土木工程专业英语文章

Text A Field Measurement of Density and Moisture Content

Control of field compaction requires frequent determination of the density and moisture content

of the compacted soil, and also measurements of the moisture content of the excavated soil to assess

its 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 method

①

. 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 Method

A hole approximately 4 in. diameter ( 8 in. diameter for coarse-grained soils) is excavated to

the 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 method

This 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 sample

is 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 of

103measurements 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 5

and 10 measurements per 1, 000 cu yd of fill constitute a reasonable number as regards the amount of

testing 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 limits

②

. Clearly this number of

tests 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 undue