Refined Model for Interpreting Shallow Penetration CPTs in Sands
Authors: Puech Alain , Foray Pierre 
Published: October 2008

Over the last 5 years the CPT (Cone Penetration Test) has become the reference geotechnical tool for assessing burial conditions along pipeline or telecommunication cable routes. At shallow penetrations in sands, the increase of the cone resistance with depth is strongly affected by the low confining pressures due to the overburden. This transitory phase is ignored in traditional CPT interpretation methods.

The paper presents a new method for interpreting results obtained from standard cone penetration tests at shallow penetration (less than 3 meters) in saturated marine sands. The method is based on experimental data obtained from a series of laboratory tests in a calibration chamber and a back analysis of hundreds of offshore CPTs performed in water depths ranging from 50 to 2000m and in a large variety of environmental conditions (North Sea, Atlantic Ocean, Pacific Ocean)

Introduction

In response to the demand of the telecommunication cable market new tools have been recently developed and introduced for burial assessment purposes which are now available to the pipeline industry. The CPT has become the reference tool for obtaining the geotechnical data required for burial engineering. It is used in conjunction with continuous profiling techniques, e.g., seismic refraction or electrical resistivity. These techniques implemented through bottom-towed sleds provide an accurate stratigraphy of the subsoil over the target burial depth and a quantitative characterization of the materials via their compressive seismic velocity (Vp) or their electrical resistivity (?). A more detailed description of the so-called e_bass (electronic burial assessment survey) technology can be found in a companion paper to this conference (OTC 14074).

Correlating geophysical parameters (Vp, ?) obtained on a continuous basis and geotechnical parameters derived from CPT measurements at discrete locations is the key to data integration and continuous assessment of burial conditions all along the route.

Extensive research has been carried out with a view of establishing relationships between CPT data and geophysical or mechanical properties of sands. The main results available concern correlations between:–cone resistance (qc), relative density (Dr) and confining pressure (?'v or ?'m), e.g., Schmertmann (1978), Lunne and Christoffersen (1983), Jamiolkowski et al. (1988);–compressive (Vp) or shear (Vs) wave velocities (alternatively small strain moduli Emax or Gmax), soil density and confining pressure e.g. Hardin (1978), Dobry et al. (XX), Lo Presti and O'Neill (1991).

Relations between seismic velocities have been inferred by combining the above two sets of correlations, e.g., Jamiolkowski and Robertson (1988), Foray and Bouguerra (1995) or by direct measurements of both quantities in calibration chamber tests, e.g., Baldi et al. (1989) or Rix and Stokoe (1991).

These works have a common limitation due to the fact that experimental data used to establish the correlations were obtained at confining pressures in excess of 50 kPa. Their applicability to low confining pressures (typically less than 30 kPa) representative of the first 2 to 3m of penetration relevant for burial engineering remains questionable.