Study of Soil Layering Effects on Lateral Loading Behavior of Piles Zhaohui Yang 1 and Boris Jeremić 2 Abstract: This paper presents results of the finite element study on the behavior of a single pile in elastic-plastic soils. Pile behavior in uniform sand and clay soils as well as cases with a sand layer in a clay deposit and a clay layer in a sand deposit were analyzed using finite element modeling. Finite element results were used to generate p-y response curves, which were cross compared to investigate the soil layering effects. DOI: 10.1061/ASCE1090-02412005131:6762 CE Database subject headings: Layered soils; Lateral loads; Piles; Finite element method; Elastoplasticity. Introduction The theory of beams on a Winkler-type subgrade Hartog 1952, also known as the p-y approach, has been widely used to design piles subjected to lateral loading. Based on that theory, the method models the lateral soil–foundation interaction with em- pirically derived nonlinear springs p-y curves. The advancement of computer technology has made it possible to study this prob- lem using more rigorous elastic–plastic finite element method FEM. Here mentioned are a few representative examples of finite element studies of pile foundations. Muqtadir and Desai 1986 studied the behavior of a pile-group using a three-dimensional 3D program with nonlinear elastic soil model. An axisymmetric model with elastic-perfectly plastic soil was used by Pressley and Poulos 1986 to study group effects. Brown and Shie 1990a,b, 1991 and Trochanis et al. 1991 conducted a series of 3D FEM studies on the behavior of a single pile and a pile group with elastic-plastic soil model. These researchers used interface ele- ments to account for pile-soil separation and slippage. Moreover, Brown and Shie derived p - y curves from FEM data, which pro- vide some comparison of the FEM results with the empirical de- sign procedures in use. Kimura et al. 1995 conducted 3D FEM analysis of the ultimate behavior of laterally loaded pile groups in layered soil profiles with the soil modeled by Drucker–Prager model and pile modeled by nonlinear beam elements. A number of model tests of free- or fixed-headed pile groups under lateral loading in homogeneous soil profiles have been simulated by Wakai et al. 1999 using 3D elastoplastic FEM. Pan et al. 2002 studied the performance of single piles embedded in soft clay under lateral soil movements. A good correlation between the experiments and the analysis has been observed in these studies. All of these results demonstrated that FEM can capture the essen- tial aspects of the nonlinear problem. Information about the lateral behavior of piles in layered soil profiles is very limited. Some analytical studies have been con- ducted by Davisson and Gill 1963 and Lee and Karunaratne 1987 to define the influence of pile length, the thickness of upper layer, and the ratio of stiffness ratio of adjacent layers on the pile response based on the assumption that the soil is elastic. Reese et al. 1981 conducted small-scale laboratory tests on a 25 mm diameter pile and a field test with 152 mm diameter pile in layered soils and found that there was a relatively good agreement between deflections measured in the tests and deflections com- puted using homogeneous p-y curves at small loads. Georgiadis 1983 proposed an approach which is currently used in the LPILE program Reese et al. 2000a,b. This method assumes that the p-y curves of the first layer are the same as those for homo- geneous soils. The effects of upper layers on the p-y curves of the lower layers are accounted for by the equivalent depth of the overlying layers based on strength parameters. To the writer’s knowledge, there are no literature reporting on the FEM study of layering effects on the behavior of laterally loaded piles in layered profiles. However, it is of great interest to investigate the layering effects since in practice, most of soil de- posits are layered systems. In a predominantly clay site with a minor sand layer, the sand layer will still be counted on to provide most of the soil resistance. In this case, the layering effects prob- ably reduction of resistance in the sand layer must be considered. Current practice is to “make an educated guess to reduce the sand p-y curves to account for the soil layering effects” Lam and Law 1996. Obviously, an educated guess might not result in an optimal design. It is very important to find out how these layers in the layered system affect each other in order to carry out a more accurate analysis of pile foundation and, therefore, provide a more effective way to design pile foundations in layered soil systems. This paper describes four 3D FEMs of a laterally loaded pile embedded in uniform and layered soil profiles, with the dimen- sions and soil parameters similar to those used in the centrifuge 1 Assistant Professor, Dept. of Civil Engineering, Univ. of Alaska Anchorage, 3211 Providence Dr., Anchorage, AK 99508. E-mail: afzy@uaa.alaska.edu 2 Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of California, One Shields Ave., Davis, CA 95616. E-mail: jeremic@ucdavis.edu Note. Discussion open until November 1, 2005. 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 pos- sible publication on February 11, 2003; approved on November 3, 2003. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 131, No. 6, June 1, 2005. ©ASCE, ISSN 1090-0241/ 2005/6-762–770/$25.00. 762 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / JUNE 2005