1、Designation: D6938 15Standard Test Methods forIn-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth)1This standard is issued under the fixed designation D6938; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、 case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method describes the procedures for measuringin-place density and moisture o
3、f soil and soil-aggregate by useof nuclear equipment. The density of the material may bemeasured by direct transmission, backscatter, or backscatter/air-gap ratio methods. Measurements for water (moisture)content are taken at the surface in backscatter mode regardlessof the mode being used for densi
4、ty. It is the intent of thissubcommittee that this standard replace D2922 and D3017.1.1.1 For limitations see Section 5 on Interferences.1.2 The total or wet density of soil and soil-aggregate ismeasured by the attenuation of gamma radiation where, indirect transmission, the source is placed at a kn
5、own depth up to300 mm (12 in.) and the detector(s) remains on the surface(some gauges may reverse this orientation); or in backscatter orbackscatter/air-gap the source and detector(s) both remain onthe surface.1.2.1 The density of the test sample in mass per unit volumeis calculated by comparing the
6、 detected rate of gamma radia-tion with previously established calibration data.1.2.2 The dry density of the test sample is obtained bysubtracting the water mass per unit volume from the testsample wet density (Section 11). Most gauges display thisvalue directly.1.3 The gauge is calibrated to read t
7、he water mass per unitvolume of soil or soil-aggregate. When divided by the densityof water and then multiplied by 100, the water mass per unitvolume is equivalent to the volumetric water content. Thewater mass per unit volume is determined by the thermalizingor slowing of fast neutrons by hydrogen,
8、 a component of water.The neutron source and the thermal neutron detector are bothlocated at the surface of the material being tested. The watercontent most prevalent in engineering and construction activi-ties is known as the gravimetric water content, w, and is theratio of the mass of the water in
9、 pore spaces to the total massof solids, expressed as a percentage.1.4 Two alternative procedures are provided.1.4.1 Procedure A describes the direct transmission methodin which the probe extends through the base of the gauge intoa pre-formed hole to a desired depth. The direct transmission isthe pr
10、eferred method.1.4.2 Procedure B involves the use of a dedicated backscat-ter gauge or the probe in the backscatter position. This placesthe gamma and neutron sources and the detectors in the sameplane.1.4.3 Mark the test area to allow the placement of the gaugeover the test site and to align the pr
11、obe to the hole.1.5 SI UnitsThe values stated in SI units are to beregarded 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 theguide for significant digits and rounding established in PracticeD602
12、6.1.6.1 The procedures used to specify how data are collected,recorded, and calculated in this standard are regarded as theindustry standard. In addition, they are representative of thesignificant digits that should generally be retained. The proce-dures used do not consider material variation, purp
13、ose forobtaining the data, special purpose studies, or any consider-ations for the users objectives; and it is common practice toincrease or reduce significant digits of reported data to becommensurate with these considerations. It is beyond the scopeof this standard to consider significant digits u
14、sed in analysismethods for engineering design.1.7 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is the1This test method is under the jurisdiction ofASTM Committee D18 on Soil andRock and is the direct responsibility of Subcommittee D18.08 on
15、 Special andConstruction Control Tests.Current edition approved Aug. 1, 2015. Published August 2015. Originallyapproved in 2006. Last previous edition approved in 2010 as D693810. DOI:10.1520/D6938-15.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100
16、Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1responsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D653 Te
17、rminology Relating to Soil, Rock, and ContainedFluidsD698 Test Methods for Laboratory Compaction Character-istics of Soil Using Standard Effort (12 400 ft-lbf/ft3(600kN-m/m3)D1556 Test Method for Density and Unit Weight of Soil inPlace by Sand-Cone MethodD1557 Test Methods for Laboratory Compaction
18、Character-istics of Soil Using Modified Effort (56,000 ft-lbf/ft3(2,700 kN-m/m3)D2167 Test Method for Density and Unit Weight of Soil inPlace by the Rubber Balloon MethodD2487 Practice for Classification of Soils for EngineeringPurposes (Unified Soil Classification System)D2488 Practice for Descript
19、ion and Identification of Soils(Visual-Manual Procedure)D2216 Test Methods for Laboratory Determination of Water(Moisture) Content of Soil and Rock by MassD2937 Test Method for Density of Soil in Place by theDrive-Cylinder MethodD3740 Practice for Minimum Requirements for AgenciesEngaged in Testing
20、and/or Inspection of Soil and Rock asUsed in Engineering Design and ConstructionD4253 Test Methods for Maximum Index Density and UnitWeight of Soils Using a Vibratory TableD4254 Test Methods for Minimum Index Density and UnitWeight of Soils and Calculation of Relative DensityD4643 Test Method for De
21、termination of Water (Moisture)Content of Soil by Microwave Oven HeatingD4718 Practice for Correction of Unit Weight and WaterContent for Soils Containing Oversize ParticlesD4944 Test Method for Field Determination of Water (Mois-ture) Content of Soil by the Calcium Carbide Gas PressureTesterD4959 T
22、est Method for Determination of Water (Moisture)Content of Soil By Direct HeatingD6026 Practice for Using Significant Digits in GeotechnicalDataD7013 Guide for Nuclear Surface Moisture and DensityGauge Calibration Facility SetupD7759 Guide for Nuclear Surface Moisture and DensityGauge Calibration3.
23、Terminology3.1 Definitions: See Terminology D653 for general defini-tions.3.2 Definitions of Terms Specific to This Standard:3.2.1 nuclear gaugea device containing one or moreradioactive sources used to measure certain properties of soiland soil-aggregates.3.2.2 wet densitysame as bulk density (as d
24、efined inTerminology D653); the total mass (solids plus water) per totalvolume of soil or soil-aggregate.3.2.3 dry densitysame as density of dry soil or rock (asdefined in Terminology D653); the mass of solid particles perthe total volume of soil or soil-aggregate.3.2.4 gamma (radiation) sourcea sea
25、led source of radio-active material that emits gamma radiation as it decays.3.2.5 neutron (radiation) sourcea sealed source of radio-active material that emits neutron radiation as it decays.3.2.6 Compton scatteringthe interaction between agamma ray (photon) and an orbital electron where the gammara
26、y loses energy and rebounds in a different direction.3.2.7 detectora device to detect and measure radiation.3.2.8 thermalizationthe process of “slowing down” fastneutrons by collisions with light-weight atoms, such as hydro-gen.3.2.9 water contentthe ratio of the mass of water con-tained in the pore
27、 spaces of soil or soil-aggregate, to the solidmass of particles in that material, expressed as a percentage(this is sometimes referred to in some scientific fields asgravimetric water content to differentiate it from volumetricwater content).3.2.10 volumetric water contentthe volume of water as ape
28、rcent of the total volume of soil or rock material.3.2.11 test count, nthe measured output of a detector for aspecific type of radiation for a given test.3.2.12 prepared blocksblocks prepared of soil, solid rock,concrete, and engineered materials, that have characteristics ofvarious degrees of repro
29、ducible uniformity.4. Significance and Use4.1 The test method described is useful as a rapid, nonde-structive technique for in-place measurements of wet densityand water content of soil and soil-aggregate and the determi-nation of dry density.4.2 The test method is used for quality control and accep
30、-tance testing of compacted soil and soil-aggregate mixtures asused in construction and also for research and development.The nondestructive nature allows repetitive measurements at asingle test location and statistical analysis of the results.4.3 DensityThe fundamental assumptions inherent in theme
31、thods are that Compton scattering is the dominant interac-tion and that the material is homogeneous.4.4 Water ContentThe fundamental assumptions inherentin the test method are that the hydrogen ions present in the soilor soil-aggregate are in the form of water as defined by thewater content derived
32、from Test Methods D2216, and that thematerial is homogeneous. (See 5.2)NOTE 1The quality of the result produced by this standard test methodis dependent on the competence of the personnel performing it, and the2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Custo
33、mer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.D6938 152suitability of the equipment and facilities used. Agencies that meet thecriteria of Practice D3740 are generally considered capable of compete
34、ntand objective testing/sampling/inspection, and the like. Users of thisstandard are cautioned that compliance with Practice D3740 does not initself ensure reliable results. Reliable results depend on many factors;Practice D3740 provides a means of evaluating some of those factors.5. Interferences5.
