All granular materials including Clause 803 Type 1 granular sub-base must be compacted near to their optimum moisture content. The optimum moisture content can vary considerably depending on the average particle size and, to a much smaller extent, on the type of mineral or rock involved. However, a laboratory compaction test is invariably carried out on a sample of material from which the larger particles have been removed. The sample is always compacted in a small smooth sided steel cylinder and the standard methods of compaction bear little similarity with current compaction plant. Experience has shown that the most commonly specified laboratory compaction test i.e. BS 1377-4, Vibrating Hammer, Method 3.7 will produce an optimum moisture content result that is typically significantly wetter than the field optimum for a granular material that is to be compacted within a trench using a vibrotamper.
Granular materials suitable for compaction by pedestrian-controlled plant can usually be identified by a simple visual examination. Typically, the test will identify materials within 1% to 1.5% of the field optimum moisture content depending on the mineral type. Experience has shown that compaction within this visual moisture range will not normally show any significant reduction in compaction performance. The test cannot indicate the actual moisture content of any material, but this is rarely of any relevance as far as an operator is concerned.
Material within the target moisture content range will show a dull sheen when viewed obliquely against the light with all fines adhering to the larger particles, and no free water will be visible. Material at the dry limit will not show the characteristic sheen, fines will not be strongly adherent and many of the fines will be free. Material at the wet limit will begin to show free moisture collecting in surface grooves or amongst the fines, fines will not be strongly adherent and many of the fines will amalgamate as soggy clusters. Any result between the wet and dry limits is acceptable provided the bulk of the sample is reasonably well graded.
Sands used as fine fill or as a regulating layer also need to be used near to the optimum moisture content and can be identified by a simple squeeze test.