Interpretation of piezocone data and its use in estimating seismic soil liquefaction potential.

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Civil Engineering
Degree name
Doctor of Philosophy
Publisher
University of Canterbury
Journal Title
Journal ISSN
Volume Title
Language
English
Date
1995
Authors
Vreugdenhil, Roger Anton
Abstract

The first half of this report documents an investigation of level ground sites of liquefaction during the 17 October, 1989, Loma Prieta earthquake. Field testing at ten liquefaction sites included piezocone probes (CPTU) and Standard Penetration Tests (SPT). Based on the comparison of ejecta and SPT sample mean grain size, liquefaction appeared to be shallow for the Loma Prieta event.

The performance of five liquefaction prediction models is assessed using the Loma Prieta CPTU data. The five models include two energy-based methods, two methods based on cyclic stress ratio, and one purely empirical method. Results from this analysis indicate that, for this data set, no single model clearly out-performed the others; the predictions of four of the five models were highly conservative. The CPT-SPT correlation is examined, as part of a wider study of model sensitivity to parameter variation.

Previously developed corrections to piezocone data for pore water pressure effects are examined, along with methods for CPTU data normalisation. Pore pressure effects, however, are far outweighed by effects of soil layering on cone resistance. An investigation of layering effects on cone resistance and sleeve friction is detailed in the second half of this report.

A simple approximate analysis is presented for interpretation of cone penetration results when cone resistance is affected by layering of soils with different stiffnesses. It is argued that the cone senses the presence of a nearby layer elastically, and an approximate elastic analysis is developed to quantify the effect on cone resistance. The approximate solution compares favourably with an exact elastic solution for the case of two layers, and with multilayered calibration chamber experimental results. The problem of misclassification of thin stiff layers embedded in softer materials is illustrated by application of the approximate solution to sections of cone resistance records from the Loma Prieta data.

Results from an investigation of layering effects on sleeve friction indicate that, when probing in asymmetrical soil layering configurations, sleeve friction resolution may exceed that of cone resistance. Values for lag between cone resistance and sleeve friction are suggested, based on results from a statistical cross-correlation analysis performed on the Loma Prieta CPTU data. Lag is found to increase with increasing soil stiffness.

An attempt is made to invert sections of real cone resistance records for layer thicknesses and stiffness ratios, by employing a Genetic Algorithm to optimise the approximate solution. Sections of cone resistance from Loma Prieta field records are successfully inverted for stiffness ratios. Partial inversion is favoured due to computation time considerations.

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All Right Reserved