The International Information Center for Geotechnical Engineers

In Situ Shear Wave Velocity Measurements in Rocks - Introduction

1.0 Introduction:

            This report contains a review of the in situ geophysical testing methods used to determine the shear wave velocity of geologic materials. Brief overviews of several geophysical techniques are provided and accompanied by examples and useful references. Three case studies utilizing shear wave velocity measurements are also discussed. Finally, the in situ Vs values for various rock types reported in the technical references herein are compiled for comparison and reference.

 

The measurement of seismic wavevelocities has become an integral component of material characterization in geotechnical and geological engineering. The propagation velocity of stress waves is related to the mass density (ρ) and small-strain stiffness of the material (Stokoe et al. 2004). The compression wave, or P-wave, velocity (Vp) is related to the material mass density by the constrained modulus (M) as shown in eq. 1. Similarly, the shear wave, or S-wave, velocity (Vs) is related to the material mass density by the shear modulus (G) as shown in eq. 2. These seismic wave velocities are related to each other through Poisson’s ratio (ν) as shown in eq. 3.

 

eq 1                                                        eq. 1

eq 2                                                           eq. 2

eq 3                                                        eq. 3

            While Vp is traditionally more commonly measured seismic velocity in rocks, it is not optimal for measurement in soils. In a fully saturated soil, the compression stiffness of the pore fluid will dominate due to water being incompressible relative to the soil skeleton. As a result, the value of Vp measured in a soil is heavily influenced by the degree of saturation (Stokoe et al. 2004). This becomes critical when considering that a site investigation may encompass both soil and rock depending on the geologic conditions, desired depth of investigation, and characterization goals. For this reason, it would be helpful, and efficient, to simultaneously measure Vs in both materials. Vs in both soil and rock is valuable due to its relation to small-strain shear modulus, use in correlations, and necessity as an input into seismic site response analyses. The Vs of a soil or rock also provides a general estimate of its stiffness and/or degree of weathering.

Add comment

NOTE: The symbol < is not allowed in comments. If you use it, the comment will not be published correctly.

Security code
Refresh
*Please insert the above-shown characters in the field below.

The Geoengineer.org Corporate Sponsors: