Sediment Description and Classification Background

 

U.S. Standard Sieves

Note that the same size mesh can be a differing sieve number depending on the Sieve manufacturer (Tyler vs. ASTM)

Mesh Size   (microns)   TYLER     ASTM-E11     BS-410     DIN-4188   µm Mesh No. Mesh mm   5 2500   2500 0.005 10 1250   1250 0.010 15 800   800 0.015 20 625   625 0.020 22       0.022 25 500   500 0.025 28       0.028 32       0.032 36       0.036 38 400 400 400   40       0.040 45 325 325 350 0.045 50       0.050 53 270 270 300   56       0.056 63 250 230 240 0.063 71       0.071 75 200 200 200   80       0.080 90 170 170 170 0.090 100       0.100 106 150 140 150   112       0.112 125 115 120 120 0.125 140       0.140 150 100 100 100   160       0.160 180 80 80 85 0.180 200       0.200 212 65 70 72   250 60 60 60 0.250 280       0.280 300 48 50 52   315       0.315 355 42 45 44 0.355 400       0.400 425 35 40 36   450       0.450 500 32 35 30 0.500 560       0.560 600 28 30 25   630       0.630 710 24 25 22 0.710 800       0.800 850 20 20 18   900       0.900 1000 16 18 16 1.0 1120       1.12 1180 14 16 14   1250       1.25 1400 12 14 12 1.4 1600       1.6 1700 10 12 10   1800       1.8 2000 9 10 8 2.0 2240       2.24 2360 8 8 7   2500       2.5 2800 7 7 6 2.8 3150       3.15 3350 6 6 5   3550       3.55 4000 5 5 4 4.0 4500       4.5 4750 4 4 3.5   5000       5.0

 

Sediment Classification based on Grain Size:

Unified Soil Classification System (USCS)

Visual logging of sediments entails estimating percentages of gravels, sands and fines (silt and clays). Practice and the use of the Geotechnical Gage will increase your confidence and ability in visually logging sediments.

Read: Visual Exam Test

Read: Field Identification Guidelines

Ultimately, sediment samples may undergo grain size analysis through sieves. Graphing the cumulative weight percent retained/passing by sieve no. or grain size will result in the sediment grain-size distribution curve. The grain-size distribution curve is used to quantitatively classify the sediment type (your visual identification is a qualitative classification).

Read: Grain Size Distribution Measurement

Grain Size Distribution Curve

The grain-size distribution curve is used with the USCS classification chart to classify the sediment type. Other measures used to describe the sediment are the sorting or gradation of the sediment. As can be seen in the above chart, a well-sorted sediment has a small range of sediment grain sizes while a poorly sorted sediment has a large range of sediment grain sizes. In the USCS classification scheme, the gradation of the sediment is used instead of the sorting. A well-graded sediment has a large range of grain sizes while a poorly or uniformly graded sediment has a small range of grain sizes.

POORLY SORTED SEDIMENT = WELL GRADED SEDIMENT

WELL-SORTED SEDIMENT = POORLY OR UNIFORMLY GRADED SEDIMENT

After sieve analysis, the data are tabulated showing the weight of sediment retained on each sieve. The cumulative weight retained is calculated starting from the largest sieve size and adding subsequent sediment weights from the smaller size sieves (see table below). The percent retained is calculated from the weight retained and the total weight of the sample. [Don't get confused by the graph - it is individual percent retained in Column 16 and cumulative percent passing in Column 17]. The cumulative percent passing in Column 17 of the table below is calculated by sequentially subtracting percent retained from 100 %. In table below, cumulative percent passing 1/4 inch sieve = 100 - 16 = 84; cumulative percent passing #4 sieve = 84-5.2 = 78.8; etc.

The cumulative percent passing is plotted on the grain-size distribution graph. The percentage passing the No. 4 and 200 sieves is used to classify the sediments as gravels (G), sands (S) or fines (must use plasiticity index to differentiate between silts and clays).

The grain-size distribution graph is used to read off the grain size at which 10% of the sample passed (D10), 30% of the sample passed (D30) and 60% of the sample passed (D60). These numbers are used to calculate several coefficients:

Hazen's effective size, D10, which will be used to estimate permeability

Uniformity Coefficient, Cu = D60/D10

In the above graph,

The uniformity coefficient is used to judge gradation.

Coefficient of Curvature, Cc

In the above graph,

In the graph below, well-graded soils (GW and SW) are long curves spanning a wide range of sizes with a constant or gently varying slope. Uniformly graded soils (SP) are steeply sloping curves spanning a narrow range of sizes. For a gap-graded soil (GP), the curve flattens out in the area of the grain-size deficiency or gap.

The USCS criteria for well-graded gravels (GW) and sands (SW) are: