For design purposes, the California Bearing Ratio (CBR) must be established on virgin ground before construction begins. On a brownfield or otherwise developed site, a methodology must be established. The council should be notified in advance of site tests to establish the subgrade strength to allow the opportunity to be present at such tests. The council must be provided with copies of all test results.
Soil-classification tests should be carried out to give the types of soil an ‘Equilibrium CBR’ based on material type, using Table 33, unless agreed otherwise by the council.
| Type of soil | Plasticity index | Equilibrium CBR % |
|---|---|---|
| Heavy clay | 50 or greater | Less than 2 |
| Heavy clay | 40 to 49 | 2 |
| Heavy clay | 30 to 39 | 2 |
| Silty clay | 20 to 29 | 3 |
| Sandy clay | 10 to 19 | 4 |
| Silt | Less than 10 | 1 |
| Sand (poorly graded) | Non-plastic | 20 |
| Sand (well graded) | Non-plastic | 40 |
| Gravel (poorly graded) | Non-plastic | 40 |
| Sandy gravel (well graded) | Non-plastic | 60 |
(a) Based on withdrawn HD 25/94 & IAN 73/06.
Soils are a finite resource and must be managed sustainably. Guidance on the protection of soil resources can be found in “Construction Code of Practice for the Sustainable Use of Soils on Construction Sites”