In soft soils, the shear strength of the upper sediments is sometimes difficult to determine accurately using a traditional cone penetrometer, due to the various corrections required. The paper describes the first offshore application of a novel T-bar penetrometer, which overcomes some of the disadvantages of the cone penetrometer, for a site investigation in 380 m of water in the Timor Sea, off the North coast of Australia. The T-bar comprises a short cylindrical bar that is attached perpendicularly to the penetrometer rods It is pushed into the soil, with the load being measured by a load cell situated immediately behind the bar. The paper presents comparative data from the site investigation, where both cone and T-bar penetrometers were used in parallel. In addition, comparative cone, T-bar, and vane shear strengths are presented from centrifuge model tests, showing the correlation between these different measurements of shear strength.
As offshore developments move into deeper waters, subsea completions are increasingly becoming the more favoured development scenario. The subsea facilities are either bed back to fixed platforms in shallower waters or to anchored floating facilities nearby. The design of foundation elements to support or anchor these facilities and also the infield flowlines is significantly influenced by the shear strength close to the seabed. Deeper waters generally Imply soft normally consolidated soils, where the strength intercept at the seabed may be very low. Accurate determination of a small strength intercept of 5 to 15 kPa, rather than zero, is critical in determining the required size of the foundation.
Recovery of high quality undisturbed samples of near surface sediments is extremely difficult at deep water sites, using either conventional down-hole investigation tools deployed through motion compensated drilling equipment, or seabed sampling techniques. Further disturbance results as very soft sod samples are extruded, thereby exacerbating the loss of accuracy of laboratory strength measurements on such material.
Alternative investigation techniques such as remote seabed test equipment have generally been limited to the use of cone penetrometer testing. However, cone penetrometers are innately inaccurate for soft soils in deep water, since they must be designed to withstand the ambient pore pressure, which may amount to several MPa, and yet measure sod strengths down to 10 kPa or lower, with net bearing resistance potentially less than 100 kPa. Furthermore, the measured cone resistance must be corrected for the effects of overburden pressure and pore pressure acting on the back face of the cone, both of which introduce significant uncertainty.
The T-bar penetrometer was first introduced at the University of Western Australia in order to improve the accuracy of strength profiling in centrifuge model tests (Stewart and Randolph, 1991,1994). The probe consists of a short cylindrical bar attached at right angles to the penetrometer rods, just below a load cell. It has two major advantages over the cone. Firstly, the load cell measures what is essentially a differential force (or net pressure) on the bar, so that no adjustment need be made for the overburden stress and ambient pore pressure.