An investigation of liquefied shear strength using novel centrifuge tests, direct simple shear tests, and field case histories

Chen, Jiarui

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https://hdl.handle.net/2142/117534 Copy

Description

Title

An investigation of liquefied shear strength using novel centrifuge tests, direct simple shear tests, and field case histories

Author(s)

Chen, Jiarui

Issue Date

2022-09-21

Director of Research (if dissertation) or Advisor (if thesis)

Olson, Scott M.

Doctoral Committee Chair(s)

Olson, Scott M.

Committee Member(s)

• Hashash, Youssef M. A.
• Tom, Joe G.
• Dewoolkar, Mandar M.
• Dubief, Yves

Department of Study

Civil & Environmental Eng

Discipline

Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• liquefaction
• flow liquefaction
• liquefied shear strength
• residual strength
• direct simple shear
• centrifuge
• cone penetration test
• standard penetration test
• case history

Abstract

Recent high-profile catastrophic ash pond and tailings dam failures, which have been classified as flow liquefaction failures, renewed interest in the mobilized shear strength during flow liquefaction failures, usually termed the liquefied shear strength, su(liq). Statistics indicate that although the number of failures and incidents at tailings storage facilities decreased in the past 30 years or so, the number of “very serious” and “serious” failures actually constituted an increasing percentage since 1960, and the average financial loss for each “very serious” failure was found to be at least $500 million. This trend in the mining and coal ash industries indicates an urgent need for safety improvement, which highly relies on the shear strength of the deposited material within the impoundment. To improve the understanding of the behavior of soils within a tailings or coal ash impoundment as well as other problematic soils and to better quantify the shear strength of the soil if liquefaction occurs, three research methodologies were adopted in this study: (1) field flow liquefaction failure back-analysis; (2) element-level direct simple shear testing; and (3) novel centrifuge modeling. Corresponding results were compared and discussed. Field cases of flow liquefaction provide valuable information for liquefied shear strength because those failures provide realistic estimates of su(liq). An updated database of liquefaction-induced flow failures (now totaling 71 cases) was compiled as part of this study, including 13 new cases identified and evaluated. Previously studied cases were reviewed and adjusted if needed to maintain consistency in the analyses for all 71 cases. Two predictive models for su(liq) based on field test indices – cone penetration test (CPT) tip resistance and standard penetration test (SPT) blow count, were developed using the best-quality cases analyzed using the most rigorous procedures. The predictive models developed in this study highlighted: (1) the importance of compressibility of the liquefied soil in the field; and (2) the relationship between su(liq) and pre-failure effective vertical stress might be slightly nonlinear. Direct simple shear (DSS) laboratory element tests under controlled drainage conditions performed were used to characterize the critical state properties of three nonplastic soils, including the one used in the centrifuge modeling. In addition, various strain rates were used in DSS tests to supplement the strain rate range used in the centrifuge modeling. These DSS tests indicated a negligible to minor strain rate effect. Moreover, various complicated loading scenarios were adopted in DSS tests to closely mimic the loadings experienced in the centrifuge modeling to provide insights for interpreting those centrifuge test results. A series of novel centrifuge tests, which involved pulling (at constant velocity) a thin metal plate (referred to as a coupon) horizontally through liquefied soil, were performed to mimic shearing along a critical slip surface in a flow liquefaction failure and to produce additional data pairs of su(liq) and CPT tip resistance. Porewater pressure along the coupon shear surface was directly measured using a miniature pressure transducer embedded in the coupon. With similar initial states, the variation of coupon velocity induced different degrees of dilation and led to two distinct shear surface porewater pressure responses. Potential void redistribution was identified for faster coupon pulls with a larger extent of dilation. Values of su(liq) were extracted from select coupon pulls where the shear-surface excess porewater pressure ratio ≥ 0.8. Coupon pull su(liq) generally were comparable to case history back-calculated su(liq) and exhibited a dependency on strain rate. Lastly, dimensionless mechanics-based parameters were investigated to interpret the physics of the liquefied soil. The adopted dimensionless parameters, Savage number, NSav, and dispersive-viscous ratio, Dv*, linked the behavior of liquefied soils at multiple scales – from element scale to centrifuge model scale to field scale. Specifically, NSav successfully explained the controversy regarding strain rate effects on su(critical) or su(liq) observed in laboratory element (including DSS tests in this study) and centrifuge tests. In addition, Dv* revealed that the shear strength of the liquefied soil, which resembled a neutrally buoyant mixture of solid grains and fluid, could increase linearly (i.e., a Bingham plastic model) or quadratically with the strain rate. The preliminary comparison of coupon pull su(liq) and case history back-calculated su(liq) suggested that the Bingham plastic model may capture the behavior of the liquefied soil in the field reasonably well. As a result, using the dimensionless parameters helped reconcile the soil mechanics interpretation and the fluid mechanics interpretation of the liquefied soil.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Jiarui Chen

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