Studio della ripetibilità dell'analisi granulometrica delle terre

This paper presents a study on the repeatability of grain size analysis of soils, determined by means of sieve and hydrometer tests. In order to evaluate the average error and the variability of the main statistical parameters, six specimens of every selected soil have been analysed. The hydrometer analysis has been executed both on bulk specimens and on the fraction finer than 0.075 mm. Results have shown that, as for the sieve analysis, the worst controlled stage is the sampling, consisting in many mixings and cone-and-quarter subdivisions alternated with each other. This manual procedure can lead to a selection of poorly representative samples, prejudicing the soil classification and, therefore, its proper use for engineering purposes. As for the hydrometer analysis, the main problem seems to be possibility of testing the bulk sample or the fraction finer than 0.0075 mm. The operator can choose between these two possibilities on the basis of his personal analysis. This choice introduces a bias error the magnitude of which can vary by changing operator. Therefore it seems that the laboratory standards would better establish that the hydrometer test has to be executed only on the fraction finer than 0.0075 mm.