Standard Test Methods for Determination of the Soil Water Chararcteristic Curve for Desorption Using a Hanging Column, Pressure Extractor, Chilled Mirror Hygrometer, and/or Centrifuge

SIGNIFICANCE AND USE
The soil water characteristic curve (SWCC) is fundamental to hydrological characterization of unsaturated soils and is required for most analyses of water movement in unsaturated soils. The SWCC is also used in characterizing the shear strength and compressibility of unsaturated soils. The unsaturated hydraulic conductivity of soil is often estimated using properties of the SWCC and the saturated hydraulic conductivity.
This method applies only to soils containing two pore fluids: a gas and a liquid. The liquid is usually water and the gas is usually air. Other liquids may also be used, but caution must be exercised if the liquid being used causes excessive shrinkage or swelling of the soil matrix.
A full investigation has not been conducted regarding the correlation between soil water characteristic curves obtained using this method and soil water characteristics curves of in-place materials. Thus, results obtained from this method should be applied to field situations with caution and by qualified personnel.
Note 1—The quality of the result produced by this standard depends on the competence of the personnel performing the test and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D 3740 are generally considered capable of competent and objective testing, sampling, inspection, etc. Users of this standard are cautioned that compliance with Practice D 3740 does not in itself ensure reliable results. Reliable results depend on many factors. Practice D 3740 provides a means of evaluating some of these factors.
SCOPE
1.1 These test methods cover the determination of soil water characteristic curves (SWCCs) for desorption (drying). SWCCs describe the relationship between suction and volumetric water content, gravimetric water content, or degree of water saturation. SWCCs are also referred to as soil water retention curves, soil water release curves, or capillary pressure curves.
1.2 This standard describes five methods (A-E) for determining the soil water characteristic curve. Method A (hanging column) is suitable for making determinations for suctions in the range of 0 to 80 kPa. Method B (pressure chamber with volumetric measurement) and Method C (pressure chamber with gravimetric measurement) are suitable for suctions in the range of 0 to 1500 kPa. Method D (chilled mirror hygrometer) is suitable for making determinations for suctions in the range of 500 kPa to 100 MPa. Method E (centrifuge method) is suitable for making determinations in the range 0 to 120 kPa. Method A typically is used for coarse soils with little fines that drain readily. Methods B and C typically are used for finer soils which retain water more tightly. Method D is used when suctions near saturation are not required and commonly is employed to define the dry end of the soil water characteristic curve (that is, water contents corresponding to suctions > 1000 kPa). Method E is typically used for coarser soils where an appreciable amount of water can be extracted with suctions up to 120 kPa. The methods may be combined to provide a detailed description of the soil water characteristic curve. In this application, Method A or E is used to define the soil water characteristic curve at lower suctions (0 to 80 kPa for A, 0 to 120 kPa for E) near saturation and to accurately identify the air entry suction, Method B or C is used to define the soil water characteristic curve for intermediate water contents and suctions (100 to 1000 kPa), and Method D is used to define the soil water characteristic curves at low water contents and higher suctions (> 1000 kPa).
1.3 All observed and calculated values shall conform to the guide for significant digits and rounding established in Practice D 6026. The procedures in Practice D 6026 that are used to specify how data are collected, recorded, and calculated are regarded as the industry standard. In addition, they are representative of...

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ASTM D6836-02(2008)e1 - Standard Test Methods for Determination of the Soil Water Chararcteristic Curve for Desorption Using a Hanging Column, Pressure Extractor, Chilled Mirror Hygrometer, and/or Centrifuge
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
´1
Designation: D 6836 – 02 (Reapproved 2008)
Standard Test Methods for
Determination of the Soil Water Characteristic Curve for
Desorption Using Hanging Column, Pressure Extractor,
Chilled Mirror Hygrometer, or Centrifuge
This standard is issued under the fixed designation D6836; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—Mercury warning and other minor changes were editorially added in November 2008.
