Department of Transportation Switches From Hydrometers to the SediGraph 5100

Department of Transportation Switches
From Hydrometers to the SediGraph 5100

In today's high speed construction market, performing a soils analysis is the last thing you would expect to hold up a construction site. But that's exactly what was happening according to the Alabama Department of Transportation (DOT) here in the US. The hydrometer technique they were using for measuring grain size of soils was taking so long to perform that it was causing construction delays while contractors waited for the results of the tests. That's when the Alabama DOT decided to take action.

They brought together research specialists from the Civil Engineering and Materials Engineering Departments of the Michigan Technological University and from the Chemical Engineering Department of the University of Alabama to study various particle size analyzers suitable for soils analysis. They wanted recommendations based on speed, automation, and accuracy of the measurement technique. Key questions were:

Which measurement technique would produce the most accurate particle size data of DOT soil samples in the shortest amount of time?

Which analyzer offers greater precision than the hydrometer method? And produces comparable data?

Which measurement technique has the sensitivity to distinguish different soil compositions consistently, yet affordably?

Particle Analyzers Researched

The research team evaluated the following four particle size instruments:

1. Leeds and Norththrup Microtrac (laser diffraction instrument)
2. Lasentec Partec-100 (instrument using laser beam reflectance)
3. Micromeritics SediGraph 5100 (instrument using X-ray absorption)
4. Quantachrome Microscan (instrument also using X-ray absorption)

The Coulter Counter, a resistive pulse technique that suspends particles in an electrically conductive liquid, was not evaluated in this research as it did not compare well for this type of soils analysis.( 1 )

The results of the research conducted for the Alabama DOT as reported by Vitton and Sadler( 2 ) show that the X-ray absorption instruments agreed much more closely with the hydrometer test results than did the laser-based instruments. They also reported that "between the two X-ray absorption devices, it was found that the SediGraph had closer agreement than did the Quantachrome device, however fewer samples were tested with the Quantachrome device." Citing earlier research by Singer et.al.( 3 ), the research team was able to corroborate their findings that the Micromeritics SediGraph X-ray absorption technique performed better than laser diffraction, resistive pulse, and photo-extinction techniques, especially for smaller silt-size and clay-size particles.

Although the hydrometer test was inexpensive, relatively easy to conduct, and had been a standard in the industry, it was too costly in terms of labor and turn-around time. Plus, the particle-size results were crude and easily prone to operator error. That, plus the research findings, led the Alabama DOT to replace the hydrometer method with the SediGraph X-ray absorption method.

Key Findings

Research tests on soil samples indicated that the SediGraph 5100:

• Produced data comparable to the hydrometer technique (i.e., no need to rework archives of hydrometer-collected data)
• Produced high-resolution analyses of DOT soil samples in only 20 minutes
• Automated the measurement process thereby minimizing operator-induced error
• Earned high marks for repeatability ("Results basically overlap," stated the research team. "And this was very consistent with all of the samples that were re-tested in the SediGraph 5100.")
• Featured an accurate sensing device that can distinguish the small scale changes in soil structure
• Permitted a more detailed investigation into a soil's geologic history
• Gave accurate results in target range: 75 to 1 um in diameter
• Would process a sufficient number of tests per year to offset cost of SediGraph with reduction in labor costs

SediGraph is a trademark of Micromeritics. All other products are trademarks of their holders.

1. R. Stien, Rapid Grain-Size Analysis of Clay and Silt Fraction by SediGraph 5000D: Comparison with Coulter Counter and Atterberg Methods , Journal of Sedimentary Petrology, Vol. 55, No. 4, 1985, pp. 590-515.

2. S.J. Vitton and L .Y. Sadler, Particle Size Analysis of Soils Using Laser Diffraction and X-ray Absorption Technology to Supplement Hydrometer Analysis , Research Report to the Alabama DOT, Project 930-290, May, 1994.

3. J.K. Singer, J.B. Anderson, M.T. Ledbetter, I.N. McCave, and P.N. Jones , An Assessment of Analytical Techniques of Fine-Grained Sediments , Journal of Sedimentary Petrology, Vol. 58, No. 3, May 1988, pp. 534-543.

To obtain a copy of the latest report, "Soil Particle-Size Analysis Using X-ray Absorption," by Vitton, Nesbitt, and Sadler, use our Product Enquiry Form and enter the title in the text area. This report covers the SediGraph operating technique, comparisons of SediGraph and hydrometer data, analysis conditions, effects of soil plasticity on data, and effects of soil sieving and dispersion, and sampling techniques.