| Summary: |
The design of geotechnical structures requires an assessment of the likely displacements during the life of the structure and so knowledge of the ground stiffness and its variations with stress and strain are crucially important. Factors that condition reference values for these constitutive laws - the very small stiffness dynamic values -, such as strain-rate, frequency, time effects and stress anisotropy are of utmost importance to quantify these properties.
This project will pursue the study of these important methodological factors and questions of principle, being directed to a natural ambient that is very much unknown and badly understood, mostly because of its meta-stable natural structure: the residual soils. A clear purpose will be to study the real influence of the fabric and, mostly, the structure conditions in these values of dynamic properties. An answer to the following question will be pursued: does these strain/frequency dependency ratios depend on the structural condition of the same material, when in preserved natural structured condition or in remoulded condition?
Several methodologies will be used for this evaluation, trying to study in laboratory conditions (over undisturbed high quality samples) and in situ (by performing seismic cross-profiles in field), with the recourse some existing facilities (in the Geotechnical Laboratories of the Faculty of Engineering of the University of Porto, FEUP, and "Instituto Superior Técnico" of the Technical University of Lisbon, IST) and, prospectively, with some complementary equipments to be acquired by the project funds. Data will be deduced from tests to be conducted on a recently launched experimental site in FEUP.
The generation of a high quality database of in situ and geophysical test results, which includes Gvh, Ghv and Ghh data evaluation situ (by using down-hole and cross-hole techniques and other complementary tests), will allow cross-correlations to be investigated in rela  |
Summary
The design of geotechnical structures requires an assessment of the likely displacements during the life of the structure and so knowledge of the ground stiffness and its variations with stress and strain are crucially important. Factors that condition reference values for these constitutive laws - the very small stiffness dynamic values -, such as strain-rate, frequency, time effects and stress anisotropy are of utmost importance to quantify these properties.
This project will pursue the study of these important methodological factors and questions of principle, being directed to a natural ambient that is very much unknown and badly understood, mostly because of its meta-stable natural structure: the residual soils. A clear purpose will be to study the real influence of the fabric and, mostly, the structure conditions in these values of dynamic properties. An answer to the following question will be pursued: does these strain/frequency dependency ratios depend on the structural condition of the same material, when in preserved natural structured condition or in remoulded condition?
Several methodologies will be used for this evaluation, trying to study in laboratory conditions (over undisturbed high quality samples) and in situ (by performing seismic cross-profiles in field), with the recourse some existing facilities (in the Geotechnical Laboratories of the Faculty of Engineering of the University of Porto, FEUP, and "Instituto Superior Técnico" of the Technical University of Lisbon, IST) and, prospectively, with some complementary equipments to be acquired by the project funds. Data will be deduced from tests to be conducted on a recently launched experimental site in FEUP.
The generation of a high quality database of in situ and geophysical test results, which includes Gvh, Ghv and Ghh data evaluation situ (by using down-hole and cross-hole techniques and other complementary tests), will allow cross-correlations to be investigated in relation to stress-strain orientation. The hypothesis explaining the better correlation of cone resistance (qc) or DMT/PMT parameters to an essentially horizontal small strain stiffness may be a sign of this anisotropy of stress-stiffness. This will require a full understanding why this should be so, but as more Ghh data become available, so it may become clearer why this is so.
These potential correlations are no more than a guide as to likely variations within a profile and then only on a site specific basis until the reasons behind them are more fully understood. This is to be systematized in laboratory controlled conditions, by using true triaxial systems mounted with multidirectional travelling and polarizing shear wave transmission on the x, y and z principal axis.
Although this problematic is very much innovative, our groups have been collaborating with relevant research units outside Portugal (the University of Western Australia and the University College of London, UK), and the continuation of this work in a specific regional ambient will give a most valuable contribution. |
| Results: |
The present study will concentrate in evaluating the validity of correlations presented in the literature between small strain stiffness measured both in situ or in laboratory over high quality block undisturbed samples, under different orientation, test method, stress level etc.
The various adjustments that have been proposed, including void ratio, trying to normalise the scatter of published data, will be tested. Knowing, however, that the real fundament for this scatter may rely on the general stress state – x, y and z-axis – this will be analysed in an extensive campaign in “true triaxial tests”, with seismic waves generators which constitutes an innovative tool, recently available in the Geotechnical Laboratory of FEUP.
The influence of strain-rate and frequency on dynamic stiffness will be most objectively studied in testing several specimens of natural samples on the Ressonant Collumn system available in IST, by adapting the new generation of bender/extender elements and shear plates and the new conditioning systems and software frameworks, implemented by FEUP group. |