ASTM D6938-06e1
(Test Method)Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth)
Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth)
SCOPE
1.1 This test method describes the procedures for measuring in-place density and moisture of soil and soil-aggregate by use of nuclear equipment. The density of the material may be measured by direct transmission, backscatter, or backscatter/air-gap ratio methods. Measurements for water (moisture) content are taken at the surface in backscatter mode regardless of the mode being used for density. It is the intent of this subcommittee that this standard replaces D2922 and D3017.
1.1.1 For limitations see Section on Interferences.
1.2 The total or wet density of soil and soil-aggregate is measured by the attenuation of gamma radiation where, in direct transmission, the source is placed at a known depth up to 300 mm (12 in.) and the detector (s) remains on the surface ( some gauges may reverse this orientation ); or in backscatter or backscatter/air-gap the source and detector (s) both remain on the surface.
1.2.1 The density of the test sample in mass per unit volume is calculated by comparing the detected rate of gamma radiation with previously established calibration data.
1.2.2 The dry density of the test sample is obtained by subtracting the water mass per unit volume from the test sample wet density ( Section 11 ). Most gauges display this value directly.
1.3 The gauge is calibrated to read the water mass per unit volume of soil or soil-aggregate. When divided by the density of water, and then multiplied by 100, the water mass per unit volume is equivalent to the volumetric water content. The water mass per unit volume is determined by the thermalizing or slowing of fast neutrons by hydrogen, a component of water. The neutron source and the thermal neutron detector are both located at the surface of the material being tested. The water content most prevalent in engineering and construction activities is known as the gravimetric water content, w, and is the ratio of the mass of the water in pore spaces to the total mass of solids, expressed as a percentage.
1.4 Two alternative procedures are provided.
1.4.1 Procedure A describes the direct transmission method in which the gamma source rod extends through the base of the gauge into a pre-formed hole to a desired depth. The direct transmission is the preferred method.
1.4.2 Procedure B involves the use of a dedicated backscatter gauge or the source rod in the backscatter position. This places the gamma and neutron sources and the detectors in the same plane.
1.5 SI Units The values stated in SI units are to be regarded as the standard. The values in inch-pound units (ft - lb units) are provided for information only.
1.6 All observed and calculated values shall conform to the guide for significant digits and rounding established in Practice D 6026.
1.6.1 The procedures used to specify how data are collected, recorded, and calculated in this standard are regarded as the industry standard. In addition, they are representative of the significant digits that should generally be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the users objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in analysis methods for engineering design.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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e1
Designation:D6938–06
Standard Test Methods for
In-Place Density and Water Content of Soil and Soil-
Aggregate by Nuclear Methods (Shallow Depth)
This standard is issued under the fixed designation D6938; 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 (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Editorial changes were made in November 2006.
1. Scope 1.4 Two alternative procedures are provided.
1.4.1 Procedure A describes the direct transmission method
1.1 Thistestmethoddescribestheproceduresformeasuring
inwhichthegammasourcerodextendsthroughthebaseofthe
in-place density and moisture of soil and soil-aggregate by use
gauge into a pre-formed hole to a desired depth. The direct
of nuclear equipment. The density of the material may be
transmission is the preferred method.
measured by direct transmission, backscatter, or backscatter/
1.4.2 Procedure B involves the use of a dedicated backscat-
air-gap ratio methods. Measurements for water (moisture)
ter gauge or the source rod in the backscatter position. This
content are taken at the surface in backscatter mode regardless
places the gamma and neutron sources and the detectors in the
of the mode being used for density. It is the intent of this
same plane.
subcommittee that this standard replaces D2922 and D3017.
