Seismic liquefaction of heterogeneous soil: mechanism and effects on structural response

Chakrabortty, Pradipta (2008) Seismic liquefaction of heterogeneous soil: mechanism and effects on structural response. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In the current practice of liquefaction prediction analysis, horizontally layered soil (with uniform properties within distinct soil layers) is usually assumed to estimate the liquefaction susceptibility of a soil deposit. However most of the soil properties of a natural deposit not only vary in the vertical direction but they could also vary in the horizontal direction, even within the so-called 'uniform' soil layers. This soil variability can be broadly classified into two main groups. They are the lithological heterogeneity (variability due to geological layers) and the small scale spatial variability. The first source of variability (variability due to layers) is considered properly in the current practice. But the second source of soil variability (small scale spatial variability), which is the subject of this research, is not properly addressed in general. -- From recent numerical research it was observed in the case of seismically induced excess pore water pressure (EPWP) generation that more EPWP is generated during an earthquake in a heterogeneous soil deposit than in a homogeneous soil with equivalent average soil relative density. EPWP is generated in the loose sand pockets first and then the water migrates into the neighbouring dense soil, and softens the dense soil by reducing the effective stress. However, to date, a limited amount of experimental verifications are available to the practicing engineering community to help in recognizing, quantifying and accepting the above-mentioned behaviour of heterogeneous soils. Therefore the first major objective of the present study is to investigate, quantify, and explain the effects of small scale soil heterogeneity on seismically induced liquefaction using both numerical modelling and centrifuge experiments. The second major objective of this research is to provide recommendations for seismic design of structures on heterogeneous soil. -- A series of three centrifuge tests were performed in this study: one on homogeneous soil and two on heterogeneous soil. The test on uniform soil was performed on a soil deposit with the soil relative density lower than the average soil relative density of heterogeneous soil. The results, such as EPWP, accelerations, and settlements were monitored and measured throughout the test duration. However, it is very difficult and expensive to monitor all this responses everywhere in the model. Therefore, a numerical model was calibrated and validated from the centrifuge test results on uniform soil first. Then, the liquefaction mechanism in heterogeneous soil was studied in more details using the numerical simulations. Experimental results support the conclusion of previous research that more EPWP is generated in a heterogeneous soil than in the corresponding homogeneous soil. From this study it is concluded that although the average soil relative density of heterogeneous soil deposit (test2 and test3) was larger than that of the uniform soil (test1), the liquefaction resistance of the heterogeneous soil was lower than that of the uniform soil due to water migration from loose to dense soil pockets in heterogeneous soil deposit. -- In the last part of this research, a parametric study was performed for finding the effects of soil heterogeneity on the structural response. The numerical model validated based on the results from the centrifuge experiments with heterogeneous soil was used in this study. Dynamic analyses were performed for various types of structures situated on heterogeneous soil. Based on the type of structure, this segment of the research is divided into two parts. The performance of a tower structure (where total settlements and base rotations were of primary interest) was studied in the first part. In the second part, the performance of a frame structure (where total and differential settlements were of primary interest) was studied. Quantitative and qualitative recommendations for geotechnical design practice are provided for structures on liquefiable heterogeneous soil deposits for a wide range of soil relative densities. The results of reliability analysis for the structure situated on heterogeneous soil are presented in the form of fragility curves and combined damage curves. -- Current design guidelines for spectral amplification of seismic motion were also verified for a range of soil properties, and updated guidelines were also provided after considering the effect of soil heterogeneity. The effect of soil variability was found to be not very significant in the studied ranges for calculating the values of spectral amplification factors. There is a less than 10% change in the values of Fₐ and Fᵥ for heterogeneous soil compared to that in equivalent homogeneous soil.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/8931
Item ID: 8931
Additional Information: Includes bibliographical references (leaves 263-280)
Department(s): Engineering and Applied Science, Faculty of
Date: 2008
Date Type: Submission
Library of Congress Subject Heading: Engineering geology; Soil liquefaction; Soil mechanics--Computer programs; Soil permeability

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