The modulus of subgrade reaction may be found by performing a plate test as described by Teller and Sutherland and by Panak, McCullough and Treybig. An approximate modulus of subgrade reaction may be obtained from the CBR value by using Figure.
A sub-base reduces the stress induced in the slab and, in the case of wheel and rack loads, assists in the distribution of the load to the subgrade. When a sub-base is provided between the slab and the subgrade, the 'apparent' modulus of subgrade reaction, i.e. the value obtained from a plate bearing test performed on the sub-base, will be greater than the modulus of subgrade reaction determined on the subgrade. This apparent modulus of subgrade reaction should be ascertained in the field and used in the design. To use the relationship between CBR and modulus of subgrade reaction in this instance would not be appropriate, because the thickness of the sub-base layer would influence the apparent modulus of subgrade reaction.
The sub-base must give uniform support and be capable of withstanding the loads imposed by the construction plant without significant deformation. To meet these requirements, the recommended depths of sub-base given in Road Note should be followed.
For subgrades consisting of clays and organic materials, additional measures should be undertaken to ensure uniform consolidation.