Storage effects on tube specimens of a highly plastic marine clay

This paper presents an experimental investigation aimed to evaluate the influence of the storage time on the compressibility and the undrained shear strength behaviour of Ballina clay, a highly plastic marine structured soft clay that represents most of the natural soft soil deposits encountered along the east and south Australian coastlines. Constant rate of strain oedometer tests and unconfined compressive strength tests were carried out on tube specimens retrieved using an open (Shelby) sampler and a fixed-piston (Osterberg-type) sampler. Tube specimens were tested between 2 and 377 days after sampling. Mechanical tests were complemented with geochemical analysis of the pore fluid as well as mercury intrusion porosimetry tests to assess the influence of the storage time on pore fluid composition and soil fabric. The paper reports a minor influence of the storage time on the mechanical properties of tube specimens of Ballina clay. When tube specimens are retrieved using fixed-piston samplers, which induce low disturbance if the recommended practice for sampling and storage is followed, the storage time (long-term effect) shows almost no influence on compressibility parameters, shear wave velocity, undrained shear strength, and pore size distribution. On the other hand, changes in mechanical properties observed in Shelby samples are primarily controlled by the mechanical disturbance caused by the penetration and extraction of the sampling tube (short-term effect). The fact that tube specimens retrieved using fixed-piston samplers may be used with confidence in the estimation of mechanical properties even after several months of storage is an important finding for practitioners and researchers considering the long testing periods required for the characterization of natural soft soil deposits. Whereas the existing experimental evidence for low-to-medium plasticity soils suggests a clear detrimental effect of the storage time on the mechanical properties of the soil, the results obtained in this study confirms that, for highly plastic soils of low/moderate sensitivity, the disturbance induced by the sampler is the main factor controlling the soil response measured in the laboratory.