An Attempt to Predict the Failure Time of Abandoned Mine Pillars

Summary  Left in place pillars of abandoned mines are subject to weathering (e.g action of water, bacteria) that degrades their mechanical strength and eventually leads to collapse. A simple weathering model is proposed, that is governed by two parameters: the rate of progression of weathered front and the rate of degradation of the compressive strength with time. Both plane strain and axisymmetric analyses are performed and closed form solutions of the variation with time of the bearing capacity of the pillar are given. Experimental data of the tests conducted on gypsum and anhydrite specimens attacked by water are presented. It is shown that in order to fit the experimental data a third parameter must be introduced. New closed form solutions are given and the data are used for estimating the time to failure of abandoned gypsum mines in Northern Italy. Authors’ address: Dr. Riccardo Castellanza, Research Assistant, Department of Structural Engineering, Milan University of Technology (Politecnico), Piazza Leonardo da Vinci 32, 20133 Milan, Italy     

An elastoplastic strainhardening model for soil allowing for hydraulic bonding–debonding effects

The paper presents a strainhardening constitutive model for unsaturated soil behaviour based on energy conjugated stress variables in the framework of superposed continua. The proposed constitutive law deals with hydro‐mechanical coupling phenomena. The main purpose is to develop within a consistent framework a model that can deal with possible mechanical instabilities occurring in partially saturated materials. The loss of capillary effects during wetting processes can, in fact, play a central role in unstable processes. Therefore, it will be shown that the bonding effects due to surface tensions can be described in a mathematical framework similar to that employed for bonded geomaterials to model weathering or diagenesis effects, either mechanically or chemically induced. The results of several simulations of common laboratory tests on partially saturated soil specimens are shown. The calculated behaviour appears to be in good qualitative agreement with that observed in the laboratory. In particular it is shown that volumetric collapse phenomena due to hydraulic debonding effects can be successfully described by the model. Finally, it will be highlighted the ability of the model to naturally capture the transition to a fully saturated condition and to deal with possible mechanical instabilities in the unsaturated regime. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

A constitutive model for sand in triaxial compression

Although there is a large number of constitutive models for sand available in the literature it is believed that a fresh approach, striking a balance between complexity and theoretical rigour, is desirable. The approach here has certain conceptual links with the Cam Clay series of elastic–plastic models, but includes the more general starting assumption that the yield function, plastic potential and failure locus should be given quite distinct mathematical expressions. Possible physical bases for the proposed forms are discussed. Ways in which the parameters required to define the model may be determined are suggested and the use of the model is then demonstrated. Firstly, it is shown that, where a limited set of experimental data is available, the model is flexible enough to be able to match the test results. Secondly, it is shown that, where a wide range of test results has been produced, it is possible to determine the model constitutive parameters from a small number of tests and proceed to make satisfactory predictionsfor other, quite different, types of test. The model is developed for sand at a single initial density, but the way in which the constitutive parameters might be expected to vary with density is discussed. The model is described for conditions of triaxial compression, and extension to more general stress states will be needed before it can be put to the test of incorporation in, for example, a finite element program.     

On the Measurement of the Tensile Strength of Soft Rocks

Summary. This paper reports on a comparative study of various types of experimental tests for measuring the tensile strength of rocks and rock-like materials. A critical assessment is presented of some widely used laboratory techniques on the basis of experimental data from the literature and from the laboratory investigation performed in this study.Tests were carried out using a triaxial apparatus recently set up at Milan University of Technology. This was designed to reduce random misalignments between specimen and loading frame, which are typical of conventional triaxial cells with external tie bars. The apparatus was then modified to perform various types of tests for determining the tensile strength of materials. An artificial building stone and a natural calcarenite of the Gravina di Puglia geological formation, sampled at Montescaglioso (Matera-Italy) were tested in this research. The experimental investigation included various types of tests, namely the uni-axial, the Brazilian, the ring, the three and four points bending and the Luong test. Specimens of both materials were also compressed to failure in unconfined conditions and loaded cyclically in unconfined tension and compression, Young’s moduli being measured by means of local instrumentation.     

A constitutive model for bonded geomaterials subject to mechanical and/or chemical degradation

The mechanical behaviour of bonded geomaterials is described by means of an elastoplastic strain‐hardening model. The internal variables, taking into account the ‘history’ of the material, depend on the plastic strains experienced and on a conveniently defined scalar measure of damage induced by weathering and/or chemical degradation. For the sake of simplicity, it is assumed that only internal variables are affected by mechanical and chemical history of the material. Despite this simplifying assumption, it can be shown that many interesting phenomena exhibited by weathered bonded geomaterials can be successfully described. For instance, (i) the transition from brittle to ductile behaviour with increasing pressure of a calcarenite with collapsing internal structure, (ii) the complex behaviour of chalk and other calcareous materials in oedometric tests, (iii) the chemically induced variation of the stress and strain state of such kind of materials, are all phenomena that can be qualitatively reproduced. Several comparisons with experimental data show that the model can capture the observed behaviour also quantitatively. Copyright © 2003 John Wiley & Sons, Ltd.     

A comparative study of bounding techniques in dynamic shakedown of elasto-plastic structures

Two conceptually different procedures to obtain upper bounds to displacements and deformations developed in the dynamic evolution of elasto-plastic structures are considered. Computationally, both methods require the dynamic analysis of a linear system and a sequence of constrained optimization problems, each roughly analogous to a limit analysis procedure in the quasi-static range, with a substantial saving in effort when compared to an incremental analysis of the plasto-dynamic response. A discrete model for frames is proposed and the two approaches are reformulated in a convenient matrix form. The approaches are then compared on the basis of the information needed (total or plastic displacements, deformations, dissipated energy), of the usefulness of results obtained and of the computational effort involved. The results seem to indicate the practical usefulness of these techniques and encourage the extension of the study to broader classes of structures.     

Regime shifts in the marine environment: the scientific basis and political context

Regime shifts in the marine environment have recently received much attention. To date, however, few large-scale meta-analyses have been carried out due to insufficient data coverage and integration between sustained observational datasets because of diverse methodologies used in data collection, recording and archival. Here we review the available data on regime shifts globally, followed by a review of current and planned policies with relevance to regime shifts.We then focus on the North and Baltic Seas, providing examples of existing efforts for data integration in the MarBEF Network of Excellence. Existing gaps in data coverage are identified, and the added value from meta-analyses of multiple datasets demonstrated using examples from the MarBEF integrated data project LargeNet. We discuss whether these efforts are addressing current policy needs and close with recommendations for future integrated data networks to increase our ability to understand, identify and predict recent and future regime shifts.     

Compatible taper and stem volume equations for <Emphasis Type="Italic">Larix kaempferi</Emphasis> (Japanese larch) species of South Korea

In this study, compatible taper and stem volume equations were developed for Larix kaempferi species of South Korea. The dataset was split into two groups: 80% of the data were used in model fitting and the remaining 20% were used for validation. The compatible MB76 equations were used to predict the diameter outside bark to a specific height, the height to a specific diameter and the stem volume of the species. The result of the stem volume analysis was compared with the existing stem volume model of Larix kaempferi species of South Korea which was developed by the Korea Forest Research Institute and with a simple volume model that was developed with fitting dataset in this study. The compatible model provided accurate prediction of the total stem volume when compared to the existing stem volume model and with a simple volume model. It is concluded that the compatible taper and stem volume equations are more convenient to use and therefore it is recommended to be applied in the Larix kaempferi species of South Korea.