In most offshore design projects, in situ penetrometer tests are first correlated with element test data in order to assess a suitable resistance factor, so that effectively the absolute value of penetration resistance is not used directly. However, given the difficulty in obtaining high quality samples from deepwater sites, the direct field measurement of penetration resistance may provide a better guide for estimating the response of pipelines and shallow foundation or anchoring systems than laboratory shear strength data obtained from specific strain paths. The key to making better use of in situ penetrometer data is to improve methods of interpretation to allow evaluation of the separate effects of the high strain rates imposed during the test, as well as the gradual softening of the soil as it is sheared. The paper explores how the penetration resistance might be ‘scaled’ for direct application in design, with appropriate adjustments to allow for the timescale of loading (or rate of straining); whether the design event involves steady-state shearing or initial shearing from a previously installed position; and the geometric scale of the design object relative to the size of the penetrometer. New approaches to interpretation are presented to facilitate this process. The question of accuracy of the in situ test data is also addressed, with particular reference to strength measurement at very shallow depths.
Much of offshore design in soft sediments hinges on a design shear strength profile, which is generally based on results from laboratory or vane shear strength data, supplemented by profiles of penetration resistance. The latter provide a means of interpolating strengths, in terms of relative values, but the resistance factor relating penetration resistance to shear strength measured in an element test is generally derived for the given site by correlation, so that the absolute value of the penetration resistance is not used directly.
Design calculations, however, are usually based on theoretical relationships, such as the standard Prandtl bearing capacity formula. There is, therefore, an inconsistency between the interpretation of data from in situ penetration tests, where the correlated resistance factor may not accord with theory or a design process based on theoretical solutions. The question may be posed as to which provides a more reliable basis for estimating the capacity of an offshore system, such as a pipeline, shallow foundation or anchoring system ? the penetration resistance measured during an in situ test, or the shear strength measured in a laboratory test or vane shear test.
Penetrometer tests may be viewed as a form of model test, but one that may not reflect precisely the timescale of loading (or rate of straining) of the design application. In addition, the soil is disturbed and partially remoulded as the penetrometer passes, so that the average shear strength reflected in the penetration resistance is less than the intact value. However, the simple geometry of full-flow devices, such as T bar and ball penetrometers, renders them amenable to analysis, incorporating constitutive models that allow for strain rate dependency of shear strength and gradual softening as the soil is sheared.