1. Scope characteristic curve at lower suctions (0 to 80 kPa for A, 0 to
120kPaforE)nearsaturationandtoaccuratelyidentifytheair
1.1 Thesetestmethodscoverthedeterminationofsoilwater
entry suction, Method B or C is used to define the soil water
characteristic curves (SWCCs) for desorption (drying).
characteristic curve for intermediate water contents and suc-
SWCCs describe the relationship between suction and volu-
tions(100to1000kPa),andMethodDisusedtodefinethesoil
metric water content, gravimetric water content, or degree of
water characteristic curves at low water contents and higher
water saturation. SWCCs are also referred to as soil water
suctions (> 1000 kPa).
retentioncurves,soilwaterreleasecurves,orcapillarypressure
1.3 All observed and calculated values shall conform to the
curves.
guideforsignificantdigitsandroundingestablishedinPractice
1.2 This standard describes five methods (A-E) for deter-
D6026. The procedures in Practice D6026 that are used to
mining the soil water characteristic curve. MethodA(hanging
specify how data are collected, recorded, and calculated are
column) is suitable for making determinations for suctions in
regarded as the industry standard. In addition, they are repre-
the range of 0 to 80 kPa. Method B (pressure chamber with
sentative of the significant digits that should generally be
volumetric measurement) and Method C (pressure chamber
retained. The procedures do not consider material variation,
with gravimetric measurement) are suitable for suctions in the
purpose for obtaining the data, special purpose studies, or any
range of 0 to 1500 kPa. Method D (chilled mirror hygrometer)
considerations for the objectives of the user. Increasing or
is suitable for making determinations for suctions in the range
reducing the significant digits of reported data to be commen-
of 500 kPa to 100 MPa. Method E (centrifuge method) is
surate with these considerations is common practice. Consid-
suitable for making determinations in the range 0 to 120 kPa.
eration of the significant digits to be used in analysis methods
MethodAtypically is used for coarse soils with little fines that
for engineering design is beyond the scope of this standard.
drainreadily.MethodsBandCtypicallyareusedforfinersoils
1.4 The values stated in SI units are to be regarded as
which retain water more tightly. Method D is used when
standard. No other units of measurement are included in this
suctions near saturation are not required and commonly is
standard.
employed to define the dry end of the soil water characteristic
1.5 Warning—Mercury has been designated by EPA and
curve(thatis,watercontentscorrespondingtosuctions>1000
many state agencies as a hazardous material that can cause
kPa). Method E is typically used for coarser soils where an
central nervous system, kidney, and liver damage. Mercury, or
appreciable amount of water can be extracted with suctions up
its vapor, may be hazardous to health and corrosive to
to 120 kPa. The methods may be combined to provide a
materials.Cautionshouldbetakenwhenhandlingmercuryand
detailed description of the soil water characteristic curve. In
mercury-containing products. See the applicable product Ma-
this application, MethodAor E is used to define the soil water
terial Safety Data Sheet (MSDS) for details and EPA’s website
(http://www.epa.gov/mercury/faq.htm) for additional informa-
ThesetestmethodsareunderthejurisdictionofASTMCommitteeD18onSoil
tion. Users should be aware that selling mercury or mercury-
and Rock and are the direct responsibility of Subcommittee D18.04 on Hydrologic
containingproducts,orboth,inyourstatemaybeprohibitedby
Properties and Hydraulic Barriers.
state law.
Current edition approved Sept. 1, 2008. Published November 2008. Originally
approved in 2002. Last previous edition approved in 2002 as D6836–02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
´1
D 6836 – 02 (2008)
1.6 This standard does not purport to address all of the 3.2.4 gravimetric water content, w—theratioofthemassof
safety concerns, if any, associated with its use. It is the water contained in the pore spaces of soil or rock to the mass
responsibility of the user of this standard to establish appro- of solid particles.
priate safety and health practices and determine the applica- 3.2.5 matric suction, c—the negative gage pressure, rela-
bility of regulatory limitations prior to use.
tive to an external gas pressure acting on the soil water, that
must be applied to a solution identical in composition to the
2. Referenced Documents
soilwatertomaintainequilibriumthroughaporousmembrane
existingbetweenthesolutionandthesoilwater.Matricsuction
2.1 ASTM Standards:
is also referred to as matric potential, capillary suction, and
D421 Practice for Dry Preparation of Soil Samples for
capillary potential. By definition, matric suction is the differ-
Particle-Size Analysis and Determination of Soil Con-
ence between the pore gas pressure, u , and the pore water
stants g
pressure, u , that is, c = u − u . In most cases the pore gas is
D425 Test Method for Centrifuge Moisture Equivalent of w g w
air.
