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Measuring soil suction—why filter paper isn't good enough

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The filter paper method is an older method for measuring soil suction or water potential that is still commonly used in geotechnical engineering today. With this method, a piece of filter paper is sealed in an airtight container with a soil sample and they are allowed to equilibrate. 

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The filter paper method (still commonly used in geotechnical engineering) is a relatively old, indirect method used to measure soil suction (or water potential). In this video, Dr. Doug Cobos explains why the filter paper method can produce large errors in the suction measurement, and what you should use instead.

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How to measure soil suction with the filter paper method

The filter paper method is an older method for measuring soil suction or water potential that is still commonly used in geotechnical engineering today. With this method, a piece of filter paper is sealed in an airtight container with a soil sample and they are allowed to equilibrate. 

Figure 1. Filter paper is sealed in an airtight container with soil and allowed to equilibrate
Figure 1. Filter paper is sealed in an airtight container with soil and allowed to equilibrate

Here’s how it works. If the filter paper is in contact with the soil (Figure 2), then it will equilibrate with the matric suction of the soil. And if it is suspended above the soil, then the total soil suction equilibrates between those two. Once the filter paper has equilibrated with the soil, then the filter paper is brought out and weighed wet, dried out, and then weighed dry to get the water content. 

Figure 2.  Putting the filter paper in contact with the soil enables it to equilibrate with the matric suction of the soil
Figure 2. Putting the filter paper in contact with the soil enables it to equilibrate with the matric suction of the soil

Once you know the water content of the filter paper, you can infer the suction of that filter paper using a calibration curve. This calibration curve is essentially the moisture characteristic curve of the filter paper. Thus it is an indirect method for measuring the soil suction. 

Measuring soil suction: Filter paper method pros and cons 

The filter paper method has both advantages and disadvantages. The following are a few attractive aspects to using the filter paper method:

Filter paper method advantages

  1. It's an inexpensive technique: All you need is Wattman number 42 filter paper, an oven, and some sealed containers in order to make these measurements. 
  2. ASTM standard ASTM D 5298 describes the filter paper method: This is an important requirement for many engineering applications.
  3. Measures soil suction all the way from saturation to air dry: The filter paper method can measure from zero suction all the way to air dry, so you can cover the entire range of soil suctions you might encounter in nature. 
Figure 3. Whatman #42 filter paper
Figure 3. Whatman #42 filter paper

Filter paper method disadvantages

Despite the positives of the filter paper method, there are large disadvantages that make using this method risky. The following problems are often overlooked:

  1. Potentially large errors in the soil suction measurement: If there is any temperature difference between the soil sample and the filter paper, it creates a large error in the suction measurement. Even a one degree temperature difference between the sample and the filter paper will give you an 8 MPa error in the soil suction measurement. And while in practice you would never expect a one degree temperature difference, a 10th of a degree temperature difference or even a 100th of a degree temperature difference between the filter paper and the sample is possible, and in fact, quite likely. This is because it's much more difficult than people understand to create truly isothermal conditions.
  2. Soil suction errors increase in wetter soils: To give you a feel for how large this error can be, if you're at permanent wilting point, which is 1.5 MPa of suction (or 4.2 PF), just a 1/10 of a degree temperature difference between your soil sample and the filter paper will give you a 55% error in your suction measurement. These errors will increase as you go wetter or toward a lower suction. 1.5 MPa is a relatively dry soil. If you're measuring in moist soils, you need temperature agreement or temperature stability within about 1/1,000 of a degree to make an accurate measurement of soil suction with this method. 
  3. ASTM standard recommends universal calibration method for different filter paper lots: The other big problem with the filter paper method is that the ASTM standard recommends a universal calibration for multiple types of filter paper and multiple lots of filter paper. This has been proven not to be an effective way to get good measurements out of the filter paper method. The literature shows that different batches and even different lots of the same type of filter paper don't have the same calibration curve. Dr. Bill Lycos and Dr. Ning Lu published a paper (Likos, 2002) comparing the differences in calibration among seven different lots of Wattman number 42 filter paper. What they found was that they needed an individual calibration for each one of those lots to get reasonable accuracy. If they applied the universal calibration to all those different lots of filter paper, they ended up with a 25 to 50% error encompassed by the 95% confidence interval.

Modern methods give more accurate soil suction measurements

The bottom line is that even if you have perfectly isothermal conditions, which are almost impossible to achieve in practice, you may still get a 25 to 50% error in your suction measurements with the filter paper technique. There are modern methods that are much more accurate and simpler in practice. The WP4C is a fast and easy way to get accurate soil suction measurements. It’s a proven first-principles method based on fundamental physics. It’s so accurate, it’s used to calibrate other methods. Visit our product page to learn more.

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Reference

Likos, William J., and Ning Lu. "Filter paper technique for measuring total soil suction." Transportation research record 1786, no. 1 (2002): 120-128. Article link

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