Engineering Properties and Behavior of Silty Soils
Advisor: Prof. Gholamreza Mesri
Abstract
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.
Key index properties such as grain size distribution, particle morphology, Atterberg limits, and
liquidity index (Il) are evaluated across more than ten representative silty soil sites worldwide.
Silty soil is defined as particle size between 2 μm and 60 μm; clayey silts contain more than
20 % clay size smaller than 2 μm and sandy silts contain more than 30 % sand size larger
than 60μm. SEM imaging reveals that particle shape and microfabric significantly influence
permeability, compressibility, and shear strength. Permeability of silty soils is between 10−9
and 10−6 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 (σ′p) with σ′p/σ′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 (C′c ) is introduced for settlement prediction when σ′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 inflection point of the effective stress path, is evaluated. The normalized undrained
shear strength ratio (su/σ′p) ranges from 0.25 to 0.40 in TC, with corresponding cone factors
1
(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
plasticity index and liquidity index, offering improved prediction of cyclic response for finegrained
soils. Empirical values for cyclic resistance ratio (CRR) are developed as functions
of Ip and σ′p/σ′v0, and shown to align with data from 22 international silt sites. CPT-based
liquefaction frameworks, including Boulanger and Idriss (2016) and ΔQ 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.