical relations such as those used in critical state soil mechanics. In this paper, a method to construct thermodynamically consistent (isotropic, three‐invariant) elasto‐plastic models based on a given stress–dilatancy relation is discussed. Extensions to cover the case of granular materials with crushable grains are also presented, based on the interpretation of some classical model parameters (e.g. the stress ratio at critical state) as internal variables that evolve according to suitable hardening laws. Copyright © 2004 John Wiley & Sons, Ltd.     

Symmetries and Rank Deficiency in the Orbit Determination Around Another Planet

We study the possible degeneracies for the normal matrix of the observations from the Earth of the motion of a satellite around a planet, and give the possible solutions to the loss of precision in the orbit determination caused by the rank deficiency. Finally we discuss the methods available to control the instability in the orbit determination resulting from the degeneracy.     

Correlation of Plio-Pleistocene deposits of the Lower Rhine Basin (North-West Germany) and the Valle Ricca pits (Central Italy)

Biostratigraphic and palaeomagnetic research has been carried out on selected profiles in North-Western Germany and Central Italy in order to correlate Plio-Pleistocene sections.Around the Réunion subchronozone, vegetation in the Valle Ricca was dominated by a mountain forest type pointing to cool climatic conditions. In the Lower Rhine Basin vegetation had a slightly warmer character, but was also dominated strongly by a mountain type of forest vegetation, characteristic of cooler summers and higher precipitation than at present.Two warmer periods have been determined in the Central Italian profiles; the youngest around the Olduvai reversal, should correspond to the youngest Tiglian beds of the Lower Rhine Basin.Deposits older than the Réunion subzone have not been found in the Valle Ricca. In the Lower Rhine Basin, on the other hand, the Gauss-Matuyama boundary is located between the uppermost Pliocene, the Reuverian C and the Praetiglian.     

Current confinement in solar coronal loops

In this paper solar coronal loops are regarded as regions of localized current flows. The main purpose is to investigate the consequences of current confinement rather than to produce a model. The physical and observational basis for this assumption are presented as well as the connection with previous studies on loop structure. A proper choice of the current profile allows us to treat quantitatively the equilibrium structure of the loops and their MHD and resistive stability properties. Regions of absolute stability against ideal kink modes are found. Explicit growth rates for the tearing-mode instability are computed. The possible relevance of other resistive effects is also discussed and the crucial importance of the small-scale geometry of the magnetic field outlined.     

Instability and geotechnical problems of the Buddha niches and surrounding cliff in Bamiyan Valley,central Afghanistan

This paper describes the slope-instability processes and geological hazards affecting the historical site of Bamiyan, central Afghanistan, a major world-cultural heritage site. Here, two standing Buddhas carved in rock during the 2nd–4th centuries A.D. were destroyed by the Taliban in March 2001. There are very evident slope instabilities resulting from both explosions and natural processes: rock slides and rock falls had already occurred in the past and most areas are prone to collapse. Under the coordination of UNESCO, field data were collected and laboratory tests were performed to determine mechanisms for potential evolution of the cliff and niches. Areas of greatest instability, based on the field survey and data interpretation for both the small and great Buddha niches and surrounding cliff, were determined. This information helps to identify the most suitable method for restoration, in accord with the high cultural value of this site.