A numerical study of flexural buckling of foliated rock slopes

The occurrence of foliated rock masses is common in mining environment. Methods employing continuum approximation in describing the deformation of such rock masses possess a clear advantage over methods where each rock layer and each inter-layer interface (joint) is explicitly modelled. In devising such a continuum model it is imperative that moment (couple) stresses and internal rotations associated with the bending of the rock layers be properly incorporated in the model formulation. Such an approach will lead to a Cosserat-type theory. In the present model, the behaviour of the intact rock layer is assumed to be linearly elastic and the joints are assumed to be elastic-perfectly plastic. Condition of slip at the interfaces are determined by a Mohr-Coulomb criterion with tension cut off at zero normal stress. The theory is valid for large deformations. The model is incorporated into the finite element program AFENA and validated against an analytical solution of elementary buckling problems of a layered medium under gravity loading. A design chart suitable for assessing the stability of slopes in foliated rock masses against flexural buckling failure has been developed. The design chart is easy to use and provides a quick estimate of critical loading factors for slopes in foliated rock masses. It is shown that the model based on Euler's buckling theory as proposed by Cavers (Rock Mechanics and Rock Engineering 1981; 14:87-104) substantially overestimates the critical heights for a vertical slope and underestimates the same for sub-vertical slopes. Copyright (C) 2001 John Wiley & Sons, Ltd.

Finite element modelling of rock alteration and metamorphic process in hydrothermal systems

We use the finite element method to simulate the rock alteration and metamorphic process in hydrothermal systems. In particular, we consider the fluid-rock interaction problems in pore-fluid saturated porous rocks. Since the fluid rock interaction takes place at the contact interface between the pore-fluid and solid minerals, it is governed by the chemical reaction which usually takes place very slowly at this contact interface, from the geochemical point of view. Due to the relative slowness of the rate of the chemical reaction to the velocity of the pore-fluid flow in the hydrothermal system to be considered, there exists a retardation zone, in which the conventional static theory in geochemistry does not hold true. Since this issue is often overlooked by some purely numerical modellers, it is emphasized in this paper. The related results from a typical rock alteration and metamorphic problem in a hydrothermal system have shown not only the detailed rock alteration and metamorphic process, but also the size of the retardation zone in the hydrothermal system. Copyright (C) 2001 John Wiley & Sons, Ltd.

A director theory for visco-elastic folding instabilities in multilayered rock

A model for finely layered visco-elastic rock proposed by us in previous papers is revisited and generalized to include couple stresses. We begin with an outline of the governing equations for the standard continuum case and apply a computational simulation scheme suitable for problems involving very large deformations. We then consider buckling instabilities in a finite, rectangular domain. Embedded within this domain, parallel to the longer dimension we consider a stiff, layered beam under compression. We analyse folding up to 40% shortening. The standard continuum solution becomes unstable for extreme values of the shear/normal viscosity ratio. The instability is a consequence of the neglect of the bending stiffness/viscosity in the standard continuum model. We suggest considering these effects within the framework of a couple stress theory. Couple stress theories involve second order spatial derivatives of the velocities/displacements in the virtual work principle. To avoid C-1 continuity in the finite element formulation we introduce the spin of the cross sections of the individual layers as an independent variable and enforce equality to the spin of the unit normal vector to the layers (-the director of the layer system-) by means of a penalty method. We illustrate the convergence of the penalty method by means of numerical solutions of simple shears of an infinite layer for increasing values of the penalty parameter. For the shear problem we present solutions assuming that the internal layering is oriented orthogonal to the surfaces of the shear layer initially. For high values of the ratio of the normal-to the shear viscosity the deformation concentrates in thin bands around to the layer surfaces. The effect of couple stresses on the evolution of folds in layered structures is also investigated. (C) 2002 Elsevier Science Ltd. All rights reserved.

