Field Estimates for Measuring Rock Fragment Content Marc Crouch, SSDQS, MO14 Professional Development Workshop Ocean City, MD October 23-26, 2000

Field Estimates for Measuring Rock Fragment Content

• Marc Crouch, SSDQS, MO14

• Professional Development Workshop

• Ocean City, MD

• October 23-26, 2000

2 Methods for Measuring Rock Fragment Content

• Weighing

• Water displacement

Objective is...

• “The percent by weight of the horizon occupied by rock fragments >10 inch and 3-10 inch in size”

• “Soil fraction passing a number 10 sieve as a weight percentage of the less than 3 inch fraction” (2-76 mm)

Materials

• Need some stuff to do the job right

Materials

• Container (bucket and/or coffee can)

• Weight scale• Tape measure• sampling scoop• Water

Sample the horizon

• Get a good representative sample from the horizon

• Use scoop, spade ??

Weighing Method

• Direct way of measuring weight and calculating as a percent of the whole horizon (> 3 inch) and the soil material less than 3 inch fraction (sieves)

Weighing Method -Step #1

• Place material in container

• If bucket, need not fill completely

• Just make sure that you have a sample representative of the horizon

Weighing Method-Step #2

• Weight the bucket + soil

• note weight as:

• W (soil) =

• ----example----

• W (soil) = 7kg

Weighing Method-Step #3• Sieve the soil with

75mm sieve to retain >3 inch (if have one)-set this aside

• Note that this is for spherical or equiaxial shapes; not for flat fragments-see SSM Table 3-11, page 143


• If do not have the 3 inch sieve, measure for fragments greater than 3 inches

Weighing Method - Step #4

• Weigh the > 3 inch. • Note weight as:

• W (>3-measured) =

• ----example----

• W (>3-measured) = 1kg

Weighing Method -Step #5

• Subtract weight of >3 inch from weight of whole sample

• Note this as weight of <3 inch fraction of the horizon

• example----

• W (soil2) = 6kg


• Sieve the < 3 inch fraction with #10 sieve to retain 2-76 mm -set this aside


• Weigh the 2-76 mm• Note weight as:

• W (2-76mm-measured.) =

• ----example----

• W (2-76mm-measured.) = 3kg

Weight Summary

• Soil w/ >3” = 7 kg• >3 inch = 1 kg• Soil w/o >3” = 6 kg• 2-76 mm = 3 kg


• Calculate weight % of > 3 inch

• W (>3) % = W (>3-meas.) / W (soil w/>3”) X 100

• ----example----

• W (>3) % = 1/7 X 100 = 14%


• Calculate weight % of 2 - 76 mm

• W (2-76mm) % = W (2-76mm-meas.) / W (soil w/o >3”) X 100

• ----example----

• W (2-76 mm) % = 3/6 X 100 = 50%

Water Displacement Method

• An indirect method of calculating weight as a percent of the whole horizon.

• Calculation equals volume

• Will need further calculations or conversion charts to estimate weight

Water Displacement-Step # 1

• Place material in container. Have several sizes of containers so that can fill the container

• Tamp the soil in the bucket to better approximate the bulk density, accounting for void space


• Sieve the soil with 75mm sieve (if have one) to retain >3 inch- Set this aside

• Note again SSM Table 3-11, page 143 for flat fragments


• If do not have the 3 inch sieve, measure for fragments greater than 3 inches


• Replace the soil in the bucket, tamp again, and add ,3 inch sample so that container is again filled

• This is for volume of the less than 3-inch part of the horizon

• H w/o


• Sieve with #10 sieve to retain 2-76 mm or the 2mm-3 inch fraction, set this aside


• Measure bucket height from bottom to the top

• Note height:

• ----example----

• H (bucket) = 10 cm


• Add water to the bucket

• Enough to anticipate covering rock fragments when they are added


• Measure from the top of the water to top of the container

• Note height as:

• ----example----

• H (w) = 7 cm


• Return the > 3 inch rock fragments to the bucket

• measure from top of water to the top of the container

• record:• ----example----

• H (>3) = 7.5 cm


• This is measurement of volume of the > 3-inch as part of the whole horizon

• remove the > 3-inch rock fragments


• Check height of water in container again

• Return the 2-76 mm rock fragments to the bucket

• Measure again from the top of water to top of container

• record

Water Displacement Measurement Examples

• Height of bucket = 25 cm

• from top of water to top of container w/o fragments = 15 cm

• from top of water to top of container with >3 inch only = 10 cm

• from top of water to top of container with 2-75mm only = 5 cm


• Calculate the volume of the rock fragment >3 inch in size

• Subtracting the value derived with >3 inch from value derived w/o rock fragments

• 15 - 10 = 5 cm (amount displaced by >3 inch fraction)

• 5 divided by 25 = .20 X 100 = 20% of container which represents the whole horizon in this case.


• Calculate the volume of the rock fragments 2-75 mm

• Subtracting the value derived with 2-75 mm from value derived w/o rock fragments

• 15 - 5 = 10 cm (amount displaced by 2-75 mm fraction)

• 10 divided by 25 = .40 X 100 = 40% of container which represents the <3 inch part of the horizon in this case.

Why do this?

• Our visual efforts to estimate volume are usually high

• Complete a certain number of actual measurements in order to calibrate your observations with actual measurements.

Where is this used?

• Where else…

• NASIS

And how do we get there?

• Conversion charts, for now

• NASIS will have a sieve calculator in the future

Documents

Field Estimates for Measuring Rock Fragment Content Marc Crouch, SSDQS, MO14 Professional Development Workshop Ocean City, MD October 23-26, 2000