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Combining dewpoint and Wind/Schindler methods to create full range SMCCs

Combining dewpoint and Wind/Schindler methods to create full range SMCCs. Doug Cobos, Colin Campbell, and Leo Rivera Decagon Devices, Inc ., Pullman , WA Washington State University, Pullman, WA. Characterizing unsaturated soils.

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Combining dewpoint and Wind/Schindler methods to create full range SMCCs

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  1. Combining dewpoint and Wind/Schindler methods to create full range SMCCs Doug Cobos, Colin Campbell, and Leo Rivera Decagon Devices, Inc., Pullman, WA Washington State University, Pullman, WA

  2. Characterizing unsaturated soils • Relationship between suction 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

  3. 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

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

  5. Background: Filter Paper • Based on work by Hamblin (1981), Al-Khafaf and Hanks (1974), and Deka et al. (1995) • Calibrated method • Filter paper in suction equilibrium with soil sample • Measure water content of filter paper • Correlated with suction through calibration relationship (SWCC of filter paper) • Provided suction measurements without difficult lab setup

  6. Background: Filter Paper • Problems • Calibrated method that relies on repeatable filter paper SWCC • Results are affected by equilibration time, hydraulic conductivity, paper contact with soil, fungal growth • Slight temperature gradient has huge effect (8 MPa/C error) • Filter paper SWCC has hysteresis • Labor and time intensive

  7. Porous plate and soil sealed in chamber Outflow at atmospheric pressure Chamber and soil at elevated pressure Can achieve much higher ΔP than under tension Axis translation

  8. Effectiveness of axis translation at low (dry) water potential routinely questioned Recent work shows that samples equilibrated at -1.5 MPa only reached -0.55 MPa Hydraulic disconnect between plate and soil sample Low Kunsat at low (dry) water potential Axis translation

  9. Or and Tuller 2002, Baker and Frydman 2009 Soil pores don’t drain the same way under positive pressure as they do under tension SMCCs with axis translation fundamentally different from those developed under tension Axis translation D. Or and M. Tuller. 2002. Cavitation during desaturation of porous media under tension. Water Resources Research38: (19-1) – (19-4)

  10. “No-man’s Land” of suction instrumentation

  11. New Measurement Methods • 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

  12. 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) Tensiometer: Suction in “wet” soil

  13. Wind/Schindler Evaporation Method

  14. SMCC with HyProp (Wind Schindler) HyProp is setup with saturated soil sample Measures sample weight and tension at two different points as sample naturally dries Typically takes 4 to 7 days

  15. HyProp Output • The average water content and the average water potential give a discrete value of the SMCC at any time.

  16. HyProp Output • hi1 - hi2and Δ mass of sample give hydraulic conductivity

  17. Air Entry Point of Ceramic • Can obtain one additional (drier) data point using the air entry point of the ceramic

  18. Suction in “Dry” range • Cool mirror until dew forms • Detect dew optically • Measure mirror temperature • Measure sample temperature with IR thermometer • Accuracy +/- 50 kPa or better Fan Optical Sensor Mirror Infrared Sensor Sample

  19. Generating SMCC with WP4C

  20. Preparing SMCC samples 1. Air dry soil 2. Grind and/or sieve with 2 mm sieve (if necessary)

  21. Preparing samples 3. Fill 10-12 stainless steel sample cups ~1/2 full of dry soil - Weigh out same mass of soil in each cup - ~2-7 g depending on density

  22. Preparing samples 4. Add ascending amount of DI water to each sample - 0, 1, 2, 4, 6, 8, 10, 14, 18, 22… drops of water works well • Amount of water added depends on soil type and range of interest

  23. Preparing samples 6. Mix samples thoroughly

  24. Preparing samples • Cap samples and allow to equilibrate overnight • Remove lids and allow to dry for 30-60 minutes • Replace lids and allow to re-equilibrate for 3-6 hours Done with preparation!

  25. Measure water potential with the WP4C Insert sample Seal chamber Wait ~5 min. and read the result

  26. Measure the water content Dry in a 105 C oven for 24 hours Weigh moist samples Weigh dry samples w = (moist soil mass – dry soil mass)/dry soil mass

  27. Construct SMCC

  28. Silt loam SWCC: Tensiometer & WP4 New WP4C: 10x better temperature measurement: 0.001o C precision Suction (MPa) Data Void: Original WP4 Water Content (g/g)

  29. Chilled mirror absolute error of wet-end suction (WP4C and WP4) Error of Original Chilled Mirror Sensor(WP4) +/- 100 kPa

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

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

  32. Schwana loamy fine sand

  33. Palouse silt loam

  34. Important considerations • Hysteresis • Hyprop always on drying leg • WP4C must be on drying leg too in fine textured soils • Soil structure (fabric) • For best measurements in wet end, WP4C should use intact samples • Matric vs. total suction • Tensiometers – Matric • WP4C – Matric + Osotic • Correction needed in salty soil

  35. 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

  36. References • Al-Khafaf, S., and Hanks, R.J. 1974. Evaluation of the filter paper method for estimation soil water potential. Soil Sci. 117:194-199 • R. Baker and S Frydman. 2009. Unsaturated soil mechanics: Critical review of physical foundations. Engineering Geology 106: 26-39. • Barbour, S.L. 1998. Nineteen Canadian geotechnical colloquium: The soil-water characteristic cure: A historical perspective. Canadian Geotechnical Journal. 35:873-894. • Bittelli, M. and Flury, M. 2008. Errors in Water Retention Curves Determined with Pressure Plates. Soil Sci. Soc. Am. J. 73:1453-1460 • Deka, R.N., Wairiu, M., Mtakwa, P.W., Mullins, C.E., Veenendaal, E.M., and Townsend, J. 2995. Use and accuracy of the filter-paper technique for measurement of soil matric potential. Eur. J. Soil Sci. 46:233-238 • Fredlund, D.G. and Rahardjo, H. 1993. Soil mechanics for unsaturated soils. John Wiley and Sons, Inc.: New York. • Gardner, W.R. 1937. A method of measuring the capillary tension of soil moisture over a wide moisture range. Soil Science. 43(4), 277-283 • Gee et. al, 2002. The influence of hydraulic disequilibrium on pressure plate data. Vadose Zone Journal. 1: 172-178. • Hamblin, A.P. 1981. Filter paper method for routine measurement of field water potential. J. Hydrol. 53:355-360

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