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Rational estimation of debris flow velocity is one of the key issues in debris hazard mitigation. Among the various procedures, back-calculation of debris flow velocity is one of the most frequently used approaches. Back-calculation includes determination of super-elevations and channel properties, and velocities are calculated using the forced vortex equation. Super-elevation and bend radius are biasing parameters in the back-calculation scheme. Therefore, an iterative approach based on simplified assumptions is developed to avoid the direct and subjective determination of bend radius. Besides, as only the highest mud prints are visible after the disaster, this misreads the actual super-elevation. To seek better ways to fix this anomaly, a series of three-dimensional numerical curved flume tests using smoothed particle hydrodynamics are carried out. Estimated velocities from highest flow marks underestimate the actual velocities near the source, while they converge on the actual velocities as the distance to source increases. A best-fit line is then proposed to adjust the mud-marks-derived velocities to the real velocities. The assumption taken during development of the iterative approach for determining bend radii is also justified from the numerical simulations. The law of similarity allows application of these findings to real debris flow disasters. Thus, two real debris flow disasters in Japan are examined for validation of the developed procedure and adjusted velocities are proven to be consistent with verbal evidences and previous analyses. Key words: debris flow, velocity, bend radius, iterative approach, smoothed particle hydrodynamics. L'estimation rationnelle de la vitesse d'ecoulement des debris est l'une des questions cles de l'attenuation des risques de debris. Parmi les differentes procedures, le retrocalcul de la vitesse d'ecoulement des debris est l'une des approches les plus frequemment utilisees. Le retrocalcul comprend la determination des super- elevations et des proprietes des canaux, et les vitesses sont calculees en utilisant l'equation du vortex force. La super- elevation et le rayon de courbure sont des parametres de biais dans le schema de retrocalcul. Par consequent, une approche iterative basee sur des hypotheses simplifiees est developpee pour eviter la determination directe et subjective du rayon de courbure. En outre, seules les plus grandes traces de boue sont visibles apres la catastrophe, ce qui fausse la super-elevation. Afin de trouver de meilleurs moyens de resoudre ce probleme, une serie de tests de canaux courbes numeriques tridimensionelles utilisant l'hydrodynamique des particules lissees est realisee. Les vitesses estimees a partir des marques d'ecoulement les plus elevees sousestiment les vitesses reelles pres de la source, alors qu'elles convergent vers les vitesses reelles a mesure que la distance a la source augmente. Une ligne de meilleur ajustement est alors proposee pour ajuster les velocites derivees des marques de boue aux vitesses reelles. L'hypothese retenue lors du developpement de l'approche iterative pour determiner les rayons de courbure est egalement justifiee a partir des simulations numeriques. La loi de similitude permet d'appliquer ces resultats aux veritables catastrophes liees aux coulees de debris. Ainsi, deux desastres reels de coulees de debris au Japon sont examines pour la validation de la procedure developpee et les vitesses ajustees sont prouvees etre compatibles avec les evidences verbales et les analyses precedentes. [Traduit par la Redaction] Mots-cles: flux de debris, vitesse, rayon de courbure, approche iterative, hydrodynamique des particules lissees., Introduction Debris flow is one of the most devastating natural disasters, mostly occurring in mountainous areas. Earthen masses on steep flanks of these mountains immediately behind urbanized areas can be [...]

Recent near-field earthquakes accompanied by large soil deformations have been clouding the notion that mountain tunnels would be safe places during earthquakes. One of the recent eye-openers was the damage to railway tunnels in the 2004 Mid-Niigata Prefecture earthquake. Changes in stresses in the interior of a half-space of stratified sedimentary rocks, as a representative of the earthquake-hit region, are obtained using the authors' previous works on co-seismic deformations of this region to study the damage mechanism of deeply embedded railway tunnels. The values of square root of the second invariant of the stress deviator tensor, [square root of [J.sub.2]], and the first invariant of Cauchy stress tensor, [I.sub.1], are compared with the reported damages along the entire stretch of selected tunnels and a very good correlation is observed between the peak values of [square root of [J.sub.2]] and the damaged sections of the tunnels. A yield surface is defined as the boundary between clusters of points for damaged and undamaged tunnel sections in the scatter diagram of [I.sub.1] and [J.sub.2]. This yield surface and rock-soil deformability can be used to examine the margin of safety of both existing and new tunnels as well as for hazard zonation in a given scenario earthquake. Key words: Mid-Niigata Prefecture earthquake, ground displacement, fault slip, railway tunnels, seismic stresses. Les recents tremblements de terre en champ proche accompagne par de grandes deformations du sol ont assombri la notion que les tunnels de montagne seraient des endroits surs pendant les tremblements de terre. L'un des derniers a revele les dommages causes aux tunnels ferroviaires dans le tremblement de terre de la prefecture de Mid-Niigata 2004. Les variations de contraintes dans l'interieur d'un demi-espace de roches sedimentaires stratifiees, en tant que representant du tremblement de la region, sont obtenues en utilisant les reuvres precedentes des auteurs de deformations sismiques de cette region afin d'etudier le mecanisme d'endommagent profond integre dans les tunnels ferroviaires. Les valeurs de racine carree de la deuxieme constante de la contrainte deviatorique, le tenseur de contrainte [square root of [J.sub.2]], et le premier invariant de tenseur de stress de Cauchy, [I.sub.1], sont compares avec les dommages signales tout au long de la partie de certains tunnels et une tres bonne correlation est observee entre les valeurs de crete [square root of [J.sub.2]], et les parties endommagees des tunnels. Le rendement d'une surface est defini comme la limite entre les grappes de points de l'etat et des sections du tunnel endommage dans le diagramme de dispersion de [I.sub.1] and [J.sub.2]. Cette surface d'ecoulement et la deformabilite des sols-roches peuvent servir a examiner la marge de securite des tunnels existants et nouveaux ainsi que des feux de zonalite dans un scenario seisme donne. [Traduit par la Redaction] Mots-cles: tremblement de terre de la prefecture de Mid-Niigata, deplacement de sol, glissement de sol, tunnels ferroviaires, stress sismique., Introduction A magnitude 6.8 Mid-Niigata Prefecture earthquake jolted central Japan on 23 October 2004 (EERI 2005). The earthquake was followed by a large number of aftershocks with four of magnitude [...]

To assess earthquake resistance of new or existing structures and predict earthquake damage of structures, it is important to properly evaluate the response of the structure at the time of the earthquake. In cold regions however, where the ground can freeze in winter, the dynamic soil-foundation interaction can change with the change in the density and stiffness of the frozen side soil. This paper examines in a quantitative manner, the influence of freezing of ground on the dynamic soil grouped piles interaction., It is shown herein that the grouped piles in the frozen ground tend to follow the motion of the ground over a wider range of frequency. Furthermore, it is shown that even when the freezing depth is much thinner than the characteristic length of piles determined by the stiffness ratio of the ground and the group pile, the pile cap stiffness at the time of freezing becomes noticeably large. It is confirmed that the freezing of the ground can greatly affect the interaction between the ground and the piles.