Cone penetration tests were performed with a piezocone and a friction cone in highly over consolidated silts offshore southern California. Measurements of pore pressure response allowed interpretation of the results to yield effective stress and undrained strength parameters agreeing reasonably well with those measured on high quality samples. Strength gain caused by dilatant tendencies during undrained shear has to be taken into account for the interpretation.
During the past decade, increasing use has been made of the cone penetration test (CPT) in offshore geotechnical site investigations. These tests--which are normally conducted offshore either by means of a seabed-resting jacking frame, or with a downhole-latching device in borings advanced by rotary drilling techniques--provide the distinct advantage of being performed in in-situ soils, in comparison to laboratory tests which are performed on samples disturbed to one or other extent by the sampling, stress relief, and retrieval process. The advantages and disadvantages associated with in-situ CPT testing relative to sampling and laboratory testing are described by Mitchell et al. (1978) and Wroth (1984) amongst others, and are not repeated here.
Over the same decade, there has also developed an increased understanding of the mechanisms of soil failure during performance of a CPT and, consequently, an improvement in the theoretical and semi-empirical methods for interpretation of the test results. Currently there are a number of methods for interpreting soil strength from measured CPT tip resistance, either in terms of friction angle,Ø, for tests performed in sands, or undrained shear strength, Su, for tests performed in clays. There does not yet exist a method specifically developed for interpreting CPT results in silts.
Accordingly, the purpose of this paper is to present data obtained with a pheromone in highly over consolidated offshore silts and to discuss methods successfully used for interpretation of the results.
Interpretation of tests in clays (Lunne and Kleven, 1981; 3amiolkowski et al., 1982; Wroth, 1984) are normally based on the equation:
(Equation available in full paper)
Values of Nk have been computed theoretically based on bearing capacity or cavity expansion theory, with computed values ranging from about 9 to 20 and have also been derived empirically, with resulting values ranging from as low as 5 to as high as 70. Schmertmann (1975), who summarized the variables that can be expected to influence Nk' recommended that site-specific correlations of this factor be developed for each investigation.
Methods for interpretation of CPT results in sands are also based either on bearing capacity equations (Durgunoglu and Mitchell, 1975; Schmertmann, 1975; Senneset et al., 1982) or cavity expansion theories (Vesic, 1972; Al-Akwati, 1975; Baligh 1975). It has been shown (Villet and Mitchell, 1981; Baldi et al., 1981) that the accuracy of the interpretive methods are significantly improved if the relationship between friction angle and confining stress is considered, either by direct measurement, or by using Baligh's (1976) equations for Mohr envelope curvature.
