Unified Soil Classification is the principal classification used by engi- 

 neers. The Wentworth classification is the basis of a classification 

 widely used by geologists, but is becoming more widely used by engineers 

 designing beach fills. 



For most shore protection design problems, typical littoral mate- 

 rials are sands with sizes between 0.1 and 1.0 millimeters or, in phi 

 units, between 3.3 and phi. According to the Wentworth classification, 

 sand size is in the range between 0.0625 cind 2.0 millimeters (4 and -1 

 phi); according to the Unified Soil Classification, it is between 0.074 

 and 4.76 millimeters (3.75 and -2.25 phi). Within these sand-size ranges, 

 engineers commonly distinguish size classes by median grain size measured 

 in millimeters, based on sieve analyses. 



Samples of typical beach sediment usually have a few relatively 

 large particles covering a wide range of diameters, and many small par- 

 ticles within a small range of diameters. Thus, to distinguish one 

 sample from another, it is necessary to consider the small differences 

 (in absolute magnitude) among the finer sizes more than the same differ- 

 ences among the larger sizes. For this reason, all sediment size classi- 

 fications exaggerate absolute differences in the finer sizes compared to 

 absolute differences in the coarser sizes. 



As shown in Figure 4-7, limits of the size classes differ. The 

 Unified Soil Classification boundaries correspond to U.S. Standard Sieve 

 sizes. The Wentworth classification varies as powers of 2 millimeters; 

 that is, the size classes have limits, in millimeters, determined by the 

 relation 2 , where n is any positive or negative whole number, including 

 zero. For example, the limits on sand size in the Wentworth scale are 

 0.0625 millimeters and 2 millimeters, which correspond to 2 "* and 2 ^ 

 millimeters. 



This property of having class limits defined in terms of whole number 

 powers of 2 millimeters led Krumbein (1936) to propose a phi unit scale 

 based on the definition: 



Phi units (0) = — log (diameter in mm.) (4-1) 



Phi unit scale is indicated by writing ^ or phi after the numerical 

 value. The phi unit scale is shown in Figure 4-7. Advantages of phi 

 units are: 



(a) Limits of Wentworth size classes are whole numbers in phi 

 units. These phi limits are the negative value of the exponent, n, in 

 the relation 2 . For example, the sand size class ranges from +4 to -1, 

 in phi units. 



(b) Sand size distributions typically are near lognormal, so that 

 a unit based on the logarithm of the size better emphasizes the small 

 significant differences between the finer particles in the distribution. 



4-13 



