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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. - PowerPoint PPT Presentation
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Combining dewpoint and Wind/Schindler methods to create full
range SMCCs
Doug Cobos, Colin Campbell, and Leo Rivera
Decagon Devices, Inc., Pullman, WAWashington State University, Pullman,
WA
Characterizing unsaturated soilsRelationship 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 likeCompacted soilsSwelling claysLow bulk density soils
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
Background: Creating the soil water characteristic curve
Soil water contentSoil suction Soil suction
Background: Filter Paper Based on work by Hamblin (1981), Al-
Khafaf and Hanks (1974), and Deka et al. (1995)
Calibrated methodFilter paper in suction equilibrium with soil
sampleMeasure water content of filter paperCorrelated with suction through calibration
relationship (SWCC of filter paper) Provided suction measurements without
difficult lab setup
Background: Filter PaperProblems
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 hysteresisLabor and time intensive
Axis translation 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 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 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
D. Or and M. Tuller. 2002. Cavitation during desaturation of porous media under tension. Water Resources Research 38: (19-1) – (19-4)
“No-man’s Land” of suction
instrumentation
New Measurement MethodsLiquid equilibrium for wet region
TensiometerWIND/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
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)
Wind/Schindler Evaporation Method
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
The average water content and the average water potential give a discrete value of the SMCC at any time.
HyProp Output
hi1 - hi
2 and Δ mass of sample give hydraulic conductivity
HyProp Output
Can obtain one additional (drier) data point using the air entry point of the ceramic
Air Entry Point of Ceramic
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
Infrared SensorMirror
Optical Sensor
Fan
Sample
Generating SMCC with WP4C
Preparing SMCC samples1. Air dry soil2. Grind and/or sieve with 2 mm sieve (if
necessary)
Preparing samples3. 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
Preparing samples4. Add ascending amount of DI water to each
sample- 0, 1, 2, 4, 6, 8, 10, 14, 18, 22… drops of water
works well
5. Amount of water added depends on soil type and range of interest
Preparing samples6. Mix samples thoroughly
Preparing samples7. Cap samples and allow to equilibrate
overnight8. Remove lids and allow to dry for 30-60
minutes9. Replace lids and allow to re-equilibrate for
3-6 hours Done with preparation!
Measure water potential with the WP4C
Insert sample
Seal chamber
Wait ~5 min. andread the result
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
Construct SMCC
0 10 20 30 40 50 60 70 80 90 1000.000
0.020
0.040
0.060
0.080
0.100
0.120
moisture characteristic curvelinear scale
water potential (-MPa)
VWC
(m3/
m3)
0.1 1 10 100 10000.000
0.020
0.040
0.060
0.080
0.100
0.120
moisture characteristic curvesemi-log plot
water potential (-MPa)
VWC
(m3/
m3)
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
Chilled mirror absolute error of wet-end suction (WP4C and WP4)
Error of Original Chilled Mirror Sensor (WP4) +/- 100 kPa
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)
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)
Schwana loamy fine sand
Palouse silt loam
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
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. 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