Multistation Methods for Geotechnical Characterization using Surface Waves

This dissertation deals with soil characterization methods based on surface wave propagation applied to geotechnical engineering purposes. This topic has gained much interest in the last decade because of the appealing possibilities given by non-invasive methods, which are at once very flexible and cost effective. An overview of the properties of Rayleigh waves in layered linear elastic and linear viscoelastic media is presented, together with their applications for site characterization, of whose the SASW (Spectral Analysis of Surface Waves) method is by far the most well-known in geotechnical engineering. The research has been mainly focused on the application of multistation methods, compared with the classical two-station approach typical of the SASW method. Results from both numerical simulations and experimental testing are reported to compare two-station and multistation methods and to clarify the advantages that can be obtained using the latter ones. In particular the research has been developed following two different directions: on the one hand the application of classical geophysical analysis tools (such as domain analysis and slant stack transform) to tests performed with impulsive sources. On the other one the possibility of obtaining from surface wave testing not only a stiffness profile, but also a damping ratio profile for the site. In this respect a new method for simultaneous measurements of Rayleigh dispersion and attenuation curves is proposed. Regarding the first topic, the necessity of a multistation approach to determine the experimental dispersion test is essentially related to the spatial variation of phase velocity. Analyses in the frequency-wavenumber domain and in the frequency-slowness domain are very powerful approaches, still there was a need of studying the effects of the change of scale from geophysical applications to geotechnical ones. Indeed because of the peculiar properties of Rayleigh waves, surface testing is strongly affected by the distance travelled by the analysed wave. The numerical simulations performed in the research show that the phase velocity obtained using multistation methods with a limited number of receivers close to the source is not a modal value as it is for geophysical applications, but an apparent phase velocity arising from modal superposition. The experimental tests showed the good performances of multistation methods when compared to the SASW method. In particular some drawbacks of the latter method, due essentially to its two-station nature, are avoided and the field-testing appears to be very promising for future applications. In particular the application of the frequency-wavenumber domain analysis can lead to much faster and more stable estimates of the experimental dispersion curve and the process is easily automated, with a great saving of time and less requirement for subjective decisions. Another important advantage is given by the stability with respect to a near field effects that lead to a better reconstruction of the dispersion curve for the low frequencies and hence to a deeper characterization. The necessity of a new method for the simultaneous determination of surface wave dispersion and attenuation curves is linked to the strong coupling existing between the two. Such coupling is extremely important for the subsequent inversion process, in a consistent method leading from the field measurements to the stiffness and damping profiles. The proposed method uses a new testing configuration, designed to measure the experimental transfer function. Successively a regression process of the complex quantity with the corresponding expression obtained modelling soil as a linear viscoelastic layered system leads to the experimental dispersion and attenuation curves. Some preliminary results are reported showing very encouraging results, also if a more extensively testing programme is required for the complete validation of the metho

RELIABILITY OF SOIL POROSITY ESTIMATION FROM SEISMIC WAVE VELOCITIES

Soil porosity is a state parameter of fundamental importance for several geotechnical problems. Geophysical testing provide appealing strategies for the determination of soil porosity, as several geophysical parameters are directly related to soil porosity. In particular the theory of wave propagation in saturated porous media, developed by Biot in the 1950s, allows the determination of soil porosity from the measured velocity of propagation of compressional and shear waves. A formal assessment of the reliability of the estimated porosity values is of primary importance to evaluate the applicability of this approach to solve practical geotechnical problems. In this paper the propagation of measurement uncertainties on the estimated values of soil porosity is theoretically evaluated. Moreover, experimental data of multiple acquisitions of cross-hole tests are considered. Data collected by different operators are also used to assess the confidence interval associated to different equipment, acquisition practices and testing methodology

Evaluation of porosity and degree of saturation from seismic and electrical data

The characterisation of unsaturated intermediate and coarse-grained soils faces some practical difficulties because undisturbed sampling is not easy. Geophysical methods provide useful information as they can be applied on site for testing geo-materials in their natural state. Moreover their repeated application over time is effective and efficient for monitoring purposes. A procedure for evaluating porosity and degree of saturation on the basis of electrical resistivity and wave velocities measurements is proposed. The approach is based on an electro-seismic model that utilises Archie's law to describe the electrical behaviour of soils and a recent formulation of elastic wave propagation in unsaturated soils. The proposed procedure is applied to laboratory data, and shows promising result

Surface wave analysis for building near surface velocity models: established approaches and new perspectives

Today, surface-wave analysis is widely adopted for building near-surface S-wave velocity models. The surface-wave method is under continuous and rapid evolution, also thanks to the lively scientific debate among different disciplines, and interest in the technique has increased significantly during the last decade. A comprehensive review of the literature in the main scientific journals provides historical perspective, methodological issues, applications, and most-promising recent approaches. Higher modes in the inversion and retrieval of lateral variations are dealt with in great detail, and the current scientific debate on these topics is reported.Abest-practices guideline is also outline

Preliminary results of P-wave and S-wave measurements by seismic dilatometer test (SPDMT) in Mirandola (Italy)

A trial seismic dilatometer-VP (SPDMT) has been recently developed to measure the compressional wave velocity VP, in addition to the shear wave velocity VS and to the DMT geotechnical parameters. The new SPDMT is the combination of the traditional mechanical flat dilatometer (DMT) with an appropriate seismic module placed above the DMT blade. The SPDMT module consist in a probe outfitted with two receivers for measuring the P-wave velocity, along with two receivers for measuring the S-wave velocity. The paper describes the SPDMT equipment, the test procedure and the interpretation of VP and VS measurements, together with some considerations on the potential geotechnical applications which can benefit from the contemporary measurement of the two propagation velocities. Finally, the paper illustrates preliminary results of P-wave and S-wave measurements by SPDMT compared to several cross-hole, down-hole and suspension logging data at the Mirandola test site (Italy), a soft alluvial site which was investigated within the InterPACIFIC (Intercomparison of methods for site parameter and velocity profile characterization) project