1、BRITISH STANDARD BS 7755-5.2: 1996 ISO 10573: 1995 Soil quality Part 5: Physical methods Section 5.2 Determination of water content in the unsaturated zone Neutron depth probe method ICS 13.080BS7755-5.2:1996 This British Standard, having been prepared under the directionof the Health and Environmen
2、t Sector Board, waspublished under the authorityof the Standards Boardand comes into effect on 15August1996 BSI 03-2000 The following BSI references relate to the work on this standard: Committee reference EH/4/5 Draft for comment 92/503316 DC ISBN 0 580 25949 8 Committees responsible for this Briti
3、sh Standard The preparation of this British Standard was entrusted by Technical Committee EH/4, Soil quality, to Subcommittee EH/4/5, Physical methods, upon which the following bodies were represented: Association of Consulting Scientists British Society of Soil Science Cambridge University Institut
4、e of Hydrology Institution of Civil Engineers Macaulay Land Use Research Institute Soil Survey and Land Research Centre University of Glasgow University of Stirling University of Ulster Amendments issued since publication Amd. No. Date CommentsBS7755-5.2:1996 BSI 03-2000 i Contents Page Committees r
5、esponsible Inside front cover National foreword ii 1 Scope 1 2 Normative references 1 3 Definitions 1 4 Principle 1 5 Apparatus 2 6 Procedure 2 7 Expression of results 4 8 Accuracy 4 9 Test report 5 Annex A (informative) Background information for the calibration of the neutron depth probe 6 Annex B
6、 (informative) Field calibration 7 Annex C (informative) Reference counts 9 Annex D (informative) Precision and applicability of field calibration curves 9 Annex E (informative) Bibliography 11 List of references Inside back coverBS7755-5.2:1996 ii BSI 03-2000 National foreword This Section of BS 77
7、55 has been prepared by Subcommittee EH/4/5 and is identical with ISO10573:1995 Soil quality Determination of water content in the unsaturated zone Neutron depth probe method, published by the International Organization for Standardization (ISO). ISO 10573 was prepared by Subcommittee 5, Physical me
8、thods, of Technical Committee ISO/TC190, Soil quality, with the active participation and approval of the UK. BS 7755 is being published in a series of Parts subdivided into Sections and Subsections that will generally correspond to particular International Standards. The Parts of BS7755 are, or will
9、 be, as follows. Part 1: Terminology and classification; Part 2: Sampling; Part 3: Chemical methods; Part 4: Biological methods; Part 5: Physical methods. Cross-references. The Technical Committee has reviewed the provisions of the draft of ISO11272 and ISO11461, to which normative reference is made
10、 in the text, and has decided that they are acceptable for use in conjunction with this standard. Textual errors. When adopting the text of the International Standard, the textual errors listed below were discovered. They have been marked in the text and have been reported to ISO in a proposal to am
11、end the text of the International Standard. In 6.2, the second sentence of paragraph 5 should read “To meet the requirement for time invariant gradients, the calibration shall not be conducted after heavy rain or irrigation applications, or immediately after the sudden beginning of very warm weather
12、.” In A.3, the first sentence in item a) should read “This is characterized by a constant vertical distribution of water content (steady-state water content profile), for given soil water potential conditions in the unsaturated soil (pressure head, h) and a given depth d of the phreatic level (groun
13、dwater level), the so-called state variables.” In A.3, paragraph 4 of item b), in lines 9 and 10 delete “hydrostatic pressure distribution”; in line 17, delete “Inversely” and substitute “Conversely”. In B.2, the second sentence of paragraph 4, delete “depth probes calibration” and substitute “calib
14、ration procedures for neutron depth probes”. In paragraph 6, delete the last two sentences and substitute “A depth interval of less than0.1m is not usually useful because of overlapping of the measuring volume (the sphere of importance of the probe) as measurements are made”. In D.1, line 3, delete
15、“final result of” and substitute “resulting”. NOTETypographical errors In 6.1 b), line 3, the reference to 6.1.1 should be to6.1 a). In A.2.1, in the first line of the paragraph below the list, “mentionned” should be “mentioned”. In A.3, in the penultimate line of paragraph1, “tabs” should be “table
16、s”; in the first sentence in paragraph4 “corresponding” was misspelt. In D.3, in the penultimate line of paragraph4, “s e. s 1 ” should be “sF . s 1 ”. In Annex E, in reference 3, “GRAECEN” should be “GREACEN”.BS7755-5.2:1996 BSI 03-2000 iii A British Standard does not purport to include all the nec
