ASTM D6836-02(2008)e2
(Test Method)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
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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Designation: D6836 − 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.
ε NOTE—Mercury warning and other minor changes were editorially added in November 2008.
ε NOTE—“1n” was editorially corrected to “ln” in Eq 12 in March 2009.
1. Scope characteristic curve for intermediate water contents and suc-
tions(100to1000kPa),andMethodDisusedtodefinethesoil
1.1 Thesetestmethodscoverthedeterminationofsoilwater
water characteristic curves at low water contents and higher
characteristic curves (SWCCs) for desorption (drying). SW-
suctions (> 1000 kPa).
CCs describe the relationship between suction and volumetric
water content, gravimetric water content, or degree of water
1.3 All observed and calculated values shall conform to the
saturation. SWCCs are also referred to as soil water retention
guideforsignificantdigitsandroundingestablishedinPractice
curves, soil water release curves, or capillary pressure curves.
D6026. The procedures in Practice D6026 that are used to
specify how data are collected, recorded, and calculated are
1.2 This standard describes five methods (A-E) for deter-
regarded as the industry standard. In addition, they are repre-
mining the soil water characteristic curve. MethodA(hanging
sentative of the significant digits that should generally be
column) is suitable for making determinations for suctions in
retained. The procedures do not consider material variation,
the range of 0 to 80 kPa. Method B (pressure chamber with
purpose for obtaining the data, special purpose studies, or any
volumetric measurement) and Method C (pressure chamber
considerations for the objectives of the user. Increasing or
with gravimetric measurement) are suitable for suctions in the
reducing the significant digits of reported data to be commen-
range of 0 to 1500 kPa. Method D (chilled mirror hygrometer)
surate with these considerations is common practice. Consid-
is suitable for making determinations for suctions in the range
eration of the significant digits to be used in analysis methods
of 500 kPa to 100 MPa. Method E (centrifuge method) is
for engineering design is beyond the scope of this standard.
suitable for making determinations in the range 0 to 120 kPa.
MethodAtypically is used for coarse soils with little fines that
1.4 The values stated in SI units are to be regarded as
drainreadily.MethodsBandCtypicallyareusedforfinersoils
standard. No other units of measurement are included in this
which retain water more tightly. Method D is used when
standard.
suctions near saturation are not required and commonly is
1.5 Warning—Mercury has been designated by EPA and
employed to define the dry end of the soil water characteristic
many state agencies as a hazardous material that can cause
curve(thatis,watercontentscorrespondingtosuctions>1000
central nervous system, kidney, and liver damage. Mercury, or
kPa). Method E is typically used for coarser soils where an
its vapor, may be hazardous to health and corrosive to
appreciable amount of water can be extracted with suctions up
materials.Cautionshouldbetakenwhenhandlingmercuryand
to 120 kPa. The methods may be combined to provide a
mercury-containing products. See the applicable product Ma-
detailed description of the soil water characteristic curve. In
terial Safety Data Sheet (MSDS) for details and EPA’s website
this application, MethodAor E is used to define the soil water
(http://www.epa.gov/mercury/faq.htm)foradditionalinforma-
characteristic curve at lower suctions (0 to 80 kPa for A, 0 to
tion. Users should be aware that selling mercury or mercury-
120kPaforE)nearsaturationandtoaccuratelyidentifytheair
containingproducts,orboth,inyourstatemaybeprohibitedby
entry suction, Method B or C is used to define the soil water
state law.
1.6 This standard does not purport to address all of the
ThesetestmethodsareunderthejurisdictionofASTMCommitteeD18onSoil
and Rock and are the direct responsibility of Subcommittee D18.04 on Hydrologic
safety concerns, if any, associated with its use. It is the
Properties and Hydraulic Barriers.
responsibility of the user of this standard to establish appro-
Current edition approved Sept. 1, 2008. Published November 2008. Originally
priate safety and health practices and determine the applica-
approved in 2002. Last previous edition approved in 2002 as D6836–02. DOI:
10.1520/D6836-02R08E02. bility of regulatory limitations prior to use.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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D6836 − 02 (2008)
2. Referenced Documents be applied to a solution identical in composition to the soil
2 water to maintain equilibrium through a porous membrane
2.1 ASTM Standards:
existingbetweenthesolutionandthesoilwater.Matricsuction
D421Practice for Dry Preparation of Soil Samples for
is also referred to as matric potential, capillary suction, and
Particle-Size Analysis and Determination of Soil Con-
capillary potential. By definition, matric suction is the differ-
stants
ence between the pore gas pressure, u , and the pore water
g
D425Test Method for Centrifuge Moisture Equivalent of
pressure, u , that is, ψ = u − u . In most cases the pore gas is
w g w
Soils
air.
D653Terminology Relating to Soil, Rock, and Contained
3.2.6 osmotic suction, ψ —the negative gage pressure de-
Fluids
o
rived from the measurement of the vapor pressure of water in
D698Test Methods for Laboratory Compaction Character-
equilibrium with a solution identical in composition with the
istics of Soil Using Standard Effort (12 400 ft-lbf/ft (600
soilwater,relativetothevaporpressureofwaterinequilibrium
kN-m/m ))
with free pure water. Osmotic suction is also referred to as
D854Test Methods for Specific Gravity of Soil Solids by
osmotic potential.
Water Pycnometer
D2216TestMethodsforLaboratoryDeterminationofWater
3.2.7 porous membrane—a porous polymeric membrane
(Moisture) Content of Soil and Rock by Mass
that can transmit water and has a air entry pressure exceeding
D3740Practice for Minimum Requirements for Agencies
the highest matric suction to be applied during a test.
