Determination of shear velocity profiles for soil and concrete by analysis of seismic surface waves

The determination of the shear modulus G as a function of depth is of primary importance to civil engineers. For instance it is an important parameter for the assessment of seismic risk and for the characterisation of the mechanical properties of soils. For concrete, it can be used to quantify the extent of surface damage. It is presently not very common to use surface waves to calculate shear velocity profiles, which are necessary to calculate G profiles. The aim of this paper is to show how surface wave analysis has some advantages as compared to the direct measurement of shear wave propagation. To characterise the efficiency of surface waves to solve such civil engineering problems, we have undertaken tests on concrete slabs. The concrete slabs are considered here as reduced scale physical models. On top of three 3mx4mx0,8m concrete slabs, a layer of light weight concrete with three different thicknesses (0.05m, 0.1m and 0.2m) has been added. As the geometry, densities and velocities are known, different measuring set-ups and processing techniques can be tested and compared. In this paper we focus on two common set-ups. The first one is the SASW method (Spectral Analysis of Surface Waves). The second one is multiple filter and phase velocity analysis as used in Earth physics and here applied to a large number of sensors. As opposed to the SASW set-up, it is applied on surface waves in the far field. The two methods are applied both to seismograms obtained on the three concrete slabs and on synthetic seismograms in models equivalent to the concrete slabs. This multiple filter and phase velocity analysis has been successfully combined to seimic refraction during a sub-surface survey in the civil engineering depth range (0-10m).