35、1 In-Place Density Interferences5.1.1 Measurements may be affected by the chemical com-position of the material being tested.5.1.2 Measurements may be affected by non-homogeneoussoils and surface texture (see 10.2). Excessive voids in theprepared test surface beneath the gauge can cause densitymeasu
36、rements that are lower than the actual soil density.Excessive use of fill material to compensate for these voidsmay likewise cause biased density measurements, or biasedwater content measurements, or both.5.1.3 Measurements in the Backscatter Mode are influencedmore by the density and water content
37、of the material inproximity to the surface.5.1.4 Measurements in the Direct Transmission mode are anaverage of the density from the bottom of the probe in the soilor soil aggregate back up to the surface of the gauge.5.1.5 Gravel particles or large voids in the source-detectorpath may cause higher o
38、r lower density measurments. Wherelack of uniformity in the soil due to layering, aggregate orvoids is suspected, the test site should be excavated andvisually examined to determine whether the test material isrepresentative of the in situ material in general and whether anoversize correction is req
39、uired in accordance with PracticeD4718.5.1.6 Oversize particles or large voids in the source-detectorpath may cause higher or lower density measurements. Wherelack of uniformity in the soil due to layering, aggregate orvoids is suspected, the test site should be excavated andvisually examined to det
40、ermine if the test material is represen-tative of the in situ material in general and if an oversizecorrection is required in accordance with Practice D4718.5.1.7 The measured volume is approximately 0.0028 m3(0.10 ft3) for the Backscatter Mode and 0.0057 m3(0.20 ft3) forthe Direct Transmission Mode
41、 when the test depth is 150 mm(6 in.). The actual measured volume is indeterminate and varieswith the apparatus and the density of the material.5.1.8 Other radioactive sources must not be within9m(30ft) of equipment in operation.5.2 In-Place Water (Moisture) Content Interferences5.2.1 The chemical c
42、omposition of the material being testedcan affect the measurement and adjustments may be necessary(see Section 10.6). Hydrogen in forms other than water andcarbon will cause measurements in excess of the true value.Some chemical elements such as boron, chlorine, and cadmiumwill cause measurements lo
43、wer than the true value.5.2.2 The water content measured by this test method is notnecessarily the average water content within the volume of thesample involved in the measurement. Since this measurementis by backscatter in all cases, the value is biased by the watercontent of the material closest t
44、o the surface. The volume ofsoil and soil-aggregate represented in the measurement isindeterminate and will vary with the water content of thematerial. In general, the greater the water content of thematerial, the smaller the volume involved in the measurement.Approximately 50 % of the typical measu
45、rement results fromthe water content of the upper 50 to 75 mm (2 to 3 in.).5.2.3 Other neutron sources must not be within 9 m (30 ft)of equipment in operation.6. Apparatus6.1 Nuclear Density / Moisture GaugeWhile exact detailsof construction of the apparatus may vary, the system shallconsist of:6.1.
46、1 Gamma SourceA sealed source of high-energygamma radiation such as cesium or radium.6.1.2 Gamma DetectorAny type of gamma detector suchas a Geiger-Mueller tube(s).6.1.3 Fast Neutron SourceA sealed mixture of a radioac-tive material such as americium, radium and a target materialsuch as beryllium, o
47、r a neutron emitter such as californium-252.6.1.4 Slow Neutron DetectorAny type of slow neutrondetector such as boron trifluoride or helium-3 proportionalcounter.6.2 Reference StandardA block of material used forchecking instrument operation, correction of source decay, andto establish conditions fo
48、r a reproducible reference count rate.6.3 Site Preparation DeviceA plate, straightedge, or othersuitable leveling tool that may be used for planing the test siteto the required smoothness, and in the Direct TransmissionMethod, guiding the drive pin to prepare a perpendicular hole.6.4 Drive PinA pin
49、of slightly larger diameter than theprobe in the Direct Transmission Instrument used to prepare ahole in the test site for inserting the probe.6.4.1 Drive Pin GuideA fixture that keeps the drive pinperpendicular to the test site. Generally part of the sitepreparation device.6.5 HammerHeavy enough to drive the pin to the requireddepth without undue distortion of the hole.6.6 Drive Pin ExtractorAtool that may be used to removethe drive pin in a vertical direction so that the pin will notdistort the hole in the extraction process.6.7 Slide Hammer, with a d
