Nason J. McCullough

THE SEISMIC PERFORMANCE OF PILES IN WATERFRONT APPLICATIONS

Earthquakes have highlighted seismic hazard concerns for ports. The design of a seismically resilient wharf requires an understanding of its performance during earthquakes. Due to the complex nature of pile-supported wharves, state of the art centrifuge modeling techniques are being used to better understand seismic performance. This paper summarizes aspects of the construction, imstrumentation, and testing of the models. Results on the seismic performance of the model are also presented, as well as the comparison between the measured centrifuge seismic performance as well as the seismic performance estimated using a standard-of-practice design method.

Centrifuge Seismic Modeling of Pile-Supported Wharves

Five pile-supported wharf models were dynamically tested in a large-scale geotechnical centrifuge at UC Davis, California. Models representing pile-supported wharf configurations common in the United States were subjected to recorded acceleration time histories. Model variations included single-lift, multi-lift, and cut-slope rock dike configurations with foundation layers of loose liquefiable sand, marine clay, or dense sand, or a combination thereof. In addition, zones of soil were placed to model soil improvement. Structural elements representing pile-supported wharf geometries were placed within the models; some models included all vertical piles, while two of the models included batter piles. In addition, single piles were placed in two of the models and subjected to static cyclic lateral load tests. All models were extensively instrumented with nearly 100 instruments recording accelerations, pore pressures, linear deformations, and pile strains. This paper summarizes the design, construction, and testing of these complex models, and includes a brief summary of the results and recommendations for future modeling.

THE BEHAVIOR OF PILES IN SLOPING ROCK FILL AT MARGINAL WHARVES

Piles are often installed in sloping rock fill for the construction of marginal wharves. Though much information is available on the lateral performance of piles in sloping sand, silt, and clay profiles, only limited information is available on the lateral performance of piles in sloping or horizontal rock fill. This paper summarizes the results of a recent research effort using centrifuge and numerical models, as well as field data, to better understand the performance of piles at marginal wharves. The results indicate that modifications to current analysis methods are required to accurately model the lateral behavior of piles in sloping rock fill. Modifications are required to account for the sloping profile, resulting in reduced p-y curve ultimate strength in the downslope direction. In addition, if the diameter of the rock fill is approximately the same as the pile diameter, the ultimate strength of the p-y curve in both the upslope and downslope directions needs to be increased to account for the discrete particle interaction between the piles and rock fill.

Influence of Batter Piles on the Dynamic Behavior of Pile-Supported Wharf Structures

Recent experience has demonstrated that waterfront structures are highly susceptible to earthquake-induced damage. In the western United States, port waterfront structures are commonly constructed using pile-supported wharves in combination with rock dike structures retaining hydraulically placed fills. Many ports use batter piles to limit deflections from lateral loads, such as ship berthing and seismic loads. Extensive earthquake-induced damage to batter piles has been observed at several ports worldwide. Consequently, batter piles are now used cautiously in the design of new wharves in seismically active regions even though many wharves with batter piles have performed adequately. They have also been used as part of the unique “structural fuse” concept that has been adopted on major projects in the western United States. The continued use of batter piles combined with the significant number of existing wharves supported with batter piles creates the need for a better understanding of their seismic performance. In order to augment the limited number of instrumented earthquake case studies for modern wharves and evaluate the performance of the soil-foundation-structure system, a series of large-scale centrifuge models have been constructed and tested with typical pile-supported wharf configurations. This paper presents the results of the final two models where batter piles were incorporated. Tests were carried out with and without the batter piles attached for each model at identical input accelerations. To the authors’ knowledge, the tests provide the first recording and quantification of seismic force distribution for pile-supported wharf structures with batter piles. This paper summarizes 1) quantification of seismic lateral loads on vertical and batter piles, 2) pile shear and moment data with emphasis on the wharf deck connection, 3) embankment displacements with comments regarding their influence on pile loading.

Soil Structure Interaction

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).

Unconservative Outcomes from Conservative Soil Properties Used in Waterfront Applications

In light of the inherent variability and soft nature of soil deposits in the marine environment, it is common geotechnical engineering practice to use conservative soil property values for analysis and design recommendations. The shear strength and modulus values often recommended are generally smaller than best estimate or mean values. The connotation that lower bound values are conservative can be related to the use of limit-equilibrium analyses for stability applications involving foundations, earth retention systems, and slopes. Current use of numerical models for performance-based design at ports has demonstrated, however, that the assumption that lower bound soil properties yield conservative results is not always accurate. This paper presents two examples in which lower values of strength and/or modulus resulted in unconservative estimates of performance for the design of port waterfront structures.