Georg H. Erharter , Santiago Quinteros, Diana Cordeiro, Matthias Rebhan, Franz Tschuchnigg

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 minimum sample mass (m_min) for the PSD-
analyses of soils with a main component of gravel or greater is based on equations
including the soil's maximum grain diameter (D_max). We claim that the recommended
m_min is overestimated as D_max does not represent the relevant large soil fraction
but only the PSD's uppermost outlier. Furthermore, the recommended m_min is not
based on a specific sampling confidence (i.e. how closely does the sample’s PSD need
to approximate the soil’s PSD?) and thus it is not clear why the m_min should even be
necessary. We conducted Monte-Carlo simulation-based sieve analyses of coarse-
grained soils and developed a new, practically applicable framework to determine
m_min based on D_90 that also includes explicit consideration of sampling confidence.
A survey was conducted that shows that there is no significant difference in how well
operators are able to assess parameters like D_90 or D_max. 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.