- Silty gravel (GM) or sand (SM)_: more than 12 percent by weight 

 are finer than No. 200 sieve, and the fines have little or no 

 plasticity. 



- Clayey gravel (GC) or sand (SC) : more than 12 percent by weight 

 are finer than the No. 200 sieve, and the fines are plastic. 



When the fraction smaller than the No. 200 sieve size is greater than 

 5 percent and less than 12 percent, a dual symbol should be used. 



Well and poorly graded gravels and sands are further defined as clean 

 gravels or sands; silty or clayey gravels and sands may be referred to as 

 dirty gravels or sands. It should also be noted that the particle shape has 

 an influence on the density and the stability of the coarse-grained soils. 



(1) Gravel (G) . The USCS defines gravel as the material whose 

 size ranges between 76.2 millimeters (3 inches) and the No. 4 Sieve. 

 Materials larger than 76.2 millimeters are designated as cobbles. Gravels 

 may be man-made (crushed stone) or may come from natural deposits (bank- 

 run) . Gravels are cohesionless materials. 



(2) Sand (S) . A material is defined as sand when its grain size 

 is between 4.76 and 0.075 millimeter (No. 4 and 200 sieves, respectively). 

 The USCS developed further classification: the sand is coarse when its 

 grain size varies between 4.76 and 2.00 millimeters (No. 4 and 10 sieves, 

 respectively); medium when between 2.00 and 0.42 millimeter (No. 10 and 

 40 sieves, respectively); and fine when between 0.42 and 0.075 millimeter 

 (No. 40 and 200 sieves, respectively). Sands are cohesionless materials, 

 however, they present an apparent cohesion when damp or moist due to 

 surface tension effects of pore fluids. These effects disappear when the 

 sand is saturated. 



b. Fine-Grained Materials . Silts and clays are known as fine-grained 

 soils. Fine-grained materials are such that 50 percent or more of the 

 materials by weight pass the No. 200 sieve. They are distinguished either 

 visually and manually or by means of the Atterberg limits. The USCS, 

 contrary to most other classification systems, does not make any size 

 distinction between silt and clay. This is because the engineering proper- 

 ties of fine-grained soil are more closely related to plasticity character- 

 istics than to grain size. 



The USCS distinguishes the following: 



- Silt, clay, and organic silt and clay having liquid 

 limits less than 50: ML, CL, and OL, respectively. 



- Silt, clay, and organic silt and clay having liquid 

 limits greater than 50: MH, CH, and OH, respectively. 



Fine-grained soils usually have a low permeability (10~ 7 to 10 -9 

 centimeter per second) with silty soils being somewhat more permeable than 

 clayey ones. Organic materials tend to lower the strength characteristics 

 of the soil, lower the maximum density, increase the time for consolidation 

 and increase the optimum water content. 



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