Das Projekt "Non - Linear Threshold for amplification analyses in moderate earthquakes in alpine valleys (NLT)" wird vom Umweltbundesamt gefördert und von Eidgenössische Technische Hochschule (ETH) Zürich, Institut für Geotechnik durchgeführt. The project Non-Linear Threshold for amplification analyses for moderate earthquakes in alpine valleys (NLT) aims to enhance the use of non-linear soil models in amplification analyses. Due to the complexity of the non-linear soil models, especially when incorporated in 2 or 3 dimensional dynamic calculations, the knowledge of threshold conditions need to be studied in detail to reduce computational efforts. The so-called non-linear soil behaviour is governed by the change of stiffness of soils under loading, deformation and in case of high water table levels by an increase in porewater pressure, which can reach values equivalent to the effective stress (liquefaction). Whereas studies exists on the threshold for stress and strain to consider nonlinearity in dry conditions (e.g. Vucetic, 1994), no such value exists for the interaction between earthquake loading causing porewater pressure build-up and subsequently softening of the material and a change of the stress state. The increase in porewater pressure depends on the incoming stresses and strains as well as the length of an earthquake. As long as particle reorientation is involved, porewater pressures increase already under low loading states, even though liquefaction might only be reached after significant numbers of load cycles. The influence of pore pressure increase, change in directivity and elongated earthquakes, as shown by Faeh et al. 2006 due to topographical effects, will be studied with a series of cyclic laboratory experiments using advanced triaxial and a hollow cylinder apparatus. These tests will be accompanied by field experiments, which have been partially funded with matching funds from the Competence Centre for Environment and Sustainability (CCES) in the project COGEAR . This study is absolutely essential for countries like Switzerland, where big earthquakes are rare whereas moderate earthquakes occur quite frequently and valley effects amplify and lengthen an earthquake in time. To be able to study the amplification further, different material models developed to describe non-linear soil behaviour in the last decades need to be judged to which extent the non-linearity caused by an increase of pore pressures can be covered with these models. While the judgment will be based on one-dimensional calculations and on the existing codes, the most promising model(s) will be transferred into a hybrid calculation building on the developments from the Swiss Seismological Survey (SED). The area of non-linearity can be separated from the linear equivalent calculations. For specific valley conditions (Visp, VS), a model is in preparation to incorporate topographical effects into the study. The Valais, and specifically the area of Visp, has been selected for its high seismic hazard and their well reported non-linear effects observed after the 1855 event. In the past, the Valais has experienced a magnitude 6 or larger event every 100 years, the last in 1946, and the region of (...).
Das Projekt "Dynamic Properties of Granular Soils and Behavior of Earth Structures under Strong Earthquake Motion" wird vom Umweltbundesamt gefördert und von Ecole Polytechnique Federale de Lausanne, Institut des sols, roches et fondations, Laboratoire de mecanique des sols durchgeführt. Seismic wave propagation in granular soils can induce large strain amplitudes in case of strong earthquakes. Seismic motions are irregular in frequency content and in amplitude, and have three different components in orthogonal directions. In this context, the main objective of this PhD research deals with nonlinear effects observed in granular soils under such complex loadings. A dynamic triaxial press was developed for dry and undrained saturated sand samples. Axial and lateral stresses can be applied independently with large amplitudes for various loading shapes. An innovative laser-based non-contact measurement technique was developed to continuously monitor the sample radius Dry and undrained cyclic tests performed on Leman Sand at various frequencies from 0.1 to 6.5 Hz show that the behaviour of this granular material is frequency-dependent at medium to large strains. Sand stiffness, which depends on stress conditions, seems to influence the extent of frequency effects on soil behaviour: for tests with lower stiffness, the soil response to low frequency is significantly amplified compared to the high frequency range. The overall rate-sensitivity may be enhanced by the angularity of the grains. Other cyclic undrained saturated tests on Leman Sand demonstrate that the superposition of two different loadings, one axial and one lateral (bidirectional tests), induce coupling effects in the nonlinear soil response. Experimental results are finally modelled with the linear equivalent method and with a multi-mechanism elastoplastic model (ECP Hujeux). Nonlinear effects observed in laboratory experiments, and particularly the increase of strain amplitude leading to cyclic liquefaction of dense sand, are well captured by the elastoplastic model. Assessing the behaviour of granular soils under earthquake loadings clearly requires to take into account the nonlinear features of sand behaviour in terms of pore pressure generation and strain amplitude.