Correlations Between Compressive Seismic Velocity And Cone Resistance At Shallow Penetration In Sands

The evaluation of burial conditions for pipelines or telecommunication cables in the seabed requires essentially two demands to be fulfilled as regards the terrain survey. Firstly, a large distance must be covered (from a few kilometres to several hundreds of kilometres) and, secondly, the survey must provide a precise identfication of the subsoil stratigraphy and a preliminary evaluation of the mechanical characteristics in the 3 to 5 metres below the seabed. In order to achieve this, two techniques are generally employed :

1. In situ geotechnical techniques such as the Cone Penetration Test (CPT) that provide detailed but punctual information concerning the zone in the immediate neighbourhood of the borehole and

2. geophysical techniques such as seismic reflaction, whereby a quasi-continuous picture of the survey route can be built up from the measurement of seismic propagation velocities (e.g. Puech and Tuenter, 2002). The harmonising of these two techniques and the research into correlations between the two sets of data are therefore of great Importance.

Numerous laboratory studies using the Cone Penetration Test (CPT) have already been carried out on soils in calibration chambers and there exists a large amount of CPT data, obtained from both research conducted in calibration chambers and m-situ surveys. Seismic measurements can also be performed on soils confined in calibration chambers m parallel with the CPT, enabling one to examine the relationship between seismic velocities and CPT data, as well as their correlation with the mechanical properties of the soil. Such correlations have been investigated by a number of researchers and have lead to publications by Stokoe et al (1991), Baldi et al. (1986) and Rix & Stokoe (1991) amongst others. However, these studies have mainly concentrated on depths and confining pressures in excess of 50kPa, which is greater than the low confining pressure (typically < 30kPa) found close to the surface of the seabed. In sands, a simple extrapolation of these results to shallow depths is not justified due to the specific transitory response of the cone resistance along the first one or two metres of penetration. Furthermore, most of the studies concerned shear wave velocity measurements and correlations between small strain shear moduli and cone resistances. Burial assessment techniques, such as the Gambas]> <![CDATA[system developed by Fugro, require a correlation with compressive waves.

The purpose of the laboratory study presented in this paper was to accumulate a database of both CPT and seismic measurements in well controlled conditions at very shallow depths in sands. The aim was firstly to compare the results with existing data from studies conducted at greater depths and coding pressures and, secondly, to establish a correlation between the resistance at the tip of the penetrometer and the compressive wave velocity.

The research program undertaken consisted of a series of tests earned out in the calibration chamber of the Laboratory 3s (Fig.1) This calibration chamber is a right, circular cylinder of depth 1.5m and of diameter 1.2m. Above the chamber a rigid frame holds in place the penetrometer, which is driven into the soil at a standard, constant rate of 20mm/s by a hydraulic system.