Compressibility and permeability of natural soft clays and surcharging to reduce settlements
Feng, Tao-Wei
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https://hdl.handle.net/2142/20417
Description
Title
Compressibility and permeability of natural soft clays and surcharging to reduce settlements
Author(s)
Feng, Tao-Wei
Issue Date
1991
Doctoral Committee Chair(s)
Mesri, Gholamreza
Department of Study
Civil and Environmental Engineering
Discipline
Civil Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Civil
Language
eng
Abstract
Soft clays, often with high compressibility and low permeability, are frequently encountered in important engineering projects. This investigation has been concerned with consolidation characteristics of natural soft clays.
The uniqueness concept of EOP void ratio-effective stress relationship is further examined by: (1) comparing laboratory EOP e $-$ log $\sigma\sp\prime\sb{\rm v}$ curves obtained from specimens with maximum drainage distance of 0.5m, 0.125m, 0.05m, and 0.025m, and (2) analyzing soil behavior during primary consolidation using laboratory data on $(\partial{e}/\partial\sigma\sp\prime\sb{v})\sb{t}$ and $(\partial{e}/\partial{t})\sb{\sigma\sp\prime\sb{v}}.$ The consolidation stress path of thick field soil sublayers is interpreted from laboratory data and compared to the EOP e $-$ log $\sigma\sp\prime\sb{\rm v}.$ It is confirmed from results of this investigation that the ILLICON methodology is the most realistic approach for settlement and pore water pressure analyses of thick field soil layers, for consolidation increments that span the preconsolidation pressure.
A strain rate of 10 times the laboratory EOP strain rate is recommended for carrying out CRS tests to determine both compressibility and permeability characteristics of natural soft clays and silts. The permeability characteristics, in terms of the initial coefficient of vertical permeability, k$\sb{\rm v0},$ and e $-$ log k$\sb{\rm v},$ were measured during oedometer tests by either falling-head or constant-head permeability tests. The measured e $-$ log k$\sb{\rm v}$ data are summarized in terms of (e/CF)/(A$\sb{\rm c}$ + 1) versus log k$\sb{\rm v},$ and described by a simple empirical equation. Based on this equation, a series of curves are generated to show the dependence of k$\sb{\rm v0}$ on void ratio, clay fraction, and activity of soft clay.
An empirical approach for prediction of post-surcharge secondary settlements is presented. Since both field and laboratory data show an initial increase in post-surcharge secondary compression index with time, post-surcharge secondary settlement is expressed in terms of a secant secondary compression index, C$\sp{\prime\prime}\sb\alpha.$ The removal of surcharge leads to rebound, including primary rebound up to t$\sb{\rm pr}$ and secondary rebound that levels off at t$\sb{\rm l},$ and is followed by secondary compression. Results of laboratory surcharging tests on 6 natural soft clays are used to develop a database for C$\sp{\prime\prime}\sb\alpha$/C$\sb\alpha$ $-$ t/t$\sb{\rm l}$ and t$\sb{\rm l}$/t$\sb{\rm pr}$ $-$ R$\sp\prime\sb{\rm s}$ correlations, where R$\sp\prime\sb{\rm s}$ is the effective surcharge ratio.
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