Infoscience

Thesis

Cyclic properties of sand: dynamic behaviour for seismic applications

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. The assumptions and simplifications usually considered for representing seismic loadings are evaluated, focusing on two main aspects: (i) cyclic stress frequency applied to the sample (ii) superposition of two independent stresses. For that purpose, the nonlinear behaviour of two different sands, Leman Sand and Fonderie Sand, is explored with cyclic and seismic triaxial tests. These tests are performed with unidirectional or bidirectional loadings, at medium to high strain amplitude, and in the earthquake frequency range. A dynamic triaxial press was developed to perform such tests, with dry and undrained saturated sand samples. Axial and lateral stresses can be applied independently with large amplitudes for various loading shapes. An innovative non-contact measurement technique was developed to continuously monitor the sample radius; this testing equipment is based on three laser sensors, set up around the triaxial cell, which detect the position of the sample surface thanks to optical triangulation. The obtained data are processed through a complex calibration system to provide the radial strain evolution at mid-height of the sample. The mounting structure supporting the sensors allows precise positioning and is equipped for manual vertical scanning of the sample profile. The first triaxial tests are performed with classical cyclic loadings, to characterize the behaviour of the two sands in pseudo-dynamic conditions. These dry and undrained saturated tests allow to describe the decrease of stiffness which leads to failure of the sand sample. Failure of undrained saturated sand occurs by liquefaction. 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 (i.e. higher strain amplitude, more pore pressure increase, etc.) 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. Bidirectional effects result in an amplification of the sand response until the occurrence of cyclic liquefaction. The phase angle between axial and lateral stresses is the key parameter influencing the coupling. Moreover, the comparison between unidirectional and bidirectional irregular seismic loadings show that bidirectional conditions slightly influence undrained sand response, with conditions of amplification very similar to cyclic tests. 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. The linear equivalent method gives a very crude approximation, even at medium strain level, and is not suitable for accurate evaluation of stiffness degradation observed during our cyclic tests. To conclude, assessing the behaviour of granular soils under earthquake loadings requires to take into account the nonlinear features of sand behaviour in terms of pore pressure generation and strain amplitude. In particular, frequency content and bidirectional loadings influence the sand response for medium to large strains. These experimental results could be considered for improving the analysis of strong ground motions. They constitute an important contribution for promoting more accurate nonlinear modelling of site effects in natural sands.

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