1.5 SI Units—The values stated in SI units are to be
1.1.1 For limitations see Section 5 on Interferences.
regarded as the standard. The values in inch-pound units (ft –
1.2 The total or wet density of soil and soil-aggregate is
lb units) are provided for information only.
measured by the attenuation of gamma radiation where, in
1.6 All observed and calculated values shall conform to the
directtransmission,thesourceisplacedataknowndepthupto
guideforsignificantdigitsandroundingestablishedinPractice
300 mm (12 in.) and the detector (s) remains on the surface (
D6026.
somegaugesmayreversethisorientation);orinbackscatteror
1.6.1 Theproceduresusedtospecifyhowdataarecollected,
backscatter/air-gap the source and detector (s) both remain on
recorded, and calculated in this standard are regarded as the
the surface.
industry standard. In addition, they are representative of the
1.2.1 Thedensityofthetestsampleinmassperunitvolume
significant digits that should generally be retained. The proce-
is calculated by comparing the detected rate of gamma radia-
dures used do not consider material variation, purpose for
tion with previously established calibration data.
obtaining the data, special purpose studies, or any consider-
1.2.2 The dry density of the test sample is obtained by
ations for the user’s objectives; and it is common practice to
subtracting the water mass per unit volume from the test
increase or reduce significant digits of reported data to be
sample wet density ( Section 11 ). Most gauges display this
commensuratewiththeseconsiderations.Itisbeyondthescope
value directly.
of this standard to consider significant digits used in analysis
1.3 The gauge is calibrated to read the water mass per unit
methods for engineering design.
volume of soil or soil-aggregate. When divided by the density
1.7 This standard does not purport to address all of the
of water, and then multiplied by 100, the water mass per unit
safety concerns, if any, associated with its use. It is the
volume is equivalent to the volumetric water content. The
responsibility of the user of this standard to establish appro-
water mass per unit volume is determined by the thermalizing
priate safety and health practices and determine the applica-
orslowingoffastneutronsbyhydrogen,acomponentofwater.
bility of regulatory limitations prior to use.
The neutron source and the thermal neutron detector are both
located at the surface of the material being tested. The water
2. Referenced Documents
content most prevalent in engineering and construction activi-
2.1 ASTM Standards:
ties is known as the gravimetric water content, w, and is the
D653 Terminology Relating to Soil, Rock, and Contained
ratio of the mass of the water in pore spaces to the total mass
Fluids
of solids, expressed as a percentage.
1 2
ThistestmethodisunderthejurisdictionofASTMCommitteeD18onSoiland For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Rock and is the direct responsibility of Subcommittee D18.08 Special and contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Construction Control Tests. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Nov. 20, 2006. Published October 2006. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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D6938–06
D698 Test Methods for Laboratory Compaction Character- 3.2.8 Thermalization—The process of “slowing down” fast
istics of Soil Using Standard Effort (12,400 ft-lbf/ft (600 neutrons by collisions with light-weight atoms, such as hydro-
kN-m/m )) gen.
D1556 Test Method for Density and UnitWeight of Soil in
3.2.9 Water Content—The ratio of the mass of water con-
Place by the Sand-Cone Method
tained in the pore spaces of soil or soil-aggregate, to the solid
D1557 Test Methods for Laboratory Compaction Charac-
mass of particles in that material, expressed as a percentage
teristics of Soil Using Modified Effort (56,000 ft-lbf/ft
(this is sometimes referred to in some scientific fields as
(2,700 kN-m/m ))
gravimetric water content to differentiate it from volumetric
D2167 Test Method for Density and UnitWeight of Soil in
water cotent).
Place by the Rubber Balloon Method
3.2.10 Volumetric Water Content—the volume of water as a
D2487 Practice for Classification of Soils for Engineering
percent of the total volume of soil or rock material.
Purposes (Unified Soil Classification System)
3.2.11 Test Count, N—The measured output of a detector
D2488 Practice for Description and Identification of Soils
for a specific type of radiation for a given test.
(Visual-Manual Procedure)
3.2.12 Prepared Blocks—Blocks prepared of soil, solid
D2216 Test Methods for Laboratory Determination of Wa-
rock, concrete, and engineered materials, that have character-
ter (Moisture) Content of Soil and Rock by Mass
istics of various degrees of reproducible uniformity.