Soils
3.2.6 osmotic suction, c —the negative gage pressure de-
D653 Terminology Relating to Soil, Rock, and Contained
o
rived from the measurement of the vapor pressure of water in
Fluids
equilibrium with a solution identical in composition with the
D698 Test Methods for Laboratory Compaction Character-
soilwater,relativetothevaporpressureofwaterinequilibrium
istics of Soil Using Standard Effort (12 400 ft-lbf/ft (600
with free pure water. Osmotic suction is also referred to as
kN-m/m ))
osmotic potential.
D854 Test Methods for Specific Gravity of Soil Solids by
3.2.7 porous membrane—a porous polymeric membrane
Water Pycnometer
that can transmit water and has a air entry pressure exceeding
D2216 Test Methods for Laboratory Determination of Wa-
the highest matric suction to be applied during a test.
ter (Moisture) Content of Soil and Rock by Mass
D3740 Practice for Minimum Requirements for Agencies 3.2.8 porous plate—aplatemadeofmetal,ceramic,orother
porous material that can transmit water and has an air entry
Engaged in Testing and/or Inspection of Soil and Rock as
Used in Engineering Design and Construction pressure exceeding the highest matric suction to be applied
during a test.
D4753 Guide for Evaluating, Selecting, and Specifying
Balances and Standard Masses for Use in Soil, Rock, and 3.2.9 pressure chamber—a vessel used to apply a gas
Construction Materials Testing pressure on the specimen and the soil pores to induce a
D5084 Test Methods for Measurement of Hydraulic Con- specified matric suction.
ductivity of Saturated Porous Materials Using a Flexible 3.2.10 saturated water content—volumetric or gravimetric
Wall Permeameter
water content when the specimen is saturated.
D6026 Practice for Using Significant Digits in Geotechni-
3.2.11 soil water characteristic curve—a graph of suction
cal Data
(matric or total) versus water content (gravimetric or volumet-
2.2 API Standard:
ric) or saturation. The soil water characteristic curve is also
API RP 40 Recommended Practice for Core-Analysis Pro-
referred to as the soil water retention curve, the soil water
cedure
release curve, and the capillary pressure curve.
3.2.12 total suction, c—the negative gage pressure derived
t
3. Terminology
from the measurement of the vapor pressure of water in
3.1 For common definitions of other terms in this standard equilibrium with water in the soil pores, relative to the vapor
pressure of water in equilibrium with free pure water. Total
see Terminology D653.
3.2 Definitions of Terms Specific to This Standard: suction is the sum of matric and osmotic suction, c = c + c .
t o
Total suction is also referred to as total potential.
3.2.1 air entry pressure—the air pressure required to intro-
duce air into and through the pores of a saturated porous plate. 3.2.13 volumetric water content, u—the ratio of the volume
3.2.2 air entry suction, c —the suction required to intro- ofwatercontainedintheporespacesofsoilorrocktothetotal
a
duce air into and through the pores of a saturated porous volume of soil and rock.
material. 3.2.14 water activity, a —the ratio of vapor pressure of
w
3.2.3 axis translation—the principle stating that a matric
water in the soil gas to the saturated vapor pressure at the
suction c can be applied to a soil by controlling the pore gas existing soil temperature. Water activity is also referred to as
pressure, u , and the pore water pressure, u , so that the
the relative humidity.
g w
difference between the pore gas pressure and pore water
pressure equals the desired matric suction, that is, c = u − u .
