First-order doubly-asymptotic formulation of the direct stiffness method for elastodynamic problems

A first-order formulation to analyze the dynamic response of layered soil profiles is presented as an alternative to the widely used second-order thin-layer method by the direct stiffness approach, including an efficient simulation of the underlaying elastic half-space. In contrast to the thin-layer method where response is expressed through a combination of second-order propagation modes, the proposed procedure uses first-order modal parameters that have the capacity to provide a good approximation in the complete wave number domain k, including the exact stiffness values for k=0 and k→∞, thus justifying its designation of doubly-asymptotic. This feature allows obtaining the exact soil profile response for static loads, while the proposed treatment of the elastic half-space reproduces naturally the radiation condition without a need of artificial damping. The capacity of the proposed formulation to solve elastodynamic problems is assessed by comparing its results with those of exact solutions available in the literature, and numerical solutions of rigid disks supported on the surface of different soil profiles.     

Experimental study of a stiff wave barrier in gelatine

Railway induced vibrations and re-radiated noise in buildings can be mitigated by means of wave barriers in the soil. Numerical simulations demonstrate that a stiff wave barrier, consisting of a material that is stiffer than the surrounding medium, can be very effective if the stiffness contrast between the barrier and the medium is sufficiently large. This paper presents results of a lab experiment that has been carried out to validate these findings, using gelatine instead of soil in order to reduce the wavelengths and thus the scale of the test setup. An expanded polystyrene beam is employed as wave barrier, while a non-contact measurement procedure is applied for visualizing the waves in the gelatine, based on reflections of a grid of laser rays. The experimental results are found to be in line with the numerical predictions, confirming the vibration reduction effectiveness of stiff wave barriers.     

New paleomagnetic data from Upper Oligocene–Lower Miocene rocks of the Northern Argentine Puna–Southern Bolivian Altiplano: Constraining the age of vertical axis rotations

Along the Central Andes a pattern of vertical axis rotations has been paleomagnetically identified. Such rotations are counterclockwise north of Arica Deflection (∼19° S) and clockwise to the south. Different hypothesis and models have been proposed to explain the Central Andean Rotation Pattern (CARP). However, the origin of the CARP is a subject of ongoing debate. Recently, different authors have proposed the possible existence of a close correlation between the time–space distribution of deformation and the amount of registered vertical axis rotations in the Southern Central Andes. In order to further investigate such relationship, new paleomagnetic studies were carried out in Upper Oligocene–Lower Miocene rocks of the Northern Argentine Puna and the Southern Bolivian Altiplano. Our results indicate that while one of the sampled localities did not undergo significant vertical axis rotations, the other two recorded clockwise vertical axis rotations larger than 30°. These results suggest the occurrence of small-block rotations in the Southern Bolivian Altiplano–Northern Argentine Puna prior to 15 Ma, which would correspond to the local accommodation of the regional deformation field.     

Hydrological impact and potential flooding of convective rain cells in a semi-arid environment

Abstract

t Spatio-temporal storm properties have a large impact on catchment hydrological response. The sensitivity of simulated flash floods to convective rain-cell characteristics is examined for an extreme storm event over a 94 km² semi-arid catchment in southern Israel. High space–time resolution weather radar data were used to derive and model convective rain cells that then served as input into a hydrological model. Based on alterations of location, direction and speed of a major rain cell, identified as the flooding cell for this case, the impacts on catchment rainfall and generated flood were examined. Global sensitivity analysis was applied to identify the most important factors affecting the flash flood peak discharge at the catchment outlet. We found that the flood peak discharge could be increased three-fold by relatively small changes in rain-cell characteristics. We assessed that the maximum flash flood magnitude that this single rain cell can produce is 175 m³/s, and, taking into account the rest of the rain cells, the flash flood peak discharge can reach 260 m³/s.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Morin, E. and Yakir, H., 2013. Hydrological impact and potential flooding of convective rain cells in a semi-arid environment. Hydrological Sciences Journal, 59 (7), 1275–1284. http://dx.doi.org/10.1080/02626667.2013.841315     

Epistemic uncertainty in on-site earthquake early warning on the use of PGV–PD3 empirical models

From the literature, we found that PGV–PD3 regressions for on-site earthquake early warning (EEW) can be quite different depending on the presumption whether or not PGV–PD3 data from different regions should be “mixable” in regression analyses. As a result, this becomes a source of epistemic uncertainty in the selection of a PGV–PD3 empirical relationship for on-site EEW. This study is aimed at examining the influence of this epistemic uncertainty on EEW decision-making, and demonstrating it with an example on the use of PGV–PD3 models developed with data from Taiwan, Japan, and Southern California. The analysis shows that using the “global” PGV–PD3 relationship for Southern California would accompany a more conservative EEW decision-making (i.e., early warning is activated more frequently) than using the local empirical model developed with the PGV–PD3 data from Southern California only. However, the influence of this epistemic uncertainty on EEW is not that obvious for the cases of Taiwan and Japan.     

