Application of the SASW-technique in geotechnical in-situ testing

With the main aim to test the applicability of the surface wave seismic technique Spectral Analysis of Surface Waves (SASW), used for determination of the shear stiffness (shear modulus - Gmax), four areas of applications have been studied. The four areas have been sites with natural geological conditions, sites where non destructive testing is necessary, pavement testing and multichannel testing. A number of sites with a wide range of different geological conditions, varying from limestone to soft clay, were investigated and compared with results from a number of reference methods, such as seismic cone (SCPT), cone penetration testing (CPT), pressuremeter and vane shear testing. Testing of sites where non-destructiveness is necessary was carried out by combining SASW results and Continuous Vertical Electric Sounding (resistivity) results, and hence a methodology where two non-destructive geophysical methods are used for both stratigraphical and mechanical characterisation of the shallow subsurface was used. The pavement testing was performed in order to study the main problems with using SASW on the typical layering with extreme stiffness variations between the adjacent top layers, and, when compared with the most common natural soil layering, an inverse stiffness profile. Possible solutions to the pavement testing problems are discussed. The large potential in using profiling SASW-technique for continuity and spatial purposes is discussed. In order to demonstrate the potential in SASW profiling an 8-channel SASW-system was designed and tested where resistivity profiling and numerous geotechnical data was available.

Several conclusions are drawn in the thesis. The SASW moduli correlates in general with moduli from SCPT, except for two occasions. First, on the silty sites a big difference in the absolute values are seen. This may be explained by different degrees of saturation on the different occasions of testing. Secondly, when a stiffer layer overlays a less stiff layer the latter is not always detected. This problem is found to be related to the inversion technique that is used, and can be avoided if accurate a priori information of the layering is available. The methodology of combining SASW-technique and the resistivity method for a “complete” geotechnical characterisation of the subsurface was found to be successful on a landfill and a lime stabilised road embankment. With a multichannel SASW-system profiling was carried out and the shear stiffness was determined continuously along a line. The multichannel data was compared with reference data from soundings, soil sampling and resistivity profiling data, and the correlation between the different methods was good. This clearly demonstrated a useful possibility in geotechnical characterisation of sites.