Although there are a wide variety of tests for assessing soils and road materials, any serious investigation will need to know the mechanical behaviour of these materials when subjected to repeated loading that simulates the effects of trafficking and under moisture conditions (water content and suction) that simulate that found in the layers of the road construction and embankment. Various devices have been developed including the k-mould (Semmelink et al., 1997) and the Springbox (Edwards et al., 2004), but the cyclic triaxial test has secured the greatest following for material assessment over many years and is now the subject of European, US and Australian standards (CEN 2004, AASHTO 2000, Standards Australia 1995). It is the use of this test that is described in this section.
In laboratory testing procedures it is well known that the size of the sample may have a very important influence on the results. If the size of the sample is not appropriate for a test procedure, the results obtained may be corrupted and not valuable. In order to have a continuum condition in the sample, it is necessary to satisfy some conditions with regard to its microstructure and sample size. For fine soils, it can be assumed that the test sample should be some centimetres in diameter, roughly from 2 to 6 cm. However, for granular materials, the sample diameter should be much higher, from about 5 to 10-15 cm, depending on maximum grain size.
