Offshore Site Characterization of Small Strain Shear Modulus Using a Seabed Based Drilling System
Authors: Lunne Tom , Kelleher Patrick , Banimahd Meysam , Degroot Don , Nguyen Hoang Quang , Senders Marc M 
Published: May 2013

The paper describes operational details and performance results of a new deepwater seismic probe for measuring the in situ shear wave velocity (Vs) of offshore soils. The system uses a seismic cone that incorporates two sets of geophones located 1 m apart in the probe so as to allow for true-interval determination of the in situ Vs profile. The system is integrated with the new generation Portable Remotely Operated Drill (PROD) seafloor based drilling system. An energy source that consists of two opposite direction hammers is mounted under the PROD's stern foot and provides opposite polarity S-wave pulses at the seabed which are subsequently recorded downhole by the seismic cone geophones. The new system was proof tested at a soft clay site in Brisbane, Australia and directly compared with Vs data obtained from a seismic dilatometer. The system was then subsequently used for two deepwater offshore site investigation programs that involved varying soil conditions. In-situ Vs measurements were performed in water depths ranging from 90 m to 600 m, in the Caspian Sea, offshore Azerbaijan and the Timor Sea, offshore North Western Australia. The seismic cone system was successfully deployed to a depth of up to 76 m below seafloor using the new generation PROD.

Piezocone Penetrometer (CPTU) testing was undertaken in boreholes immediately adjacent to the seismic probe boreholes. Piston samples were collected from the Caspian Sea sites and resonant column and bender element testing was performed on selected samples reconsolidated to the estimated in-situ effective stress state. The field measurements of Vs were found to generally be in good agreement with the laboratory results with due account taken for the effects of sample disturbance.

The results presented in this paper demonstrate that the seismic probe system, deployed in conjunction with a stable, quiet seabed drilling system provided by the PROD, can provide high quality direct in-situ measurement of shear wave velocity.

It is recommended that field and laboratory measurements of Vs and Gmax be part of standard practice for important offshore site investigations. Gmax in particular is a critical input parameter for several applications including static and dynamic analysis of foundations systems, soil liquefaction analysis, input for advanced constitutive soil models, and non-destructive evaluation of sample quality.

Introduction

The shear wave velocity Vs and the corresponding small strain shear modulus Gmax are important soil parameters for several offshore geophysical and geotechnical engineering applications. Gmax is directly related to Vs based on the elastic relationship

Gmax = ???tVs2 [1]

where ???t is the total soil density.

Gmax is an essential parameter for analysis and design of foundations subjected to both static and dynamic loading and that involve elastic and elastoplastic deformations (e.g., Burland 1989, Anderson et al. 2008, and Puzrin, A. 2012). Vs is widely used in soil liquefaction assessment (e.g., Robertson et al. 1992 and Andrus and Stokoe 2000) and more recently has been used as a non-destructive method for evaluating sample quality of clays (e.g., Donohue and Long 2007 and Landon et al. 2007). Furthermore, direct in situ measurement of Vs can be used for calibrating shear wave velocities determined indirectly using other geophysical methods (e.g., Peuchen et al. 2002).

Vs and Gmax can be measured using several in situ and laboratory methods. The seismic piezocone (CPTU) and seismic dilatometer are in situ tools that provide a down hole measurement of Vs while in the laboratory Gmax is directly measured in the resonant column device (e.g., Drnevich et al. 1978) and bender elements can be used in a consolidation or shear device (e.g., triaxial) to measure Vs (e.g., Dyvik and Madshus 1985).