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The severity of surficial liquefaction manifestation was significantly over-predicted for a large subset of case histories from relatively recent earthquakes that impacted the Canterbury region of New Zealand. Such over-predicts generally occurred for profiles having predominantly high fines-content (FC), high-plasticity soil strata. Herein, the liquefaction case histories from the Canterbury earthquakes are used to investigate the performances of three different manifestation severity index (MSI) models. The prevalence of high FC, high-plasticity strata in a profile is quantified through the soil behavior type index averaged over the upper 10 m of a profile ([I.sub.c10]). It is shown that for each MSI model (1) the threshold MSI value distinguishing cases with and without manifestation increases as [I.sub.c10] increases and (2) the ability of the MSI to segregate cases with and without manifestation decreases with increasing [I.sub.c10]. Additionally, probabilistic models are proposed for evaluating the severity of surficial liquefaction manifestation as a function of MSI and [I.sub.c10]. The approaches presented in this study allow better interpretations of predictions made by existing MSI models, although their efficacy decreases at sites with high [I.sub.c10]. An improved MSI model is ultimately needed that better accounts for the effects of high-FC, high-plasticity soils more directly. Key words: liquefaction, earthquake, manifestation severity index, Canterbury earthquakes, Introduction The objective of this study is to investigate the effect of high fines-content (FC), high-plasticity soils on the prediction of the occurrence and severity of surficial liquefaction manifestations at [...]

Tools for characterizing thin layering and groundwater table conditions are evaluated at silty soil sites being assessed for liquefaction. Thin interlayered stratigraphy and groundwater table fluctuation are two potential causes for inconsistencies observed during the Canterbury earthquake sequence, wherein liquefaction did not manifest at several silty soil sites, despite simplified liquefaction assessment procedures indicating severe manifestations would be expected. Site investigations should capture these features to allow for improved assessment of liquefaction potential at silty soil sites. Cone penetration tests (CPTs), mini-CPTs, and sonic borings do not adequately capture thin layering. However, detailed logging of high-quality samples captures the actual in situ layering that may help explain the limitations of simplified liquefaction assessment procedures at these sites, revealing the need to understand underlying limitations in current site investigation techniques. Piezometers, sonic borings, high-quality sampling, crosshole testing, and regional groundwater maps are evaluated to assess their ability to capture complex groundwater conditions. Multiple groundwater measurement methods are typically required to characterize groundwater fluctuations. An approach to using enhanced site characterization tools is recommended for liquefaction assessments at silty soil sites with thin layering and groundwater fluctuations. Key words: site characterization, thin layering, silty soils, groundwater, liquefaction. Les outils de caracterisation des couches minces et de l'etat de la nappe phreatique sont evalues sur des sites de sols limoneux en cours d'evaluation pour la liquefaction. La stratigraphie a couches minces intercalees et la fluctuation de la nappe phreatique sont deux causes possibles d'incoherences observees pendant la sequence du seisme de Canterbury, ou la liquefaction ne s'est pas manifestee sur plusieurs sites de sols limoneux, malgre des procedures simplifiees d'evaluation de liquefaction indiquant que des manifestations graves seraient a prevoir. Les etudes sur le terrain devraient tenir compte de ces caracteristiques afin de permettre une meilleure evaluation du potentiel de liquefaction sur les sols limoneux. Les essais de penetration au cone (CPT), mini-CPT et forages soniques ne capturent pas correctement les couches minces. Toutefois, la realisation de coupe detaillee d'echantillons de haute qualite permet de saisir les couches in situ reelles qui peuvent aider a expliquer les limites des procedures simplifiees d'evaluation de la liquefaction sur ces sites, ce qui revele la necessite de comprendre les limites sousjacentes des techniques actuelles d'investigation des sites. Les piezometres, les forages soniques, l'echantillonnage de haute qualite, les essais en sondage transversal et les cartes regionales des eaux souterraines sont evalues pour evaluer leur capacite a capter les conditions complexes des eaux souterraines. De multiples methodes de mesurage des eaux souterraines sont generalement necessaires pour caracteriser les fluctuations des eaux souterraines. Il est recommande d'utiliser des outils ameliores de caracterisation des sites pour l'evaluation de la liquefaction des sols limoneux en couches minces et des fluctuations des eaux souterraines. [Traduit par la Redaction] Mots-cles: caracterisation du site, couches minces, sols limoneux, eaux souterraines, liquefaction., Introduction This paper investigates the geotechnical characterization of silty soil sites for liquefaction assessment. The study focuses on sites in Christchurch, New Zealand, where post-earthquake liquefaction manifestations were not observed [...]

