Determination of the material damping ratio and the shear wave velocity with the Seismic Cone Penetration Test

The Seismic Cone Penetration Test (SCPT) is used to determine the variation with depth of the shear (and longitudinal) wave velocity at various sites in Belgium. The cross correlation technique together with a posteriori increase of the sampling frequency proves to be a reliable method to determine wave velocities by the SCPT. Additionally, the Spectral Ratio Slope (SRS) method is applied to Fourier transforms of the measured response to determine the variation of the material damping ratio with depth. This method is applied to synthetic signals computed for three different soil profiles with known characteristics and to the horizontal acceleration time histories that have been measured during SCPT on two sites. The influence of some signal processing techniques commonly applied in the SRS method is studied. A remarkable influence of the window length, applied on the predicted signals, on the computed material damping ratio is found, especially its scattering. It is therefore concluded that the use of a window function should be avoided wherever possible. On one of the two test sites, results of Resonant Column Tests and Free Torsion Pendulum Tests on undisturbed soil samples have been used to evaluate the values of the shear wave velocity and the material damping ratio as obtained in the in situ test.