259 Manuscript received January 30, 2019; revised May 20, 2019; accepted July 15, 2019. 1 Graduate Student, Glenn Department of Civil Engineering, Clemson University, 123 Lowry Hall, Clemson, SC 29634, U.S.A.. 2* Associate Professor (corresponding author), Glenn Department of Civil Engineering, Clemson University, 202A Lowry Hall, Clemson, SC 29634, U.S.A. (e-mail: [email protected]). 3D NONLINEAR FINITE ELEMENT ANALYSIS OF PILED-RAFT FOUNDATION FOR TALL WIND TURBINES AND ITS COMPARISON WITH ANALYTICAL MODEL Shweta Shrestha 1 and Nadarajah Ravichandran 2 ABSTRACT Geotechnical design of piled-raft foundation is typically performed using simplified semi-empirical equations that do not consider the interaction between structural components and supporting soil and the effect of bending moment on the differential settlement of piled-raft. In this study, the settlements and rotations computed using an analytical and linear and nonlinear finite element methods were compared. First, a piled-raft foundation for supporting a 130 m-tall wind turbine was designed using simplified analytical method and then a nonlinear finite element model was created in ABAQUS and analyzed. In the finite element modeling, the stress-strain behavior of the soil was represented by linear elastic (LE) and nonlinear elastoplastic Drucker-Prager (DP) models. The interfaces between structural components and soil were modeled as two bodies in the contact that allows slipping and separation at the interfaces. The results showed that the vertical and the horizontal displacements from the analytical procedure were significantly higher than that of the nonlinear finite element method. At the same time, the differential settlement and rotation were lower than that of ABAQUS. The parametric study conducted by varying the wind speed and undrained shear strength of the soil indicates that the difference between the predicted responses decreases when the load is large and/or soil is soft. From the finite element analyses, it was observed that the separation and slip between the soil and pile were negligible. It was also found that the piles contributed more in reducing vertical settlement, raft contributed more in reducing horizontal displacement, and only piles were contributing to reduce differential settlement. Key words: Piled-raft foundation, finite element analysis, ABAQUS, soil-pile interaction, differential settlement, Drucker-Prager. 1. INTRODUCTION The importance of meeting the energy demand through clean and sustainable sources has been well recognized in recent years. Among the many sustainable sources, the wind is gaining popularity around the world particularly in the USA and Europe. The wind energy production can be increased either by building taller turbine towers to access steadier and higher wind speed or by building many turbines. Selection of site for building a wind farm depends on site-specific wind characteristics and subsurface condition that affects the design and construction of the founda- tion for supporting the wind turbines. In some areas, the wind characteristics may be favorable, but the subsurface condition may not be suitable for transferring the larger vertical load, hori- zontal load and moment to the subsurface soil. This will result in a larger and uneconomical foundation, especially when the foun- dation must support tall turbines that induce larger moment at the base of the tower. Mat foundation, pile group foundation, and piled-raft foun- dation are commonly used for supporting wind turbines. Out of these three foundation types, the piled-raft foundation (shown in Fig. 2 later) that has a large mat at the top of a number of deep foundations is economical for tall onshore wind turbine, espe- cially when the subsurface soil is weak (Shrestha and Ravichan- dran 2016). Higher bearing resistance is derived from the mat foundation while higher settlement resistance is derived from the deep foundation. Although the combined mat and deep founda- tion is better for meeting the safety and serviceability require- ments effectively, the geotechnical design of piled-raft founda- tion is complicated because of the complex load transfer and soil-structure interaction mechanisms. The load sharing between the piles and raft are not well understood especially when the piled-raft is for supporting wind turbine that induces shear and moment loads in addition to the vertical load. There are a few analytical methods available for the design of piled-raft founda- tion in the literature (Poulos and Davis 1980; Poulos 2001; Ran- dolph 1994; Burland 1995; Hemsley 2000). The details of these methods are given in the analytical design section. Although these simplified methods can be used to perform designs to a reasonable extent for certain geometric and loading conditions, the literature still lacks a reliable method that consid- ers the complex load transfer and interaction mechanisms accu- rately. In such situations, a numerical method can be used for gaining insights into the behavior of piled-raft foundation sub- jected to complex loading conditions. With the rapid advance- ment in computer technology and efficient algorithm develop- ment for accurately representing the interaction between contact- ing surfaces, computer models of piled-raft foundation can be developed and analyzed within a reasonable time. Ruel and Randolph (2003) presented a comparative study of a 3D finite Journal of GeoEngineering, Vol. 14, No. 4, pp. 259-276, December 2019 http://dx.doi.org/10.6310/jog.201912_14(4).5
3D NONLINEAR FINITE ELEMENT ANALYSIS OF PILED-RAFT FOUNDATION FOR TALL WIND TURBINES AND ITS COMPARISON WITH ANALYTICAL MODEL
Jun 14, 2023
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