17、essary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages
18、1to 12, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.iv blankBS7755-5.2:1996 BSI 03-2000 1 WARNING Neutron depth probes contain radioactive
19、sources which will present health and environmental hazards if a probe is improperly used, stored or disposed of. National and international legislation and regulations must be complied with. 1 Scope This International Standard specifies an in situ method for the determination of water content in th
20、e unsaturated zone of soils using a neutron depth probe. It is applicable when investigations into the water storage, water balance and water distribution in the unsaturated zone of the soil are carried out. Because the method is non-destructive, it is particularly suitable for repeated measurements
21、 at fixed locations. Water content profiles can be obtained by measuring at a series of depths down to any depth within the range of the phreatic level at the site. The advantage of the method compared with some others, for example the gamma probe method, is the rapidity with which measurements can
22、be carried out. A disadvantage, however, is the relatively poor depth resolution of the measurements. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the edition
23、s indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently va
24、lid International Standards. ISO 11272:, Soil quality Determination of dry bulk density 1) . ISO 11461:, Soil quality Determination of soil water content calculated on a volume basis Gravimetric method 1) . 3 Definitions For the purposes of this International Standard, the following definition appli
25、es. 3.1 water content volume fraction, the volume of water evaporating from soil when dried to constant mass at105 C, divided by the original bulk volume of the soil NOTE 1The water content may be expressed as a percentage by volume or a volume fraction. NOTE 2In this International Standard, water c
26、ontent as defined above may also be referred to as “free water”. NOTE 3The procedure for drying soil to constant mass at105 C is described in ISO11461. NOTE 4The procedure for determination of the bulk volume of soil is described in ISO11272. 4 Principle A neutron depth probe, consisting of a neutro
27、n source and detector, is lowered into a vertical access tube in the soil. The neutron source, usually of the 241 Am-Be type, emits neutrons of high kinetic energy. The neutrons lose part of their energy when they collide with atomic nuclei. After several collisions, their energy level is reduced to
28、 the thermal energy level corresponding to the prevailing temperature. This level is reached most rapidly when neutrons collide with hydrogen nuclei because their masses are almost equal. The thermal neutrons form a stable cloud, the concentration of which is determined by the detector in the probe.
29、 The number of thermal neutrons registered by the detector per unit time (the count rate) is therefore a measure of the concentration of hydrogen nuclei in the soil around the probe. In general, the majority of those nuclei are in water molecules and therefore the count rate is also a measure of the
30、 soil water content. A calibration curve is used to convert the neutron count rate to soil water content. NOTE 5The neutron count rate obtained is influenced by the presence of all the atomic nuclei in the soil. However, the count rate at a given water content may be increased in some soils because
31、of the thermalization of neutrons by collisions with nuclei of certain soil elements, or because much hydrogen is present in substances other than free water. However, the count rate may be decreased because of absorption of neutrons by nuclei with a large atomic absorption cross-section. SeeAnnex A
32、. NOTE 6The soil volume (measuring volume) to which the measurement refers approximates a sphere. For a given type of neutron probe, the radius of the sphere depends on the total density of atomic nuclei in the soil. For the majority of probes used in practice, the radius of the volume from which95%
33、 of the neutrons counted by the detector are generated (“the sphere of importance” 1 ) can vary from0,1m to0,2m in wet soil to0,8m or more in dry (sandy) soil. Consequently, the measurement obtained at a given depth is influenced by the water content distribution within the measuring volume at that
34、time, and by any other gradients in soil composition. Therefore, reproduction of the measurement of a given water content at a certain depth is only possible when the distributions of water content and of soil composition within the measuring volume are time-invariant. This requirement (local time-i
