Liquefação de solos à luz da mecânica aplicada

Há uma crescente convicção da vantagem em encarar o fenómeno de liquefação de solos como passível de ser considerado como um comportamento elasto-plástico que é modelável a partir de conceitos baseados em estados críticos, enquanto se reconhece que pode ocorrer num largo espectro de materiais e condições. Estes assuntos são desenvolvidos neste artigo, atendendo a que as ferramentas de estados críticos têm sido estendidas a outros materiais para além das areias. Esta abordagem integra o conhecimento da influência que a micromecânica das partículas e dos seus contactos tem no comportamento dos solos, e tem em consideração os efeitos da quebra progressiva das partículas e as alterações do grau de uniformidade dos solos no decurso do carregamento. Os objetivos do dimensionamento com base em comportamento são apresentados à luz de ensaios em laboratório e de campo que permitem identificar o risco de se espoletar o fenómeno de liquefação, tanto em condições cíclicas, como estáticas. Também será discutida a forma como os resultados desses ensaios podem ser interpretados para se poder prever o fenómeno, à luz de uma abordagem mecânica global.The advantage of looking at soil liquefaction as an elasto-plastic mechanical behaviour that is well modelled by critical state concepts is well accepted, while recognising that it takes places in a wide range of materials and conditions. These issues are outlined in this paper, as the critical state framework has now been extended to other materials apart from sands. This approach integrates the knowledge of the influence of the micromechanics of particles and their contacts on the observed behaviour, and takes into account the effects of continued particle breakage and change in uniformity. The objectives of performance-based design are presented in the light of laboratory and field tests that permit to identify the risk of triggering both cyclic and static liquefaction. It is also discussed how those tests can be performed and their results interpreted to predict these phenomena, under a global mechanical modelling approach

A Framework Interpreting Bender Element Tests, Combining Time-Domain and Frequency-Domain Methods

Bender element (BE) testing is a powerful and increasingly common laboratory technique for determining the shear S-wave velocity of geomaterials. There are several advantages of BE testing, but there is no standard developed for the testing procedures or for the interpretation of the results. This leads to high degree of uncertainty and subjectivity in the interpretation. In this paper, the authors review the most common methods for the interpretation of BE tests, discuss some important technical requirements to minimize errors, and propose a practical framework for BE testing, based oil the comparison of different interpretation techniques in order to obtain the most reliable value for the travel time. This new procedure consist,, of the application of a methodical, systematic, and objective approach for the interpretation of the results, in the time and frequency domains. I he use of an automated tool enables unbiased information to be obtained regarding variations in the results to assist in the decision of the travel time. Two natural soils were tested: residual soil from Porto granite, and Toyoura sand. Specimens were subjected to the same isotropic stress conditions and the results obtained provided insights on the effects of soil type and confining stress on the interpretation of BE results; namely, the differences in testing dry versus saturated soils, and in testing uniform versus well-graded soils

Preliminary displacement-based assessment procedure for buildings on liquified soil

This paper provides a simple preliminary procedure for estimating the performance of a building on liquefiable soil. The procedure directly accounts for damage related to ground shaking and in-directly accounts for settlements. Additionally, it also considers the change in shaking demand and changes to the natural vibration modes of the systems due to liquefaction. The proposed procedure makes use of a displacement-based assessment procedure that considers nonlinear soil-foundation-structure interaction and extends it to include the effects of liquefaction. The extensions rely on several assumptions about the behaviour of the soil, site response and the structure, which require further research to improve the robustness of the assessment. Two small studies are conducted: one explores at what time the peak displacement of a system occurs during shaking, and the second explores the potential changes in site amplification due to liquefaction, which provide some justification to the proposed assumptions for the procedure. The procedure is applied to a six-storey two-bay case study reinforced concrete frame building to demonstrate the influence of various effects of liquefaction. For the case study building, the role of shaking damage was large and the estimated reduction in shaking demand was important to the estimated level of ductility demand, highlighting the importance of quantifying the expected site response for the assessment of building performance

Cyclic and Dynamic Behavior of Sand-Rubber and Clay- Rubber Mixtures

In this paper, the possibility of using fine scrap tyre rubber to improve the mechanical properties of soil subjected to cyclic loading is addressed. Ground rubber (0.1-0.8 mm) in various proportions (0, 9, 33% and 100% by weight) was mixed with a uniform river sand and a lean clay. Cyclic triaxial tests with bender elements were executed to observe the behaviour of the materials and also to determine damping and shear stiffness parameters. The results have shown that the addition of rubber has significantly decreased the density and shear stiffness of both types of soils, which favours mitigation of vibrations. The shear stiffness degradation at shear strains higher than 10−3 was lower in specimens containing more rubber. Within this strain range, addition of rubber decreased the damping ratio, but increased the normalized accumulated absolute strain energy absorbed by the material. Higher rubber content in sandy specimens resulted in more elastic behaviour, with lower strain accumulation in each loading cycle, eventually resulting in a higher number of loading cycles before failure. The positive effect of rubber presence was not observed in compacted clay-rubber mixture, which sustained less loading cycles than clay alone. The influence of rubber addition in the p′-q stress space was expressed in the form of lower pore pressure generation which shifted the stress path further from the failure envelope. (c) 2021, The Author(s), under exclusive licence to Springer Nature Switzerland AG part of Springer Nature

