Minimum Required Length for Geotechnical Lateral Stability of RockSocketed Pile Shafts

The original strong-rock (SR) p-y curves for rock-socketed shafts exhibit a brittle response where the post-peak resistance drops to approximately zero. This brittle response could result in a progressive failure of the rock p-y curves which, in turn, results in excessive pile lengths. This paper proposes a modification to the strong-rock (SR) p-y curves where the post-peak residual strength is equal to 20% of the ultimate resistance (0.2*pult). This residual resistance is proposed based on the assumption that the lateral resistance of cracked rock (after the peak point) should not be smaller than the lateral resistance of heavily weathered rock estimated from the weak-rock (WR) and Liang et al. p-y curves. The effectiveness of the modified SR curves is evaluated against the results of a lateral load test. The modified SR curves are compared against back-calculated p-y curves from the lateral load test

The Effects of Long-Duration Subduction Earthquakes on Inelastic Behavior of Bridge Pile Foundations Subjected to Liquefaction-Induced Lateral Spreading

Effective-stress nonlinear dynamic analyses (NDA) were performed for a large-diameter reinforced concrete (RC) pile in multi-layered liquefiable sloped ground. The objective was to assess the effects of earthquake duration on the combination of inertia and liquefaction-induced lateral spreading. A parametric study was performed using input motions from subduction and crustal earthquakes covering a wide range of motion durations. The NDA results showed that the pile head displacements increased under liquefied conditions, compared to nonliquefied conditions, due to liquefaction-induced lateral spreading. The NDA results were used to develop a displacement-based equivalent static analysis (ESA) method that combines inertial and lateral spreading loads for estimating elastic and inelastic pile demands

Cyclic Porewater Pressure Generation in Intact Silty Soils

The results of cyclic strain-controlled, constant volume direct simple shear (CDSS) tests and field shaking tests have been evaluated for intact, natural, low-plastic silts from six different fine-grained soils with 54%–100% fines content, 47%–83% silt content, and plasticity indices (PI) ranging from nonplastic to 16. These tests constitute a subset of a larger archive of CDSS tests performed on silt deposits from the Pacific Northwest, British Columbia, and Alaska collected and analyzed by the co-authors. The cyclic data are presented in this paper for two objectives: (a) to characterize cyclically-induced excess pore pressure generation in intermediate soils with various soil index properties and stress histories, and (b) to provide calibrated Vucetic and Dobry model parameters for simulating excess pore pressure generation in the silt soils based on the data and trends presented in the first objective. The CDSS test results showed that excess pore pressure ratios decrease with PI over the narrow range of PI evaluated and decrease with overconsolidation ratio. The cyclic threshold shear strain amplitude for pore pressure generation extracted from field shaking tests on silts were within the range proposed in the literature, confirming that the cyclic threshold shear strain amplitude is a fundamental soil property. Calibrated Vucetic and Dobry model parameters for these intermediate, fine-grained silts were significantly different than those reported for sands in the literature and were heavily influenced by the overconsolidation ratio. The calibrated parameters obtained in this study can be used as a benchmark in selecting model parameters for silts

A Finite Difference Method for Off-fault Plasticity throughout the Earthquake Cycle

We have developed an efficient computational framework for simulating multiple earthquake cycles with off-fault plasticity. The method is developed for the classical antiplane problem of a vertical strike-slip fault governed by rate-and-state friction, with inertial effects captured through the radiationdamping approximation. Both rate-independent plasticity and viscoplasticity are considered, where stresses are constrained by a Drucker-Prager yield condition. The off-fault volume is discretized using finite differences and tectonic loading is imposed by displacing the remote side boundaries at a constant rate. Time-stepping combines an adaptive Runge-Kutta method with an incremental solution process which makes use of an elastoplastic tangent stiffness tensor and the return-mapping algorithm. Solutions are verified by convergence tests and comparison to a finite element solution. We quantify how viscosity, isotropic hardening, and cohesion affect the magnitude and off-fault extent of plastic strain that develops over many ruptures. If hardening is included, plastic strain saturates after the first event and the response during subsequent ruptures is effectively elastic. For viscoplasticity without hardening, however, successive ruptures continue to generate additional plastic strain. In all cases, coseismic slip in the shallow sub-surface is diminished compared to slip accumulated at depth during interseismic loading. The evolution of this slip deficit with each subsequent event, however, is dictated by the plasticity model. Integration of the off-fault plastic strain from the viscoplastic model reveals that a significant amount of tectonic off-set is accommodated by inelastic deformation (~0.1 m per rupture, or ~10% of the tectonic deformation budget)

Numerical Modeling of a Pile-Supported Wharf Subjected to Liquefaction-Induced Lateral Ground Deformations

Fully-coupled nonlinear dynamic analysis is increasingly used for assessing the seismic performance of pile-supported wharf structures subjected to liquefaction-induced lateral ground deformations. Several numerical challenges exist for analysis of this highly nonlinear soil-structure interaction, which require robust, yet practical, solutions that are validated with experimental data. This study presents a numerical model of a pile-supported wharf and evaluates the applicability of a soil constitutive model, and modeling assumptions and methods by using recorded data from a well-instrumented, large-scale centrifuge test. The objectives of this study include: (a) evaluating the performance of a recently developed pressure-dependent multi-yield surface constitutive soil model (PDMY03) to simulate the behavior of a liquefiable sand (b) assessing the effectiveness of a soil–pile interaction modeling approach in capturing the kinematic displacement demands on piles from a laterally spreading ground, and (c) evaluating the effectiveness and limitations of the 2D numerical model in approximating the 3D behavior of a wharf structure supported by multiple rows of piles, including the dynamic response of the centrifuge container. The implications of these assumptions and lessons learned from this study provide guidance for researcher modelers and practitioners for numerical modeling of similar soil–structure systems