The results of an experimental investigation of riprap stability at wing-wall abutments are reported. The aim of the experiments was the determination of the size of riprap stones necessary to resist shear failure by the flow. The results, which were obtained under a wide range of mobile-bed conditions, are compared with predictions of riprap stone size given by existing equations. Two existing methods, with the addition of suitable factors of safety, are shown to give stable stone size for riprap layers at wing-wall abutments under mobile-bed conditions. It is noted that consideration needs also to be given to filter protection, edge effects, and the effects of bed-form undermining in the design of such riprap layers. DOI: 10.1061/(ASCE)0733-9429(2007)133:11(1265) CE Database subject headings: Bridge abutments; Erosion; Scour; Revetments; Sediment transport; Bedforms; Open channels; Hydraulics.

An experimental study of scour countermeasures for wing-wall abutments under mobile-bed conditions is reported. The purpose of the study was to determine the required apron size to protect a wing-wall abutment from scour under mobile-bed conditions, for different flow depths, flow velocities, and apron placement level. Riprap and cable-tied block countermeasures were investigated. It was observed that scour at the abutment undermines the apron, causing the outer edge of the apron to settle. Scour at the abutment can be a combination of localized scour (i.e., contraction scour and local scour) and scour due to the troughs of the bed forms that migrate past the abutment, the latter being the focus of the study presented. As the flow depth and flow velocity are increased, the outer edge of the apron settles into the sand, to a depth proportional to the height of the bed forms developed in the flume. Localized scour may additionally influence the depth of settlement. When the apron width is increased, the depth to which the outer edge settles remains reasonably constant. When the apron is preburied below the average bed level, the settlement of the outer edge is less, but the depth of the outer edge of the apron below the average bed level is the same as for the corresponding apron placed on top of the bed. Riprap and cable-tied block aprons respond in different ways. Stones at the outer edge of riprap aprons tend to roll away from the abutment as they settle, pushing the erosion zone further away from the abutment. Conversely, because cable-tied block mats remain attached to the abutment, apron settlement allows the scour to occur closer to the abutment face. It is concluded that the scour at abutments under mobile-bed conditions is often governed by the size of the bed forms that pass the abutment. Therefore, the design of armored scour countermeasures for abutments under mobile-bed conditions should be based upon the largest expected bed forms, which can be predicted based on existing expressions, together with any effects of localized scour. This will enable the settlement of the outer edge of the apron to be determined, and the required apron width to be chosen, depending on the tolerable distance between the deepest scour and the abutment. An equation is given for prediction of the minimum width of apron remaining horizontal after erosion. DOI: 10.1061/(ASCE)0733-9429(2006) 132:6(563) CE Database subject headings: Abutments; Erosion; Scour; Revetments; Sediment transport; Open channels; Hydraulics; Bedforms.

An experimental study of flow-induced failure of cable-tied blocks is presented. The particular failure mechanism studied is overturning and rolling up of the leading edge of a cable-tied block mat. Individual blocks were investigated also. The block size, flow depth, and block (mat) protrusion above the surrounding bed were systematically varied. The results are presented in terms of the critical dimensionless shear stress [[theta].sub.c] for block (mat) failure. A relationship between [[theta].sub.c] block size and block protrusion, and flow depth is given. DOI: 10.1061/(ASCE)0733-9429(2006)132:3(324) CE Database subject headings: Sediment transport; Erosion; Revetments; Entrainment; Shear stress; Blocks; Failures.

An experimental study of scour countermeasures for spill-through abutments situated on the flood plain of a compound channel is reported. The purpose of the study was to determine the variations in the scour hole geometry under clear water conditions by varying the compound channel and abutment geometries, and to determine the extent and type of scour countermeasure toe protection provided. This approach avoids one of the inherent difficulties in conducting scour countermeasure experiments--that is, the subjectivity of determining whether the countermeasure used in the experiment is a success or a failure. Riprap and cable-tied block countermeasures are incorporated. The results show that for most cases, as the countermeasure apron width (i.e., the extent of toe protection) is increased, the scour hole is deflected further away from the abutment and reduces in size. However, for abutment and compound channel configurations where the scour hole forms close to the main channel bank, the scour hole increases in size as the apron width is increased. The results also show that cable-tied block mats allow the scour hole to form closer to the abutment than equivalent riprap aprons and result in deeper scour holes. A suggested design methodology for the extent of apron protection is presented. The method is an improvement on the current, rather-simplified practice of providing aprons of fixed width equal to twice the flow depth. DOI: 10.1061/(ASCE)0733-9429(2006) 132:3(235) CE Database subject headings: Spills; Abutments; Scour; Flood plains; Experimentation.