Evaluation of soil suction as an indicator of sample quality for a soft saturated marine clay
This thesis presents results and conclusions from a research program that developed a nondestructive indicator of sample disturbance in saturated soft clays using soil suction. A portable, field deployable, suction probe was designed and built for the research using primarily off-the-shelf parts and the procedures developed for its use in the field are relatively simple to perform.
The probe, once placed in contact with an unconfined soil sample, requires approximately 10 to 20 minutes to reach equilibrium with the suction in the sample. The measurements were found to be accurate and repeatable based on laboratory proof testing of the device and its subsequent use in the field during sampling operations.
A test site located in northeast Massachusetts was used to evaluate the suction probe. The site contains a 10 m layer of Boston Blue Clay with a thin, stiff upper crust followed by a medium to low overconsolidation ratio soft layer. A series of samples ranging from high quality to poor quality were collected including Sherbrooke block samples (high quality), 76 mm fixed piston tube samples with modified tube geometry, 76 mm free piston tube samples, and 76 mm split spoon samples (very poor quality).
Soil suction was measured on all samples in the field and subsequently over time in the laboratory. Sample quality was evaluated using the well established but destructive volumetric strain method (i.e., ϵvol or Δe/e 0 at σ′vo) from the consolidation phase of CRS, CAUC, and DSS tests. The suction values were found to generally mirror that of the volumetric strain measurements, with higher suction values correlating with lower volumetric strains, and hence better quality samples. Normalization of the suction values by σ′ vo provides better resolution of the data and is hence used for development of a suggested nondestructive sample quality criterion for clays using soil suction.