Investigation of Possible Relationship between Undrained Shear Strength and Shear Wave Velocity for Normally Consolidated Clays
Authors: Blake W.D. , Gilbert R.B. 
Published: October 2008

An empirical model is presented to infer undrained shear strength from shear wave velocity in normally consolidated clays. The uncertainty in the inferred undrained shear strength is estimated based on a statistical analysis of test data. This uncertainty is manageable provided accurate shear wave velocity measurements can be obtained. An example application of the model is presented to demonstrate how it could be used to infer the profile of undrained shear strength to depth at a site. Potential benefits of the method include being able to adjust laboratory data for sample disturbance, and to extrapolate boring data laterally.

Introduction

A method for inferring design parameters of offshore soils from geophysical data would be particularly valuable as the construction of new structures moves into deeper waters. Using geophysical testing techniques to determine the strength of offshore soils would avoid problems with sample disturbance associated with conventional testing techniques. Further, such a testing method would be better able to provide a three dimensional picture of the sea floor over large regions, and would potentially be more cost-effective than conventional methods. Information from geophysical techniques could also be used to supplement information from borings. Undrained shear strength, Su, is typically the most sought after parameter for foundation design in normally consolidated clays. This paper will focus on how to infer undrained shear strength from shear wave velocity, vs, in normally consolidated clays.

Conventional geophysical testing of offshore soils involves the measurement of compression waves. Compression wave velocity, vp, in high void ratio soils (e &gt; l) is largely a function of the compression wave velocity of water, and is unrelated to the soil structure (Shumway, 1966). Most offshore normally consolidated clays have void ratios greater than one; therefore, the undrained shear strength is not sensitive to the compression wave velocity in these soils. For example, Figure I shows undrained shear strength and compression wave velocity versus depth at a site comprised of normally consolidated clays (Schultheiss, 1985). These Su and vp data were obtained from laboratory measurements on undisturbed samples.

Measurements of shear wave velocity, vs, may be more valuable than compression wave velocity measurements in normally consolidated clays. Shear wave velocity, even at high void ratios, is related primarily to the soil structure. For example, Figure 2 shows how Vs and Su vary together with depth at a site from Schultheiss (1985). As with the data on Figure I, the Vs, data were obtained from laboratory measurements on undisturbed samples. There are a number of ways of measuring the shearwave velocity in situ of offshore soils, including cross-hole methods, down-hole methods, and the Seismic Analysis of Surface Waves (SASW) method (Stokoe et al, 1990).

In this paper, an empirical model is presented for inferring undrained shear strength from shear wave velocity in normally consolidated clays. The uncertainties in the given undrained shear strength versus shear wave velocity relationship are then estimated based on statistical analyses of available test data. These preliminary results indicate a promising potential for the use of shear wave velocity measurements in the design of offshore foundations.