32 teask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



clay, and colloid are universally adopted for classifying individual 

 constituents by size. Fairly good agreement regarding boundary be- 

 tween size groups has been achieved, particularly for sand particles, 

 which almost everybody agrees range between 64 microns (0.064 milli- 

 meter) and 2 millimeters in diameter. The lower limit for silt gen- 

 erally is set at 4 or 5 microns. Workers interested in classifying sedi- 

 ments into size groups whose upper and lower limits have geometric 

 dimensions prefer 4 microns as the upper limit for clay because it 

 makes the classification so much easier to handle statistically; but 

 some workers interested in the physical properties of clays prefer 

 to use 5 microns for the upper limit of clay because that size marks a 

 division point between recognizable physical properties, based prin- 

 cipally on the properties of clay minerals, which commonly do not 

 reach much beyond 5 microns in size. 



Most people set the boundary between clay and colloid between 0.5 

 and 1 micron; but, as particles of this dimension are difficult to meas- 

 ure accurately and as the actual content of colloids is not yet a matter 

 of major importance for most practical applications of sediments, 

 people have not argued much over the lower limit of clay size. 



Sediments are composed of particles of many different sizes. Geolo- 

 gists are accustomed to classify the deposit upon the basis of the di- 

 ameter of the average (median) particle, the D 50 of the engineer. The 

 median is the diameter for which one half of the weight of the sedi- 

 ment is composed of particles of larger size and one half of particles 

 of smaller size. It represents the mid-point on the grading curve. If 

 the sample is well sorted, that is, if most of the particles are close to 

 the median, little difficulty in classification exists, but, if the sample 

 is poorly sorted and contains a large amount of material larger or 

 smaller than the median, the tendency is to use a modifying adjective. 

 Thus a sample having a median diameter of 15 microns and containing 

 25 percent clay would be called a clayey silt. Difference of opinion ex- 

 ists about how much of the lower or larger grade size need be included 

 to warrant the use of a modifying adjective. In practice the amount 

 ranges between 10 and 30 percent. As a rule workers use the name 

 that most conveniently fits the purpose for which the work is done. 

 Casagrande (1947) has adopted an arbitrary series of limits of par- 

 ticle size and has given corresponding names to sediments of given 

 size distributions. Soil mechanics engineers find this classification 

 useful. 



The shape of the constituents is used in classifying sediments com- 

 posed chiefly of particles larger than sand in size (Wadell, 1935). If 

 rounded, the deposits are called gravel ; if angular, they are called brec- 