D2937 Test Method for Density of Soil in Place by the
Drive-Cylinder Method
4. Significance and Use
D3740 Practice for Minimum Requirements for Agencies
Engaged in theTesting and/or Inspection of Soil and Rock
4.1 The test method described is useful as a rapid, nonde-
as Used in Engineering Design and Construction structive technique for in-place measurements of wet density
D4253 TestMethodsforMaximumIndexDensityandUnit
and water content of soil and soil-aggregate and the determi-
Weight of Soils Using a Vibratory Table nation of dry density.
D4254 TestMethodsforMinimumIndexDensityandUnit
4.2 The test method is used for quality control and accep-
Weight of Soils and Calculation of Relative Density
tance testing of compacted soil and soil-aggregate mixtures as
D4643 Test Method for Determination ofWater (Moisture)
used in construction and also for research and development.
Content of Soil by the Microwave Oven Method
Thenon-destructivenatureallowsrepetitivemeasurementsata
D4718 Practice for Correction of Unit Weight and Water
single test location and statistical analysis of the results.
Content for Soils Containing Oversize Particles
4.3 Density—The fundamental assumptions inherent in the
D4944 Test Method for Field Determination of Water
methods are that Compton scattering is the dominant interac-
(Moisture) Content of Soil by the Calcium Carbide Gas
tion and that the material is homogeneous.
Pressure Tester
4.4 Water Content—The fundamental assumptions inherent
D4959 Test Method for Determination ofWater (Moisture)
inthetestmethodarethatthehydrogenionspresentinthesoil
Content of Soil By Direct Heating Method
or soil-aggregate are in the form of water as defined by the
D6026 Practice for Using Significant Digits in Geotechni-
water content derived from Test Methods D2216, and that the
cal Data
material is homogeneous. (See 5.2)
D7013 Guide for Nuclear Surface Moisture and Density
Gauge Calibration Facility Setup
NOTE 1—The quality of the result produced by this standard test
method is dependent on the competence of the personnel performing it,
3. Terminology
andthesuitabilityoftheequipmentandfacilitiesused.Agenciesthatmeet
the criteria of Practice D3740 are generally considered capable of
3.1 Definitions - See Terminology D653 for general defi-
competent and objective testing/sampling/inspection, and the like. Users
nitions.
of this standard are cautioned that compliance with Practice D3740 does
3.2 Definitions of Terms Specific to This Standard:
not in itself assure reliable results. Reliable results depend on many
3.2.1 Nuclear Gauge—A device containing one or more
factors; Practice D3740 provides a means of evaluating some of those
radioactive sources used to measure certain properties of soil
factors.
and soil-aggregates.
3.2.2 In-place Density—The total mass (solids plus water) 5. Interferences
per total volume of soil or soil-aggregates measured in place.
5.1 In-Place Density Interferences
3.2.3 Gamma (Radiation) Source—A sealed source of ra-
5.1.1 Measurements may be affected by the chemical com-
dioactive material that emits gamma radiation as it decays.
position of the material being tested.
3.2.4 Neutron (Radiation) Source—A sealed source of ra-
5.1.2 Measurements may be affected by non-homogeneous
dioactive material that emits neutron radiation as it decays.
soils and surface texture (see 10.2).
3.2.5 Compton Scattering—The interaction between a
5.1.3 MeasurementsintheBackscatterModeareinfluenced
gamma ray (photon) and an orbital electron where the gamma
more by the density and water content of the material in close
ray loses energy and rebounds in a different direction.
proximity to the surface.
3.2.6 Detector—A device to detect and measure radiation.
3.2.7 Source Rod—Ametal rod attached to a nuclear gauge 5.1.4 MeasurementsintheDirectTransmissionmodearean
in which a radioactive source or a detector is housed. The rod average of the density from the bottom of the probe in the
can be lowered to specified depths for testing. ground back up to the surface of the gauge.
