Numerical modeling and simulation of seismic settlements on dense sands

Numanoglu, Ozgun Alp

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https://hdl.handle.net/2142/106460 Copy

Description

Title

Numerical modeling and simulation of seismic settlements on dense sands

Author(s)

Numanoglu, Ozgun Alp

Issue Date

2019-12-05

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

Hashash, Youssef MA

Doctoral Committee Chair(s)

Hashash, Youssef MA

Committee Member(s)

• Olson, Scott M
• Elbanna, Ahmed E
• Rutherford, Cassandra JR

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)

• constitutive model
• numerical modeling
• plasticity
• dense sand
• seismic settlements
• finite element analysis
• I-soil
• earthquake
• shear behavior
• volumetric behavior
• contraction
• dilation

Abstract

Heavy components of nuclear power plant structures, in absence of rock stratum, may be founded on dense compacted granular materials. The seismic performance of such structures constructed on natural or compacted, dense sands depends on the cyclic shear stress - shear strain – volumetric strain response of the underlying soil. Dense natural or compacted coarse-grained soils are not expected to liquefy during an earthquake. However, accumulated volumetric strains within a deep deposit during multi-directional seismic loadings may lead to non-trivial settlements. In these situations, reliable estimations of seismic settlements are essential for assessing safety and serviceability of the structures. However, such settlements are not yet well understood due to limited number of studies for dense sands and the effects of multi-directionality. This dissertation describes the development of a new, three-dimensional constitutive model and application of three-dimensional nonlinear finite element analyses to estimate settlements in sands with and without structure conditions under uni/bi-directional seismic loadings. Constitutive model performance for liquefaction problems is also evaluated. The first part (Chapter 1 and Chapter 2) defines the engineering problem and reviews selected existing constitutive models, and a new, three-dimensional, distributed element plasticity-based, effective mean stress-dependent constitutive model (I-soil) is introduced for dense to very dense sands (Chapter 3). This constitutive model captures: (1) both Masing and non-Masing type hysteretic behavior; (2) small strain nonlinearity; and (3) shear-induced volumetric behavior including seismic settlements and excess porewater pressure generation/dissipation. I-soil uses distributed element plasticity (DEP) framework and does not require kinematic hardening rule. Thus, the mathematical formulation and numerical implementation is simple and efficient. The model is implemented in soil-fluid coupled dynamic finite element analysis platform LS-DYNA. The model parameters, defining shear stress – shear strain behavior, are determined using soil index properties, shear wave velocity, a normalized strain-dependent modulus reduction curve, and damping curve. Shear induced volumetric response is modelled using two additional parameters. Calibration of these parameters were achieved using 168 laboratory tests and 2730 calibration simulations utilizing statistical method for generating a near-random sample of parameter values (Chapter 4). Part two presents application of the developed model for boundary value problems including shear beam type free-field (Chapter 5) and three-dimensional soil -structure interaction (SSI) conditions with fluid coupling (Chapter 6). A single calibration was used for both free-field and SSI simulations. A unique centrifuge “case histories” were used to evaluate the model performance. The results show that the numerical models capture the uni and bi-directional responses in a shear beam and SSI in terms of energy intensities, peak ground accelerations and spectral acceleration with depth. Only two additional parameters for shear induced volumetric response allows finite element simulations to capture measured settlements, excess porewater pressures and effect of multi-directional loading on shear and volumetric behavior. In addition, energy intensities and three-dimensional strain information extracted from finite element analyses are shown to provide reliable information to proposed hybrid empirical and semi-empirical models on estimation of seismic settlements in free-field and with structure conditions. Run times for 3-D SSI simulations are within 6-22 hours using two logical core per event in a computer server. Part three (Chapter 7) presents finite element analysis of a liquefying site. Port Island case history was used to assess the constitutive and numerical model performance on estimating recorded response in a downhole array. The selected case history is the Port Island array response during the 1995 Kobe Earthquake. The recorded acceleration time histories and spectral response were compared to the computed counterparts. The numerical model was observed to capture the certain recorded behavior such as long period acceleration time histories and amplification in spectral accelerations at longer periods as well as de-amplification of PGA due to liquefaction. Stress-strain behavior observed from computed response show that the constitutive model is capable of producing “banana loops” which is result of effective mean stress dependent response of the constitutive model along with its capability of representing contractive-dilative tendency.

Graduation Semester

2019-12

Type of Resource

text

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http://hdl.handle.net/2142/106460

Copyright and License Information

Copyright 2019 Ozgun Alp Numanoglu

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