ALTERNATIVES TO TRADITIONAL PARTICLE SIZE ANALYTICAL METHODS: A CRITICAL COMPARISON

Methods measuring the particle size distribution of unconsolidated sediments were first described in the late 19th Century. By early 20th Century heated debates raged championing certain procedures and denigrating others. In spite of the fact that sophisticated electronic methods have been developed over the past 25 years, several older procedures have stood the “test of time” and have endured Sieve analysis is one such method and in spite of strong criticism directed by Mitscherlich in 1905 (who pointed out that sieves sort grains not only according to size, but also according to shape), sieve analysis continues to be recognized as a viable method(ASTM D-422. Historically sieves have been used for gravel- and sand-sized particles but to obtain a “complete” distribution, the analysis must be combined with a method providing information on the silt-clay fraction. This has always been identified as a flaw in the analytical procedure because, if either a hydrometer or pipette analysis is used for the silt-clay fraction, then error is introduced because the latter methods are dependent on particle density and are predicated on an assumption of “spherical” particle morphology. The use of a sonic sifter can largely avoid this problem because precision sieves are now available that allow particle sizes to be measured to as small as 5 microns. Sieves in the sub-sand size are expensive, however, and differences were noted between results obtained using sieves to measure silt-sized particle data versus those obtained by traditional hydrometer or pipette methods. Arguments, similarly, persist as to the “accuracy” of the Boyocous (hydrometer) method versus the pipette method. Both are acceptable ASTM procedures and both, in fact, yield similar results. Many advocates of the pipette method may well be surprised to learn that the standard procedure for carrying out pipette analyses used by many geology and engineering laboratories actually produces results that are different from those obtained using the venerable precision pipette method developed by Andreason in 1929. The bottom line is simply that there is no “best” method. Even modern laser and X-ray methodologies have weaknesses and the best advice is to simply select a method, understand its weaknesses and, above all, be consistent and realize that different results may occur if different methods are used.