4. Summary of Methods
g w
4.1 Methods A-C—MethodsA-C yield soil water character-
istic curves in terms of matric suction. Various suctions are
applied to the soil and the corresponding water contents are
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
measured. Two different procedures are used to apply the
Standards volume information, refer to the standard’s Document Summary page on
suction.InMethodA,thematricsuctionisappliedbyreducing
the ASTM website.
theporewaterpressurewhilemaintainingtheporegaspressure
Available from American Petroleum Institute (API), 1220 L. St., NW, Wash-
ington, DC 20005-4070, http://www.api.org. at the atmospheric condition. In Methods B and C, the pore
´1
D 6836 – 02 (2008)
water pressure is maintained at atmospheric pressure, and the 5. Significance and Use
pore gas pressure is raised to apply the suction via the axis
5.1 The soil water characteristic curve (SWCC) is funda-
translation principle.
mentaltohydrologicalcharacterizationofunsaturatedsoilsand
4.1.1 For all three methods, saturated soil specimens are
isrequiredformostanalysesofwatermovementinunsaturated
placed in contact with a water saturated porous plate or
soils. The SWCC is also used in characterizing the shear
membrane. The matric suction is applied by one of the two
strength and compressibility of unsaturated soils. The unsatur-
aforementioned procedures. Application of the matric suction
ated hydraulic conductivity of soil is often estimated using
causes water to flow from the specimen until the equilibrium
properties of the SWCC and the saturated hydraulic conduc-
water content corresponding to the applied suction is reached.
tivity.
Equilibriumisestablishedbymonitoringwhenwaterceasesto
5.2 This method applies only to soils containing two pore
flow from the specimen. Several equilibria are established at
fluids: a gas and a liquid. The liquid is usually water and the
successive matric suctions to construct a soil water character-
gas is usually air. Other liquids may also be used, but caution
istic curve.
must be exercised if the liquid being used causes excessive
4.1.2 The water content corresponding to the applied suc-
shrinkage or swelling of the soil matrix.
tion is determined in one of two ways. For MethodsAand B,
5.3 A full investigation has not been conducted regarding
the volume of water expelled is measured using a capillary
the correlation between soil water characteristic curves ob-
tube.Thewatercontentisthendeterminedbasedontheknown
tained using this method and soil water characteristics curves
initial water content of the specimen and the volume of water
of in-place materials. Thus, results obtained from this method
expelled. For Method C, the water content is measured
should be applied to field situations with caution and by
gravimetrically by weighing the specimen after removal from
qualified personnel.
the apparatus.
NOTE 1—Thequalityoftheresultproducedbythisstandarddependson
4.2 Method D—Method D yields a soil water characteristic
the competence of the personnel performing the test and the suitability of
curveintermsoftotalsuction.IncontrasttoMethodsA-C,the
the equipment and facilities used. Agencies that meet the criteria of
water content of the soil is controlled in Method D, and the
Practice D3740 are generally considered capable of competent and
corresponding suctions are measured. Two different ap-
objective testing, sampling, inspection, etc. Users of this standard are
proaches are commonly used. In one approach, a set of
cautioned that compliance with Practice D3740 does not in itself ensure
specimens are prepared that are essentially identical, but have
reliable results. Reliable results depend on many factors. Practice D3740
provides a means of evaluating some of these factors.
different water contents. Water contents are selected that span
the range of water contents that will be used to define the soil
6. Apparatus
water characteristic curve. In the other approach, a single
specimen is used. The specimen is tested, dried to a lower
6.1 Hanging Column Apparatus (Method A)—A hanging
water content, and then tested again. This process is repeated
column apparatus consists of three parts: a specimen chamber,
until suctions have been measured at all of the desired water
an outflow measurement tube, and a suction supply (Fig. 1).
contents.
The specimen chamber consists of a glass or rigid plastic
4.2.1 In Method D, the water activity of the pore water is funnelcontainingaporousplatethatislargeenoughtocontain
measured using a chilled mirror hygrometer (also known as a thespecimenbeingtested.Suchfunnelsarecommonlyreferred
chilled mirror psychrometer) and then the total suction is to as “Buchner” funnels. A photograph of a funnel used for a
computedusingtheKelvinequation.Inmanycases,MethodD hanging column apparatus is shown in Fig. 2. Water expelled
is used to determine only that portion of the soil water from the specimen during the test is measuring using a
characteristiccurvecorrespondingtohighersuctions(typically capillary tube connected to the outflow end of the funnel. The
>1000kPa)andlowerwatercontents.Undertheseconditions, other end of this capillary tube is connected to suction supply
the osmotic
...

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