Step 1 - When a geo - technician performs a field density measurement using the nuclear density/moisture gauge on clay fill, he is measuring the field wet density of the material ie. 1.750 t/m3
Step 2 - He is then required to dig up a sample from beneath the instrument to perform a laboratory compaction test. There are several test methods used depending on the contract specification and the situation of the likely loading of the soil.
Step 3 - The Hilf standard compaction test is a rapid method of determining the laboratory compaction characteristics of the soil and involves breaking the field sample into small pieces.
Step 4 - The prepared sample is then split into three sub-samples which is to be used in the test. Each split sample will be treated differently. It may either have water added, water dried out or used in its current condition depending on the moisture condition of the field sample. Three separate moisture conditions will need to be mixed into the soil.
Step 5 - Each sample is then compacted into a metal 1 litre mould using three equal layers and compacted with a small hand held compaction rammer. Each layer will receive 25 blows of the rammer.
Step 6 - Any excess soil that is protruding from the mould is struck off and mould and soil is weighed to determine the bulk density of the soil. Differing moisture conditions will result in a different bulk density.
Step 7 - The bulk density of the soil will reach a maximum level of compaction at an ideal moisture level. This is moisture level is called the Optimum Moisture Content (OMC). The peak bulk density =1.770 t/m3 will be achieved at this OMC.
Step 8 - The field bulk density reading taken form the nuclear density/moisture meter will then be compared to the laboratory maximum compaction and expressed as a percentage. ie. (1.750 / 1.770 = 99%)
The methods used for testing soils or crushed rock are basically the same, with variation existing in the number of layers used in the laboratory test and the size of the compaction rammer.