Scott J. Brandenberg

Seismic Design of Pile Foundations for Liquefaction Effects

Procedures for the seismic design of pile foundations for liquefaction effects are presented with emphasis on the conditions relevant to bridges. Two local subsystems for a bridge are discussed in detail: (1) pile groups in laterally spreading ground away from the abutments and (2) pile groups at the abutments where the restraining or “pinning” effects of the piles and bridge superstructure can be advantageous. The recommended design procedures involve equivalent static analyses using beam on nonlinear Winkler foundation models. Guidance for these design procedures was derived from a combination of dynamic centrifuge model tests and associated nonlinear dynamic finite element studies. The design procedures, their basis, and other issues for design of bridges for liquefaction effects are discussed.

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...

Centrifuge Modeling Studies of Site Response in Soft Clay over Wide Strain Range

AbstractCentrifuge 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 1.0 s. High-intensity base motions produced shear strains higher than 10%, mobilizing shear failure in clay at stresses larger than the undrained monotonic shear strength. The authors attribute these high mobilized stresses to rate effects, which should be considered in strength-parameter selection for nonlinear analysis. This nonlinear response deamplified short-period spectral accelerations and lengthened the...

Kinematic Framework for Evaluating Seismic Earth Pressures on Retaining Walls

AbstractDuring earthquake ground shaking earth pressures on retaining structures can cyclically increase and decrease as a result of inertial forces applied to the walls and kinematic interactions between the stiff wall elements and surrounding soil. The application, based on limit equilibrium analysis, of a pseudostatic inertial force to a soil wedge behind the wall [the mechanism behind the widely-used Mononobe–Okabe (M–O) method] is a poor analogy for either inertial or kinematic wall–soil interaction. This paper demonstrates that the kinematic component of interaction varies strongly with the ratio of wavelength to wall height (λ/H), asymptotically approaching zero for large λ/H, and oscillating between the peak value and zero for λ/H<2.3. Base compliance, represented in the form of translational and rotational stiffness, reduces seismic earth pressure by permitting the walls to conform more closely to the free-field soil displacement profile. This framework can explain both relatively low seismic pre...

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...

Characterization of Seismic Levee Fragility Using Field Performance Data

We characterize the seismic fragility of levees along the Shinano River system in Japan using field performance data from two M 6.6 shallow crustal earthquakes. Levee damage is quantified based on crack depth, crack width, and crest subsidence for 3,318 levee segments each 50 m long. Variables considered for possible correlation to damage include peak ground velocity (PGV), geomorphology, groundwater elevation, and levee geometry. Seismic levee fragility is expressed as the probability of exceeding a damage level conditioned on PGV alone and PGV in combination with other predictive variables. The probability of damage (at any level) monotonically increases from effectively zero for PGV < 14 cm/s to approximately 0.5 for PGV ≈ 80 cm/s. Of the additional parameters considered, groundwater elevation relative to the levee base most significantly affects fragility functions, increasing and decreasing failure probabilities (relative to the PGV-only function) for shallow and deep groundwater conditions, respectively.

FEM Analysis of Dynamic Soil-Pile- Structure Interaction in Liquefied and Laterally Spreading Ground

A two-dimensional nonlinear dynamic finite element (FE) model was developed and calibrated against dynamic centrifuge tests to study the behavior of soil-pile-structure systems in liquefied and laterally spreading ground during earthquakes. The centrifuge models included a simple structure supported on pile group. The soil profiles consisted of a gently sloping clay crust over liquefiable sand over dense sand. The FE model used an effective stress pressure dependent plasticity model for liquefiable soil and a total stress pressure independent plasticity model for clay, beam column elements for piles and structure, and interface springs that couple with the soil mesh for soil-structure interaction. The FE model was evaluated against recorded data for eight cases with same set of baseline parameters. Comparisons between analyses and experiments showed that the FE model was able to approximate the soil and structural responses and reproduce the lateral loads and bending moments on the piles reasonably well.

p -Wave Reflection Imaging of Submerged Soil Models Using Ultrasound

An ultrasonic p -wave reflection imaging system is used to noninvasively image submerged soil models with embedded anomalies and complex geometric layer contacts. The ultrasonic transducers emit compressive waves into water that subsequently transmit into the underlying soil, and measurements of the reflections are used to construct the images. The properties of the transducers and data acquisition hardware and software are explained. Fast signal stacking is used to improve signal-to-noise ratio and provide clearer images. Transducer directivity is explained as a wave passage effect, and transfer functions are derived for square and circular transducers to quantify directivity. The transfer functions agree reasonably with measured amplitude data. The cause of errors in the imaged position of dipping reflectors is explained, and a Kirchhoff migration algorithm is implemented to correct these errors. A soil model consisting of embedded high- and low-impedance anomalies, dipping soil layer contacts, and an u...

Dynamic Response of a Model Levee on Sherman Island Peat: A Curated Data Set

A model levee resting atop soft compressible peaty organic soil in the Sacramento/San Joaquin Delta was shaken by forced vibration to study the seismic deformation potential of the underlying peat and measure dynamic levee–peat interaction. Forced vibration testing occurred over a frequency range of 0 Hz to 5 Hz and produced force amplitudes applied to the embankment crest that induced elastic to nonlinear levee-foundation responses. Available data include acceleration records from sensors mounted on the model levee and on the ground surface near the model levee, and acceleration and pore pressure measurements from sensors embedded in the underlying peat. A remote data acquisition system measured settlements and pore pressures over a span of more than a year, encompassing time before and after the dynamic testing. Small pore pressures were generated in the peat during testing although embankment settlements from cyclic loading were small.

Fragility Functions for Bridges in Liquefaction-Induced Lateral Spreads

Fragility functions are generated for bridges in liquefied and laterally spreading ground using equivalent static global nonlinear finite element analyses. Bridges are classified based on structural configurations and vintage. Probability density functions are assigned to both structural and geotechnical properties of bridges. Nonlinear equivalent static analyses are conducted with inputs sampled randomly using the Monte Carlo simulation method. Cumulative distribution functions are fitted to the simulated data, and define the probability of exceeding various engineering demand parameters (pier column curvature ductility, pile cap displacement, abutment displacement, etc.) conditioned on the maximum free-field lateral spreading ground surface displacement. Correlations among EDPs are presented to facilitate risk assessment based on a vector of EDPs. The derived fragility functions, combined with seismic hazard analysis, liquefaction potential, and lateral spreading estimation, are useful in the context of performance-based earthquake engineering and risk assessment of current bridge inventory in California.