35、nvariant gradients) is important for the calibration of the neutron depth probe. SeeAnnex A. NOTE 7The shape and parameters of the calibration curve depend on the following (see 2inAnnex E): the chemical composition of the soil horizon considered and its bulk density; the gradients in this compositi
36、on that occur within the measuring volume; the gradients in soil water content that occur within the measuring volume; 1) To be published.BS7755-5.2:1996 2 BSI 03-2000 the method of access tube installation; the characteristics of the access tubing; the specifications of the apparatus used. The cali
37、bration curve usually differs for each soil layer. In homogeneous layers that are thicker than the measuring volume, calibration curves are generally linear, their parameters depending on the soil composition. In the case of thin or non-homogeneous soil layers, however, calibration curves will often
38、 be non-linear due to the different effects of gradients in soil composition and water content under wet and dry conditions. 5 Apparatus 5.1 Neutron depth probe, consisting of a fast neutron source and a thermal neutron detector combined with a read-out unit. 5.2 Thin-walled access tubing, with an i
39、nner diameter slightly larger than that of the neutron probe. The tubing shall consist of material that is very “transparent” to fast and thermal neutrons (e.g. aluminium, aluminium alloy) and which is resistant to chemical corrosion and to deformation due to installation activities. Stainless steel
40、, galvanized iron and plastics (polyethylene) are also suitable, though less transparent to neutrons. 5.3 Equipment for installing access tubes 5.4 Equipment for drying and cleaning the access tubes, if necessary, a dummy probe for testing the tubing performance. 5.5 Calibration curves, for conversi
41、on of count rate to water content. 5.6 Usual apparatus for taking soil samples, for carrying out a field calibration to determine the volumetric water content gravimetrically according to ISO11461. 6 Procedure 6.1 Installation of access tubes The location shall be representative of the immediate sur
42、roundings and care shall be taken to avoid surface water from concentrating on the spot. Use a platform to prevent damage to surrounding vegetation and compaction of the soil surface whilst installing a tube. Ensure that radial soil compaction around the tube, compaction below it and the creation of
43、 voids adjacent to it are prevented as far as possible. Install access tubes by either of the following methods. a) Push the tube into the soil using a hammer and empty it using an auger. It is recommended that the lower end of the tube be closed with quick drying cement or a stopper, to prevent inf
44、iltration of ground water. b) Push the tube into a prepared hole of the same or slightly smaller diameter and of the required depth, then seal the lower end as in 6.1.1. Alternatively, the lower end of the tube may be sealed before insertion. Holes can be prepared using a guide tube or an auger or b
45、y a combination of these two methods. Close the top of the tube with a tight rubber stopper to keep out rain or surface water. The tubing shall always be dry inside. NOTE 8It is recommended that access tubes be cut to protrude above the soil surface as little as the apparatus permits, so as to minim
46、ize the radiation dose received by the operator when lowering the probe. NOTE 9More specific guidelines for installation are given in 3 and 4 inAnnex E. After installation, take great care to minimize disturbance of the soil and vegetation at the site whilst conducting measurements in the access tub
47、e. 6.2 Calibration In most cases, calibration curves supplied by neutron probe manufacturers, and those published in the literature, give only a rough indication of the absolute soil water content, because no or insufficient recognition can be given to the specific influences of the site mentioned i
48、n note7 in clause4 (see alsoAnnex A). The influence of chemical composition and bulk density (seeA.2) is accounted for in calibrations derived theoretically from the macroscopic neutron-interaction cross-section of the soil concerned (see 1, 4, 9 inAnnex E). The combined influence of gradients in wa
49、ter content, chemical composition and bulk density is only accounted for by a field calibration. Therefore an in situ field calibration is necessary for accurate measurements of absolute water content. The field calibration is based on simultaneous determination of the neutron count rate and sampling for the determination of the volumetric water content of each soil layer in accordance with ISO11461, under several different hydrological conditions, to derive a calibration curve for each layer. NOTE 10The subdivision,
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