Strain rate and stress relaxation in simulated pressuremeter testing in clays

Arumugam Skandarajah, Purdue University

Abstract

The research described in this thesis was undertaken to answer issues related to cavity expansion and in situ testing using the pressuremeter. Emphasis in the research focused upon the fundamental concepts underlying the cylindrical cavity expansion theory. Tests were performed in a cuboidal shear device at strain rates between 0.01% and 5.00% per minute on artificially sedimented kaolin clay and kaolin-silica mixture whose plasticity indices range similar to the most of the natural clays. It was found from the experiments that the normalized shear strength (with respect to 0.01%/min) increases linearly with the logarithm of strain rate. The increase in undrained shear strength in the pressuremeter stress path is about 14.3% per log cycle for kaolin clay and 15.3% for kaolin-silica mixture. The undrained shear strength in conventional triaxial tests increase about 8 to 10% for a tenfold increase in strain rate. Therefore, it can be concluded that the undrained shear strength increases about 40-50% more in pressuremeter stress path tests than in triaxial stress path tests. A procedure to improve existing cavity expansion based models to incorporate the influence of decreasing strain rate in the surrounding soil mass was developed. The findings from the strain rate tests to simulate the pressuremeter were used to estimate differences in the interpreted undrained shear strength. Although the actual peak undrained shear strength increases with strain rate, the interpreted large strain value from the PMT expansion curve is lower due to greater strain softening behavior predicted with the model. Model parametric studies showed that there was little influence on the interpreted strength value below a threshold strain rate of 0.001%/min. From the simulated pressuremeter test it was found that, for kaolin clay, irrespective of the initial strain level or strain rate, the relaxation time was about 200 to 250 minutes for undrained and drained conditions. Theoretical models show how to obtain creep/relaxation parameters from pressuremeter creep/relaxation tests conducted in a borehole. Several correlations are also identified in order to obtain relaxation parameters from creep parameters and empirically from plasticity index. These creep/relaxation parameters are required to predict the time-dependent behavior of soil.

Degree

Ph.D.

Advisors

Frost, Purdue University.

Subject Area

Civil engineering

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