October 27, 2026
This week’s photo is courtesy of- Western Hydro Engineering- Cochise, AZ, USA .
Safety Focus
Part 2 of 4: Respiratory Protection Program
1910.134(a)(2) puts it plainly: "A respirator shall be provided to each employee when such equipment is necessary to protect the health of such employee." To know when that is, the standard requires the employer to "identify and evaluate the respiratory hazard(s) in the workplace." That evaluation is the decision point.
On a drilling job, the hazards to assess often include dry cuttings and rock dust, cement and other powders at the mixing station, and vapors from chemical well treatments. Those are field examples of what to look at, not an OSHA checklist. Dusts call for particulate filters. Gases and vapors call for the right cartridge, and Wednesday covers how long a cartridge lasts.
Silica has its own path. The construction silica standard's Table 1 lists tasks and controls, including vehicle-mounted drilling rigs for rock and concrete, with no respirator required when the listed controls are fully and properly in place. When a task is not in Table 1, or its controls are not fully in place, the employer assesses exposure instead. Respirators are then required where exposure exceeds the permissible exposure limit (PEL): while controls are being installed, during tasks where controls are not feasible, or when all feasible controls still fall short. Whether a given job fits a Table 1 entry is a call for your exposure control plan.
One hazard no filter fixes: low oxygen. The standard treats an atmosphere below 19.5 percent oxygen by volume as oxygen deficient. An air-purifying respirator cleans the air you have. It does not add oxygen. Vaults, pits, and tanks get tested before anyone goes in.
Voluntary use has rules too. If the employer lets someone wear a dust mask (a filtering facepiece) on their own, the employer must give them the Appendix D information sheet, but the full program is not required. Voluntary use of other respirators still requires a medical evaluation and keeping the respirator clean.
Key Takeaways:
• A respirator is required when it is necessary to protect health, and the employer must evaluate the hazard to know when that is.
• Dust, cement and other powders, and treatment chemicals are field examples to assess; match the filter or cartridge to the hazard.
• On silica work, Table 1 decides the respirator; off Table 1, exposure above the PEL does.
• An air-purifying respirator does not add oxygen; test the atmosphere first.
Sources:
Knowledge Share
Part 2 of 4: Formation Sieve Analysis
FORMULA SPOTLIGHT: UNIFORMITY COEFFICIENT
Cu = D60 / D10
• D60 = the grain size that 60 percent of the sample, by dry weight, is finer than
• D10 = the grain size that 10 percent of the sample, by dry weight, is finer than (also called the effective size)
• Cu = uniformity coefficient (no units)
The grain-size curve is a picture. The D-numbers turn it into numbers you can design with. D10 is the grain size that 10 percent of the sample, by dry weight, is finer than. D60 is the same idea at 60 percent. That is how the USDA Natural Resources Conservation Service defines them, and it is the standard way to read a curve.
D10 should sound familiar. It is the effective size from Week 2, the one Hazen used to estimate K.
The uniformity coefficient puts the two together: Cu = D60 / D10. Read it off the curve above. D60 lands right at the #30 sieve, .023". D10 falls between the #100 and #50 sieves at about .007". So Cu = .023 / .007, about 3.3.
What the number tells you: Cu describes how spread out the grain sizes are. A Cu of 1 means every grain is the same size. The higher Cu goes, the wider the range from fine to coarse in the same sample. Two sands can have the same D10 and behave very differently because one is uniform and the other is not.
Now the catch. Soils labs following ASTM D6913 report percent passing, which is how the curve above reads on its left axis. Some well-design references, including Groundwater and Wells, plot cumulative percent retained instead, the right axis. On a retained curve, D10 is the 90 percent retained size. Read the wrong axis and you are picking a number from the wrong end of the sample.
What to check before you order:
• Confirm whether the curve is plotted percent passing or percent retained before you read a single D-number.
• Read D10 and D60 off the curve, interpolating between sieves, not off the nearest sieve.
• Write D10, D60, and Cu on the design sheet. Next week's slot selection starts there.
Sources:
• USDA NRCS: National Engineering Handbook, Part 633, Chapter 26: Gradation Design of Sand and Gravel Filters
Recommended Print References:
• Hydraulics of Wells: American Society of Civil Engineers
• Handbook of Ground Water Development- Roscoe Moss Company
• Groundwater & Wells, Third Edition- Edited by Robert J. Sterrett, Johnson Screens
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