Engaged in Testing and/or Inspection of Soil and Rock as
3.2.8 porous plate—aplatemadeofmetal,ceramic,orother
Used in Engineering Design and Construction
porous material that can transmit water and has an air entry
D4753Guide for Evaluating, Selecting, and Specifying Bal-
pressure exceeding the highest matric suction to be applied
ances and Standard Masses for Use in Soil, Rock, and
during a test.
Construction Materials Testing
3.2.9 pressure chamber—a vessel used to apply a gas
D5084Test Methods for Measurement of Hydraulic Con-
pressure on the specimen and the soil pores to induce a
ductivity of Saturated Porous Materials Using a Flexible
specified matric suction.
Wall Permeameter
D6026Practice for Using Significant Digits in Geotechnical 3.2.10 saturated water content—volumetric or gravimetric
Data water content when the specimen is saturated.
2.2 API Standard:
3.2.11 soil water characteristic curve—a graph of suction
API RP 40Recommended Practice for Core-Analysis Pro-
(matric or total) versus water content (gravimetric or volumet-
cedure
ric) or saturation. The soil water characteristic curve is also
referred to as the soil water retention curve, the soil water
3. Terminology
release curve, and the capillary pressure curve.
3.1 For common definitions of other terms in this standard
3.2.12 total suction, ψ—the negative gage pressure derived
t
see Terminology D653.
from the measurement of the vapor pressure of water in
equilibrium with water in the soil pores, relative to the vapor
3.2 Definitions of Terms Specific to This Standard:
pressure of water in equilibrium with free pure water. Total
3.2.1 air entry pressure—the air pressure required to intro-
suction is the sum of matric and osmotic suction, ψ = ψ + ψ .
duce air into and through the pores of a saturated porous plate.
t o
Total suction is also referred to as total potential.
3.2.2 air entry suction, ψ —the suction required to intro-
a
3.2.13 volumetric water content, θ—the ratio of the volume
duce air into and through the pores of a saturated porous
ofwatercontainedintheporespacesofsoilorrocktothetotal
material.
volume of soil and rock.
3.2.3 axis translation—the principle stating that a matric
3.2.14 water activity, a —the ratio of vapor pressure of
suction ψ can be applied to a soil by controlling the pore gas
w
water in the soil gas to the saturated vapor pressure at the
pressure, u , and the pore water pressure, u , so that the
g w
existing soil temperature. Water activity is also referred to as
difference between the pore gas pressure and pore water
the relative humidity.
pressure equals the desired matric suction, that is, ψ = u − u .
g w
3.2.4 gravimetric water content, w—theratioofthe mass of
4. Summary of Methods
water contained in the pore spaces of soil or rock to the mass
of solid particles. 4.1 Methods A-C—MethodsA-C yield soil water character-
istic curves in terms of matric suction. Various suctions are
3.2.5 matric suction,ψ—thenegativegagepressure,relative
applied to the soil and the corresponding water contents are
to an external gas pressure acting on the soil water, that must
measured. Two different procedures are used to apply the
suction.InMethodA,thematricsuctionisappliedbyreducing
theporewaterpressurewhilemaintainingtheporegaspressure
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
at the atmospheric condition. In Methods B and C, the pore
Standards volume information, refer to the standard’s Document Summary page on
water pressure is maintained at atmospheric pressure, and the
the ASTM website.
pore gas pressure is raised to apply the suction via the axis
Available from American Petroleum Institute (API), 1220 L. St., NW,
Washington, DC 20005-4070, http://www.api.org. translation principle.
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D6836 − 02 (2008)
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 unsatu-
aforementioned procedures. Application of the matric suction rated 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.
NOTE1—Thequalityoftheresultproducedbythisstandarddependson
4.2 Method D—Method D yields a soil water characteristic
the competence of the personnel performing the test and the suitability of
the equipment and facilities used. Agencies that meet the criteria of
curveintermsoftotalsuction.IncontrasttoMethodsA-C,the
Practice D3740 are generally considered capable of competent and
water content of the soil is controlled in Method D, and the
objective testing, sampling, inspection, etc. Users of this standard are
corresponding suctions are measured. Two different ap-
cautioned that compliance with Practice D3740 does not in itself ensure
proaches are commonly used. In one approach, a set of
reliable results. Reliable results depend on many factors. Practice D3740
specimens are prepared that are essentially identical, but have
provides a means of evaluating some of these factors.
different water contents. Water contents are selected that span
6. Apparatus
the range of water contents that will be used to define the soil
6.1 Hanging Column Apparatus (Method A)—A hanging
water characteristic curve. In the other approach, a single
column apparatus consists of three parts: a specimen chamber,
specimen is used. The specimen is tested, dried to a lower
an outflow measurement tube, and a suction supply (Fig. 1).
water content, and then tested again. This process is repeated
The specimen chamber consists of a glass or rigid plastic
until suctions have been measured at all of the desired water
funnelcontainingaporousplatethatislargeenoughtocontain
contents.
thespecimenbeingtested.Suchfunnelsarecommonlyreferred
4.2.1 In Method D, the water activity of the pore water is
to as “Buchner” funnels. A photograph of a funnel used for a
measured using a chilled mirror hygrometer (also known as a
hanging column apparatus is shown in Fig. 2. Water expelled
chilled mirror psychrometer) and then the total suction is
from the specimen during the test is measuring using a
computedusingtheKelvinequation.Inmanycases,MethodD
capillary tube connected to the outflow end of the funnel. The
is used to determine only that portion of the soil water
other end of this capillary tube is connected to suction supply
characteristiccurvecorrespondingtohighersuctions(typically
consisting of two reservoirs. The relative elevation of the two
>1000kPa)andlowerwatercontents.Undertheseconditions,
res
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