Determination of Grain Size Distribution

For assistance contact Tom Neuenfeldt, Soil Data Quality Specialist.

The following order should always be user in determining sieve entries:

• Step #1
  • Item 1a — Determine Fraction 3 - 10 Inches (PC)
  • Item 1b — Determine Fraction Greater Than 10 Inches (PCT)
  • Item 2 — Determine Fraction Passing #10 Sieve (PCT)
  • Item 3 — Determine Fraction Passing #4 Sieve (PCT)
• Step #2
  • Item 4 — Determine Fraction Passing #200 Sieve (PCT)
• Step #3
• Item 5 — Determine Fraction Passing #40 Sieve (PCT)

Step #1

Item 1a

Determine Fraction Greater Than 3 Inches

FRACT. 3-10 IN (CPT)--The fraction between 3-10 inches is the weight percent of the whole soil that is between 3 and 10 inches in diameter.

The fraction of soil greater than 3 inches is not included in the engineering classification system except that it is usually added as descriptive terms to the group name (e.g., poorly graded gravel with silt, sand, cobbles and boulders).

• For more precise estimates, where data are lacking, values may be estimated to the nearest 5 percent.
• If there are no rock or woody fragments for the 3-10 inch entry, enter zero "0", dashes are not allowed.
• For Histosols enter volume of woody fragments.
• If volume percent of rock fragments is not given in the soil description, use high-low range for series within range of characteristics. Use Guide Sheet 10 "Terms to describe Rock Fragments within Horizons".

DETERMINATION:

1. SOURCE OF INFORMATION: The fraction from 3 to 10 inches, and the 10 inches or greater are usually obtained from volume estimates. Estimates are usually made by visual means on the basis of percent by volume.
2. From the series description:
   • Determine both the percent volume of coarse fragments >3 inch and the total percent volume of coarse fragments for each layer.
   • Determine total percent of 3-10 and greater than 10 inches. Use Rock Fragments Volume to Weight Conversion Curves, Guide Sheet 8, to determine the 3-10 inch fraction and percent passing #10 sieve.

   The "Chart for Estimating Rock Fragments" is a good tool. Also use Guide Sheet 9 "conversion of volume to weight for cobbles and stones and adjusting percent gravel by volume to material passing #10 sieve".

Rock Fragment Volume to Weight Conversion Curves

The rock fragment conversion curves can be used for converting all rock fragments (stones, cobbles, and gravel) from volume to weight and obtain appropriate entries for the >3" fraction and percent of fine earth passing the #10 sieve. These curves are based on the assumption the specific gravity of rock fragments is 2.7 and bulk density of the fine earth fraction is 1.5. They also take into account the two separate bases of 100% that must be used to make these computations: the first 100% base is the whole soil, and the second 100% base is gravel plus fine earth fraction.

Procedure:

1. Summate the estimated total rock fragment content of the whole soil in percent by volume (stones, cobbles, and gravel) of a major soil horizon and enter at the appropriate percent by volume value on the horizontal axis (abscissa) of the curves.
2. Move vertically along the percent by volume value line obtained in 1 above until intersecting the appropriate curve representing the percent by volume of rock fragments >3" (stones and cobbles) that are present in the whole soil. For soils having percent by volume values of rock fragments >3" ending in 5, interpolate between curves and along the vertical line.
3. Move horizontally from point obtained in 2 above to the vertical axis (ordinate) and read value. This value, in percent, is the amount of gravel by weight retained on the #10 sieve, based on the gravel plus fine earth fraction. Subtract this value from 100 to obtain percent of fine earth materials passing the #10 sieve.
4. To obtain the entry for percent by weight of rock fragments >3" present in the whole soil enter at the appropriate percent by volume of estimated rock fragments >3" on the horizontal axis (abscissa) of the curves.
5. Repeat steps 2 and 3 above but use 0 percent cobbles and stones curve. The value obtained is the percent by weight of >3" fraction (stones and cobbles) present in the whole soil. Enter this value in the >3" fraction column.

Example: A soil contains 35% by volume of stones and cobbles and 35% by volume of gravel. Determine entries for >3" fraction and fine earth fraction passing #10 sieve.

Solution:

1. Enter horizontal axis (abscissa) at 70% (estimated total rock fraction in the whole soil by volume).
2. Move vertically along 70% line to intersection of interpolated point 1/2 distance between 30% and 40% cobbles and stones by volume curves.
3. Move horizontally to the vertical axis (ordinate) and read value of 67%. This value is the percent by weight of gravel retained on the #10 sieve based on the gravel plus fine earth fraction. Subtract this value for 100%. The resultant value, 33%, is the amount by weight of fine earth fraction passing the #10 sieve.
4. Enter horizontal axis (abscissa) at 35% (estimated total stones and cobbles in the whole soil). Move vertically along 35% line to intersection with 0% stones and cobbles curve. Move horizontally to vertical axis (ordinate) and read value of 50%. This is the percent by weight value entry for rock fragments >3" based on the whole soil.

Note: Weight values for % rock fragments >3" fraction and % gravel may often total in excess of 100 if appreciable stones, cobbles and gravel are present in the whole soil because there are two different bases of 100% being utilized.

Chart for Estimating Rock Fragments and Textural Cclass Modifier

Instructions - This chart can be used to:

1. Estimate the percent gravel by volume from estimates of percent of fragments passing the No. 10 sieve and percent larger than 3 inches.
2. Convert weight to volume or volume to weight for rock fragments (columns 1 and 2), based on dry bulk density 1.4 g/cc.
3. Check or determine the textural class modifier that coincides with percent passing No. 10 sieve and percent larger than 3 inches on SCS Form 5.
1. less than 15% rock fragments - nongravelly, noncobbly, or nonstony
2. 15 to 35% rock fragments - gravelly, cobbly, or stony
3. 35 to 60% rock fragments - extremely gravelly, extremely cobbly, or extremely stony.
4. Estimate percent passing No. 10 sieve (weight) for various combinations of rock fragment percentages by volume (top).

The A line separates the nongravelly, noncobbly, or nonstony soils from the gravelly, cobbly, or stony soils (15% rock fragments by volume).

The B line separates the gravelly, cobbly, or stony soils from the very gravelly, very cobbly, or very stony soils (35% rock fragments by volume).

The C line separates the very gravelly, very cobbly, or very stony soils from the extremely gravelly, extremely cobbly, or extremely stony soils (60% rock fragments).

The D line shows the break between gravelly size and cobbly size modifiers-2.0 times or more as much gravel as cobble to use gravelly if more than 35% rock fragments, and 1.5 times or more as much gravel as cobble to use gravelly if 15 to 35% rock fragments.

The E line shows the break between gravelly size and stony size modifiers-2.5 times or more as much gravel as stone to use gravelly if more than 35% rock fragments, and 2.0 times or more as much gravel as stone to use gravelly if 15 to 35% rock fragments.

All conversion of rock fragments from volume to weight and percent passing No. 10 sieve are based on dry bulk density of 1.5 g/cc. If desired, adjustments can be made in estimates by adding or subtracting 3% for each 0.2 g/cc change in bulk density. Add for lower bulk density; subtract for high bulk density.

Example A

Step 1: If 20% by weight is composed of cobble and stone (rock fragments greater than 3 inches, col. 2) and 40 passes No. 10 sieve, where the columns intersect shows 39% is gravel by volume.

Step 2: The 20% cobble and stone by weight (col. 2) equals 13% cobble and stone by volume (col. 1). And 13% cobble and stone (vol.) + 39% gravel (vol.) (step 1) equals 52% rock fragments by volume.

Step 3: The intersection of 13% cobble and stone and 39% gravel by volume is above the D line, and occurs in the blue area (35 to 60% rock fragments by volume), so the textural class modifier is very gravelly.

Example B

If field estimates for rock fragments show 10 percent cobble and stone and 30 percent gravel (vol.), enter the chart from 10% col. 1 (15% co. 2) go horizontally to 31 (closest to 30% gravel) and then vertically to estimate that 50% passes No. 10 sieve. This soil is very gravelly because it is in the blue and above lines D and E.

Example C

If the soil is designated as very gravelly then it should fall within the blue color and above line D, it should show that 30 to 55% passes a No. 10 sieve, and it should show that 0 to 30% or less by weight is larger than 3 inches.

In some locations along the A, B, C lines, the figure falls exactly on the 15, 35, and 60%. Where these cases occur they have in general been included with lower limits (i.e. 0-15, 16-35, 36-60, etc.). However, these borderline estimates need to be given special consideration in evaluations of the percent passing No. 10 sieve and the textural class modifiers.

