Daniel W. Wilson

Nonlinear Seismic Soil‐Pile Structure Interaction

Analytical design tools for evaluation of soil-pile-structure interaction during seismic events are evaluated and modified. Several implementations of the “Beam on Nonlinear Winkler Foundation” (BNWF) method were used to predict results of centrifuge model tests of single piles in a soft clay soil profile. This paper shows that calculations from these computer codes can be sensitive to the details of the arrangement of nonlinear springs and linear viscous dashpots. Placing the linear viscous dashpots (representing radiation damping in the far field) in series with the hysteretic component of the p-y elements (representing the nonlinear soil-pile response in the near field) is shown to be technically preferable to a parallel arrangement of the viscous and hysteretic damping components. Preliminary centrifuge data is reasonably modeled by the numerical calculations using this implementation of damping, but additional field or physical model data are needed to fully evaluate the reliability of BNWF procedures.

Electron waveguiding characteristics and ballistic current capacity of semiconductor quantum slabs

Semiconductor slab electron waveguides with and without spatially varying effective mass are analyzed using the single-band effective-mass equation. Starting with ballistic electron incidence on a potential energy/effective mass interface, expressions for the phase shift, the lateral shift, and the time delay upon total internal reflection are found. It is shown that heterostructure wells, homostructure voltage-induced wells, and heterostructure barriers can act as waveguides for ballistic electrons, and that the waveguiding is described by a single dispersion relation. The guided mode wave functions, dispersion curves, cutoffs, group velocity. effective mass, density of states, and ballistic guided current density are determined. >

Fabrication, Operation, and Health Monitoring of Bender Elements for Aggressive Environments

Bender elements are commonly used to monitor the shear wave velocity of soils in various tests, including triaxial, consolidation, and centrifuge tests. When used in aggressive soil environments, electromagnetic crosstalk can distort the received bender element signal, preventing accurate shear wave velocity measurements. Aggressive soil environments are defined herein as conductive soils with high relative permittivity. Under these conditions, the electrical source is transmitted from source to receiver bender, dominating any received shear wave signal propagating through the soil. Careful attention must be paid to reducing the transmission of the electromagnetic signal, particularly in aggressive soil environments. When the waterproof coating of a bender element degrades and the inner and outer electrodes become electrically connected in a saturated environment, the bender element will no longer operate. However, when the waterproofing material is degraded so that only a single electrode on the source element is exposed, electric current can enter the pore fluid and affect the received signal. Further, even if the waterproofing coating is intact, electromagnetic crosstalk from the induced electrical field generated by the transmitting bender element can still affect the received signal when the conductivity of the pore fluid is high. Bender elements can be constructed so as to greatly reduce the electromagnetic crosstalk, and simple tests can be performed to help ensure that the bender element system is not susceptible to crosstalk. The objective here is to present details and practical guidelines regarding the fabrication, operation, and health monitoring of bender elements that will help ensure clear shear wave velocity measurements in aggressive soil environments. The fabrication steps presented improve on previous recommendations. Bender element operation (including signal form, frequency, and amplitude) also affects signal quality and the accuracy of the measured travel time. Finally, recommendations for monitoring the health of the bender elements throughout the transducer life are outlined.

Examination and Reevalaution of SPT-Based Liquefaction Triggering Case Histories

AbstractA standard penetration test (SPT)-based liquefaction case history database for liquefaction triggering criteria is reexamined and reevaluated. The updated database incorporates a number of additional case histories, replaces prior estimates of earthquake magnitudes with current estimates of their moment magnitudes, uses improved estimates of peak ground accelerations when available, and includes a reexamination of the selection and computation of representative SPT (N1)60 values for most case histories. The approach used to select and compute representative SPT (N1)60cs values is illustrated using select case histories. The distribution of the case history data relative to the Idriss-Boulanger triggering correlation is examined for any bias with respect to various parameters and to identify the conditions that are, and are not, well covered by available case history data.

Weighted Residual Numerical Differentiation Algorithm Applied to Experimental Bending Moment Data

A weighted-residual approach for differentiating one-dimensional discrete data is presented and applied to an experimental program in which distributions of bending moment were measured along a model pile foundation in a centrifuge test. The weighted-residual approach is validated by first differentiating a sinusoidal bending moment distribution, and errors in first and second derivatives associated with various ratios of wavelength to sampling interval are computed. A bending moment distribution from a finite-element simulation of a pile foundation is differentiated using the weighted-residual technique, by fitting cubic splines, and by polynomial regression, and second derivatives are compared with the recorded subgrade reaction distributions. The influence of adding noise to the sampled bending moment distribution prior to differentiation is explored and is found to be most influential when sampling intervals are small. Bending moment data recorded during the centrifuge experiment are double differenti...

