weights have been suggested, the weights recommended are for the standard 

 8-inch screens. Obviously the weight used on an 8-inch screen should not be 

 used on a 3-inch screen. 



In general, the type of cementing material governs selection of the dis- 

 aggregation process; therefore, it is recommended that each sample be tested 

 first to determine the most logical method of disaggregation. For example, 

 dilute hydrochloric acid treatment aids in disaggregation of calcareous sands. 

 Soaking in water will often break down argillaceous sands. Grinding with a 

 rubber-tipped pestle can be attempted, but should be minimized to avoid ex- 

 cessive grain breakage. 



If the sample contains an appreciable amount of clay and fine silt particles, 

 it is best to remove this material because it has a tendency to form small 

 pellets of clay and silt which will be retained on the coarser meshed screens. 

 The fine material may be removed by decanting or wet sieving on a 250-mesh 

 screen. The fine particles should be retained and added to the material passing 

 the 250-mesh during the regular sieving process. 



Separation Procedure 



After the sample has been dried and weighed, it is placed in the nested 

 screens and shaken by hand or in a mechanical shaker such as the Tyler auto- 

 matic Ro-Tap. Ten to fifteen minutes in a mechanical shaker usually gives 

 consistent fractionation. The material retained on each screen then is weighed 

 and recorded. Often a slight weight loss is noted after shaking. This loss is 

 attributed to grains adhering to the screens and fine particles lost as dust. If 

 the sample is principally sand, the weight error may be proportioned among the 

 various screens; however, if the sample has a high clay-and-silt content, the 

 largest loss will be usually in this size range. 



The screens should be cleaned thoroughly after running each sample by 

 tapping the rim of the screen. If many grains remain in the mesh, gentle rubbing 

 with a moderately stiff brush will dislodge most of them. 



Table 6-1 shows that the Tyler 250-mesh or the U. S. 230-mesh screens mark 

 the dividing line between very fine sand and silt. It is common practice to use 

 these screens as the smallest in the series and to catch the silt and clay particles 

 in the lower pan. If there is considerable silt or clay, these particles then may be 

 separated into size groups by one of the various methods outlined by Krumbein 

 and Pettij ohn (1938) and Twenhofel and Tyler (1941). 



Presentation of Results 



The results of size analyses are tabulated as shown in Table 6-II. From these 

 data, histograms and cumulative-frequency curves (figs. 6-1, 6-2, and 6-3) 

 are prepared. 



98 



