Summary
Recently a need has developed for new methods of soil testing in the offshore industry as the construction of new structures has moved into deeper waters. In the past thirty years, the maximum water depth of construction has moved from the shoreline to 5000 feet. In the next ten years, the maximum depth is expected to increase to 10,000 feet. Conventional testing techniques are less effective at these greater depths than in shallow water: drilling bore holes in deep water can be extremely problematic and expensive. Further, testing the samples from these bore holes can be difficult because of the high pressures at depth. Upon being brought to the surface, the samples often experience disturbance due to the pressure change, thus bringing the accuracy of their laboratory-measured properties into question. (Doyle,1994).
The new structure designs necessitated by the deeper water environment also demand new soil testing procedures able to cover wider areas. Many of the new rig designs involve tension cables which expand over areas considerably wider than those covered by the more shallow structures. The data from one bore hole may not be considered enough for the foundation designs of the deeper structures when the different parts of the foundation are separated by thousands instead of hundreds of feet. For the newer designs, using conventional testing techniques, the engineer would either have to design the structure without measuring the lateral variability in strength, or observe it at very high cost.
The most critical geotechnical parameter in offshore pile design is undrained shear strength, su . It has been proposed that indirect methods of soil testing, including seismic methods, back calculations of strength from bathymetry observations, and engineering judgment could each provide useful information on the strength of the sea floor in deep water. A method that combines these sources of information in a systematic way could provide a powerful method in the analysis of deep-water sites.
A critical issue in developing such a method concerns the first of the aforementioned sources of indirect soil testing, geophysical testing; namely, what is the relationship between geophysical parameters and undrained shear strength for soils? The thesis investigates this question for clays. The results presented herein address the potential for correlations between undrained shear strength and compression and shear wave velocity for such soils.
The methods of dynamic testing include those that produce and measure compression and shear waves. Compression waves create particle motion parallel to the direction of wave propagation, while shear waves create particle motion perpendicular to the direction of wave propagation. Compression and shear wave velocities are denoted by vp and vs, respectively.
Methods that produce compression waves have been widely used in the offshore industry for years for exploration and engineering purposes, and research into their use continues today. The velocities of shear waves in situ are typically measured via down-hole or cross-hole techniques, or via the Spectral Analysis of Surface Waves method. The latter is effective at evaluating the shear wave properties of near-surface offshore
Related Publications: Blake, W.D. and Gilbert, R.B., “Investigation of Possible Relationship Between Undrained Shear Strength and Sheer Wave Velocity for Normally Consolidated Clays,” Proceedings Offshore Technology Conference, pp. 411-420, 1997.
