44 Field density
COMMENTARY ON 44.1
The bulk density of soil can be measured by a range of field tests which comprise the removal of a representative sample of soil from the site and then the determination of its mass and the volume it occupied before being removed. The variations lie in the several procedures used for measuring the volume and these depend upon the nature of the soil being tested. In coarse grained soils it forms the "field" element of the relative density test, the other two elements often being carried out in the laboratory.
Safe physical access to the soil in situ should be provided for all test methods in 44.2 to 44.5. The water content of the sample should be representative. Ideally, the weighing should be done on-site; if this is not possible, the entire sample should be preserved until it can be weighed, taking care to avoid loss of water.
NOTE The tests can be of limited accuracy and it might be necessary to take the average of at least three determinations to obtain a significant result.
44.2 Sand replacement test method
Sand replacement tests should be carried out in accordance with BS 1377-9, which describes two test variations (see BS 1377-9:1990, 2.1 and 2.2). The first is used for fine and medium-grained soils, as defined in BS 1377. The second is suitable for fine, medium and coarse-grained soils. These test methods are unsuited to soils containing a high proportion of coarse gravel or larger particles. The method should not be used in soils where the volume of the hole cannot be maintained constantly. It also loses accuracy in soils where it is difficult to excavate a smooth hole because the test sand added into the hole cannot easily occupy the full volume.
44.3 Water replacement test method
The water replacement test method should be carried out in accordance with BS 1377-9:1990, 2.3. The method is normally used in coarse and very coarse soils (including rockfill) when the other methods for determining the field density are unsuitable because the volume excavated would be unrepresentative. It consists of excavating a hole large enough to obtain a representative sample, lining the hole with flexible polyethylene or similar sheeting and then determining the volume of water required to fill the hole.
The accuracy of the results of this test can be enhanced by attention to the following details:
- a) the hole should be made as large as possible;
- b) the sides of the hole should be made as smooth as possible;
- c) as thin a gauge of polyethylene as possible should be used, consistent with it not puncturing too easily.
44.4 Core cutter test methods
The core cutter test method should be carried out in accordance with BS 1377-9:1990, 2.4. The method depends upon being able to drive a cylindrical cutter into the soil without a significant change of density and retaining the sample inside it so that the known internal volume of the cylinder is completely filled. It is, therefore, restricted to fine soils that do not contain gravel and are sufficiently cohesive for the sample not to fall out, and to chalk soils. It might be preferable in cohesive and sensitive soils to trim and push the cutter rather than drive it.
44.5 Nuclear test methods
Nuclear test methods should be carried out in accordance with BS 1377-9:1990, 2.5. These do not measure density directly; calibration curves should be established for each soil type, which involves measuring the densities of representative samples of the soils concerned by the container method or one of the conventional in-situ methods given above. Once this has been done and provided there are no significant changes in soil type, the method is very much faster than the others. It is, therefore, most suited to jobs where there is a continuous need for density determinations over a long period and where the soil types do not vary to any significant extent. The density determined by these methods is not necessarily the average density within the volume involved in the measurement. The equipment utilizes radioactive materials and appropriate safety precautions should be taken.
NOTE The major use of this test is in the control of the compaction of earthworks. It Is also used in connection with the design of road and airfield pavements and in the control of the compaction of sub-grades on which they rest. It can be used for the determination of natural in-situ density, where it is difficult or impossible to take undisturbed samples.
44.6 Electrical test method
COMMENTARY ON 44.6
Electrical test methods are beginning to become available and, as with the nuclear methods in 44.5, they do not measure the density directly. Some measure the electrical dielectric properties and moisture levels of compacted soil using high, radio frequency traveling between darts driven into the soil being tested, others adopt electromagnetic techniques. The results are correlated with estimates of the in-situ density using in-built "soil models".
Site-specific checks using alternative methods should be undertaken to ensure reliability of the results.
Testing of this type should be undertaken in accordance with the manufacturers' guidance (as many are not currently standardized).
NOTE The major use of this test is in the control of the compaction of earthworks. It is also used in connection with the design of road and airfield pavements and in the control of the compaction of sub-grades on which they rest. It can be used for the determination of natural in-situ density, where it is difficult or impossible to take undisturbed samples.