Guide Sheet 9

Guide Sheet 10

PERCENTAGE OF ROCK FRAGMENTS WITHIN HORIZONS

Gravel Only

Percent (volume) Descriptive term Example: 10% gravel.; 10% cobbles, 30% stones

Total volume of mixed Rock fragments is = to 50%

15 to 35 gravelly Determine column to use. In this case use Column for

35 to 60 very gravelly mostly gravel and stones. Minimum gravel for very gravely

over 60 extremely gravelly is 35%. We have only 10% and the name will be very stony.

Rock fragments of mixed sizes ranging from 15 to 35 percent (volume)

Mostly gravel and cobbles--Must be 1.5 or more times as much gravel as cobbles to name as gravelly, otherwise use cobbly.

Mostly gravel and stones--Must be 2 or more times as much gravel as stones to name as gravelly, otherwise use stony.

Rock fragments of mixed sizes comprise more than 35 percent (volume)

Mostly gravel and cobbles--Must be 2 or more times as much gravel as cobbles to use the modifier gravelly. If this ratio is met and total rock fragments is 35 to 60 percent, use very gravelly; if more than 60 percent, use extremely gravelly. If ratio is less than 2 and total rock fragments is 35 to 60 percent, use very cobbly; if more than 60 percent, use extremely cobbly.

Mostly gravel and stones--Must be 2.5 or more times as much gravel as stones to use modifier gravelly. If this ratio is met and total rock fragments is 36 to 60 percent, use very gravelly; if over 60 percent, use extremely gravel. If the ratio is less than 2.5 and the total rock fragments is 35 to 60 percent, use very stony; is more than 60 percent, use extremely stony.

Item 1b:

FRACT. > 10 IN (PCT): -- Fraction greater than 10 inches is the weight percent of the whole soil that is larger than 10 inches in diameter. The upper limit is undefined but for practical purposes is not larger than a pedon (up to 10 meters square)."

ALLOWABLE ENTRIES: Enter the range in weight percentage of whole soil material greater than 10 inches, e.g., 30-60. For more precise estimates, where data are lacking values may be estimated to the nearest 5 percent. If there are no rock or woody fragments for the >10 inch entry, enter zero "0", dashes are not allowed. For Histosols enter volume of woody fragments.

DETERMINATION: Use the percent volume calculated in Item 1a above, and make conversions as previously indicated.

Items 2 and 3:

FRACT. PASSING NO. 10 AND NO. 4--"The percent passing sieve numbers 4, and 10 is the weight of the material less than 3 inches that passes these sieves, expressed as a percentage."

"Data for the percent passing sieve numbers are used to classify the soil in the engineering classification and to make judgements on soil properties and performance. Use Guide Sheet 12 for well graded material.

ALLOWABLE ENTRIES:

A. Enter the range or whole number in weight percentages passing each of the sieve sizes, e.g., 100, 95-100, 75-100 or 80-100. For more precise estimates where data is not available, values will be measured or estimated to the nearest 5 percent.

B. Estimates are on 100 percent of the soil. The whole soil includes stones, cobbles, gravels and fine earth fraction.

DETERMINATION: You should have this figure from Step 1. However, if you have not done so; use the total percent volume of coarse fragments measured or estimated in Step 1 compare with

Use Guide Sheet 8 to determine percent passing the #10 sieve. If there are no coarse fragments in the layer, enter "100" for the #10 entry. If the layer is organic or thixotropic, consider the percent passing to determine the entry value. Enter a dash "-" for the #10 sieve if nothing is passing. If the layer is thixotropic-skeletal, enter the appropiate range of material passing the sieve.

If volume percent of rock fragments is not given in the soil descriptions, use high-low range for series within range of characteristics. Use Guide Sheet 10 "Percent rock fragments within horizons".

From your knowledge of the soil, determine what percent of the total gravel content (weight) is larger than approximately 0.2 inches and what percent is less. The percent of material passing the #4 sieve = 100 - the percent gravel larger than approximately 0.2 inches. The percent passing #4 sieve must be greater than or equal to that passing the #10 sieve.

C. If there are no coarse fragments in the layer greater than approximately 0.2 inches, enter "100" for the #4 entry. If the layer is organic or thixotropic consider the percent passing to determine the entry value. Enter a dash "-" for the #4 sieve if nothing is passing. If the layer is thixotropic-skeletal, enter the appropriate range of material passing the sieve.

