Text A Field Measurement of Density and Moisture ContentControl of field compaction requires frequent determination of the density and moisture contentof the compacted soil, and also measurements of the moisture content of the excavated soil to assessits suitability as filling material. There is a number of standard methods of measuring field dry density, the two most commonly used being the sand-replacement method and the core-cutter method①. Both these methods are outlined briefly below; in each case the bulk density of the soil is determined by finding the weight of material occupying a known volume and, by measuring the moisture content of a representative sample, the dry density can then be calculated.Sa nd-r epla cement MethodA hole approximately 4 in. diameter ( 8 in. diameter for coarse-grained soils) is excavated tothe depth of the layer to be tested and the material removed is weighed. The volume of the hole is then determined by running in dry, closely-graded sand of known bulk density from a suitable container, the volume of sand used being found by weighing the sand-filled container before and after the test. The test may be performed on all types of soils, but is liable to give too high a value for the bulk density of wet granular materials as a result of the surrounding soil flowing into the sample hole during excavation.Cor e-cutter methodThis method is useful for soft, cohesive soils which are free from stones. A 4 -in. internal diameter cutter of known weight and volume is driven into the soil and then dug out. The soil sampleis trimmed flush with the ends of the cutter and weighed to determined the bulk density.With any of the standard methods for dry density, it is necessary to make a number of103measurements over a comparatively small area in order to obtain a representative average value,since considerable variations occur as a result of testing errors. It has been suggested that between 5and 10 measurements per 1, 000 cu yd of fill constitute a reasonable number as regards the amount oftesting time involved, but recent work indicates that about 40 dry density and moisture content measurements per 1, 000 cu yd of fill are necessary to obtain a mean value with an accuracy not poorer than ±2 for the percentage air voids at the 90% confidence limits②. Clearly this number oftests would be impracticable using the standard methods of measurement, but efforts are at present being made to develop methods which can be carried out with greater rapidity.One possible approach is based on the fact that the scattering and absorption of gamma radiation is a function of the bulk density of the material. Briefly, a radioactive source ( caesium 137 ) contained in the end of a stainless-steel probe is inserted in the soil to a depth of up to 6 in. and a Geiger-Muller tube which detects gamma radiation is placed on the surface about 8 in. from the probe to measure the rate of transmission of radiation through the material. Alternatively, in soils containing large stones which make it difficult to insert the probe without causing unduedisturbance,both the source and detector may be placed on the surface separated by lead shielding and the backscatter method used, although this method suffers from the disadvantage that measurements arelargely confined to the upper 2 in. of soil.Since this equipment records the bulk density, it is necessary to determine also the moisture content of the material before the dry density can be calculated. This may also be done with nucleartechniques using a radium/ beryllium source of fast neutrons placed on the ground surface alongside aboron trifluoride counter which records the number of slow neutrons scattered back after collisionshave occurred between the fast neutrons and nuclei of hydrogen present in the soil moisture. This measurement is also made on a unit volume basis and therefore the difference of the two readings give the dry density of the soil.Wor ds and Expr essionsmeasurement n. 测量density n. 密度moisture content 含水量compaction n. 压实, 夯实access v. 估计, 评价suitability n. 适用性sand-replacement method 换砂法core-cutter method 钻心法bulk density 毛体积密度sample n. 试样run in 倒入, 注入liable a . 易于……的, 有……倾向的104土木工程专业英语granular a . 粒状的, 粒料的trim v. 修整, 创平flush v. 使齐平; a . 齐平的flush with 与……齐平as regards 关于, 至于confidence n. 信任, 信心confidence limit 置信界限( 用于数理统计)scatter v. 散射, 分散absorption n. 吸收gamma n. 伽玛( 希腊字母)radiation n. 辐射, 放射radioactive a . 放射性的caesium n. 铯( 化学元素)probe n. 探头disturbance n. 扰动, 妨碍shield vt. 用盾掩护; n. 防护nuclear a . 核的, 原子核的radium n. 镭( 化学元素)beryllium n. 铍( 化学元素)neutron n. 中子boron n. 硼( 化学元素)trifluoride n. 三氟化合物collision n. 碰撞nucleus n. [ 复] nuclei 核, 原子核Notes①There is a number of standards of methods of measuring field dry density, the two most commonly used being the sand-replacement method and the core-cutter method. 本句可译为: 野外量测干密度的标准方法有多种, 其中最常用的两种是换砂法和钻芯法。