Field Estimates for Measuring Rock Fragment Content

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

2. 2 Methods for Measuring Rock Fragment Content • Weighing • Water displacement

3. 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)

4. Materials • Need some stuff to do the job right

5. Materials • Container (bucket and/or coffee can) • Weight scale • Tape measure • sampling scoop • Water

6. Sample the horizon • Get a good representative sample from the horizon • Use scoop, spade ??

7. 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)

8. 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

9. Weighing Method-Step #2 • Weight the bucket + soil • note weight as: • W (soil) = • ----example---- • W (soil) = 7kg

10. 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

11. Weighing Method-Step #3 • If do not have the 3 inch sieve, measure for fragments greater than 3 inches

12. Weighing Method - Step #4 • Weigh the > 3 inch. • Note weight as: • W (>3-measured) = • ----example---- • W (>3-measured) = 1kg

13. 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

14. Weighing Method-Step #6 • Sieve the < 3 inch fraction with #10 sieve to retain 2-76 mm -set this aside

15. Weighing Method - Step #7 • Weigh the 2-76 mm • Note weight as: • W (2-76mm-measured.) = • ----example---- • W (2-76mm-measured.) = 3kg

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

17. Weighing Method - Step #7 • Calculate weight % of > 3 inch • W (>3) % = W (>3-meas.) / W (soil w/>3”) X 100 • ----example---- • W (>3) % = 1/7 X 100 = 14%

18. Weighing Method - Step #8 • 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%

19. 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

20. 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

21. Water Displacement-Step # 2 • 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

22. Water Displacement-Step # 2 • If do not have the 3 inch sieve, measure for fragments greater than 3 inches

23. Water Displacement-Step # 3 • 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

24. Water Displacement-Step # 4 • Sieve with #10 sieve to retain 2-76 mm or the 2mm-3 inch fraction, set this aside

25. Water Displacement-Step # 5 • Measure bucket height from bottom to the top • Note height: • ----example---- • H (bucket) = 10 cm

26. Water Displacement-Step # 6 • Add water to the bucket • Enough to anticipate covering rock fragments when they are added

27. Water Displacement-Step # 7 • Measure from the top of the water to top of the container • Note height as: • ----example---- • H (w) = 7 cm

28. Water Displacement-Step # 8 • 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

29. Water Displacement-Step # 8 • This is measurement of volume of the > 3-inch as part of the whole horizon • remove the > 3-inch rock fragments

30. Water Displacement-Step # 9 • 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

31. 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

32. Water Displacement-Step # 10 • 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.

33. Water Displacement-Step # 11 • 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.

34. 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.

35. Where is this used? • Where else… • NASIS

36. And how do we get there? • Conversion charts, for now • NASIS will have a sieve calculator in the future