# Your search: "author:Brandenberg, Scott J"

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## Scholarly Works (47 results)

In Part I of this dissertation, equivalent static analysis (ESA) procedures for computing foundation demands during lateral spreading are applied to two parallel bridges that were damaged during the 2010 M 7.2 El Mayor-Cucapah earthquake in Baja California, Mexico. A railroad bridge span collapsed, whereas the adjacent highway bridge survived with one support pier near the river having modest flexural cracking of cover concrete. Cone penetration and geophysical test results are presented along with geotechnical and structural conditions evaluated from design documents. ESA using a beam-on-Winkler foundation model is found to accurately predict observed responses when liquefaction-compatible inertia demands are represented as spectral displacements that account for resistance from other bridge components. Pier columns for the surviving bridge effectively resisted lateral spreading demands in part because of restraint provided by the superstructure. Collapse of the surviving bridge is incorrectly predicted when inertial demands are computed for the individual bent in isolation from other components, and are represented by forces that do not consider global restraint.

In Part II, results of a parametric study of the influence of kinematic pile-soil interaction on foundation-input motions (FIM) are presented. One-dimensional nonlinear ground response analysis was used to define free-field motions, which were subsequently imposed on a beam-on-nonlinear-dynamic-Winkler-foundation pile model. The free-field ground surface motion (FFM) and top-of-pile “foundation-input motion” (FIM) computed from these results were then used to compute transfer functions and spectral ratios for use with the substructure method of seismic analysis. A total of 1,920 parametric combinations of different pile sizes, soil profiles, and ground motions were analyzed.

Results of the study show that significant reductions of the FFM occur for stiff piles in soft soil, which could result in a favorable reduction in design demands for short-period structures. Group effects considering spatially-variable (incoherent) ground motions are found to be modest over the footprint of a typical bridge bent, resulting in an additional reduction of FFM by 10 percent or less compared to an equivalent single pile. This study aims to overcome limitations of idealistic assumptions that have been employed in previous studies such as linear-elastic material behavior, drastically simplified stratigraphy, and harmonic oscillations in lieu of real ground motions. In order to capture the important influence of more realistic conditions such as material nonlinearity, subsurface heterogeneity, and variable frequency-content ground motions, a set of models for predicting transfer functions and spectral ratios has been developed through statistical regression of the results from this parametric study. These allow foundation engineers to predict kinematic pile-soil interaction effects without performing dynamic pile analyses.

Part I of this dissertation describes a simple experimental technique that can be used to obtain the slopes of the plastic potential and yield functions during shear based on the deformation theory of plasticity. The method imposes small perturbations in the direction of the stress increment by closing the drainage valve, thereby abruptly switching from drained to undrained loading conditions during plastic loading. Elastoplastic moduli are obtained immediately before and after the perturbation from the measured deviatoric stress, mean effective stress, deviatoric strains, and volumetric strains for the stress paths immediately before and after closing the drain valve. During drained shear, samples were sheared while the mean effective stress was maintained constant. Combining tests performed at several confining stresses, the proposed method can map yield and plastic potential surfaces and predict their evolution for a wide range of stresses.

Part II of the dissertation focuses on providing insights into the effects of changes in clay mineralogy and pore-fluid chemistry on cyclic behavior of low-plasticity fine-grained soils. A series of cyclic and monotonic direct simple shear experiments was conducted on three low-plasticity fine-grained mixtures of non-plastic silt with either bentonite or kaolinite clay minerals blended with fresh deionized water or saline water. The clay fractions were adjusted to achieve a plasticity index of PI = 9 for all three mixtures to study differences or similarities in their behavior and to examine the effectiveness of index properties in identifying the observed differences in lab results.

Even though all three blends have the same plasticity index, significant differences in their cyclic response were observed. Results indicate that plasticity index is an insufficient indicator of the cyclic behavior of low-plasticity fine-grained soils, and corrections for pore fluid chemistry and clay minerology may be needed for future liquefaction susceptibility and cyclic softening assessment procedures. Site-specific cyclic and monotonic testing for important projects remains to be the recommended approach for properly characterizing the seismic behavior of such soils with the current level of understating of their complex behavior.

The collected experimental data, visualization and post-processing tools, as well as documented experimental procedures of Part I are curated and published for public access on http:www.DesignSafe-ci.org. The data from Part II is being curated and will be published online in the near future.

Peat is a highly compressible organic material with unique properties that differ from inorganic mineral soils, which poses a challenge in their constitutive modeling. The main specific challenge addressed in this dissertation include matching dynamic properties (i.e., modulus reduction and damping behavior). Constitutive models used in 1D site response typically use modulus reduction and damping curves as input parameters, and usually introduce a misfit of the desired behavior, particularly at high strains. This is problematic for peat because large strains are expected to develop during cyclic loading due to the peat softness.

Nonlinear one dimensional ground response models generally present a compromise between fitting the backbone curve or the hysteretic damping curve. Fitting the damping curve depends on unloading and reloading rules. Most of the models use Masing rules or extended Masing to correct the overdamping at high strains resulting from using Masing rules. Frequency dependent Rayleigh damping is used to introduce damping at low strains. I present a new formulation of unloading and reloading rules completely departing from Masing rules. The main idea is to rotate the axis of the stress strain curve and change the point of reference to calculate the stress at the next time step. The small strain damping is made hysteretic by increasing the initial departure tangent modulus when unloading, in a way consistent with what has been observed in laboratory tests. The unloading-reloading rule is implemented in a nonlinear code and is able to match any backbone and hysteretic damping without Rayleigh damping.

Dynamic curves are typically not used in 2D or 3D models because their inclusion in a plasticity framework is complicated due to their dependence on confining pressure, which can change during earthquake loading (e.g. when excess pore pressure develops under undrained loading). Hence, the damping behavior is not an input of current 3D constitutive models. In order to facilitate the inclusion of dynamic curves in constitutive models, I present a new concept that plots modulus reduction and damping curves against stress ratio instead of shear strain. This results in pressure-independent modulus reduction and damping curves for three empirical relationships commonly-used to derive modulus reduction and damping curves. This finding is useful for implementation in one-dimensional effective stress ground response analysis codes for undrained loading conditions, and in advanced plasticity models.

I then extend the developed unloading-reloading rule and include it in a 3D constitutive model that uses modulus reduction and damping curves that are plotted against stress ratio by using the aforementioned concept. The formulation of the model allows to match dynamic properties (i.e., modulus reduction and damping curves), in 1D and 2D site response. At large strains the strength is controlled by a bounding surface algorithm following the formulation from Dafalias and Manzari (2004). The volumetric response is controlled by a dilation surface that introduces plastic volumetric strains based on deviatoric plastic strains. Most of the input parameters are well-known engineering properties easily measured in laboratory tests. Default values are defined for the input parameters that are not easily measured. I present the implementation of the model in FLAC and some typical predictions of the model through simulations of cyclic triaxial and simple shear tests. Finally, I present the calibration of the model for Sherman Island peat based on laboratory tests, and the performance of the model in 1D site response simulations.

The current best practice in geotechnical engineering in determining lateral capacity of piles is to replace the soil reaction with a series of independent springs. Basically, the model uses beam theory to represent the pile and uncoupled, non-linear load transfer functions, called p-y curves to represent the soil.

Most of the existing methods for determining p-y curves are highly empirical, based on a limited number of cases of laterally loaded piles, which were instrumented, enabling to measure the pile deflection in discrete depth intervals subject to different lateral load (i.e. Matlock 1970, Reese 1975). In essence, these methods have their own limitations, and are mainly applicable for the conditions similar to the tested conditions.

Although later, more detailed investigations by different people addressed some of the problems, still the basis of the existing design programs such as LPILE, or procedures introduced in applicable codes such as API (American Petroleum Institute), is the same original recommendations made by Matlock and Reese during seventies.

In recent era, demand in employment of in-situ direct-pushed based methods using multi-measurement in-situ devices, such as the seismic cone penetration test with pore water measurement (SCPTu) and Seismic Flat Dilatometer Test (SDMT) is significantly increased.

The main objective of this research is to introduce a unified CPT-based approach for determining p-y curves and pile responses to lateral loads. The suggested approach will provide explicit and defined steps/criteria to develop p-y curves for piles subjected to lateral loads using CPT data. CPT data will be used to determine soil strength parameters. Recent developments in relating CPT data to soil basic parameters using Critical State Soil Mechanics (CSSM) framework will be implemented in the suggested model.

