Development of a non-destructive sample quality assessment method for soft clays
This research investigates the development of a non-destructive sample quality assessment method for soft clays using shear wave velocity (Vvh) and suction (ur) (i.e., negative pore water pressure) for unconfined samples. Sampling programs were conducted at four soft clay sites in the US, Canada, Australia, and Norway, where a range of sample types (e.g., block, tube, and split spoon) were collected and seismic cone penetration (SCPTU) profiling performed to measure in situ Vvh (VSCPTU ). Vvh and ur were measured on samples in the field and laboratory using portable bender element and suction probe devices developed at the University of Massachusetts Amherst. Quality assessment criteria were developed using comparisons of sample Vvh and ur with in situ Vvh and vertical effective stress (σ'v0 ): Vvh/VSCPTU and ur/σ'v0.
Laboratory tests were performed on each sample to measure undrained shear strength (su), key aspects of consolidation behavior (e.g., preconsolidation stress, σ'p), and reconsolidation volume change (Δe/e 0 at σ'v0) for conventional sample quality assessment. Results show samples of progressively poorer quality yield greater reconsolidation volume change, lower estimates of both su and σ'p, and lower Vvh/VSCPTU and ur/σ' v0 ratios. Vvh values were found to correlate strongly with conventional Δe/e0 quality assessment and changes in engineering parameters, while ur correlates poorly. Vvh was measured during laboratory simulation of "perfect" sampling for high quality block samples to determine Vvh/VSCPTU for "perfect" samples. Additionally, the relationship of shear modulus and Vvh to soil state (i.e., stresses, void ratio, etc.) was fundamentally investigated for each soil to determine empirical shear modulus relationships and analyze reductions in Vvh due to soil destructuring during sampling.
Tentative sample quality assessment guidelines for advanced laboratory testing developed from the results indicate that samples are of acceptable quality when Vvh/VSCPTU > 0.6, should be used with caution when 0.35 > Vvh/VSCPTU > 0.6, and are unacceptable when Vvh/VSCPTU < 0.35. This method provides a much needed, non-destructive tool for field/laboratory assessment of sample quality, offering the ability to screen samples in real time for sampling program improvement and selection of samples for advanced testing that will yield more reliable estimates of engineering parameters.