Seismic tomography imaging of an unstable embankment

Seismic tomography imaging was employed to make a diagnosis and choose a remedy for an embankment supported by a retaining wall showing clear evidence of structural instability. The geometry and structural characteristics of the wall, the inside geometry of cracks, and the physical parameters of the underground materials were the primary objectives of the geophysical survey. Seismic data were acquired along two vertical sections each one delimited by the lines of the sources within an inclined borehole and of the receivers lying on the ground. For each section, a total of 744 travel times were inverted to obtain compressional-wave velocities on a regular rectangular grid of squared cells (1 m × 1 m) using an inversion algorithm which is based on the simultaneous iterative reconstruction technique (SIRT). Ultrasonic tests carried out in laboratory on intact specimens, together with other supplemental site information, improved tomography resolution, allowing global and node constraints that forced the solution of the inverse problem to match known boundary values. Tomography imaged two high-velocity zones separated by a curved-shape low-velocity zone. The first ones were found to be compatible with the concrete retaining wall and with the schist materials, ranging from highly weathered to intact schist, while the low-velocity zone was interpreted as filling materials and/or completely decomposed rock schist. The combination of seismic tomography and laboratory measurements allowed extrapolation of important parameters over a large volume of rock mass, otherwise only representative of small rock samples near boreholes. It also enabled engineering characterization of subsurface rock mass, providing useful and accurate information to design a remedy for the embankment.     

An ANN algorithm for automatic, real-time tsunami detection in deep-sea level measurements

The present paper looks at algorithms to be implemented in the software of bottom pressure recorders (BPRs) for the automatic, real-time detection of a tsunami within recorded signals. The structure of an algorithm based on the use of an artificial neural network (ANN) is presented and compared to the one developed under the Deep-ocean Assessment and Reporting of Tsunamis (DART) program run by the U.S. National Oceanic and Atmospheric Administration (NOAA). The performance and efficiency of the two algorithms are compared using both synthetic and actually measured time series. Results show that an improvement in detection performance can be obtained by using the ANN algorithm.     

Bed morphology and generation of step–pool channels

Flume experiments have been carried out to study the formation processes and the bed morphology of step–pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step–pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sand‐bed rivers. This observation points out that step–pool field‐data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand‐ and gravel‐bed rivers or step–pool units in boulder‐bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self‐stabilization by modifying the step–pool structure towards a geometry that provides maximum flow resistance and maximum bed stability. Copyright © 2008 John Wiley & Sons, Ltd.     

Design of passive systems for control of inelastic structures

A design strategy for control of buildings experiencing inelastic deformations during seismic response is formulated. The strategy is using weakened, and/or softened, elements in a structural system while adding passive energy dissipation devices (e.g. viscous fluid devices, etc.) in order to control simultaneously accelerations and deformations response during seismic events. A design methodology is developed to determine the locations and the magnitude of weakening and/or softening of structural elements and the added damping while insuring structural stability. A two‐stage design procedure is suggested: (i) first using a nonlinear active control algorithm, to determine the new structural parameters while insuring stability, then (ii) determine the properties of equivalent structural parameters of passive system, which can be implemented by removing or weakening some structural elements, or connections, and by addition of energy dissipation systems. Passive dampers and weakened elements are designed using an optimization algorithm to obtain a response as close as possible to an actively controlled system. A case study of a five‐story building subjected to El Centro ground motion, as well as to an ensemble of simulated ground motions, is presented to illustrate the procedure. The results show that following the design strategy, a control of both peak inter‐story drifts and total accelerations can be obtained. Copyright © 2008 John Wiley & Sons, Ltd.     

Conceptual seismic design in performance-based earthquake engineering

A principal aspect of seismic design is the verification of performance limit states, which help ensure satisfactory behaviour within a performance-based earthquake engineering framework. However, it is increasingly acknowledged that while ensuring life safety is a suitable basic design requirement, more meaningful metrics of seismic performance exist. Expected annual loss (EAL) has gained attention in recent years but tends to be limited to seismic assessment. This article proposes a novel conceptual design framework that employs EAL as a design tool and requires very little building information at the design outset. This means that designers may commence from a definition of required EAL and arrive at a number of feasible structural solutions without the need for any detailed design calculations or numerical analysis. This works by transforming the building performance definition to a design solution space using a number of simplifying assumptions. A suitable structural response backbone is subsequently determined and used to identify feasible building typologies and associated structural geometries. The assumptions made to implement such a conceptual design framework are discussed and justified herein followed by a case study application. This proposed design framework is intended to form the first step in seismic design to identify suitable typologies and layouts before subsequent member detailing and design verification. This way, engineers, architects, and clients can make more informed decisions that target certain performance goals at the beginning of design before further refinement.     

Differentiating past events on a cone influenced by debris‐flow and snow avalanche activity – a dendrogeomorphological approach

Dendrogeomorphology was used to investigate past events on a cone affected by both debris flows and snow avalanches. We report on results of 520 cores from 251 injured Larix decidua Mill. and Picea abies (L.) Karst. trees sampled on the Birchbach cone (Swiss Alps). Detailed analysis of tree‐ring sequences allowed dating of 561 growth disturbances in individual trees for a 252 yr period, extending from 1750 to 2002, which could be attributed to 30 different event years. We then localized the position of rows of traumatic resin ducts (TRDs) within the tree ring so as to assess the intra‐seasonal position of damage. In agreement with data on the local growth period, TRDs located at the beginning of the new growth ring were considered the result of avalanche impacts that occurred during the dormant season or in earliest earlywood between late October and early May. In contrast, TRD found in late earlywood or within latewood were considered the result of periglacial debris‐flow activity, as these layers of the tree ring are locally formed between July and early October. For nine out of the 30 reconstructed event years, the intra‐seasonal timing of TRDs indicated that reactions must be the result of past snow avalanche activity. In 19 other event years, TRDs showed that damage has been caused between July and early October and, thus, through debris flows in the Birchbach torrent. Finally, the spatial patterns of trees showing reactions as a result of particular events were assessed so as to approximate the extent of past debris flows and snow avalanches. Copyright © 2006 John Wiley & Sons, Ltd.     

Some SH-wave seismic reflections from depths of less than three metres

Shallow SH-wave reflections are far from routine, although their study can provide insights into important properties of near-surface materials that cannot be inferred from P-wave data alone. Difficulties in separating SH-wave reflections from Love waves are generally considered the major obstacle to progress in shallow SH-wave seismic reflection. This may be the case in surveys undertaken at great depths, but it is not necessarily true for reflection data gathered at shallow and ultra-shallow depths. This paper shows that when SH-wave data possess wavelengths greater than the thickness of the superficial layer, Love waves are not greatly dispersed. In this case, misinterpretation between parts of reflection hyperbolae and waveguide arrivals is sufficiently limited. In a one-layer model earth, which well approximates typical situations of the near-surface underground, the most energetic modes (the lowermost modes) of the dispersed surface waves have a dominant frequency band that falls below the wavelet spectrum of the shallow reflections; therefore, they can be filtered out in the frequency domain. Higher modes, although their spectral content overlaps that of the reflections, exhibit small amplitudes on seismograms and leave strong reflections unaffected.We present field examples from three different sites where we were able to obtain ultra-shallow reflections (< 3 m) in unconsolidated sediments. The high level of resolution (vertical resolution up to 15 cm) suggests that SH-wave reflection imaging has the potential to complement other high-resolution techniques, such as P-wave reflection and ground-penetrating radar (GPR) imaging, allowing a better and more complete characterization of the near-surface environments.     

Simplified seismic performance assessment and implications for seismic design

The last decade or so has seen the development of refined performance-based earthquake engineering (PBEE) approaches that now provide a framework for estimation of a range of important decision variables, such as repair costs, repair time and number of casualties. This paper reviews current tools for PBEE, including the PACT software, and examines the possibility of extending the innovative displacement-based assessment approach as a simplified structural analysis option for performance assessment. Details of the displacement-based s+eismic assessment method are reviewed and a simple means of quickly assessing multiple hazard levels is proposed. Furthermore, proposals for a simple definition of collapse fragility and relations between equivalent single-degree-of-freedom characteristics and multi-degree-of-freedom story drift and floor acceleration demands are discussed, highlighting needs for future research. To illustrate the potential of the methodology, performance measures obtained from the simplified method are compared with those computed using the results of incremental dynamic analyses within the PEER performance-based earthquake engineering framework, applied to a benchmark building. The comparison illustrates that the simplified method could be a very effective conceptual seismic design tool. The advantages and disadvantages of the simplified approach are discussed and potential implications of advanced seismic performance assessments for conceptual seismic design are highlighted through examination of different case study scenarios including different structural configurations.     

Including root reinforcement variability in a probabilistic 3D stability model

Forests play a significant role in protecting people, settlements in mountainous terrains from hydrogeomorphic hazards, including shallow landslides. Although several studies have investigated the interactions between forests and slope instabilities, a full understanding of them has not yet been obtained. Additionally, models that incorporate forest stand properties into slope failure probability analyses have not been developed. In principle, physical‐based models, which are powerful tools for landslide hazard analyses, represent an appropriate approach to linking stand properties and slope stability. However, the reliability of these models depends on numerous parameters that describe highly complex geotechnical and hydrological processes (e.g. potential failure depth, saturation ratio, root reinforcement, etc.) that are difficult to measure and model. In particular, the spatial heterogeneity of root reinforcement remains a problem, and the use of physically based models from a forest management perspective has been limited. This paper presents a procedure for assessing slope stability in terms of the Factor of Safety that accounts for forest stand characteristics such as tree density, average diameter at breast height and minimum distance between trees. The procedure combines a three‐dimensional (3D) slope stability model with an evaluation of the variability of root reinforcement in terms of a probability distribution, according to forest characteristics. Monte Carlo simulation is used to account for the residual uncertainties in both stand characteristics and 3D stability model parameters. The proposed method was applied in a subalpine catchment in the Italian Alps, mainly covered by coniferous forest and characterized by steep slopes and high landslide risk. The results suggest that the procedure is highly reliable, according to landslide inventory maps [area under the ROC curve (AUC) is 0.82 and modified success rate (MSR) is 0.70]. Thus, it represents a promising tool for studying the role of root reinforcement in landslide hazard mapping and guiding forest management from a slope stability perspective. Copyright © 2017 John Wiley & Sons, Ltd.