Selection of subgrade resilient modulus

The selection of the resilient modulus, Mr, of the subgrade, the other necessary data for the determination of layer thickness, is carried out after representative sampling and laboratory 

Sampling and testing of soil materials are carried out according to established procedures. It is recommended to test all subgrade material within 600 mm of the planned elevation and in embankment areas to test the expected source of the fill material. Apart from the resilient modulus test (AASHTO T 307 2007 Asphalt Institute 1997), all other tests required 

If a sample from a location gives low M, indicating an extremely weak soil, it is recommended to take and test additional samples to determine the boundaries of the area with weak soil. Such areas require replacement with improved subgrade material. 

The design value is not the average of the values determined but the value that is lower than a certain percentile of all test values. This percentile depends on the level of traffic volume and is taken as 60% for 10 ESAL, 75% for 10 -10 ESAL or 87.5% for 10 ESAL. 

The procedure for selecting the design Mr value that corresponds to a certain percentile is explained by an example in Asphalt Institute (1999). 

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