Advances in measurements of unsaturated soils 1,2 Colin S. Campbell, 1 Gaylon S. Campbell, 1,2 Douglas R. Cobos, and 1 Bryan T. Wacker 1 Decagon Devices, Inc., Pullman, WA 2 Washington State University, Pullman, WA

Characterizing unsaturated soils Relationship between water potential and water content defines soil water characteristic curve (SWCC) Soil water characteristic curve (SWCC) is central to the behavior of unsaturated soils (Fredlund and Rahandjo, 1993; Barbour, 1998) Key in understanding unsaturated soils like Compacted soils Swelling clays Low bulk density soils

Characterizing unsaturated soils Measurements Water content is relatively easy to measure Suction requires more sophisticated and time-consuming methods Goal Investigate two improved methods for obtaining SWCC

Background: Creating the soil water characteristic curve Soil water content Soil suction

Background: Filter Paper Filter paper method for suction measurement Based on work by Hamblin (1981), Al-Khafaf and Hanks (1974), and Deka et al. (1995) Calibrated method Measured water content of filter paper Correlated with suction Standard vapor pressure method and tensiometer Generated SWCC Provided suction measurements without difficult lab setup Problems Calibrated method that relies on repeatable SWCC Results are affected by equilibration time, hydraulic conductivity, paper contact with soil, fungal growth Filter paper SWCC has hysteresis Labor and time intensive

Background: Pressure Plate Introduced in 1930s by L.A. Richards Equilibrate pressure above soil sample with water in sample Forces water out of the sample so soil suction is equivalent to air pressure Equilibration times Wet samples (up to 100 kPa suction) < 1 day Dry sample (100 kPa to 1500 kPa): 1 week to never Problem Range from 100 kPa to 1000 kPa important to SWCC Axis Translation may not ever fully equilibrate in upper range (Bittelli and Flury, 2008)

“No-man’s Land” of suction instrumentation

Measurement Methods Evaluate SWCC Liquid equilibrium for wet region Tensiometer WIND/SCHINDLER integrated tensiometer and scale evaporation method Vapor pressure method for dry region Simple, fast (5 to 15 min) Evaluate consistency between wet and dry regions

Tensiometer: Suction in “wet” soil Equilibrates water under tension with soil water through a porous cup Measures pressure of water Highest accuracy, but limited range (Suction: 0 to 80 kPa) Must be measured in representative sample (compaction)

“Wet” Soil SWCC using WIND/SCHINDLER Evaporation Method

Suction in “Dry” range  Cool mirror until dew forms  Detect dew optically  Measure mirror temperature  Measure sample temperature with IR thermometer  Suction is approximately linearly related to Ts - Td Infrared Sensor Mirror Optical Sensor Fan Sample

Let stand 24 h Add water Mix Fill sample cup

Reading the Suction Insert sample Seal chamber Wait 5-10 min. and read the result (up to 15 min for very wet samples)

Silt loam SWCC: Tensiometer & WP4 Data Void: Original WP4 Suction (MPa) Water Content (g/g) New WP4C: 10x better temperature measurement: o C precision

Results

Chilled mirror absolute error of wet- end suction (WP4C and WP4) Error of Original Chilled Mirror Sensor: WP4

Soils

Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #1 Suction (kPa)

Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #2 Suction (kPa)

Schwana loamy fine sand

Kiona fine sandy loam

Palouse silt loam

WIND/SHINDLER measurements Wolfgang Durner, Personal Communication

WIND/SHINDLER SWCC and hydraulic conductivity function Wolfgang Durner, Personal Communication

Summary New techniques make determining soil water characteristic curves easier and more accurate Improved measurement range Faster and less time consuming measurements New chilled mirror measurements bridge traditional “no man’s land” Measurements at low suctions match nicely with tensiometer WIND/SCHINDLER method allows automation of “wet” range SWCC and unsaturated hydraulic conductivity Simple drying procedure Software fits SWCC and gives hydraulic conductivity function

References Al-Khafaf, S., and Hanks, R.J Evaluation of the filter paper method for estimation soil water potential. Soil Sci. 117: Barbour, S.L Nineteen Canadian geotechnical colloquium: The soil- water characteristic cure: A historical perspective. Canadian Geotechnical Journal. 35: Bittelli, M. and Flury, M Errors in Water Retention Curves Determined with Pressure Plates. Soil Sci. Soc. Am. J. 73: Deka, R.N., Wairiu, M., Mtakwa, P.W., Mullins, C.E., Veenendaal, E.M., and Townsend, J Use and accuracy of the filter-paper technique for measurement of soil matric potential. Eur. J. Soil Sci. 46: Fredlund, D.G. and Rahardjo, H Soil mechanics for unsaturated soils. John Wiley and Sons, Inc.: New York. Gardner, W.R A method of measuring the capillary tension of soil moisture over a wide moisture range. Soil Science. 43(4), Hamblin, A.P Filter paper method for routine measurement of field water potential. J. Hydrol. 53: