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Seismic stability of gravity-type quay walls and prevention of their large distortion are of major concern from a disaster prevention view point as well as in the sense of successful restoration after strong seismic events. There are, however, many existing walls which are of limited seismic resistance and would not be safe under increasing magnitude of design earthquakes. The present study conducted shaking model tests in both 1-g and 50-g centrifugal fields in order to demonstrate the efficiency of available mitigation technologies. Test results suggest that soil improvement in the loose foundation sand can reduce the quay wall damage to a certain extent when the intensity of shaking is around 0.30g. In contrast, under stronger shaking, the centrifugal tests manifested that those measures are not promising because of the increased effects of seismic inertia force.
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During strong seismic excitation, passive earth pressure at the abutments may provide resistance to longitudinal displacement of the bridge deck. The dynamic pressure component may also contribute to undesirable abutment movement or damage. Current uncertainty in the passive force-displacement relationship and in the dynamic response of abutment backfills continues to motivate large-scale experimentation. In this regard, a test series is conducted to measure static and dynamic lateral earth pressure on a 1.7 meter high bridge abutment wall. Built in a large soil container, the wall is displaced horizontally into the dense sand backfill, in order to record the passive force-displacement relationship. The wall-backfill system is also subjected to shake table excitation. In the conducted tests, lateral earth pressure on the wall remained close to the static value during the low to moderate shaking events (up to about 0.5g). At higher levels of input acceleration, a substantial portion of the backfill inertial force started to clearly act on the wall.
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Soil liquefaction during strong ground shaking results in almost a complete loss of strength and stiffness in the liquefied soil, and consequent large ground deformation. Characteristics of the liquefied soils and loads on piles are significantly different during the cyclic phase and in the subsequent lateral spreading phase. Thus, it is necessary to separately consider these two phases in the simplified analysis of piles. This paper describes a practical procedure for preliminary assessment of piles subjected to lateral spreading. Effects of a crust of non-liquefied soil at the ground surface, properties of liquefied soils and pile groups are discussed in relation to their modelling in the simplified pseudo-static analysis approach. Particular attention is given to the treatment of unknowns and uncertainties involved in the simplified analysis and the need for parametric studies.
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The evaluation of earthquake-induced landslides in natural slopes is often based on an estimate of the permanent sliding displacement due to earthquake shaking. Current procedures for estimating sliding displacement do not rigorously account for the significant uncertainties present in the analysis. This paper presents a probabilistic framework for computing the annual rate of exceedance of different levels of displacement such that a hazard curve for sliding displacement can be developed. The analysis incorporates the uncertainties in the prediction of earthquake ground shaking, in the prediction of sliding displacement, and in the assessment of soil properties. Predictive models for sliding displacement that are appropriate for the probabilistic framework are presented. These models include a scalar model that predicts sliding displacement in terms of a single ground motion parameter (peak ground acceleration) and the earthquake magnitude, as well as a vector model that incorporates two ground motion parameters (peak ground acceleration and peak ground velocity). The addition of a second ground motion parameter results in a significant reduction in the standard deviation of the sliding displacement prediction. Comparisons are made between displacement hazard curves developed from the current scalar and vector models and previously developed scalar models that do not include earthquake magnitude. Additionally, an approximation to the vector hazard assessment is presented and compared with the rigorous vector approach. Finally, the inclusion of the soil property uncertainty is shown to increase the mean hazard at a site.
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Considerable knowledge and experience has been developed over the past 40 years in the engineering profession regarding the seismic performance and analysis of dams for earthquake shaking. However, comparatively limited experience is available regarding the evaluation of dams for the effects of foundation fault rupture during earthquakes. This paper examines the factors to be considered in the evaluation of embankment dams for foundation faulting, and illustrates the analysis of dam response under foundation faulting by means of a case history, the seismic evaluation of Aviemore Dam.
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Forward-Directivity (FD) in the near-fault region can produce intense, pulse-type motions that differ significantly from ordinary ground motions that occur further from the ruptured fault. Near-fault FD motions typically govern the design of structures built close to active faults so the selection of design ground motions is critical for achieving effective performance without costly over-design. Updated empirical relationships are provided for estimating the peak ground velocity (PGV) and period of the velocity pulse (Tv) of near-fault FD motions. PGV varies significantly with magnitude, distance, and site effects. Tv is a function of magnitude and site conditions with most of the energy being concentrated within a narrow-period band centred on the pulse period. Lower magnitude events, which produce lower pulse periods, might produce more damaging ground motions for the stiff structures more common in urban areas. As the number of near-fault recordings is still limited, fully nonlinear bi-directional shaking simulations are employed to gain additional insight. It is shown that site effects generally cause Tv to increase. Although the amplification of PGV at soil sites depends on site properties, amplification is generally observed even for very intense rock motions. At soft soil sites, seismic site response can be limited by the yield strength of the soil, but then seismic instability may be a concern.
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The Mw 6.7 George Sound earthquake of October 15, 2007, occurred only a few kilometres offshore of Fiordland, within a region where the subduction zone of the Australian Plate beneath the Pacific Plate intersects the offshore extension of the Alpine Fault. Rapid response deployments of portable seismographs, a strong motion recorder and GPS receivers relatively close to the epicentre soon after the main shock allowed us to relate the event to thrusting at the subduction interface. The main shock moment tensor solution places the event at a shallow depth of 21 km. The sequence of aftershocks that followed the main event presents predominantly reverse faulting mechanisms with depths of 20 to 25 km. Earthquake re-locations using data recorded by the portable seismometers reveal a cluster of aftershocks at 17 to 25 km. This cluster defines a steeply SE-dipping plane, while another cluster at about 7-12 km depth images a NW-dipping plane within the overlying plate. Preliminary results from the seismic, geodetic and near-field strong motion geophysical data are consistent with rupture on an east dipping fault plane, presumed to be the subduction interface.
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A ground shaking amplification map of New Zealand has been compiled from data held by GNS Science. The resulting map is being used in RiskScape, a tool for comparing risks at a given site from a variety of hazards by estimating potential losses.
A GIS-based geological map with national coverage has been composed from several sources, and is used as the base data. Geological maps from the QMAP project (an ongoing project to digitally compile 1:250,000 geological maps for all of New Zealand) have been used where available, supplemented with detailed geological maps at scales ranging from 1:25,000 to 1:50,000 for the larger urban areas. Gaps in the QMAP series have been filled by the 1:1,000,000 ‘Geological Map of New Zealand’.
Every geological polygon in the composite geological map has been assigned one of the ground shaking amplification (or site) classes from the New Zealand Standard for Structural Design Actions – Earthquake actions (NZS 1170.5) to produce the result map. These conform to the site class definitions in NZS 1170.5, which describes five classes with respect to ground shaking amplification. Assignment of these classes was straightforward for rock sites but more involved for soils where, for example, at boundaries between weak rock and deep soil sites a buffer zone of shallow soil was applied.
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Within a nonlinear static analysis procedure perspective for the assessment of structures, one of the key issues is the employment of a demand spectrum that takes also into account, through an adequate reduction of its spectral ordinates, the hysteretic energy dissipation capacity of the structure being assessed. There are certainly a relatively large number of past parametric studies dedicated to the validation of different approaches to translate such structural energy dissipation capacity into spectral reduction factors, however such studies have focused mainly, if not exclusively, on single-degree-of-freedom (SDOF) systems. It seems, therefore, that verification on full structural systems, such as complete bridges, is conspicuously needed in order to verify the adequacy of using existing SDOF-derived relationships in the assessment of multiple-degree-of-freedom (MDOF) systems. In this work, eleven different spectral reduction proposals, involving diverse combinations of previously proposed equivalent damping and spectral reduction equations, are evaluated, for various intensity levels, using a preliminarily validated nonlinear static procedure. A wide set of bridges, covering regular and irregular configurations as well as distinct support conditions, is used. The accuracy of the results is checked by direct comparison with Time-History Analyses performed with ten real ground motion records. Overall conclusions are then presented with the purpose of providing practitioners and researchers with indications on the most adequate spectral reduction schemes to be employed in nonlinear static analysis of bridges.
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In order to incorporate seismic risk of facilities into a decision making framework, procedures are needed to quantify such risk for stakeholders. Seismic loss estimation methods combine seismic hazard, structural response, damage fragility, and damage consequences to allow quantification of seismic risk. This paper presents a loss estimation methodology which provides various measures of seismic risk for a specific facility. The methodology is component-based and can therefore distinguish between different structural configurations or different facility contents and is consistent with state-of-the-art loss assessment procedures. Loss is measured in the forms of direct structural and non-structural repair costs, and although not considered in the example, business disruption and occupant injuries can also be considered. This framework has been packaged in a computer code available for future dissemination in the public domain so that users need only to have a basic understanding of the methodology and the input data that is required. Discussion is given to the flexibility of the framework in terms of the rigour which can be employed at each of the main steps in the procedure. Via a case study of a high-rise office building, the use of the methodology in decision-making is illustrated. Methodological requirements and further research directions are discussed.
