advances in measurements of unsaturated soils
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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, Inc ., Pullman , WA 2 Washington State University, Pullman, WA. Characterizing unsaturated soils. - PowerPoint PPT PresentationTRANSCRIPT
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Advances in measurements of unsaturated soils
1,2Colin S. Campbell, 1Gaylon S. Campbell, 1,2Douglas R. Cobos, and
1Bryan T. Wacker1Decagon Devices, Inc., Pullman, WA
2Washington State University, Pullman, WA
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Characterizing unsaturated soilsRelationship 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 likeCompacted soilsSwelling claysLow bulk density soils
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Characterizing unsaturated soilsMeasurements
Water content is relatively easy to measure
Suction requires more sophisticated and time-consuming methods
GoalInvestigate two improved methods
for obtaining SWCC
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Background: Creating the soil water characteristic curve
Soil water contentSoil suction Soil suction
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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
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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)
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“No-man’s Land” of suction
instrumentation
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Measurement MethodsEvaluate SWCC
Liquid equilibrium for wet regionTensiometerWIND/SCHINDLER integrated tensiometer
and scale evaporation methodVapor pressure method for dry region
Simple, fast (5 to 15 min)Evaluate consistency between wet
and dry regions
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Tensiometer: Suction in “wet” soilEquilibrates 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)
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“Wet” Soil SWCC using WIND/SCHINDLER Evaporation Method
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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 SensorMirror
Optical Sensor
Fan
Sample
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Let stand 24 hAdd water
Mix
Fill sample cup
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Reading the Suction
Insert sample Seal chamber
Wait 5-10 min. and read the result (up to 15 min for very wet samples)
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0.001
0.01
0.1
1
10
100
0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350
Wat
er p
oten
tial (
-Mpa
)
VWC (m3/m3)
all
dry range
wet range
Silt loam SWCC: Tensiometer & WP4
Data Void: Original WP4Suct
ion
(MPa
)
Water Content (g/g)
New WP4C: 10x better temperature
measurement: 0.001o C precision
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Results
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Chilled mirror absolute error of wet-end suction (WP4C and WP4)
Error of Original Chilled Mirror Sensor: WP4
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Soils
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Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #1
0
0.2
0.4
0.6
0.8
1
1 10 100 1000 10000 100000
grav
imet
ric w
ater
cont
ent
(g/g
)
water potential (-kPa)
Soil B2
WP4C dewpoint
T5 tensiometer
Campbell and Shiozawa
Suction(kPa)
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Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #2
0
0.2
0.4
0.6
0.8
1
1 10 100 1000 10000 100000
grav
imet
ric w
ater
cont
ent
(g/g
)
water potential (-kPa)
Soil B4
WP4C dewpoint
T5 tensiometer
Campbell and Shiozawa
Suction(kPa)
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Schwana loamy fine sand
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Kiona fine sandy loam
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Palouse silt loam
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WIND/SHINDLER measurements
Wolfgang Durner, Personal Communication
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WIND/SHINDLER SWCC and hydraulic conductivity function
Wolfgang Durner, Personal Communication
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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
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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