Comparison of in Situ and Ship-Board Vane Measurements on a Deep-Sea Clay

Abstract

Analysis of most marine geotechnical engineering problems requires information on the shear strength of the natural sediment. One approach is to test samples from borings or cores using conventional geotechnical procedures, such as miniature vane, triaxial compression or direct shear tests. However, the disturbance caused by sampling, pressure release, handling, transportation, etc. can be severe, or at least undetermined, and hence it is often difficult to assess the in situ properties from the results. The problems are probably greater in deep water where sampling equipment is usually tethered on a long cable and the dynamics of coring are difficult to control. In addition, the volume expansion on bringing the sample to an atmospheric condition is 2 to 3% in the deep ocean basins and this would presumably cause changes in effective stresses and microstructural alterations. The expansion of gases greatly increases this problem and in many cases can render samples essentially useless for strength determinations. For these reasons, there has been a growing interest in conducting more in situ geotechnical tests. The vane shear method is only one of several which can be used to determine the in situ strength of an ocean sediment. Some others that are being used are the cone penetro-meter, the pressuremeter and even the standard penetration test. There are instances where a particular method or device may not be well suited to the situation. For example, the vane shear method is intended for use with fine-grained (cohesive) material and it cannot be expected to give meaningful results for anything coarser than a very fine sand or silt. Yet the vane method is used almost routinely in testing oceanographic core samples without regard for the texture of the material. Conversely, the cone penetro-meter method is probably not as reliable with clay as it is with sand.