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The re-liquefaction resistance of cemented sands under multiple liquefaction events such as pre-shock, main-shock, and after-shock earthquakes is a complex phenomenon because the response may alter due to bond breakage. A series of multistage liquefaction-re-consolidation soil element tests under undrained stress-controlled cyclic loading condition using cyclic triaxial were carried out to assess the liquefaction and re-liquefaction resistance of cemented sands with varying degrees of cementation. Lightly cemented specimens were reconstituted using Sydney sand and high early strength Portland cement with cement content ranging from 0.25% to 1% and unconfined compression strength from 15 to 80 kPa. The results showed that the re-liquefaction resistance of cemented sands with different amounts of cement decreased after the first liquefaction event and then increased for succeeding liquefaction events. While the trend of residual excess pore water pressure ratio and cyclic stiffness degradation index of untreated sand under successive liquefaction events remained consistent, the corresponding responses for cemented sands altered for the second to the fifth liquefaction events. In fact, the residual excess pore water pressure ratio and cyclic stiffness of cemented sand increased and degraded faster during the early cycles of loading for the second to fifth liquefaction events. Key words: liquefaction, re-liquefaction, post-liquefaction behaviour, lightly cemented sand, multistage soil element tests La resistance a la re-liquefaction des sables cimentes sous de multiples evenements de liquefaction tels que les tremblements de terre pre-choc, choc principal et post-choc est un phenomene complexe, car la reponse peut changer en raison de la rupture des liens. Une serie d'essais par elements de sol de liquefaction-reconsolidation en plusieurs etapes dans des conditions de chargement cyclique controlees par des contraintes non drainees en utilisant le triaxial cyclique a ete realisee pour evaluer la resistance a la liquefaction et a la re-liquefaction des sables cimentes avec differents degres de cimentation. Des specimens legerement cimentes ont ete reconstitues a l'aide de sable de Sydney et de ciment Portland a haute resistance initiale, avec une teneur en ciment allant de 0,25 % a 1 %, pour une resistance a la compression non confinee de 15 a 80 kPa. Les resultats montrent que la resistance a la re-liquefaction des sables cimentes avec differentes quantites de ciment diminue apres le premier evenement de liquefaction et augmente ensuite pour les evenements de liquefaction successifs. Alors que la tendance du rapport de l'exces de pression d'eau interstitielle residuelle et de l'indice de degradation de la rigidite cyclique du sable non traite lors d'evenements de liquefaction successifs est restee coherente, les reponses correspondantes pour les sables cimentes ont change du deuxieme au cinquieme evenement de liquefaction. En fait, le rapport residuel d'exces de pression d'eau interstitielle et la rigidite cyclique du sable cimente ont augmente et se sont degrades plus rapidement pendant les premiers cycles de chargement pour les deuxiemes a cinquiemes evenements de liquefaction. [Traduit par la Redaction] Mots-cles : liquefaction, re-liquefaction, comportement post-liquefaction, sable legerement cimente, essais par elements de sol a plusieurs niveaux, Introduction The liquefaction phenomenon that occurs during an earthquake is the main cause of damage to manmade structures or for natural disasters such as landslides (Brandenberg et al. 2007; Haskell [...]

The rotation of principal stress direction experienced by the soil elements in a railway track substructure during a train passage influences the magnitude of accumulated settlement. However, the existing methods to evaluate the track response under repeated train loads disregard the influence of principal stress rotation (PSR). This article presents a novel approach for assessing the behavior of ballasted railway tracks incorporating the contribution of PSR on track deformation. The proposed technique employs a geotechnical rheological model to evaluate the track behavior, in which the material plasticity is captured through plastic slider elements. The influence of PSR is accounted for by extending an existing constitutive relationship for the slider elements for the substructure layers, which is successfully validated against experimental data reported in the literature. The results reveal that PSR causes significant cumulative deformation in the substructure layers, and disregarding it in the analysis leads to inaccurate predictions. The proposed approach is then applied to an open track-bridge transition with heterogeneous support conditions, in which the differential settlement is found to be largely influenced by PSR. The findings from this study highlight the importance of including the effect of PSR in predictive models for a reliable evaluation of track performance. Key words: principal stress rotation, geotechnical rheological model, railway tracks, irrecoverable deformation, critical zones, differential settlement La rotation de la direction de la contrainte principale subie par les elements du sol dans une infrastructure de voie ferree pendant le passage d'un train influence l'ampleur du tassement accumule. Cependant, les methodes existantes pour evaluer la reponse de la voie sous les charges repetees des trains ne tiennent pas compte de l'influence de la rotation des contraintes principales (PSR). Cet article presente une nouvelle approche pour evaluer le comportement des voies ferrees ballastees en incorporant la contribution du PSR sur la deformation de la voie. La technique proposee utilise un modele rheologique geotechnique pour evaluer le comportement de la voie, dans lequel la plasticite du materiau est capturee par des elements de glissement plastique. L'influence du PSR est prise en compte par l'extension d'une relation constitutive existante pour les elements coulissants des couches de la sous-structure, qui est validee avec succes par rapport aux donnees experimentales rapportees dans la litterature. Les resultats revelent que le PSR provoque une deformation cumulative importante dans les couches de la sousstructure, et que le fait de ne pas en tenir compte dans l'analyse conduit a des previsions inexactes. L'approche proposee est ensuite appliquee a une transition voie-pont ouverte avec des conditions de support heterogenes, dans laquelle le tassement differentiel s'avere etre largement influence par le PSR. Les resultats de cette etude soulignent l'importance d'inclure l'effet du PSR dans les modeles predictifs pour une evaluation fiable des performances des voies. [Traduit par la Redaction] Mots-cles : rotation des contraintes principales, modele rheologique geotechnique, voies ferrees, deformation irrecuperable, zones critiques, tassement differentiel, Introduction A soil element in the track substructure experiences a complex change in the vertical, horizontal, and shear stresses during a train passage, which results in a rotation of the [...]

