A Robust Macroelement Model for Soil–Pile Interaction under Cyclic Loads Ertugrul Taciroglu 1 ; ChangSoon Rha 2 ; and John W. Wallace, M.ASCE 3 Abstract: The principal focus of this study is the development of a robust macroelement model for soil–pile interaction under cyclic loads. The model incorporates frictional forces and formation of gaps at the soil–pile interface as well as hysteretic behavior of the soil. The plastic envelope of the soil behavior is modeled via the so-called p–y approach, outlined in American Petroleum Institute’s guidelines for design of foundation piles for offshore platforms. The macroelement is an intuitive assembly of various basic elements, each of which incorporating a particular aspect of the soil–pile interaction. The modular structure of this macroelement allows straightforward adaptation of improved constitutive models for its building blocks. Herein, we focus on large-diameter, cast-in-drilled-hole reinforced concrete piles piers that are partially or fully embedded in soil. These types of piles are frequently used as support structures in highway construction. Consequently, the numerical robustness of the interaction model is assessed with parametric studies on pile systems and soil types relevant to this type of construction. Both elastic and inelastic pile behaviors are considered in the parametric studies. The results indicate that the proposed interaction element is numerically robust, and thus, amenable to routine structural analysis. DOI: 10.1061/ASCE1090-02412006132:101304 CE Database subject headings: Soil-structure interaction; Piles; Drilled shafts; Piers; Cyclic loads. Introduction Cast-in-drilled-hole CIDH piers piles, columns, shafts provide an economical option for highway construction. Usually consist- ing of a continuous column with a prismatic cross section, these support structures are common in urban regions because they do not require significant space and eliminate the complexities of a column-to-footing connection. Inelastic deformations of a CIDH pier typically occur below grade; therefore, the overall lateral behavior of the system is significantly influenced by the interac- tion between the pile and the surrounding soil. This interaction involves a variety of complex phenomena including the relative motion between the pile and the free-field soil, radiation damping, as well as frictional contact drag and gap formation at the soil– pile interface. Although continuum finite element models of the pile and soil may be used see, for example, Brown et al. 1989; Brown and Shie 1991; Trochanis et al. 1991; and Yang and Jeremic 2002 to account for the soil–structure interaction, the degree of uncer- tainty associated with the specification of model parameters as well as laborious mesh generation and interpretation of results, often renders the finite element method a secondary option in engineering practice. Consequently, nonlinear soil–pile interac- tion is typically considered through a Winkler-type beam on in- elastic foundation approach, commonly referred as the “p–y method,” where p denotes the soil reaction per unit length and y denotes the lateral pile deflection API 1993. The basic p–y method does not explicitly address the specific aspects of soil–pile interaction, such as gapping and drag or elas- tic reloading and unloading cycles. However, it is possible to incorporate these by creating a single degree-of-freedom macro- element, whereby subelements are used for modeling a particular process of the interaction, and are properly assembled into a com- posite element. The macroelement can then be attached to struc- tural beam, plate, shell or continuum finite element models of the embedded structure. Examples of this approach include inter- action elements by Nogami et al. 1992, who combined springs and dashpots to incorporate damping into the basic elastoplastic soil response represented by a p–y model. A more sophisticated model by Boulanger et al. 1999 is capable of simulating the drag forces as well as the formation of gaps. Although these mod- els are promising, details of their implementation and numerical robustness for a broad range of soil and pile properties have not been fully addressed, which is the focus of the present study. The proposed interaction element comprises a robust gap ele- ment, which provides a smooth transition between contact and no-contact phases Taciroglu and Hjelmstad 1999, and various elastoplastic elements based on well-established mathematical formulation and computational algorithms Simo and Hughes 1998. The modular and intuitive structure of the proposed ele- ment enables the adaptation of improved constitutive models for a particular aspect of the soil–structure interaction without altering the others. For example, it is quite straightforward to consider soil-types possessing different plastic envelope curves or alter the 1 Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Los Angeles, 5731E Boelter Hall, Los Angeles, CA 90095-1593 corresponding author. E-mail: etacir@seas.ucla.edu 2 Postdoctoral Researcher, Dept. of Civil and Environmental Engineering, Univ. of Los Angeles, 5731E Boelter Hall, Los Angeles, CA 90095-1593. E-mail: csrha@seas.ucla.edu 3 Professor, Dept. of Civil and Environmental Engineering, Univ. of Los Angeles, 5731E Boelter Hall, Los Angeles, CA 90095-1593. E-mail: wallace@seas. ucla.edu Note. Discussion open until March 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on October 20, 2004; approved on October 19, 2005. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132, No. 10, October 1, 2006. ©ASCE, ISSN 1090- 0241/2006/10-1304–1314/$25.00. 1304 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2006