Nonlinear seismic response of tall concrete-faced rockfill dams in narrow canyons

The seismic behavior of tall concrete face rockfill dams in narrow canyons is investigated, based on numerical simulation of the staged construction, creep settlements, reservoir impoundment and seismic shaking of the dam. The study takes into account the flexibility of the canyon rock, the hydrodynamic effects and potential dynamic rockfill settlements. The static analysis uses a hyperbolic model for the rockfill, whereas the dynamic analysis uses a nonlinear hysteretic model, which accounts for the initial dynamic stiffness and produces hysteresis loops in agreement with the experimental data regarding the shear modulus and damping ratio. A damage plasticity model is used for the reinforced concrete, whereas frictional contact behavior is considered at the base and vertical walls of the concrete slab panels. An existing 150-m-high dam is used to investigate some key issues on the seismic behavior of such dams subjected to upstream−downstream and vertical excitation. Emphasis is placed on the evaluation of the tensile stresses within the slab panels, the compressive stresses at the slab-to-slab vertical interfaces and the opening of the joints. Moreover, the effect of potential dynamic settlements on both the slab stresses and joint openings is investigated. Recommendations for increasing the dam safety and reducing the water leakage through the dam body are given.     

On the 3D dynamics and morphology of inner rings

We argue that inner rings in barred spiral galaxies are associated with specific 2D and 3D families of periodic orbits located just beyond the end of the bar. These are families located between the inner radial ultraharmonic 4 : 1 resonance and corotation. They are found in the upper part of a type-2 gap of the x1 characteristic, and can account for the observed ring morphologies without any help from families of the x1-tree. Due to the evolution of the stability of all these families, the ring shapes that are favoured are mainly ovals, as well as polygons with 'corners' on the minor axis, on the sides of the bar. On the other hand, pentagonal rings, or rings of the NGC 7020-type hexagon, should be less probable. The orbits that make the rings belong in their vast majority to 3D families of periodic orbits and orbits trapped around them.     

The influence of pumping parameters in fluid‐driven fractures in weak porous formations

In this work, we investigate the main pumping parameters that influence a fluid‐driven fracture in cohesive poroelastic and poroelastoplastic weak formations. These parameters include the fluid viscosity and the injection rate. The first parameter dominates in the mapping of the propagation regimes from toughness to viscosity, whereas the second parameter controls the storage to leak‐off dominated regime through diffusion. The fracture is driven in weak permeable porous formation by injecting an incompressible viscous fluid at the fracture inlet assuming that the fracture propagates under plane strain conditions. Fluid flow in the fracture is modeled by lubrication theory. Pore fluid movement in the porous formation is based on the Darcy law. The coupling follows the Biot theory, whereas the irreversible rock deformation is modeled with the Mohr–Coulomb yield criterion with associative flow rule. Fracture propagation criterion is based on the cohesive zone approach. Leak‐off is also considered. The investigation is performed numerically with the FEM to obtain the fracture opening, length, and propagation pressure versus time. We demonstrate that pumping parameters influence the fracture geometry and fluid pressures in weak formations through the viscous fluid flow and the diffusion process that create back stresses and large plastic zones as the fracture propagates. It is also shown that the product of the propagation velocity and fluid viscosity, µv that appears in the scaling controls the magnitude of the plastic zones and influences the net pressure and fracture geometry. These findings may explain partially the discrepancies in net pressures between field measurements and conventional model predictions for the case of weak porous formation. Copyright © 2014 John Wiley & Sons, Ltd.     

The Impact of Arctic Ozone Depletion on Northern Middle Latitudes: Interannual Variability and Dynamical Control

We have investigated the effect of the export of Arctic ozone loss, or`dilution', on mid-latitude ozone depletion during the 1990s, and its relation tointerannual meteorological variability. A stratospheric chemical-transport modelincorporated a simple gas-phase ozone scheme with the addition of a parameterisation ofpolar depletion which depended only on temperature and duration of sunlight. Themodel was forced with the U.K. Meteorological Office analyses from 1991 to 1999 covering eight Northern Hemisphere winters. The modelled Arctic ozone column losses wereabout half the magnitude of those in the Antarctic and showed a considerablevariation from year to year. The northern middle latitudes (40°–60° N)were mainly affected through dilution and experienced a variable 5–20%depletion. Year-round there is a depletion of about 1% in northern middle latitudes due toactivation at the pole but there is no evidence that this depletion increases with timeduring this integration. A series of inert tracer experiments for the winters from 1996 to 1999 showed that the dilution occurs primarily at the 560 K and 465 K isentropic levels where up to 30% of the airoriginating northward of 67° N on 1 March is found at 47° N later in spring. Thestrength and persistence of the Arctic vortex were crucial in determining the severity and the timing of the ozone dilution every year by influencing, respectively, the magnitude of the high-latitude depletion and the effectiveness of mixing to lower latitudes. This spring dilution was correlated with the winter/spring planetary wave activity indicating the important role of dynamical processes in regulating the polar-driven mid-latitude ozone depletion.     