Pile-Supported Wharves Subjected to Inertial Loads and Lateral Ground Deformations. II: Guidelines for Equivalent Static Analysis

An equivalent static analysis (ESA) procedure is proposed for the design of pile-supported wharves subjected to combined inertial and kinematic loads during earthquakes. The accuracy of the ESA procedure was evaluated against measurements from five large-scale centrifuge tests. The wharf structures in these tests were subjected to a suite of recorded ground motions and the associated superstructure inertia, as well as earthquake-induced slope deformations of varying magnitudes. It is shown that large bending moments at depths greater than 10 pile diameters were primarily induced by kinematic demands and can be estimated by applying soil displacements only (i.e., 100% kinematic). In contrast, the large bending moments at the pile head are primarily induced by wharf deck inertia and can be estimated by applying superstructure inertial loads at the pile head only (i.e., 100% inertial). Large bending moments at depths shallower than 10 pile diameters are affected by both inertial and kinematic loads; therefore, evaluation of pile performance should include soil displacements and a portion of the peak inertial load at the pile head that coincides with the peak kinematic loads. Ranges for inertial and kinematic load combinations in uncoupled analyses are provided for different soil profiles. The details on the back-calculated load combination factors are provided in the companion paper

Inertial and Liquefaction-Induced Kinematic Demands on a Pile-Supported Wharf: Physical Modeling

Results of a centrifuge test on a pile-supported wharf were used to investigate the time-, depth-, and row-dependent nature of kinematic and inertial loading on wharf piles in sloping rockfill. P-y models were calibrated against recorded bending moments in different piles and different depths. It was found that full kinematic demands and full superstructure inertia should be combined to estimate bending moments at pile head and shallow depths (less than 10 diameters below the ground surface). On the contrary, it was found that applying full kinematic demands alone was adequate to estimate pile bending moments at large depths (greater than 10 diameters deep)

Pile-Supported Wharves Subjected to Inertial Loads and Lateral Ground Deformations. I: Experimental Results from Centrifuge Tests

Five dynamic, large-scale centrifuge tests on pile-supported wharves were used to investigate the time- and depth-dependent nature of kinematic and inertial demands on the deep foundations during earthquake loading. The wharf structures in the physical experiments were subjected to a suite of recorded ground motions and imposed superstructure inertial demands on the piles. Partial to full liquefaction in loose sand resulted in slope deformations of varying magnitudes that imposed kinematic demands on the piles. It was found that the wharf inertia and soil displacements were always in phase during the critical cycle when bending moments were at their maximum values. The test results were analyzed to provide the relative contributions of peak inertial loads and peak soil displacements during critical cycles, and the data revealed the depth dependency of these factors. The results of this study are used in a companion paper to provide recommendations for the design of pile-supported wharves subjected to foundation deformations

Evaluation of Site Effects Utilizing Cascadia Subduction Zone Ground Motions

The potential Cascadia Subduction Zone (CSZ) megathrust earthquake is recognized as one of the major natural hazards affecting the Pacific Northwest of the United States. The estimation of expected ground-motions is complicated by the long-duration motions from CSZ as well as site effects from deep sedimentary basins in northwest Oregon. This study evaluates the combined effects of long duration motions and basin effects on site amplification factors due to propagation of earthquake waves in surficial soils. Nonlinear and equivalent linear one-dimensional site response analyses were performed using broadband synthetic CSZ ground motions from the M9 Project. A web-based tool was developed to synthesize the vast data and geographically visualize the M9 ground motions as well as the subsequently computed response spectra. Five soil profiles representing a range of site classes from Site Class C to Site Class E were considered for the analyses. Ground motions were extracted at three locations inside major basins in Northwest Oregon (Portland basin, Tualatin basin, and North Willamette basin) and three comparable locations outside the basins. The effect of basin on soil amplification factor was characterized by comparing the soil amplifications inside and outside basins for the same soil profiles. The basin amplification factors calculated for CSZ broadband synthetic ground motions at bedrock (Site Class B/C) for selected sites within the three basins in Oregon were found to be noticeably larger than the ratios calculated from empirical correlations that are incorporated in New Generation Ground Motion Attenuation Models (NGA-West2). The soil amplification ratios calculated from the site response analyses were generally within the envelope of code-based site coefficients in ASCE 7, except for very short periods (<0.5 seconds). The effect of basins on soil amplification ratios ranged from 50% increase to 30% decrease at periods close to the natural period of the basin (generally between 1 sec and 2 sec). The implication of these findings on the use of code-based site coefficients and advantages of performing site-specific site response analysis are discussed

Development of a Design Guideline for Pile Foundations Subjected to Liquefaction-Induced Lateral Spreading

Past earthquakes confirmed that seismically induced kinematic loads from soil lateral spreading and inertial loads from structure can cause severe damages to pile foundations. The research questions are: How to combine inertial and kinematic loads in design of pile foundations in liquefied soil? How the combination of inertia and kinematics changes with depth? How this combination is affected by long-duration earthquakes? How this combination affects inelastic demands in piles