Large amplitude folding in finely layered viscoelastic rock structures

We analyze folding phenomena in finely layered viscoelastic rock. Fine is meant in the sense that the thickness of each layer is considerably smaller than characteristic structural dimensions. For this purpose we derive constitutive relations and apply a computational simulation scheme (a finite-element based particle advection scheme; see MORESI et al., 2001) suitable for problems involving very large deformations of layered viscous and viscoelastic rocks. An algorithm for the time integration of the governing equations as well as details of the finite-element implementation is also given. We then consider buckling instabilities in a finite, rectangular domain. Embedded within this domain, parallel to the longer dimension we consider a stiff, layered plate. The domain is compressed along the layer axis by prescribing velocities along the sides. First, for the viscous limit we consider the response to a series of harmonic perturbations of the director orientation. The Fourier spectra of the initial folding velocity are compared for different viscosity ratios. Turning to the nonlinear regime we analyze viscoelastic folding histories up to 40% shortening. The effect of layering manifests itself in that appreciable buckling instabilities are obtained at much lower viscosity ratios (1:10) as is required for the buckling of isotropic plates (1:500). The wavelength induced by the initial harmonic perturbation of the director orientation seems to be persistent. In the section of the parameter space considered here elasticity seems to delay or inhibit the occurrence of a second, larger wavelength. Finally, in a linear instability analysis we undertake a brief excursion into the potential role of couple stresses on the folding process. The linear instability analysis also provides insight into the expected modes of deformation at the onset of instability, and the different regimes of behavior one might expect to observe.

Environmental influence on the stress corrosion cracking of rock bolts

In order to understand rock bolt Stress Corrosion Cracking (SCC), a series of experiments have been performed in Linearly Increasing Stress Test (LIST) apparatus. One series of experiments determined the threshold stress of various bolt metallurgies (900 MPa for Steel A, and 800 MPa for Steel B and C). The high values of threshold stress suggest that SCC begins in rock bolts when they are sheared by moving rock strata. Typical crack velocity values have been measured to be 2.5 x 10(-8) m s(-1), indicating that there is not much benefit for rock bolt steel of higher fracture toughness. Another series of experiments were performed to understand the environmental conditions causing SCC of steel A and galvanised Steel A rock bolt steel. SCC only occurred for environmental conditions for which produce hydrogen on the sample surface, leading to hydrogen embrittlement and SCC. Fracture surfaces of LIST samples failed by SCC were found to display the same fracture regions as fracture surfaces of rock bolts failed in service by SCC: Tearing Topography Surface (TTS), Corrugated Irregular Surface (CIS), quasi Micro Void Coalescence (qMVC) and Fast Fracture Surface (FFS). Water chemistry analysis were carried out on samples collected from various Australian mines in order to compare laboratory electrolyte conditions to those found in underground mines.

Fragmentation of a Neotropical migratory fish population by a century-old dam

Loss of connectivity in impounded rivers is among the impacts imposed by dams, and mitigation measures such as fish passages might not accomplish their purpose of reestablishing an efficient bi-directional gene flow in the fish populations affected. As a consequence, fish populations remain fragmented, and a new interpopulational structure may develop, with increased risk of reduced genetic diversity and stochastic extinction. In order to evaluate the effects of the Gavio Peixoto Dam, which was constructed almost a century ago on the Jacar,-Gua double dagger u River in the Upper Parana River basin, Brazil, a comparative morphometric study was undertaken on the populations of the Neotropical migratory characid fish Salminus hilarii living up- and downstream of this dam. Population dynamics, spatial segregation, and habitat use by different age classes were monitored for 2 years. We found that segregation caused by the dam and long periods with no efficient connection by fish passages have led to fragmentation and interpopulational structuring of S. hilarii, as revealed by canonical variable analysis of morphometric features. The fish populations occupying the up- and downstream sections have succeeded in performing short-distance reproductive migrations in the main river and tributaries, have found suitable habitats for completing their life cycle, and have been able to maintain distinct small-sized populations so far.

Program sustainability of a community-based intervention to prevent falls among older Australians

Multi-strategy interventions have been demonstrated to prevent falls among older people, but studies have not explored their sustainability. This paper investigates program sustainability of Stay on Your Feet (SOYF), an Australian multi-strategy falls prevention program (1992-1996) that achieved a significant reduction in falls-related hospital admissions. A series of surveys assessed recall, involvement and current falls prevention activities, 5 years post-SOYF in multiple original SOYF stakeholder groups within the study area [general practitioners (GPs), pharmacists, community health (CH) staff shire councils (SCs) and access committees (ACs)]. Focus groups explored possible behavioural changes in the target group. Surveys were mailed, except to CH staff and ACs. who participated in guided group sessions and were contacted via the telephone, respectively. Response rates were: GPs. 67% (139/209); pharmacists, 79% (53/67); CH staff, 63% (129/204); SCs, 90% (9/10); ACs, 80% (8/10). There were 73 older people in eight focus groups. Of 117 GPs who were practising during SOYF 80% recalled SOYF and 74% of these reported an influence on their practice. Of 46 pharmacists operating a business during SOYF, 45% had heard of SOYF and 79% of these reported being 'somewhat' influenced. Of 76 community health staff (59%) in the area at that time, 99% had heard of SOYF and 82% reported involvement. Four SCs retained a SOYF resource, but none thought current activities were related. Seven ACs reported involvement, but no activities were sustained. Thirty-five focus group participants (48%) remembered SOYF and reported a variety of SOYF-initiated behaviour changes. Program sustainability was clearly demonstrated among health practitioners. Further research is required to assess long-term effect sustainability.

