Discussion of “Assessment of the Coefficient of Consolidation with Queensland Data”

Alexander Litwinowicz

This paper presents a discussion of “Assessment of the coefficient of consolidation with Queensland data” by Burt G. Look. https://doi.org/10.56295/AGJ5825.

The efforts of the Author are acknowledged in assembling a large dataset and attempting to make sense of the inherent noise. A limitation however is the anonymity of the locations from which the data was derived as it is very likely that over the 40 years of experience with soft soils in Queensland the Discusser (and perhaps others) would have been able to contribute with specificity.

It is the Discusser’s view that the generic relationships in the public domain referenced in the Paper are of limited if any utility and has never seen practitioners making use of them on real projects where the consequences were significant.

Some of the authoritative conclusions drawn and commentary in the Paper are worthy of further consideration: 1. the absolute rejection of U50 samples; 2. the variation in cv due to adoption of t50 or t90; 3. the expected range of cv; and 4. the singular bias and preference for cv guesstimated from piezocone dissipation tests.

Before discussing these, it is instructive to consider whether a regular or consistent pattern should be expected with the data presented given the inevitable variations, repeatability and reproducibility of any or all of the following:

  • Soil structure, void ratio, Liquidity Index and sensitivity.
  • Sample type and size.
  • Sampling procedure.
  • Transportation procedure.
  • Storage procedure and conditions.
  • Test specimen preparation.
  • Test specimen parity – same material or adjacent tested for multi-test correlation purposes.
  • Test procedure.
  • Stress increments selected.
  • Stress increments straddling the yield point by varying amounts.
  • Interpretation and reporting.

In addition, real soft soils display a widely contrasting Jekyll and Hyde response depending on the stress level imposed i.e., whether stressed within the overconsolidated (OC) range or into the normally consolidated (NC) range post ruin of the soil structure on yield (Tavenas and Leroueil (1980)). Then there is the no small matter of Terzaghi’s simple consolidation theory premised on specimen vertical displacement alone assuming constant properties throughout the sample throughout the stress excursion during a consolidation test loading stage even though real soils behave differently. Given the above and when considering cv as a function of LL, it is unsurprising that graphs which the Discusser would colloquially describe as ‘measles plots’ are the result, indeed it is to be expected.

It is also no surprise that there are a significant number of extreme outliers if basic statistics as reported in the paper are applied to what is inherently a multitude of statistical data sets (or samples), rather than identifiable, statistically significant populations. The Discusser concurs with the paper’s general finding that such an approach is futile given the significant vagaries in the data.