Advances in measurements of unsaturated soils 1,2 Colin S. Campbell, 1 Gaylon S. Campbell, 1,2...
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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,
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
Slide 2
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
Slide 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
Slide 4
Background: Creating the soil water characteristic curve Soil
water content Soil suction
Slide 5
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
Slide 6
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)
Slide 7
No-mans Land of suction instrumentation
Slide 8
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
Slide 9
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)
Slide 10
Wet Soil SWCC using WIND/SCHINDLER Evaporation Method
Slide 11
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
Slide 12
Let stand 24 h Add water Mix Fill sample cup
Slide 13
Reading the Suction Insert sample Seal chamber Wait 5-10 min.
and read the result (up to 15 min for very wet samples)
Slide 14
Silt loam SWCC: Tensiometer & WP4 Data Void: Original WP4
Suction (MPa) Water Content (g/g) New WP4C: 10x better temperature
measurement: 0.001 o C precision
Slide 15
Results
Slide 16
Chilled mirror absolute error of wet- end suction (WP4C and
WP4) Error of Original Chilled Mirror Sensor: WP4
WIND/SHINDLER measurements Wolfgang Durner, Personal
Communication
Slide 24
WIND/SHINDLER SWCC and hydraulic conductivity function Wolfgang
Durner, Personal Communication
Slide 25
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 mans 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
Slide 26
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 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 Hamblin, A.P. 1981. Filter paper method for routine
measurement of field water potential. J. Hydrol. 53:355-360