Proceedings of the Institution of Civil Engineers Geotechnical Engineering 159 July 2006 Issue GE3 Pages 233–241 Paper 14420 Received 06/09/2005 Accepted 20/03/2006 Keywords: dynamics/field testing & monitoring/piles & piling A. Boominathan Professor, Department of Civil Engineering, Indian Institute of Technology, Madras, India R. Ayothiraman Assistant Professor, Department of Civil Engineering, Indian Institute of Technology, Guwahati, India Dynamic response of laterally loaded piles in clay A. Boominathan PhD and R. Ayothiraman PhD The behaviour of single piles under lateral dynamic loading is critical, and has been an important field of research since the 1950s. Many analytical or semi- analytical linear and non-linear models are available to estimate the dynamic lateral stiffness, but it is essential to determine the dynamic characteristics of the soil–pile system through full-scale lateral dynamic pile load tests for important structures and for validation of existing models. This paper presents the results of field lateral dynamic load tests conducted on 33 piles of varying types–driven precast concrete, driven cast-in-situ concrete and bored cast-in-situ concrete–at different petrochemical complexes in India. The results indicate that driven precast concrete piles have stiffnesses that are four to five times higher than those of driven cast in situ piles. The lateral stiffness was also estimated using the computer program PILAY for all piles and compared with the stiffness determined from the field tests. The estimated stiffness shows good agreement with the field values for stiff clay sites, but greatly overestimates the values for soft clay sites. NOTATION a x measured horizontal acceleration A x dynamic displacement amplitude A m amplitude of vibration at mth cycle A mþ1 amplitude of vibration at (m + 1)th cycle e eccentricity of rotating mass oscillator f forcing frequency f n natural frequency of soil–pile system F d magnitude of dynamic force G max maximum dynamic shear modulus k hp dynamic lateral stiffness of soil–pile system m e mass of eccentrically rotating body N uncorrected SPT-N value N avg average uncorrected SPT-N value V s shear wave velocity ä st static displacement amplitude ì magnification factor r mass density of soil î x damping factor ø frequency of rotating mass oscillator 1. INTRODUCTION Pile foundations are commonly employed in industrial situations, such as power plants, petrochemical complexes, oil refineries and compressor stations, to support a range of structures. These piles are subjected to dynamic lateral loads from operating machinery, wind and earthquakes in addition to static loads, and hence the dynamic response of piles and pile groups to lateral shaking has received considerable attention from designers and researchers. The lateral capacity and stiffness of piles depend mainly on the characteristics of the top layers of soil (within a few metres of the surface), which may be very soft to stiff in nature. At some industrial sites the top layer is found to be soft clay or loose sand with a thickness that may vary from 5 . 0 m to 30 . 0 m or more. Therefore the lateral load criterion rather than the vertical load often dictates the pile design. Hence evaluation of the lateral stiffness of a single pile under dynamic loading becomes a crucial step in the satisfactory design and performance of pile foundations. Although many sophisticated linear and non-linear models— theoretical, semi-analytical and numerical—have been proposed, 1–18 there are scant experimental data available to confirm the reliability of these models. The limited field testing carried out on piles embedded in clay and sandy clay sites by various authors 19–27 (details are presented in Table 1) does not provide satisfactory calibration for the existing models. Furthermore, designers have insisted upon the need to evaluate dynamic pile parameters by means of in situ tests for important and sensitive structures such as nuclear power plants and industrial structures. This paper discusses the results of lateral dynamic vibration tests carried out on 33 piles located at various petrochemical complex and oil refinery sites in India: the motor spirit quality (MSQ) unit and cogeneration (COGEN) sites at Mathura (Uttar Pradesh), the independent power producer (IPP) plant site at Panipat (Haryana), the MSQ unit and hydro cracker unit (HCU) sites at Haldia (West Bengal), and the pure teriphthalic acid unit 3 (PTA-3) site for Reliance at Hazira near the city of Surat (Gujarat). The dynamic lateral stiffnesses determined from the field tests are also compared with stiffness estimated from the computer program PILAY. 28 2. SOIL PROFILE Table 2 summarises the site conditions explored through a subsoil investigation, together with the measured average standard penetration test (SPT) values (N avg ) for the various sites. It can be seen from the table that the MSQ unit and COGEN sites at Mathura consist predominantly of stiff silty Geotechnical Engineering 159 Issue GE3 Dynamic response of laterally loaded piles in clay Boominathan • Ayothiraman 233