procedure assists in normalizing the grain size distribution and allows compu- 

 tation of other size statistics based on the normal distribution. 



Grain-size analysis involves a series of procedures to determine what 

 proportion of material in a given sample is in each grain size class. An 

 important aspect of the laboratory analysis program, which must be designed 

 into the field sampling scheme, is to obtain sufficient sediment to adequately 

 determine the sediment population characteristics (Table 10). The requirement 

 for obtaining adequate amounts of each sample underscores the importance of 

 some prior knowledge of the field conditions or of conducting a preliminary 

 field reconnaissance before undertaking a rigorous field sampling program. 

 Large samples should be divided using a sample splitter to prevent clogging of 

 sieves. Particle aggregates, especially those in the silt-clay range which show 

 cohesive properties, should be separated and dispersed by gentle grinding and 

 use of a chemical dispersant (sodium hexametaphospate) before analysis. 



Table 10 



Minimum Weight of Sample Required Given Maximum Particle Size 



in Population 



Maximum Particle Size 



Minimum Weight of Sample, g 



3.0 in. 



64,000 



2.0 in. 



19,000 



1.5 in. 



8,000 



1.0 in. 



2,400 



0.75 in. 



1,000 



0.50 in. 



300 



0.38 in. 



150 



No. 4 sieve 



50 



Laboratory techniques used to estimate sediment diameter depend in part 

 on the grain size. Pebbles and coarser sediments can be directly measured 

 with calipers or by coarse sieves. The grain-size distribution of sand is deter- 

 mined directly by sieve analysis, sedimentation tubes, or Coulter counter. For 

 silt and clay-sized material, grain-size distribution is determined indirectly by 

 hydrometer or pipette analysis, or the use of a Coulter counter. The size 

 distribution of mixed sediments is determined by using a combination of sieve 

 and hydrometer or pipette analyses. Practical procedures for conducting 

 laboratory tests on sediment samples are covered by Folk (1980) and Lewis 

 (1984). Laboratory manuals more oriented towards engineering applications 

 include American Society for Testing and Materials (1964), Bowles (1986), 

 and HQUSACE (1970). Although they describe some tests specific to 

 geotechnical engineering practice, many procedures, such as grain size 

 analyses, are universal. 



Coastal sediments reflect the relative importance of various source areas, 

 and thus differences in the relative importance of the process mechanisms in 



68 



Chapter 4 Laboratory Techniques and Approaches 



