New methodology for dynamic soil characterization using the free-decay response in resonant column testing Fernando J. Tallavo, Giovanni Cascante, Mahesh D. Pandey & Sriram Narasimhan Department of Civil and Environmental Engineering – University of Waterloo, Waterloo, Ontario, Canada ABSTRACT The resonant-column (RC) test is an ASTM standard commonly used for dynamic characterization of soils under different isotropic confinements and shear strain levels. The free-decay response of a soil specimen shows a nonlinear behaviour when the strain levels exceed the threshold shear strain. However, the dynamic parameters are commonly determined assuming a linear single-degree-of-freedom (SDOF) model. This paper presents a new analysis methodology, based on the complex exponential method (CEM), for the nonlinear dynamic characterization of soil specimens using the free-vibration response in RC testing. The effectiveness of the new methodology is demonstrated in a dry-sand specimen tested under isotropic loading and unloading conditions, two confining pressures (40 kPa and 100 kPa), and different shear-strain levels (10  ≤  ≤ 1.7 × 10  ). The results from the CEM are compared with the results from traditional SDOF methods (transfer function and free vibration). The results show that the damping ratio can be underestimated up to 80% at large strains when computed using SDOF models. In a nonlinear free-vibration test analyzed using the CEM, it is possible to observe that the dry-sand specimen becomes denser (stiffer) after large-strain cycles. Thus, the shear wave velocity (resonant frequency) increases at the low shear strain levels imposed during the same test. RESUMEN El ensayo de columna resonante (RC) es un estándar ASTM comúnmente usado para la caracterización dinámica de suelos sometidos a diferentes presiones de confinamiento y niveles de deformación por corte. La respuesta de vibración libre de un espécimen de suelo ensayado en RC muestra un comportamiento no-lineal cuando los niveles de deformación exceden el umbral de deformación por corte. Sin embargo, los parámetros dinámicos son comúnmente determinados asumiendo un modelo de un solo grado de libertad (SDOF). Este paper presenta una nueva metodología de análisis basada en el método exponencial complejo (CEM) para la caracterización dinámica no-lineal de especímenes de suelo usando la respuesta de vibración libre en ensayos de RC. La eficacia de la nueva metodología es demostrada en un espécimen de arena, en condición seca, ensayado bajo condiciones que carga y descarga, dos presiones de confinamiento (40 kPa y 100 kPa) y diferentes niveles de deformación por corte (10  ≤  ≤ 1.7 × 10  ). Los resultados del CEM son comparados con los valores obtenidos usando los métodos tradicionales (función de transferencia y vibración libre para un SDOF). Los resultados muestran que el factor de amortiguamiento puede ser subestimado hasta en 80 % a elevadas deformaciones por corte cuando éste es calculado usando los métodos tradicionales. Adicionalmente, la arena se hace más densa (rígida) después de la aplicación de ciclos a elevados niveles de deformación por corte. Como consequencia, la velocidad de onda de corte (frecuencia de resonancia) aumenta a bajo niveles de deformación por corte impuestos durante la misma prueba. 1 INTRODUCTION The resonant-column (RC) test is an ASTM standard that has been extensively used for dynamic characterization of soils under different confinement and shear strain conditions (Wilson and Dietrich 1960; Hardin and Richart 1963). In this device, a fixed-free cylindrical soil specimen is excited in torsion. The response of the specimen is measured using a piezoelectric accelerometer located at the driving plate. The shear wave velocity of the soil specimen is estimated by solving the equation of wave motion of a rod with an attached mass at the top (Hardin and Richart 1963). The shear modulus is then determined from the shear wave velocity. The damping ratio can be obtained either from the analysis of the input excitation and the response of the specimen (frequency domain) or from the analysis in the time domain of the free-decay response when the specimen is subjected to an impulsive excitation. The free-decay response of a soil specimen in the time domain shows a nonlinear behaviour when the strain levels exceed the threshold shear strain; however, the dynamic parameters are commonly determined assuming a linear single-degree-of-freedom (SDOF) model. This paper presents a new analysis methodology, based on the complex exponential method (CEM, Osborne and Smyth 1995), for the nonlinear dynamic characterization of soil specimens using the free-decay response in RC testing. The nonlinear response of the soil specimen is represented as a superposition of damped exponential functions. The use of the free-decay response is also beneficial because it reduces the number of