Undrained Stability of Dual Circular Tunnels

In this paper,numericallimit analysis and semianalytical rigid blocktechniques are used toinvestigate the effectof the tunnelspac- ingonthestabilityoftwocirculartunnelsexcavatedsidebyside.Thetunnelsaremodeledunderplane-strainconditions,whichimpliesthatthey are assumed to be infinitely long. Bounds on the stability of the tunnels are obtained using finite-element limit analysis, the numerical formu- lation of which is based on the upper and lower bounds theorems of classical plasticity. Solutions are obtained using advanced conic program- mingschemes tosolvethe resultingoptimization problems,and upperand lowerboundestimates on thestabilityof thetunnels areobtained for arangeofgeometries.Thesebounds,whichbracketthetruecollapseloadfromaboveandbelow,arefoundtodifferbyatmost5%forthecases where the solution does not approach zero. Results from this study are summarized in stability charts for use by practitioners. DOI: 10.1061/ (ASCE)GM.1943-5622.0000288.

SOIL-PILE-SUPERSTRUCTURE INTERACTION IN LIQUEFIABLE SAND

Soil-pile-superstructure interaction in liquefiable sand is evaluated using dynamic centrifuge model tests and pseudostatic p-y analyses. Select recordings from a recent centrifuge test are presented to illustrate typical behavior with and without liquefaction in an upper sand layer. Pseudostatic p-y analyses of single-pile systems in two recent centrifuge model tests show that the apparent reduction in p-y resistance due to liquefaction was strongly affected by changes in the relative density of the sand and drainage conditions.

Quantum well, voltage-induced quantum well, and quantum barrier electron waveguides : mode characteristics and maximum current

It is shown that finite‐potential heterostructure wells, homostructure voltage‐induced wells, and heterostructure barriers can act as waveguides for ballistic electrons and that waveguiding is described by a single dispersion relation and can occur at energies above all band edges. The guided mode cutoffs, electron velocity, effective mass, density of states, and ballistic current density (applicable to 2D electron gases) are presented. The maximum ballistic guided current flowing in a given direction for a 10 monolayer Ga0.75Al0.25As/GaAs/Ga0.9Al0.1As waveguide is found to be 2.3 mA per μm of waveguide width–allowing considerably greater currents than in single‐mode quantum wires.

Supermode analysis of electron wave directional coupling using a multilayer waveguide approach

Electron wave directional coupling between parallel semiconductor electron waveguides is analyzed by calculating the supermode (superposition of eigenmodes) of the complete structure. A transfer‐matrix technique is used to calculate the eigenmodes of multiple‐layer structures having arbitrary potential‐energy/effective‐mass profiles that form the coupled electron waveguides. It is shown that the eigenmodes of electron waveguides having spatially varying effective mass satisfy an orthogonality relation that involves the effective‐mass profile. As determined by supermode simulations, coupled‐mode analyses give inaccurate directional‐coupling transfer lengths when the coupling is strong or the electron energy is low. The influence of various parameters and structure asymmetries on the directional coupling process is investigated. It is shown that asymmetry severely decreases the ability to transfer current from one waveguide to the other, except in the case of effective‐mass asymmetry where it causes an ener...

p-y Plasticity Model for Nonlinear Dynamic Analysis of Piles in Liquefiable Soil

AbstractLiquefiable soil-structure interaction material models are developed and implemented in the open-source finite-element modeling platform OpenSees. Inputs to the free end of the p-y materials include the ground motion and mean effective stress time series from a free-field soil column. Example simulations using a single p-y element attached to a soil element demonstrate key features. The models are then used to analyze centrifuge experiments of a single pile in a level liquefiable profile and a six-pile group in a sloping liquefiable profile that resulted in lateral spreading. Measured displacements and mean effective stress time series are used as inputs to isolate the response of the material models from predictive uncertainties in free-field ground motion and excess pore pressure. The predicted pile response agrees reasonably well with measurements. The cyclic mobility behavior of sand in undrained loading is shown to be an important mechanism affecting bending moments in the piles; neglecting t...