My lab is evaluating the liquid, plastic, and shrinkage limits of some soil samples and we are following this video provided by the Missouri University of Science and Technology. The video indicates we should conduct at least 3 tests where it takes 15-20, 20-25, and 25-30 hits of the Casagrande Apparatus to close the gap that we make via the grooving tool. And this video also explains that the liquid limit is defined as the moisture content in which it takes 25 hits of the Casagrande Apparatus to close the divide made by the grooving tool. We have been doing our best to turn our soil sample into a "uniform paste" through adding water as the video indicates, and even when that is the case, on a consistent basis, it takes fewer than 10 hits for the gap to be closed. Bottom line: My question is could there be soil samples where their liquid limits are lower than what the Atterberg Limit and Casagrande Apparatus method could detect? I do recognize the possibility of user error on our part, that we can continue to refine our technique on making the soil sample a "uniform paste." Thanks in advance!
You have to run the test at various moisture contents. Plot the number of hits vs. moisture content and then extrapolate to 25 hits to find the moisture content at the liquid limit.
The ASTM D4318 standard should be followed. However, there is always some operator-dependence and an alternative is the cone penetration test described in BS 1377-1/2.
According to BS EN ISO 17892-12:2018 there are three methods to determine liquid limit and plasticity index of soils. Previously, BS 1377 PART 2:1990 used the definitive method of a cone penetrometer. This method superceded the Cassagrande method after 1975 (Smith's Elements of Soil Mechanics, 9th Ed, 2014, pp11-13)
The problem with the Cassagrande method is that it is highly dependant on the person doing the test and therfore far less reliable than the cone penetrometer test.
Considering the variable results of the Cassagrande method, I would hazard a statement of "yes" because there are soils that cannot release their water content completely due to their molecular alignment and ionic bonds which are also affect by Van der Waal's (pronounced fun-dur-Varl's) forces too.