Title: Coupled Experimental and Numerical Approaches Toward Reliable Dynamic Characterization of Multi-phase Geomaterials
Principal Investigator: Ionut Dragos Moldovan (Instituto Superior Técnico, University of Lisbon)
Co-Principal Investigator: António Gomes Correia (Escola de Engenharia, University of Minho)
Total funding: €233,723.06 (funded by Fundação para a Ciência e a Tecnologia, grant PTDC/EAM-GTC/29923/2017)
Starting date: October, 1st, 2018
End date: March, 30th, 2022
In the lab, seismic wave testing of geomaterials is performed using mainly resonant column apparatus or piezoelectric transducers. As compared to the resonant column, piezoelectric transducers are cheaper, faster to install, easier to use and yield similar results. However, the wave propagation induced by the transducers is more complex, hindering the standardization of the testing setup and the interpretation of the output signal.
Regarding the experimental setup, open issues include the best location of the transducers, their optimal shape, and the nature of the excitation. Regarding the output signal, its interpretation is hindered by the high frequency damping, energy radiation and boundary reflections that take place as the seismic wave propagates.
The objective of this project is to change the way piezoelectric testing is approached, by coupling innovative experimental and numerical modelling techniques to optimize the testing setup and automatize the interpretation of the output signal.
The research team combines complementary competences and equipment from four research centres in Portugal and Spain. Professor David Muir Wood, a world renowned expert in the field, enters as a consultant.
The measurement of the dynamic properties of soils are instrumental to the analysis and design of Civil Engineering structures. Out of the various experimental setups that are available for such measurements, bender elements are one of the most versatile, due to their small scale and possibility to be installed in triaxial testing devices. Bender elements consist of a small piezoelectric cantilever which is used to induce a vibration (shear, or S, wave) in a soil sample. The velocity at which this wave travels through the soil is calculated by measuring the time required for the wave to reach the other end of the soil sample. The shear modulus of the soil is then derived directly from this velocity.
Typical installation of bender elements
Bender elements are a cheap, versatile and reliable alternative to resonant column apparatus. A full bender element equipment costs ~30 times less than a resonant column, can be installed in both oedometers and triaxial devices and yields measurements that are consistent with those of resonant column and field tests.
On the downside, the wave propagation induced by piezoelectric transducers is physically more complex, hindering the standardization of the experimental setup and the interpretation of the output signal, and limiting their use in the industry. Regarding the experimental setup, open issues include the best location of the transducers, their optimal geometry and the nature of the excitation. Regarding the interpretation of the output signal, the difficulties are related with three main issues: the dissimilarity of the input and output signals, due to high frequency damping and spurious boundary reflections (back into the sample) and radiation (into the surroundings); the residual motion of the emitter transducer, which continues to vibrate due to its own inertia after the electrical signal ends; and the solid continuum assumption in the moduli computation, which may be inadequate for multi-phase materials. Consequently, the purely experimental approach to the piezoelectric transducers testing only offers (partial) knowledge of the input and output signals and essentially leaves the analyst to guess what goes on between the input and output readings.
The objective of this project is to change the way piezoelectric testing is approached, by coupling experimental and numerical techniques toward a reliable characterization of wave propagation in geomaterials. Numerical modelling will offer insight that cannot be acquired by simply analyzing the lab data, and serve as a tool for assessing the impact of various setup options on the quality of the output signal.
The conclusions, once confirmed experimentally, will enable the optimization of the experimental setup, paving the way for its future CEN standardization. Moreover, the geometry of the transducers will be optimized in order to reduce the residual motion of the emitter and to increase the quality of the readings of the receiver. Prototypes will be produced and tested, and the best solutions patented. Finally, model updating techniques will be used to study the correlation between the simulation and experimental data for the automatic recovery of the shear modulus. This feature will be included in a computational toolbox, to be made available to the geotechnical community.