High resolution spatiotemporal analysis of erosion risk per land cover category in Korçe region,Albania

Some recent land use changes in Albania, such as deforestation, cropland abandonment, and urban sprawl, have caused serious increase of erosion risk. The main objective of this study was to map erosion risk in Korçe region and assess the degree at which every land use is concerned. The G2 erosion model was applied, which can provide erosion maps and statistical figures at month-time intervals using input from free European and global geodatabases. The mapping results in Korçe region were derived at a 30-m cell size, which is an innovation for G2. Autumn-winter months were found to be the most erosive, with average erosion rates reaching the maximum in November and December, i.e. 2.62 and 2.36 t/ha, respectively, while the annual rate was estimated at 10.25 t/ha/yr. Natural grasslands, shurblands, mixed forests, and vineyards showed to exhibit the highest mean erosion rates, while shrublands, broad-leaved forests and natural grasslands were found to be the most extended land covers risky for non-sustainable erosion rates (i.e. >10 t/ha/yr). A detailed examination of the detected hot spots is now necessary by the competent authorities, in order to apply appropriate, site-specific conservation measures. Notably, use of SPOT VGT data did not prevent the maps from having extended gaps due to cloudiness. Sentinel-2 time series, freely available by the European Space Agency (ESA), have the potential to improve spatiotemporal coverage of V-factor, thus further empowering the G2 model, in the near future.     

Evaluation of a spatial rainfall generator for generating high resolution precipitation projections over orographically complex terrain

Space–time variability of precipitation plays a key role as driver of many environmental processes. The objective of this study is to evaluate a spatiotemporal (STG) Neyman–Scott Rectangular Pulses (NSRP) generator over orographically complex terrain for statistical downscaling of climate models. Data from 145 rain gauges over a 5760-km² area of Cyprus for 1980–2010 were used for this study. The STG was evaluated for its capacity to reproduce basic rainfall statistical properties, spatial intermittency, and extremes. The results were compared with a multi-single site NRSP generator (MSG). The STG performed well in terms of average annual rainfall (+1.5 % in comparison with the 1980–2010 observations), but does not capture spatial intermittency over the study area and extremes well. Daily events above 50 mm were underestimated by 61 %. The MSG produced a similar error (+1.1 %) in terms of average annual rainfall, while the daily extremes (>50-mm) were underestimated by 11 %. A gridding scheme based on scaling coefficients was used to interpolate the MSG data. Projections of three Regional Climate Models, downscaled by MSG, indicate a 1.5–12 % decrease in the mean annual rainfall over Cyprus for 2020–2050. Furthermore, the number of extremes (>50-mm) for the 145 stations is projected to change between ?24 and +2 % for the three models. The MSG modelling approach maintained the daily rainfall statistics at all grid cells, but cannot create spatially consistent daily precipitation maps, limiting its application to spatially disconnected applications. Further research is needed for the development of spatial non-stationary NRSP models.     

A method for reliability-based optimization with multiple non-normal stochastic parameters: a simplified airshed management study

We develop methodologies to enable applications of reliability-based design optimization (RBDO) to environmental policy setting problems. RBDO considers uncertainty as random variables and parameters in an optimization framework with probabilistic constraints. Three challenges in environmental decision-making problems not addressed by current RBDO methods are efficient methods in handling: (1) non-normally distributed random parameters, (2) discrete random parameters, and (3) joint reliability constraints (e.g., meeting constraints simultaneously with a single reliability). We propose a modified sequential quadratic programming algorithm to address these challenges. An active set strategy is combined with a reliability contour formulation to solve problems with multiple non-normal random parameters. The reliability contour formulation can also handle discrete random parameters by converting them to equivalent continuous ones. Joint reliability constraints are estimated by their theoretical upper bounds using reliability indexes and angles of normal vectors between active constraints. To demonstrate the methods, we consider a simplified airshed example where CO and NOx standards are violated and are brought into compliance by reducing the speed limits of two nearby highways. This analytical example is based on the CALINE4 model. Results show the potential of this approach to handle complex large-scale environmental regulation problems.     

Finite element analysis of buried steel pipelines under strike-slip fault displacements

The present paper investigates the mechanical behavior of buried steel pipelines, crossing an active strike-slip tectonic fault. The fault is normal to the pipeline direction and moves in the horizontal direction, causing stress and deformation in the pipeline. The interacting soil–pipeline system is modelled rigorously through finite elements, which account for large strains and displacements, nonlinear material behavior and special conditions of contact and friction on the soil–pipe interface. Considering steel pipelines of various diameter-to-thickness ratios, and typical steel material for pipeline applications (API 5L grades X65 and X80), the paper focuses on the effects of various soil and pipeline parameters on the structural response of the pipe, with particular emphasis on identifying pipeline failure (pipe wall wrinkling/local buckling or rupture). The effects of shear soil strength, soil stiffness, horizontal fault displacement, width of the fault slip zone are investigated. Furthermore, the influence of internal pressure on the structural response is examined. The results from the present investigation are aimed at determining the fault displacement at which the pipeline fails and can be used for pipeline design purposes. The results are presented in diagram form, which depicts the critical fault displacement, and the corresponding critical strain versus the pipe diameter-to-thickness ratio. A simplified analytical model is also developed to illustrate the counteracting effects of bending and axial stretching. The numerical results for the critical strain are also compared with the recent provisions of EN 1998-4 and ASCE MOP 119.     

Behavior of cemented sands—I. Testing

This paper is accompanied by a study on constitutive modelling issues of cemented sands. The concentration here is on experimental issues related to the triaxial testing of cemented sands. A preliminary investigation is performed aiming to identify potential effects of specimen size and slenderness on the stress–strain–strength characteristics of cemented sands. A comprehensive experimental study follows where clean sand specimens, as well as specimens with 2, 4 and 6 per cent cement content, are tested. The aim of the study is to examine the effects of cement content and confinement on the shear strength, stiffness, softening and dilation characteristics of cemented sand. © 1997 John Wiley & Sons, Ltd.