Falls prevention in rural general practice: what stands the test of time and where to from here?

Objective: General practitioner recall of the 1992-96 'Stay on Your Feet'(SOYF) program and its influence on practice were surveyed five years post-intervention to gauge sustainability of the SOYF General Practice (GP) component. Methods: A survey assessed which SOYF components were still in existence, current practice related to falls prevention, and interest in professional development. All general practitioners (GPs) situated within the boundaries of a rural Area Health Service were mailed a survey in late 2001. Results: Response rate was 66.5% (139/ 209). Of 117 GPs in practice at the time of SOYF, 80.2% reported having heard of SOYF and 74.4% of those felt it had influenced practice. Half (50.9%) still had a copy of the SOYF GP resource and of those, 58.6% used it at least 'occasionally'. Three-quarters of GPs surveyed (75.2%) checked medications 'most/almost all' of the time with patients over 60 years; 46.7% assessed falls risk factors; 41.3% gave advice; and 22.6% referred to allied health practitioners. GPs indicated a strong interest in falls prevention- related professional development. There was no significant association between use of the SOYF resource package and any of the current falls prevention practices (all chi(2)>0.05). Conclusions and implications: There was high recall of SOYF and a general belief that it influenced practice. There was little indication that use of the resource had any lasting influence on GPs' practices. In future, careful thought needs to go into designing a program that has potential to affect long-term change in GPs' falls prevention practice.

Material influence on the stress corrosion cracking of rock bolts

Rock bolt stress corrosion cracking (SCC) has been investigated using the linearly increasing stress test (LIST). One series of experiments determined the threshold stress of various bolt metallurgies (900 MPa for 1355AXRC, and 800 MPa for MAC and MA840B steels). The high values of threshold stress suggest that SCC begins in rock bolts when they are sheared by moving rock strata. SCC only occurred for environmental conditions which produce hydrogen on the sample surface, leading to hydrogen embrittlement and SCC. Different threshold potentials were determined for a range of metallurgies. Cold work was shown to increase the resistance of the steel to SCC. Rock bolt rib geometry does not have a direct impact on the SCC resistance properties of the bolt, although the process by which the ribs are produced can introduce tensile stresses into the bolt which lower its resistance to SCC. (C) 2004 Elsevier Ltd. All rights reserved.

Accumulation and Gaps in the Research on Argentine Rock

Based on a comprehensive study in research centers and libraries, a panorama of the critical response to Argentine rock in academic circles has been traced in various fields of knowledge. Research works including books, trade magazines, and academic papers on this musical movement spanning four decades have been consulted. The results show that the criticism has generated a ""developing tradition"" around the movement, mainly in the Social and Communication Sciences, and it has presented the following predominant traits: approaching rock essentially as a determinant of ""social identities""; periodization in the genre according to political regime: and analysis of the lyrics based solely on content. These traits contribute to a delimitation of rock as an exclusive phenomenon of the mass media, which relegates to secondary importance its aesthetic function and its relationship with other artistic series. Furthermore, we have observed a scarcity of approaches to rock lyrics as a linguistic-discursive surface.

Numerical simulation of double-diffusion driven convective flow and rock alteration in three-dimensional fluid-saturated geological fault zones

Numerical methods are used to simulate the double-diffusion driven convective pore-fluid flow and rock alteration in three-dimensional fluid-saturated geological fault zones. The double diffusion is caused by a combination of both the positive upward temperature gradient and the positive downward salinity concentration gradient within a three-dimensional fluid-saturated geological fault zone, which is assumed to be more permeable than its surrounding rocks. In order to ensure the physical meaningfulness of the obtained numerical solutions, the numerical method used in this study is validated by a benchmark problem, for which the analytical solution to the critical Rayleigh number of the system is available. The theoretical value of the critical Rayleigh number of a three-dimensional fluid-saturated geological fault zone system can be used to judge whether or not the double-diffusion driven convective pore-fluid flow can take place within the system. After the possibility of triggering the double-diffusion driven convective pore-fluid flow is theoretically validated for the numerical model of a three-dimensional fluid-saturated geological fault zone system, the corresponding numerical solutions for the convective flow and temperature are directly coupled with a geochemical system. Through the numerical simulation of the coupled system between the convective fluid flow, heat transfer, mass transport and chemical reactions, we have investigated the effect of the double-diffusion driven convective pore-fluid flow on the rock alteration, which is the direct consequence of mineral redistribution due to its dissolution, transportation and precipitation, within the three-dimensional fluid-saturated geological fault zone system. (c) 2005 Elsevier B.V. All rights reserved.