In this research, the finite-element numerical software COMSOL Multiphysics is used to simulate the electric pulse rock-breaking process, and the novel numerical model takes into account the multiple fields of electrical, thermal, and mechanical physics. The electric field strength inside the rock under the action of the electric pulse is updated by the full coupling function of COMSOL (version 6.1) software, and the electric damage variable χ is used to describe the process of electric breakdown inside the rock, to simulate the formation of plasma channel in the process of electric pulse rock-breaking. The voltage change curves and plasma channel trajectories in the electric breakdown process of the model in this paper are compared with the literature to verify the accuracy of the model. The formation process of plasma channels captures the temperature and stress changes during the whole process of electric pulse rock-breaking, to reveal the mechanism of the electric pulse rock-breaking process. With the presence of conductive particles inside the rock, the particles can effectively promote the formation of plasma channels, increase the area of electrical damage, and improve rock-breaking efficiency. Before the formation of the plasma channel, the internal temperature of the rock is about 600 K, and the stress is about 10−2 MPa; when the channel is formed, the energy of the electric pulse is mainly concentrated in the plasma channel, and the temperature of the plasma channel rises to 104 K. When the maximum stress exceeds the critical stress of the rock, the rock undergoes fracture. Furthermore, an appropriate increase in the rise time of the electric pulse increases the speed and area of rock-breaking, subsequently improving the efficiency of rock fracture. The results of the numerical modeling in this paper help to better understand the mechanism of the electric pulse rock-breaking process, which is of great engineering significance for the development of new electric pulse rock-breaking technology in the realm of geotechnical engineering. [ABSTRACT FROM AUTHOR]

Nano-SiO2, a highly acclaimed nanomaterial for enhancing cemented soil, has shown remarkable improvements in the physical properties and microstructure of cemented soil. The organic matter content in soil plays a crucial role in determining the engineering quality of cemented soil, regardless of whether it is in a freshwater or seawater environment. Therefore, when employing Nano-SiO2 as an amendment for cemented soil, it is crucial to consider the influence of different soil types and environmental conditions on the effectiveness of the enhancement. This study presents a scientific approach for enhancing the consolidation of cemented soil by incorporating Nano-SiO2 as an additive in both freshwater and seawater environments. To ensure consistency with practical construction practices, the experiments were divided into freshwater preparation and curing groups, as well as seawater preparation and curing groups. In soils with distinct characteristics, we utilized five different gradient levels of Nano-SiO2 additives and subjected the cemented soil specimens to a 60-day immersion curing process. Subsequently, unconfined compressive strength (UCS) tests were performed on samples that had reached the specified curing age to investigate the alterations in the mechanical properties of cemented soil caused by Nano-SiO2. The internal microstructure and chemical composition of the cemented soil were analyzed utilizing scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The UCS and deformation modulus of cement soil composed of silty clay A and silty clay D with low organic matter content in fresh water environment increased by 109%, 24.7% and 117%, 43% respectively after adding 3.2% Nano-SiO2; In freshwater environments, the cemented soil composed of high-organic-matter content mucky silty clay B, muddy soil C, and dredged silt E experienced respective increases of 16% and − 27%, 2% and 42%, 6% and − 6% in UCS and deformation modulus after adding 3.2% Nano-SiO2; The UCS and deformation modulus of cemented soil with high organic matter content in seawater and freshwater environment increased by 3% and 6% (Soil B), 12% and 67% (Soil C), 23% and 27% (Soil E), respectively, after adding 3.2% Nano-SiO2; The increase of UCS and deformation modulus of cement soil by adding Nano-SiO2 is negatively correlated with the content of organic matter. in the case of cemented soil with high organic matter content in a seawater environment, the specific type of soil not only impacts UCS and deformation modulus of the soil but also influences the effectiveness of enhancement. Due to the organic matter, the rate of hydration reaction in cemented soil is reduced, resulting in a decrease in the formation of pozzolanic reaction products with SiO2. As a result, the improvement in Nano-SiO2 effectiveness is diminished. [ABSTRACT FROM AUTHOR]