First Advisor

Diane Moug

Term of Graduation

Winter 2026

Date of Publication

2-5-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Civil & Environmental Engineering

Department

Civil and Environmental Engineering

Language

English

Subjects

Cyclic, Direct shear test, Fine-grained, Non-plastic silt, SHNASEP, Silt

Physical Description

1 online resource (xiv, 221 pages)

Abstract

Low-plasticity silts are "transitional" soils prevalent in many seismically active regions, including the Pacific Northwest. Most research in the literature has historically focused on sand materials or clay materials, while low plastic silt does not fit either traditional "sand-like" or "clay-like" frameworks, resulting in a knowledge gap regarding this type of soil. Knowing the behavior of this type of soil is more important because it is susceptible to liquefaction or cyclic softening. Existing research on low-plastic silts is mostly based on either intact soil, where some parameters, such as the overconsolidation ratio (OCR) and plasticity index (PI), cannot be accurately estimated or kept constant over several tests, or on reconstituted soil using manufactured silt, which may not be representative of natural soil.

This dissertation addresses the knowledge gap of the behavior of natural transitional soils through a systematic laboratory investigation. Natural fine-grained soil from a Columbia River floodplain deposit was mechanically separated into its non-plastic (silt) and plastic (clay) fractions using a novel process. These fractions were then remixed to create reconstituted slurry specimens with controlled PI of 0, 3, 7, 15, and 57. The monotonic and cyclic behavior of these mixtures was evaluated using one-dimensional (1D) consolidation and direct simple shear (DSS) tests across a range of effective vertical stresses (σ’vc) and OCR.

Monotonic testing revealed that the normally consolidated natural silts, including the non-plastic (PI=0) mixture, exhibited contractive, "clay-like" shear behavior, and the undrained shear strength (Su) for all mixtures tested can be normalized using SHANSEP parameters. The SHANSEP parameters, such as the S parameter, increase from 0.22 (PI=0) to 0.30 (PI=30), while the m parameter decreases from 0.86 (PI=0) to 0.67 (PI=57), as PI increases from 0 to 57.

Cyclic DSS (CDSS) testing demonstrated that cyclic behavior is also related to PI. The cyclic resistance ratio (CRR), normalized cyclic strength (τcyc/Su), post-cyclic shear strength (Su-PC), and post-cyclic volumetric strain (εv-PC) all increased with increases in PI. A transition in pore pressure generation was also observed; low-plasticity soils (PI=0-7) reached 6% shear strain double amplitude (γDA=6%) as a failure criteria at excess pore pressure ratios (ru) greater than 0.90, whereas ru for the PI=15 and PI=57 reached failure criteria lower than 0.87 and 0.8, respectively.

Finally, a study was conducted to compare two common consolidation approaches for DSS testing on specimens prepared from intact Shelby tube samples of low-plasticity silt, i.e., SHANSEP consolidation and Recompression, and then to compare them with the slurry deposition method. The study found that the SHANSEP consolidation approach yielded higher CRR, around 60% to 250% than the Recompression method. Furthermore, the ratio of CRR from SHANSEP to CRR from Recompression loading appears to increase with OCR and decrease with PI. However, CRR curves from the reconstituted method with the same soil were lower than the SHANSEP method but higher than the recompression method. This suggests the SHANSEP approach may be more representative of in-situ stresses, while the Recompression method may not replicate the in-situ lateral stress ratio (Ko).

These research findings provide a dataset for estimating engineering parameters of silts in the Portland area, particularly fine-grained Columbia River soil deposits. The study also contributes to a fundamental understanding of how natural soil shear behavior changes with PI.

Rights

In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Comments

This work was funded by the Pacific Northwest Transportation Consortium (PacTrans; grant number 69A3552348310), a U.S. DOT University Transportation Center, and by the National Science Foundation CAREER Grant CMMI-2340596.

Persistent Identifier

https://archives.pdx.edu/ds/psu/44568

Download

Share

COinS