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Title:Element-level behavior of dense coarse grained soils under multidirectional dynamic loading
Author(s):Bhaumik, Lopamudra
Director of Research:Rutherford, Cassandra J; Hashash, Youssef M. A.
Doctoral Committee Chair(s):Rutherford, Cassandra J; Olson, Scott M
Doctoral Committee Member(s):Elbanna, Ahmed
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
simple shear
direct simple shear device
dense sand
silty sand
volumetric strain
porewater pressure
liquefaction resistance
specimen preparation
Abstract:In 2011, the Tohoku earthquake generated a tsunami that led to the devastating Fukushima nuclear power plant (NPP) disaster in Japan. This accident prompted a global effort towards re-evaluation of the seismic safety criteria for NPPs. NPP structures, in absence of a rock stratum, may be founded on densified granular material to mitigate liquefaction related issues. Seismic deformations of these dense sands are generally considered to be within structure tolerable limits and are not incorporated in standard dynamic soil-structure interaction analyses of NPPs. However, research has shown that dense, compacted, or natural sands can experience considerable vibration-induced settlement when dry due to repeated loading cycles or reconsolidation settlement when saturated. Present state-of-practice in seismic deformation analysis is primarily based on unidirectional tests, chiefly due to the lack of specialized devices that can apply complex multidirectional loading paths that occur during an earthquake. The limited studies that consider bi-directional shaking do not investigate volumetric strains in very dense sands or consider the effect of fines often present in natural sands. This work seeks to address these gaps in knowledge. This dissertation describes and benchmarks the newly constructed Illinois multidirectional cyclic direct simple shear (I-mcDSS) device, which, for the first time, brings together the following capabilities: servo-hydraulic control than can apply stress- or strain-based monotonic, cyclic (e.g., sinusoidal, saw tooth, square), and high frequency broadband loads at the actual loading rate, improving over previous devices with pneumatic control; uni- and bidirectional loading; bender elements to measure shear wave velocity, to partly overcome small-strain (~10-2%) measurement limits of the device; a cell for applying different confinements (apart from K0), and back-pressure saturation; and a multi-directional load cell on the top of the specimen, to minimize the effect of compliance and friction of the device components on the load measurements. The effect of mechanical device compliance on the recorded response is explored with a comprehensice comparison to existing direct simple shear devices in literature. Recommendations are made for improving measurements at small shear strains in bidirectional devices. A new unique database consisting of more than 650 drained and undrained multidirectional cyclic direct simple shear tests is developed to examine the effect of various material, state, and loading factors on volumetric response, i.e., drained volumetric strain and porewater pressure generation. The examined factors include loading frequency, duration, amplitude, path, multidirectionality, soil density, overburden, overconsolidation, prior shaking history, specimen preparation method, non-plastic and low-plasticity fines content, and gradation. The cyclic tests consisted of unidirectional, circular, figure-8 and broadband loading tests on medium dense to very dense clean, silty, and clayey sands at varying consolidation stresses. Different state factors that can be used for unifying clean sands and sands with fines were explored. It was observed that the coefficient of volume compressibility measured in Oedometer tests was most promising. Multidirectionality factors for drained and reconsolidation volumetric strain, and porewater pressure that can be readily used in engineering practice are evaluated with respect to each of the aforementioned factors and recommended. Several demand and capacity parameters for estimating volumetric strain in medium to very dense sands with and without fines under multidirectional drained and undrained loading were examined including shear strain, cyclic shear stress, peak ground velocity, cumulative shear strain, dissipated energy (Ws), shear wave velocity, and coefficient of volume compressibility (mv). The relationship between (mv)(Ws) and volumetric strain appeared to be unique irrespective of soil type, soil density, number of loading directions, loading path, and loading duration. Thus, (mv)(Ws) is a promising parameter that potentially can be used to estimate volumetric strain for all soil types.
Issue Date:2018-12-03
Rights Information:Copyright © 2018 Lopamudra Bhaumik
Date Available in IDEALS:2019-02-08
Date Deposited:2018-12

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