Clay in the inner suburbs of Melbourne behaves very differently from the sandy gravels found on the northern fringe. That contrast makes a general soil classification insufficient for pavement design. Our laboratory CBR test measures the soaked bearing capacity of compacted samples under controlled conditions. We run the test on material passing the 19 mm sieve, compacted to standard or modified Proctor effort before soaking for 96 hours. The plunger penetration at 2.54 mm gives the CBR value used in flexible pavement thickness charts. For subgrades with high fines content, we also check swell after the soaking period. This data feeds directly into the Austroads pavement design procedure. Before sampling, it pays to run a granulometría to confirm the fraction below 19 mm is representative.
A soaked CBR below 2 % in Melbourne's plastic clays signals high swell potential and requires subgrade improvement before pavement placement.