A crucial component for the successful foundation design of all offshore structures is accurate interpretation of soil type. This paper presents a quantitative soil-type interpretation model to assist the practising design engineer to make more reliable assessments of soil-type from CPT results.
In 1935 the Department of Public in the Netherlands developed the first Cone Penetration Test (CPT) device in a form that would be recognisable today (Barentson, 1936). The cone was manually pushed into the ground to measure penetration resistance over discrete depth intervals and, therefore, could be used to assist foundation design. In the 1950s a system for measuring localised fiction behind the cone was incorporated into the tool by Begemann (1965). The objective of this "friction cone" was to allow comparison of the cone resistance and local skin friction m order to infer details of soil type within the soil profile.
Although, the first electric cone was produced in 1948 for research purposes, it was 1970 before the first commercially available electric friction cones were introduced. The electric friction cone offered a number of advantages over its mechanical precursor, including simultaneous measurement of end resistance and sleeve fiction, continuous measurement of soil data and reduced operator dependence, leading to greater accuracy and repeatability of test results. A number of authors took advantage of the greater repeatability and resolution of electric fiction cones and published improved relationships between cone end resistance, local skin fiction and soil type (Schmertmann, 1978; Douglas and Olsen 1981).
In the 1970s development of a cone that could also measure pore-water pressures induced during cone penetration was undertaken in Sweden by Torstensson (1975) and m America by Wissa et al. (1975) However, these cones were only capable of measuring cone resistance and pore-water pressure, and it was not until 1984 that three channel "piezocones", that also measured local skin fiction, became commercially available Figure 1 presents a schematic diagram of typical piezocone layouts, showing the two main locations of the pore-water pressure sensor. Piezocones have been used commercially in the North Sea since 1985 using methods of deployment similar to those shown in Figure 2.
Pore-water pressure measurements continued to improve the profiling capability of the tool, and a further series of soil-type interpretation models were published making use of the additional pore-water pressure information (Robertson, 1990, Senneset and Janbu, 1985). However, over the last ten years or so, there have been improvements in the quality and reliability of the CPT electronics, and there is a larger database of CPT results This larger and more reliable database has enabled an improved soil type interpretation model to be developed This model is considered by the author to be more reliable for soil-type interpretation m Quaternary soils, which are the soils most often of interest for foundation designers in the North Sea.