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Input of the USDA Texture and USCS Texture fields is optional. It is recommended that the texture be automatically determined using the classification algorithms present in SVSOILS. The algorithms are activated when the user clicks on the Apply fit button in the Grain-size dialog. It is suggested that the user first input the grain-size distribution and proceed to initiate either the unimodal or bimodal fit of the grain-size distribution. The soil will then be automatically classified using the “best-fit” of the USDA and the USCS method fits. The USCS method also requires the input of Atterberg Limits. Abbreviations for each soil textural classification are presented to the right of the soil texture.

Proper classification of a soil requires the percentages of clay, silt, and sand that make up a soil. The percentage of coarse, sand, silt and clay fractions of a soil requires that particle size limits be defined. For example, clay-size particles are typically assumed to be particles with a diameter less than 0.002 mm. The percentage of coarse, sand, silt and clay depend on the particle size divisions that are assumed for each soil category. SVSOILS incorporates two definitions of particle size boundaries to account for the two classification systems.


The two definitions of particle size limits are proposed by the USCS (Unified Soil Classification System) and the USDA (United States Department of Agriculture) methods of soil classification. The USCS method of classification is also the standard method of classification adopted by ASTM D 2487. The definitions of particle-size limits used by each method are shown below.



USCS (ASTM D 2487)        



x < 0.002 mm

x < 0.002 mm


0.002 < x < 0.074 mm        

0.002 < x < 0.05 mm


0.074 < x < 4.75 mm

0.05 < x < 2 mm


4.75 < x < 300 mm

2 < x < 300 mm


A USDA and USCS (ASTM) textural classification is automatically determined each time the grain-size distribution is fit with an equation. The %clay, %silt, %sand, %coarse, D10, D20, D30, D50, and D60 variables are also automatically calculated from the fit of the grain-size distribution curve. The equation (unimodal or bimodal) used to calculate the aforementioned variables depends on the accuracy of the fit which is measured in terms of (R2). SVSOILS automatically bases its classification and calculations on the equation that best represents the grain-size distribution.