Development Of a Bottom-Hole Device For Offshore Shear Wave Velocity Measurement
Authors: Stokoe K.H. , Arnold E.J. , Hoar R.J. , Shirley D.J. , Anderson D.G. 
Published: October 2008

In situ testing to determine shear moduli of soil is presently not performed in offshore geotechnical investigations because of the lack of a production-type method. Limitations of existing onshore seismic methods which generally preclude the use of these methods in offshore investigations are discussed. When combined with laboratory or empirical moduli, low-amplitude in situ moduli are shown to be valuable in estimating in situ shear moduli at higher shearing strains. As a result of this absence of valuable in situ moduli measurements offshore, the concept for a new borehole device with which such measurements can routinely be made is presented. The device will be placed at the bottom of an offshore boring and measurements will be made in terms of shear wave velocity in the soil beneath the borehole bottom. The primary components of the device are a source mechanism and receivers. Results of studies performed to select these components along with ambient noise level measurements at the bottom of offshore borings are presented.

INTRODUCTION

One of the key parameters required when analyzing the response of offshore soil-structure systems subjected to dynamic loading is the shear modulus of the soil and its variation with shearing strain amplitude. Although shear moduli of offshore soils can be determined from soil samples using laboratory testing methods, a more accurate representation of in situ moduli can be obtained by conducting field tests. This approach is not without precedence. For important onshore geotechnical studies, it is now common practice to perform seismic tests to obtain in situ shear moduli at low-amplitude shearing strains (less than 0.001 percent). Moduli determined during these onshore tests serve as direct input for low amplitude vibration studies and as a reference level for adjusting laboratory and empirical moduli for higher amplitude vibration studies.

Despite the importance and cost of many offshore installations, procedures for obtaining shear moduli by in situ testing methods are not currently available to the geotechnical engineer. Onshore methods have proven to be either technically unfeasible or too costly for offshore applications. As a result, it has been necessary to rely on laboratory testing methods tempered by engineering judgement. Although such an approach may have been considered adequate in the past, it can no longer be considered adequate both for technical and economic reasons. Conservative estimates of shear moduli may not lead to a safe design during dynamic loading. Accurate determination of in situ moduli is crucial to increasing confidence levels in predicting the dynamic performance of offshore soil structure systems. In view of the tremendous costs associated with these facilities, it is also desirable to minimize costly over designs through increased knowledge wherever possible. Furthermore, the need for a means of determining accurate, in situ values of moduli has gained even more importance recently as development and the potential for development has increased in seismically active areas.