In all current common models, pre-determination of the soil behavior and the model to be used (e.g. Matlock clay, 1970 or Reese sand, 1975), will become warranted even before commencement of the analysis. On the contrary, in the proposed model, the need for the said pre-determination of soil behavior is eliminated. As discussed in Section 2.3.5, soil behavior in the model is being classified into four broad and general groups: drained-dilative, drained- contractive, undrained-dilative and undrained-contractive

The main factor driving the suggested analytical approach is Soil Behavior Type Index, Ic. In the proposed approach, the SBT index, Ic, will be used to determine the in-situ characteristics and behavior of the soil. Based on the value of Ic calculated from CPT data, it could be determined that the soil behaves as a sand-like or a clay-like soil, and during the shearing would behave in undrained or drained condition. The measured shear wave velocity during field test using seismic cone penetration test or other methods such as SASW (Spectral Analysis of Surface Waves) or Cross-Hole logging, may be used to determine the small strain shear modulus, G0, which corresponds to the initial stiffness of the linear part of the p-y curve.

In this research, the proposed model will be verified using collected case histories of laterally loaded piles with available CPT data at the same site. The p-y curves, and pile force-head displacements determined from the model will be compared to the field-resulted p-y curves and pile head displacement measurements available from the case histories.

Non-linear (NL) ground motion amplification functions have been developed for fine grained and highly organic soils using ground response analysis for two profiles in the Kushiro area of Hokkaido, Japan. The NL ground motion amplification functions will support a broader research effort to develop fragility functions for levees resting on organic soils in Hokkaido. The ground motion amplification functions were developed using one-dimensional (1D) NL ground response simulations in DEEPSOIL (Hashash et al. 2016) based on a suite of earthquake ground motions compiled by Baker (Baker et al. 2011). Corrections to the small-strain Darendeli (2001) modulus reduction curve for fine grained soil recommended by Yee et al. (2013) were used to realistically represent the large-strain portion of backbone curves by asymptotically approaching the shear strength at large strains. The GQ/H material model (Groholski, 2016) in DEEPSOIL was utilized to preserve the input undrained shear strength while still providing the ability to represent small-strain stiffness nonlinearity. The resulting ground motion amplification values, defined as the spectral acceleration of the surface motion divided by the spectral acceleration of the input outcrop motion, was regressed using the functional form for NL site ground motion amplification by Stewart et al. (2014).

Ground motion amplification functions for organic soils in the Sacramento / San Joaquin Delta were previously developed by Kishida, et al. (2006 and 2009a) using 1D equivalent-linear (EL) ground response analysis (GRA) that were subsequently regressed to derive ground motion amplification functions. These simulations utilized linear and nonlinear regression models for dynamic properties of highly organic soils through ongoing research on this topic that was ultimately concluded by Kishida et al. (2009b). However, it is not clear whether these ground motion amplification functions are applicable to the Hokkaido system due to differences in the Peat layers in Hokkaido compared with the Delta, nor is it clear the extent to which the equivalent linear assumption influenced the ground motion amplification functions. This thesis also presents a brief comparison between the NL ground motion amplification functions developed herein and the EL ground motion amplification functions for highly organic soils presented by Kishida et al. (2009a).

Centrifuge models of soft clay deposits were shaken with suites of earthquake ground motions to study site response over a wide strain range. The models were constructed in an innovative hinged-plate container to effectively reproduce one dimensional ground response boundary conditions. Dense sensor arrays facilitate back-calculation of modulus reduction and damping values that show modest misfits from empirical models. Low amplitude base motions produced nearly elastic response in which ground motions were amplified through the soil column and the fundamental site period was approximately 1.0s. High intensity base motions produced shear strains higher than 10%, mobilizing shear failure in clay at stresses larger than the undrained monotonic shear strength. I attribute these high mobilized stresses to rate effects, which should be considered in strength parameter selection for nonlinear analysis. The nonlinearity in spectral amplification is parameterized in a form used for site terms in ground motion prediction equations to provide empirical constraint unavailable from ground motion databases.

The nonlinear site response is covered by total stress simulations of centrifuge models involving soft clay, and effective stress simulations of centrifuge models including liquefiable sand layers. Primary conclusions from the total stress analysis are (1) unreasonable shear strength values may arise from extrapolating modulus reduction curves to large strains, and properly modeling the shear strength by adjusting the high-strain region of the modulus reduction curve is essential for accurate nonlinear site response modeling, and (2) the shear strength must be adjusted for strain rate effects to capture the measured ground motions. The primary conclusion from the effective stress simulations is that ground motions following liquefaction triggering are significantly under-predicted using a modeling procedure in which the backbone stress-strain behavior is degraded as pore pressures develop in accordance with a pore pressure generation function. These models fail to capture the dilatancy behavior of liquefied sand that manifests as a transient stiffening in undrained loading, and enables propagation of high amplitude high frequency acceleration pulses. Constitutive models capturing the dilatancy behavior are demonstrated to have the capability to replicate these acceleration pulses, but the resulting ground motions are highly sensitive to input parameters.

The Sacramento-San Joaquin Delta is the hub of California's water distribution system, which consists of below sea-level islands surrounded by levees. Delta levees are constructed of local fill, have typically been unengineered and are notorious for breaching, causing flooding of the islands inside. One major concern is the seismic performance of Delta levees, which have not experienced a significant seismic event in over a century. Many of these levees are founded on local peaty organic soils, and the seismic performance of these levees is poorly understood.

As part of a collaborative research investigation to study to study the seismic performance of Delta levees, a series of dynamic field tests were performed on a model levee constructed on very soft and compressible peaty organic soils on Sherman Island. This first-of-its-kind test applied dynamic loads to the model levee-peat system using the large NEES@UCLA MK-15 eccentric mass shaker mounted on the levee crest. Two sets of tests were performed in 2011 and 2012.

Geotechnical and geophysical investigations performed at the site found a 11m thick peat deposit rests atop permeable Pleistocene sand. The peaty soils consist of 9m of soft saturated peat with a Vs of 30 m/ and a 2m stiff desiccated crust with a Vs of 60 m/s lying atop the soft peat. Artesian pressures exist in the soft saturated peat due to hydraulic connection with the nearby Sacramento River, with a zero effective stress condition existing at the peat-sand interface. Remote data monitoring measured settlement and pore pressure dissipation of the levee using embedded piezometers and a slope inclinometer. The remote monitoring found fast dissipation of pore pressures underneath the levee and continued settlement of the levee due to a high rate of secondary compression. Prior to the 2012 tests, a berm was constructed around the levee and the ground was flooded, to create more realistic soil conditions in the unsaturated crust.

Dynamic base shear-displacement and moment-rotation relations were made for the levee. The model levee translated and rotated visibly during testing, demonstrating a response that differs from the one-dimensional wave propagation assumption used to analyze seismic levee response. High radiation damping was observed, and translation of the levee was found to go out-of-phase at peak shaker frequencies. Complex-valued stiffness of the levee-peat system was analyzed and compared to analytical solutions for a rigid foundation on an elastic halfspace. Little agreement was found between the field test results and the analytic solution, suggesting that the levee-peat foundation is flexible.

Dynamic shear strains measured underneath the levee crest and toe measured an average value of shear strains at the bottom of the stiff crust and top of the soft peat. Peak shear strains measured during testing went up to 0.4%, with higher shear strains occurring underneath the levee toe, due to the rocking behavior. Comparison of residual pore pressure ratios generated during testing show a trend in increasing residual pore pressure with increasing shear strain. Comparison of field test results with dynamic laboratory testing showed very little increase in residual pore pressures from field tests, suggesting that pore pressures underneath the levee dissipated quickly due to high horizontal permeability.

A series of finite element simulations were performed with elastic isotropic materials to compare different hypothetical soil conditions and loading scenarios. Good agreement in shear strains between the field test and the finite element simulations were found. Higher shear strains were found to exist beneath the levee for softer soils and uniform base excitation. A study investigated the development of shear stresses within the levee fill, and found an increase in peak shear stresses compared to shear stresses calculated for a simple shear case. This has implications for liquefaction triggering analysis, and the finite element simulations suggest that the current methodology used in evaluating seismic demand may be underestimating shear stresses within the levee fill.