Measuring Sea Bed Properties using Static and Dynamic Penetrometers: Fid-Bau Portal

Measuring Sea Bed Properties using Static and Dynamic Penetrometers

Stoll, R. D.

During recent experiments, which were sponsored by the Office of Naval Research, a number of different kinds of test were carried out to measure the insitu strength of the sea bed. These included quasi-static cone penetration tests, tests utilizing a freely-falling probe with a 1 to 2 m long shaft and tests with a small, expendable penetrometer. During the course of these tests significant differences were noted between the penetration resistance recorded by the quasi-static probe as compared with the output of the dynamic probes even though repetitive tests were done at the same site and conventional strain-rate factors were applied. Subsequent laboratory tests, as well as comparisons using the results of the field tests, suggest that the rapid penetration rate of the dynamic probes plays a key role in determining their response in different types of sediment. For example, in granular sediment such as sand, deformation involving shear strain can result in volumetric strain owing to dilation with the result that there is a significant change in pore-water pressure, inter-granular normal stress and, therefore, shearing strength. The exact nature of this dilative response for a given set of experimental conditions is both strain-rate and permeability dependent. Results of various penetrometer tests are presented and criteria are proposed for estimating shear strength when loading is carried out dynamically. This information is important when predicting the impact burial of mines and other objects and in a variety of other activities where the geotechnical properties of the sediment must be determined.