Dynamic analysis of a rigid circular foundation on a transversely isotropic half-space under a buried inclined time-harmonic load

The dynamic analysis of a surface rigid foundation in smooth contact with a transversely isotropic half-space under a buried inclined time-harmonic load is addressed. By virtue of the superposition technique, appropriate Green׳s functions, and employing further mathematical techniques, solution of the mixed-boundary-value problem is expressed in terms of two well-known Fredholm integral equations. Two limiting cases of the problem corresponding to the static loading and isotropic medium are considered and the available results in the literature are fully recovered. For the static case, the results pertinent to both frictionless and bonded contacts are obtained and compared. With the aid of the residue theorem and asymptotic decomposition method, an effective and robust approach is proposed for the numerical evaluation of the obtained semi-infinite integrals. For a wide range of the excitation frequency, both normal and rotational compliances are depicted in dimensionless plots for different transversely isotropic materials. Based on the obtained results, the effects of anisotropy are highlighted and discussed.     

Dynamic analysis of flexible cantilever wall retaining elastic soil by a modified Vlasov–Leontiev model

Dynamic response of a flexible cantilever wall retaining elastic soil to harmonic transverse seismic excitations is determined with the aid of a modified Vlasov–Leontiev foundation model and on the assumption of vanishing vertical displacement of the soil medium. The soil–wall interaction is taken into consideration in the presented model. The governing equations and boundary conditions of the two unknown coupled functions in the model are derived in terms of Hamilton׳s principle. Solutions of the two unknown functions are obtained on the basis of an iterative algorithm. The present method is verified by comparing its results with those of the existing analytical solution. Moreover, a mechanical model is proposed to evaluate the presented method physically. A parametric study is performed to investigate the effects of the soil–wall system properties and the excitations on the dynamic response of the wall.     

Effects of deep excavation on seismic vulnerability of existing reinforced concrete framed structures

In this paper the effects of deep excavation on seismic vulnerability of existing buildings are investigated. It is well known that deep excavations induce significant changes both in stress and strain fields of the soil around them, causing a displacement field which can modify both the static and dynamic responses of existing buildings. A FEM model of a real case study, which takes into account geometry, non-linear soil behavior, live and dead loads, boundary conditions and soil–structure interaction, has been developed in order to estimate the soil displacements and their effects on seismic behavior of a reinforced concrete framed system close to deep excavation. Considering a significant accelerometric seismic input, the non-linear dynamic responses of the reinforced concrete framed structure, both in the pre and post-excavation configurations, have been evaluated and, then, compared to estimate the modification in seismic vulnerability, by means of different seismic damage indices and inter-story drifts.     

Inertial effects during irreversible meniscus reconfiguration in angular pores

In porous media, the dynamics of the invading front between two immiscible fluids is often characterized by abrupt reconfigurations caused by local instabilities of the interface. As a prototype of these phenomena we consider the dynamics of a meniscus in a corner as it can be encountered in angular pores. We investigate this process in detail by means of direct numerical simulations that solve the Navier–Stokes equations in the pore space and employ the Volume of Fluid method (VOF) to track the evolution of the interface. We show that for a quasi-static displacement, the numerically calculated surface energy agrees well with the analytical solutions that we have derived for pores with circular and square cross sections. However, the spontaneous reconfigurations are irreversible and cannot be controlled by the injection rate: they are characterized by the amount of surface energy that is spontaneously released and transformed into kinetic energy. The resulting local velocities can be orders of magnitude larger than the injection velocity and they induce damped oscillations of the interface that possess their own time scales and depend only on fluid properties and pore geometry. In complex media (we consider a network of cubic pores) reconfigurations are so frequent and oscillations last long enough that increasing inertial effects leads to a different fluid distribution by influencing the selection of the next pore to be invaded. This calls into question simple pore-filling rules based only on capillary forces. Also, we demonstrate that inertial effects during irreversible reconfigurations can influence the work done by the external forces that is related to the pressure drop in Darcy’s law. This suggests that these phenomena have to be considered when upscaling multiphase flow because local oscillations of the menisci affect macroscopic quantities and modify the constitutive relationships to be used in macro-scale models. These results can be extrapolated to other interface instabilities that are at the origin of fast pore-scale events, such as Haines jumps, snap-off and coalescence.