1、Designation: D6938 17aStandard 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 th
2、e 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
3、of soil and soil-aggregate by useof nuclear equipment (hereafter referred to as “gauge”). Thedensity of the material may be measured by directtransmission, backscatter, or backscatter/air-gap ratio methods.Measurements for water (moisture) content are taken at thesurface in backscatter mode regardle
4、ss of the mode being usedfor density.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 known depth up to300 mm (12 in.) and the detector
5、(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 detected rate of gamma radia-tion with previou
6、sly 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 the water mass per unitvolume of soil or soil-ag
7、gregate. 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, a component of water.The neutron source and th
8、e 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 pore spaces to the total massof solids, expres
9、sed 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 preferred method.1.4.2 Procedure B involves the u
10、se 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 probe to the hole.1.5 SI UnitsThe values stated i
11、n 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 PracticeD6026.1.6.1 The procedures used to specify how data
12、 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, purpose forobtaining the data, special purpose stud
13、ies, 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 used in analysismethods for engineering design.1
14、.7 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.
15、1.8 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to
16、 Trade (TBT) Committee.1This test method is under the jurisdiction ofASTM Committee D18 on Soil andRock and is the direct responsibility of Subcommittee D18.08 on Special andConstruction Control Tests.Current edition approved Nov. 1, 2017. Published December 2017. Originallyapproved in 2006. Last pr
17、evious edition approved in 2017 as D693817. DOI:10.1520/D6938-17A.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance
18、with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.12. Referenced Documents2.1 ASTM Standa
19、rds:2D653 Terminology 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
20、 Compaction 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
21、for Description and Identification of Soils(Visual-Manual Procedures)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 AgenciesEngage
22、d in Testing 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
23、Method for Determination of Water Content ofSoil and Rock 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 PressureTe
24、sterD4959 Test Method for Determination of Water Content ofSoil 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.
25、Terminology3.1 DefinitionsSee 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 gamma (radiation) sourcea sealed source
26、 of radio-active material that emits gamma radiation as it decays.3.2.3 neutron (radiation) sourcea sealed source of radio-active material that emits neutron radiation as it decays.3.2.4 Compton scatteringthe interaction between agamma ray (photon) and an orbital electron where the gammaray loses en
27、ergy and rebounds in a different direction.3.2.5 detectora device to detect and measure radiation.3.2.6 gravimetric water contentsame as water content (asdefined in Terminology D653), a nomenclature used in somescientific fields to differentiate it from volumetric water con-tent.3.2.7 thermalization
28、the process of “slowing down” fastneutrons by collisions with light-weight atoms, such as hydro-gen.3.2.8 volumetric water contentthe volume of water as apercent of the total volume of soil or rock material.3.2.9 test count, nthe measured output of a detector for aspecific type of radiation for a gi
29、ven test.3.2.10 prepared blocksblocks prepared of soil, solid rock,concrete, and engineered materials, that have characteristics ofvarious degrees of reproducible uniformity.4. Significance and Use4.1 The test method described is useful as a rapid, nonde-structive technique for in-place measurements
30、 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-tance testing of compacted soil and soil-aggregate mixtures asused in construction and also for research and development.The nondestructive natu
31、re allows repetitive measurements at asingle test location and statistical analysis of the results.4.3 DensityThe fundamental assumptions inherent in themethods are that Compton scattering is the dominant interac-tion and that the material is homogeneous.4.4 Water ContentThe fundamental assumptions
32、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 from Test Methods D2216, and that thematerial is homogeneous. (See 5.2)NOTE 1The quality of the result produced by this standard test methodis de
33、pendent on the competence of the personnel performing it, and thesuitability of the equipment and facilities used. Agencies that meet thecriteria of Practice D3740 are generally considered capable of competentand objective testing/sampling/inspection, and the like. Users of thisstandard are cautione
34、d 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. Interferences5.1 In-Place Density Interferences5.1.1 Measurements may be affected by the chemical com-positi
35、on of the material being tested.5.1.2 Measurements may be affected by non-homogeneoussoils and surface texture (see 10.2). Excessive voids in the2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards
36、volume information, refer to the standards Document Summary page onthe ASTM website.D6938 17a2prepared test surface beneath the gauge can cause densitymeasurements that are lower than the actual soil density.Excessive use of fill material to compensate for these voidsmay likewise cause biased densit
37、y measurements, or biasedwater content measurements, or both.5.1.3 Measurements in the Backscatter Mode are influencedmore by the density and water content 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 th
38、e 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 or lower density measurments. Wherelack of uniformity in the soil due to layering, aggregate orvoids is suspected, the test site should be excava
39、ted andvisually examined to determine whether the test material isrepresentative of the in situ material in general and whether anoversize correction is required in accordance with PracticeD4718.5.1.6 Oversize particles or large voids in the source-detectorpath may cause higher or lower density meas
40、urements. Wherelack of uniformity in the soil due to layering, aggregate orvoids is suspected, the test site should be excavated andvisually examined to determine if the test material is represen-tative of the in situ material in general and if an oversizecorrection is required in accordance with Pr
41、actice 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 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.
42、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 composition of the material being testedcan affect the measurement and adjustments may be necessary(see Section 10.6). Hydrogen in forms other th
43、an water andcarbon will cause measurements in excess of the true value.Some chemical elements such as boron, chlorine, and cadmiumwill cause measurements lower 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 th
44、esample involved in the measurement. Since this measurementis by backscatter in all cases, the value is biased by the watercontent of the material closest to the surface. The volume ofsoil and soil-aggregate represented in the measurement isindeterminate and will vary with the water content of thema
45、terial. In general, the greater the water content of thematerial, the smaller the volume involved in the measurement.Approximately 50 % of the typical measurement 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 equipmen
46、t in operation.6. Apparatus6.1 Nuclear Density / Moisture GaugeWhile exact detailsof construction of the apparatus may vary, the system shallconsist of:6.1.1 Gamma SourceA sealed source of high-energygamma radiation such as cesium or radium.6.1.2 Gamma DetectorAny type of gamma detector suchas a Gei
47、ger-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, or a neutron emitter such as californium-252.6.1.4 Slow Neutron DetectorAny type of slow neutrondetector such as boron trifluoride or helium-3 pr
48、oportionalcounter.6.2 Reference StandardA block of material used forchecking instrument operation, correction of source decay, andto establish conditions for a reproducible reference count rate.6.3 Site Preparation DeviceA plate, straightedge, or othersuitable leveling tool that may be used for plan
49、ing 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 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 Extracto