Step #2

Item 4:

FRACT. PASSING SIEVE NO. 200--"The percent passing sieve number 200 is the weight of the material less than 3 inches that passes these sieves, expressed as a percentage."

ALLOWABLE ENTRIES: Enter the range in weight percentage passing the sieve size, e.g., 85-100. Values should be estimated to the nearest 5 percent.

DETERMINATION:

A. From your knowledge of the soil, and the textures and clay content ranges you have already listed; Use Soil Texture and Textural Triangle, to determine the silt, very fine sand, fine sand, medium sand, coarse sand, and very coarse sand fraction.

B. If the #10 entry is 100, the #200 entry = clay + silt + 1/2 very fine sand. Otherwise, the #200 entry will have to be adjusted to compensate for the coarse fragment content.

Example 1: If #10 entry is 100 and the range for the sum of clay+silt+1/2vfs = 70 - 90, then the #200 entry is 70 - 90.

Example 2: If the #10 entry is 40 - 50 and the range of the sum of clay+silt+1/2vfs = 70 - 90, then the #200 entry (X-Y) is as follows:

X = 70 × 40/100 Y = 90 × 50/100

= 28 = 45

If you round these values to the nearest 5, gives a #200 entry of 30 - 45.

NOTE: If the layer is organic or thixotropic, enter a dash "-".

Step #3

Item 5:

FRACT. PASSING SIEVE NO. 40--"The percent passing sieve number 40 is the weight of the material less than 3 inches that passes the sieve, expressed as a percentage."

ALLOWABLE ENTRIES: Enter the range in weight percentage passing the sieve size, e.g., 90-100. Values should be estimated to the nearest 5 percent.

DETERMINATION:

A. In Step 3 you determine the silt, very fine sand, fine sand, medium sand, coarse sand, and very coarse sand fraction.

1. If the #10 entry is 100, the #40 entry = clay + silt + very fine sand + fine sand + 1/2 medium sand. Otherwise, the #40 entry will have to be adjusted to compensate for the coarse fragment content.
2. Example 1: If #10 entry is 100 and the range for the sum of clay+silt+vfs+fs+1/2 ms = 85 - 95, then the #40 entry is 85 - 95.

Example 2: If the #10 entry is 40 - 50 and the range of the sum of clay+silt+vfs+fs+1/2 ms = 85 - 95, then the #200 entry (X-Y) is as follows:

X = 85 x 40/100 Y = 95 x 50/100

= 34 = 47.5

If you round these values to the nearest 5, gives a #40 entry of (35 - 50).

Guide Sheet 13 can be used to determining the maximum and minimum allowable percent passing the sieves for each texture. If there is no gravel, use the line with zero "0" percent gravel as an aid to estimation. The #10 (sieve size) column converts the volume of gravel to a weight.

ANOTHER WAY TO DETERMINE FRACTION LESS THAN 3" PASSING SIEVE NO. 4, 10, 40 AND 200.

1. Soils lacking rock fragments (100% passing #10 sieve).
   1. Identify texture and various sand-size seperates. See Guide Sheet 11 , "A comparison of Grain-size Limits in the Three Classification Systems, or "texture triangle."
   2. The percent passing #200 sieve = 1/2 VFS + SI + C.
   3. The percent passing #40 sieve = 1/2 MS + FS + VFS + Si + C or 100 - (VCS +CS + 1/2 MS).
2. Soils having gravel and fine earth fraction only (100% soil material passing 3-inch sieve).
   1. Using known volume of gravel, calculate the percent by weight of the soil material passing the #10 sieve. Use Guide Sheet 8 or Guide Sheet 9.
   2. The percent passing #4 sieve: Adjust value in (1) above, as per your knowledge of the soil. 2/
   3. The percent passing #200 sieve - (see procedure a.2 above. Then use the formula:
      
      X = (A+B) - 100
      
      A = percent passing #10 sieve (based on whole soil)
      B = percent passing #200 sieve (based on 100% soil by weight without rock fragments)
   4. The percent passing #40 sieve - (see procedure a.3 above. Then use the formula:
      
      X = (A+C) / 100
      
      A = percent passing #10 sieve (based on whole soil)
      C = percent passing #40 sieve (based on 100% soil by weight without rock fragments)

GUIDE SHEET 13

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