How much is enough? Uncertainty aware sample mass determination of coarse-grained soils for particle size analyses

Determining particle size distributions (PSD) of soils is a basic first step in many geotechnical analyses and guidance is given in different national standards. For ambiguous reasons, the recommended required minimum sample mass (m_min) for the PSD-analyses of soils with a main component of gravel or greater is always based on equations including the soil's maximum grain diameter (D_max). We claim that the recommended m_min is overestimated in many cases as D_max does not represent the relevant large soil fraction but only the PSD's uppermost outlier. Furthermore, sampling confidence is not considered in the recommended m_min and thus it is not clear why certain sample masses should even be necessary. We conducted Monte-Carlo simulation-based sieve analyses of coarse-grained soils and developed a new, practically applicable equation to determine m_min based on d90 that also includes explicit consideration of sampling confidence. Real sieve tests performed on three different sands and gravels corroborate the theoretical results and show that substantially lower sample masses yield PSDs with only marginal differences to PSDs from samples according to the standards. While the results are promising, they open up for new research questions about which geotechnical application requires which soil sampling confidence.