Influence of structure induced by compaction into the modulus of a granitic sandy soil

Este artigo apresenta um conjunto de resultados experimentais realizados sobre um saibro granítico, em particular ensaios triaxiais cíclicos no domínio das pequenas deformações (10-5 – 10-4) complementados por medições de velocidades de ondas sísmicas (Vs e Vp) com recurso à técnica de “bender-extender elements” (na medição de G0, para deformações muito pequenas, 10-6). Foram ensaiadas várias condições de estado do solo. Deste modo, procurou-se estudar a sensibilidade das propriedades de deformabilidade (E) a algumas condições de referência, comummente estabelecidas em Cadernos de Encargos de plataformas rodoviárias e ferroviárias. Estes módulos foram normalizados em relação ao índice de vazios, permitindo inferir conclusões relevantes, nomeadamente a da grande influência que a organização interparticular, obtida pela compactação, tem nos valores do módulo de Young em condições cíclicas no domínio das pequenas deformações (e=10-4).This paper presents a set of experimental results on a granitic sandy soil, in particular cyclical triaxial tests in the domain of the small strains (10-5 - 10-4) complemented by measurements of velocity of seismic waves (Vs and Vp) using "bender-extender elements" technique (in the measurement of G0, for very small strains 10-6). Soil was tested in some state conditions. This will allow to study the sensitivity of modulus (E) to some reference conditions commonly established in technical notes of road and railways platforms. These module have been normalized in relation to the void ratio, allowing interesting findings, namely the great influence that the inter-particular packing obtained by compacting has in the values of the Young's modulus deduced under cyclic conditions in the domain of the small strains (e=10-4)

Human-driven Machine-automation of Engineering Research

This paper presents a framework for efficiently producing engineering research in a global collaborative effort in a rigorous scientific manner. The proposed framework reduces subjective analysis, automates several mundane research tasks and provides a suitable formal structure for efficient information sharing and collaboration. The implementation of the framework involves multiple research groups setting up different web-servers that can perform the steps of the scientific method and automatically determine the quality and value of new research by directly communicating between servers via public and private Application Programming Interfaces (APIs) using a set of object-oriented protocols. The automation of many mundane research tasks (e.g. data manipulation), would allow researchers to focus more on the novel aspects of their research efforts. The increased clarity around the quality and value of research would allow the research efforts of individuals and available research funding to be better disbursed. The paper discusses the major aspects of the scientific method, object-orientated programming, the application of the proposed research framework for experimental/analytical/numerical engineering research, some of the potential benefits and drawbacks, as well as the current state of implementation

Analysis of simplified time of liquefaction triggering methods by laboratory tests, physical modelling and numerical analysis

The damage resulting from earthquakes can result from the combination of seismic excitation and/or due to a build-up of excess pore pressure in the soil (liquefaction). These two effects are related since the reduction of soil stiffness due to a decrease in effective stress, modifies the seismic response of the soil deposit. Therefore, the expected level and type of damage is dependent on the amount of seismic energy reaching the ground surface before liquefaction. The development and validation of simplified liquefaction assessment methods to provide reasonable estimates of the build-up of excess pore pressure is essential for improving estimates of the level of seismic demand (ground shaking and permanent ground deformation) that may be experienced by a building. This paper presents two methods, one based on equivalent cyclic stress loading, and another based on the cumulative strain energy, which are used to predict the evolution of the pore pressure build up throughout time. The centrifuge tests performed in ISMGEO (Italy) during the LIQUEFACT project (www.liquefact.eu) were used as a benchmark to evaluate the predictive performance of the methods. Additionally, a series of one dimensional soil column effective stress and total stress analyses and single soil element simulations were run. Available laboratory tests were used to calibrate the parameters of the simplified methods, as well as input parameters for the numerical simulations. The results showed that both simplified methods had considerable bias. A direct comparison of the effective stress analyses, a set of effective stress analyses with limited drainage, and the centrifuge results, highlighted that the centrifuge experiments exhibited significant pore water flow that was not captured in the simplified models. Comparisons between the irregular loading in the one dimensional analyses compared to the uniform loading in the element tests highlighted shortfalls in the conversion from irregular to equivalent uniform loading. Comparisons between stress demands from total stress, effective stress and the simplified methods illustrated the limitations of relying on the total stress acceleration to estimate demands on a soil in a liquefying deposit