Engineering properties and behavior of silty soils

Silty soils, which exhibit engineering behavior intermediate between sands and clays, pose persistent challenges in geotechnical engineering due to their variable fabric, partial drainage behavior, and difficulty in classification and sampling. This dissertation presents a comprehensive investigation into the engineering properties, in-situ behavior, and seismic response of natural silty soils from diverse geological and geographical settings, including post-glacial, fluvial, deltaic, aeolian, and lagoonal environments. These data were compiled from multiple investigators to include the widest possible range of silty soil deposits. Key index properties such as grain size distribution, particle morphology, Atterberg limits, and liquidity index (LI) are evaluated across more than ten representative silty soil sites worldwide. Silty soil is defined as particle size between 2 and 60 microns; clayey silts contain more than 20% clay size smaller than 2 microns and sandy silts contain more than 30% sand size larger than 60 microns. SEM imaging reveals that particle shape and microfabric significantly influence permeability, compressibility, and shear strength. Permeability of silty soils is between 1e-9 and 1e-6 m/s, with measured anisotropy (kh0/kv0) values as high as 5 in laminated deposits. Compressibility is characterized using a newly developed modified energy method for determining preconsolidation pressure (sigma′p) with sigma′p/sigma′v0 in the range of 1 to 3, allowing consistent assessment of yield behavior from incremental loading tests even in the absence of a clear breakpoint between recompression and compression range. Compression indices (Cc), between 0.1 and 0.4, are correlated with natural water content, and coefficient of consolidation (cv and ch) values span 10–6000 m2/year depending on soil type and structure. A secant compression index (Cc′) is introduced for settlement prediction when sigma′p is difficult to identify. Monotonic shear strength is interpreted from high-quality undisturbed samples under triaxial and direct simple shear conditions. Yield strength criteria, such as the phase transformation point (PT) at the inflection point of the effective stress path, is evaluated. The normalized undrained shear strength ratio (su/sigma′p) ranges from 0.25 to 0.40 in triaxial compression, with corresponding cone factors (Nk(TC)) decreasing from 24 for sandy silt to 14 for clayey silt. Rigidity index (IR) ranges from 70 to 300 and supports improved interpretation of piezocone dissipation tests. Cyclic shear strength and liquefaction potential are investigated through a combination of laboratory testing and case history validation. A modified “Chinese criteria” is proposed based on the plasticity index and liquidity index, offering improved prediction of cyclic response for fine-grained soils. Empirical values for cyclic resistance ratio (CRR) are developed as functions of LI and sigma′p/sigma′v0, and shown to align with data from 22 international silt sites. CPT-based liquefaction frameworks, including Boulanger and Idriss (2016) and DeltaQ method by Saye et al. (2021), are evaluated for silty soils and benchmarked against Christchurch earthquake case data. These comparisons highlight the limitations of standard CPT methods in partially drained or fabric-sensitive deposits. This research advances the understanding of silty soil behavior by integrating laboratory findings, in-situ test interpretation, and empirical modeling into a unified framework. The findings support the development of more reliable predictive tools for design applications involving silty soils, especially under seismic loading and complex drainage conditions.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Civil Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Zhang, Lingfeng

Contributors dc:contributor

• Mesri, Gholamreza
• Stark, Timothy D
• Olson, Scott M
• Popovics, John S

• Silt
• Engineering Property
• Permeability
• Compressibility
• Shear Strength

Statement dc:rights

• Copyright 2025 Lingfeng Zhang

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132625

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132625