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D6938–06
5.1.5 Oversizeparticlesorlargevoidsinthesource-detector 6.4 Drive Pin—A pin of slightly larger diameter than the
path may cause higher or lower density measurements. Where probe in the Direct Transmission Instrument used to prepare a
lack of uniformity in the soil due to layering, aggregate or hole in the test site for inserting the probe.
voids is suspected, the test site should be excavated and 6.4.1 Drive Pin Guide—A fixture that keeps the drive pin
visually examined to determine if the test material is represen- perpendicular to the test site. Generally part of the site
tative of the in-situ material in general and if an oversize preparation device.
correction is required in accordance with Practice D4718. 6.5 Hammer—Heavyenoughtodrivethepintotherequired
depth without undue distortion of the hole.
5.1.6 The measured volume is approximately 0.0028
3 3 3 3
6.6 Drive Pin Extractor—Atoolthatmaybeusedtoremove
m (0.10 ft ) for the Backscatter Mode and 0.0057 m (0.20 ft )
the drive pin in a vertical direction so that the pin will not
for the Direct Transmission Mode when the test depth is 150
distort the hole in the extraction process.
mm (6 in.). The actual measured volume is indeterminate and
6.7 Slide Hammer, with a drive pin attached, may also be
varies with the apparatus and the density of the material.
used both to prepare a hole in the material to be tested and to
5.1.7 Other radioactive sources must not be within9m(30
extract the pin without distortion to the hole.
ft.) of equipment in operation.
5.2 In-Place Water (Moisture) Content Interferences
7. Hazards
5.2.1 Thechemicalcompositionofthematerialbeingtested
7.1 These gauges utilize radioactive materials that may be
can affect the measurement and adjustments may be necessary
hazardous to the health of the users unless proper precautions
(see Section 10.6). Hydrogen in forms other than water and
are taken. Users of these gauges must become familiar with
carbon will cause measurements in excess of the true value.
applicable safety procedures and government regulations.
Somechemicalelementssuchasboron,chlorine,andcadmium
7.2 Effective user instructions, together with routine safety
will cause measurements lower than the true value.
procedures and knowledge of and compliance with Regulatory
5.2.2 The water content measured by this test method is not
Requirements, are a mandatory part of the operation and
necessarily the average water content within the volume of the
storage of these gauges.
sample involved in the measurement. Since this measurement
is by backscatter in all cases, the value is biased by the water
8. Calibration
content of the material closest to the surface. The volume of
8.1 Calibration of the gauge will be in accordance with
soil and soil-aggregate represented in the measurement is
Annex A1 and Annex A2.
indeterminate and will vary with the water content of the
8.2 For further reference on gauge calibration, see Guide
material. In general, the greater the water content of the
D7013, Standard Guide for Nuclear Surface Moisture and
material, the smaller the volume involved in the measurement.
Density Gauge Calibration Facility Setup.
Approximately 50% of the typical measurement results from
the water content of the upper 50 to 75 mm (2 to 3 in.). 9. Standardization
5.2.3 Other neutron sources must not be within 9 m (30 ft)
9.1 Nuclear moisture density gauges are subject to long-
of equipment in operation.
term aging of the radioactive sources, which may change the
relationship between count rates and the material density and
6. Apparatus
water content. To correct for this aging effect, gauges are
calibrated as a ratio of the measurement count rate to a count
6.1 Nuclear Density / Moisture Gauge—Whileexactdetails
ratemadeonareferencestandardortoanair-gapcount(forthe
of construction of the apparatus may vary, the system shall
backscatter/air-gap ratio method).
consist of:
9.2 Standardization of the gauge shall be performed at the
6.1.1 Gamma Source—A sealed source of high-energy
start of each day’s use, and a record of these data should be
gamma radiation such as cesium or radium.
retained for the amount of time required to ensure compliance
6.1.2 Gamma Detector—
...
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