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Multi‐hazard reliability assessment of offshore wind turbines

This is a summary of the soil characteristics and site activities over the past 20 years for the National Geotechnical Experimentation Sites at Texas A&M University Additional reports have been published for this site and their references can be found in the Reference section of this document

The National Geotechnical Experimentation Sites at Texas A&M University: Clay and Sand, A Summary

Large pile groups, defined as pile groups containing a large number of closely spaced vertical piles, were examined using a three-dimensional finite-difference based numerical modeling approach. The specific case of a large pile group subject to only translational loading at the groundline was considered, assuming that a rigid pile cap, whose base is located at the groundline, was present to enforce equal horizontal displacements of all pile heads. Research efforts focused on local pile-soil interaction using p-y curves as the primary assessment tool and p-multipliers to characterize group effects. Analysis efforts were preceded by an extensive review of lateral pile-soil interaction to provide an assessment of the existing state of knowledge, and a critical review of the three-dimensional modeling approach in terms of its formulation and application to simulating laterally loaded piles and pile groups. Rationalization of a large pile group into a two-pile in-line configuration and a single pile with periodic boundaries was undertaken for the purpose of the research representing typical leading and immediately trailing piles, and internal piles, respectively. Factors considered were: (a) soil type; (b) pile type; (c) initial soil stress states; (d) pile head restraint; and (e) pile spacing. Isolated pile models have provided a benchmark for both the in-line and periodic models. A total of 30 analyses were completed. Overall, the large pile group study indicated that initial stress state, pile type and pile head restraint resulted in some differences, but these were relatively weak compared with the influence of soil behavior and movement. Marked decreases in lateral resistance for interior piles were attributed to the different stiffness and strength characteristics of the soil models, and effects resulting from the boundary conditions employed. Much lower p-multipliers compared with current small pile group recommendations are therefore recommended for large pile groups, implying a comparatively softer translational stiffness for design. While the study enabled greater insight into the mechanics of large pile group lateral stiffness, various issues such as installation effects, pile, pile head and soil conditions remain, ensuring that the task of assessing lateral group stiffness remains a challenging endeavor.

Modeling Pile Behavior in Large Pile Groups Under Lateral Loading

The nonlinear behavior of a laterally loaded monopile foundation is studied using the finite element method (FEM) to account for soil-pile interactions. Three-dimensional (3D) finite element modeling is a convenient and reliable approach to account for the continuity of the soil mass and the nonlinearity of the soil-pile interactions. Existing simple methods for predicting the deflection of laterally loaded single piles in sand and clay (e.g., beam on elastic foundation, p-y method, and SALLOP) are assessed using linear and nonlinear finite element analyses. The results indicate that for the specific case considered here the p-y method provides a reasonable accuracy, in spite of its simplicity, in predicting the lateral deflection of single piles. A simplified linear finite element (FE) analysis of piles, often used in the literature, is also investigated and the influence of accounting for the pile diameter in the simplified linear FE model is evaluated. It is shown that modeling the pile as a line with beam-column elements results in a reduced contribution of the surrounding soil to the lateral stiffness of the pile and an increase of up to 200% in the predicted maximum lateral displacement of the pile head.

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Modeling Laterally Loaded Single Piles Accounting for Nonlinear Soil-Pile Interactions

Lateral load-deflection behaviour of single piles is often analysed in practice on the basis of methods of load-transfer P–Y curves. The paper is aimed at presenting the results of the interpretation of five full-scale horizontal loading tests of single instrumented piles in two sandy soils, in order to define the parameters of P–Y curves, namely the initial lateral reaction modulus and the lateral soil resistance, in correlation with the pressuremeter test parameters. P–Y curve parameters were found varying as a power of lateral pile/soil stiffness, on the basis of which hyperbolic P–Y curves in sand were proposed. The predictive capabilities of the proposed P–Y curves were assessed by predicting the soil/pile response in full-scale tests as well as in centrifuge tests and a very good agreement was found between the computed deflections and bending moments, and the measured ones. Small-sized database of full-scale pile loading tests in sand was built and a comparative study of some commonly used P–Y curve methods was undertaken. Moreover, it was shown that the load-deflection curves of these test piles may be normalised in a practical form for an approximate evaluation of pile deflection in a preliminary stage of pile design. At last, a parametric study undertaken on the basis of the proposed P–Y curves showed the significant influence of the lateral pile/soil stiffness on the non-linear load-deflection response.

Single piles under horizontal loads in sand: determination of P – Y curves from the prebored pressuremeter test

A method is presented to predict the behavior of piles subjected to monotonic lateral loads on the basis of the results of pressuremeter tests performed in prebored holes. The method is used to predict the pile head response of 17 laterally loaded piles including driven and bored piles ranging from 0.32 m to 1.37 m in diameter and from 3 m to 21 m in length. The predictions are compared with the load test results. For the covering abstract of the conference see IRRD 287689. (Author/TRRL)

A pressuremeter method for laterally loaded piles. proceedings of the eleventh international conference on soil mechanics and foundation engineering, san francisco, 12-16 august 1985

L M Tucker

Papers

A pressuremeter method for laterally loaded piles. proceedings of the eleventh international conference on soil mechanics and foundation engineering, san francisco, 12-16 august 1985