Timing of renal replacement therapy and clinical outcomes in critically ill patients with severe acute kidney injury

Purpose: The aim of this study is to evaluate the relationship between timing of renal replacement therapy (RRT) in severe acute kidney injury and clinical outcomes. Methods: This was a prospective multicenter observational study conducted at 54 intensive care units (ICUs) in 23 countries enrolling 1238 patients. Results: Timing of RRT was stratified into ""early"" and ""late"" by median urea and creatinine at the time RRT was started. Timing was also categorized temporally from ICU admission into early (<2 days), delayed (2-5 days), and late (>5 days). Renal replacement therapy timing by serum urea showed no significant difference in crude (63.4% for urea <= 24.2 mmol/L vs 61.4% for urea >24.2 mmol/L; odds ratio [OR], 0.92; 95% confidence interval [CI], 0.73-1.15; P = .48) or covariate-adjusted mortality (OR, 1.25; 95% CI, 0.91-1.70; P = .16). When stratified by creatinine, late RRT was associated with lower crude (53.4% for creatinine >309 mu mol/L vs 71.4% for creatinine <= 309 mu mol/L; OR, 0.46; 95% CI, 0.36-0.58; P < .0001) and covariate-adjusted mortality (OR, 0.51; 95% CI, 0.37-0.69; P < .001).However, for timing relative to ICU admission, late RRT was associated with greater crude (72.8% vs 62.3% vs 59%, P < .001) and covariate-adjusted mortality (OR, 1.95; 95% CI, 1.30-2.92; P = .001). Overall, late RRT was associated with a longer duration of RRT and stay in hospital and greater dialysis dependence. Conclusion: Timing of RRT, a potentially modifiable factor, might exert an important influence on patient survival. However, this largely depended on its definition. Late RRT (days from admission) was associated with a longer duration of RRT, longer hospital stay, and higher dialysis dependence. (C) 2009 Elsevier Inc. All rights reserved.

Arterial pulse pressure variation predicting fluid responsiveness in critically ill patients (Retracted article. See vol. 31, pg. 542, 2009)

In critically ill patients, it is important to predict which patients will have their systemic blood flow increased in response to volume expansion to avoid undesired hypovolemia and fluid overloading. Static parameters such as the central venous pressure, the pulmonary arterial occlusion pressure, and the left ventricular end-diastolic dimension cannot accurately discriminate between responders and nonresponders to a fluid challenge. In this regard, respiratory-induced changes in arterial pulse pressure have been demonstrated to accurately predict preload responsiveness in mechanically ventilated patients. Some experimental and clinical studies confirm the usefulness of arterial pulse pressure as a useful tool to guide fluid therapy in critically ill patients.

A randomized controlled trial comparing a computer-assisted insulin infusion protocol with a strict and a conventional protocol for glucose control in critically ill patients

Purpose: The objective of this study is to evaluate blood glucose (BG) control efficacy and safety of 3 insulin protocols in medical intensive care unit (MICU) patients. Methods: This was a multicenter randomized controlled trial involving 167 MICU patients with at least one BG measurement +/- 150 mg/dL and one or more of the following: mechanical ventilation, systemic inflammatory response syndrome, trauma, or burns. The interventions were computer-assisted insulin protocol (CAIP), with insulin infusion maintaining BG between 100 and 130 mg/dL; Leuven protocol, with insulin maintaining BG between 80 and 110 mg/dL; or conventional treatment-subcutaneous insulin if glucose > 150 mg/dL. The main efficacy outcome was the mean of patients` median BG, and the safety outcome was the incidence of hypoglycemia (<= 40 mg/dL). Results: The mean of patients` median BG was 125.0, 127.1, and 158.5 mg/dL for CAIP, Leuven, and conventional treatment, respectively (P = .34, CAIP vs Leuven; P < .001, CAIP vs conventional). In CAIP, 12 patients (21.4%) had at least one episode of hypoglycemia vs 24 (41.4%) in Leuven and 2 (3.8%) in conventional treatment (P = .02, CAIP vs Leuven; P = .006, CAIP vs conventional). Conclusions: The CAIP is safer than and as effective as the standard strict protocol for controlling glucose in MICU patients. Hypoglycemia was rare under conventional treatment. However, BG levels were higher than with IV insulin protocols. (C) 2009 Elsevier Inc. All rights reserved.