Interpretation of Cone Penetrometer Data for Offshore Sands

ABSTRACT The use of the cone penetration test (CPT) for offshore soil investigations is particularly important in sands where it proves problematic and often impossible to take undisturbed samples. CPT data can provide reliable indications of in situ soil properties and/or in situ stress conditions. The geotechnical literature, however, abounds with different procedures for interpreting CPT data in sands. A comprehensive data base for sands was established mainly from large scale calibration chamber tests performed at several institutions. The data base also included results from field tests described in the literature and from NGI's experience. The paper presents an evaluation and updated recommendation of Schmertmann's method for estimating relative densities from recorded cone resistance. It also evaluates various procedures for interpreting the drained friction angle from cone resistance and presents correlations between the constrained deformation modulus and cone resistance. Finally, the paper recommends other in situ tests to complement and enhance the interpretation of CPT data in sand, and points out the necessity for simultaneous sampling and laboratory testing. INTRODUCTION The use of the cone penetration test (CPT) is particularly important in sands where it proves problematic and often impossible to take undisturbed samples. However, the complex mechanism of continuous penetration of a cone into a soil is very difficult to model theoretically. Therefore theoretical relationships modified with empirical data have often underlain the interpretation of CPT results in terms of engineering parameters. Purely empirical relationships are also widely used. In an attempt to clarify the cone interpretation methods in sands, Exxon Production Research Company initiated a study conducted at the Norwegian Geotechnical Institute (NGI) to evaluate and possibly update the existing procedures. Well documented references and NGI in-house research data from large calibration chamber laboratory tests have been synthesized to form consistent data sets of soil parameters and CPT results for several sand types. The data sets have been used to evaluate existing theories and correlations among CPT results and engineering parameters, mainly relative density, shear strength and compression modulus. Where appropriate, new or updated correlations have been developed. Although field tests represent more closely in situ behaviour, the present study considers predominantly cone penetrometer results from large calibration chambers, where test conditions and soil characteristics are better controlled than in situ and measured cone penetrations can be correlated to well controlled sand densities and geotechnical parameters from more conventional laboratory tests. Due to the emphasis placed on calibration chamber tests. These are described in more detail in the following chapter. LARGE CALIBRATION CHAMBER TESTS From 1975 to 1977, NGI carried out an extensive series of laboratory calibration tests on large sand samples using the electric friction cone penetrometer Similar investigations have been undertaken in Australia 2'3, in Florida 4t5 and in Italy.6,7 Calibration chamber tests enable the investigation of the cone penetrometer under laboratory conditions where one can carefully control and measure soil properties, stresses and strains.