How to identify Sub-base and base course by its natural appearance?

[agbronto]

Hi Guys!
I am Alain from philippines. I just browsed your forum regarding earthworks activities and this thing got my interest. I am currently involve in road construction, i have a difficulty of identifying between sub-base and base course without sending those items in testing center. I would be grateful if someone could help me please. Thank you very much.

 

[Ron]

Alain...depending on the design, the subbase and base courses might actually be essentially the same materials, just slightly different stabilities and thicknesses. Most often though, the subbase is a weaker, finer stabilized material that is likely to be native to the site and mixed with another material to provide additional stabilization; whereas the base material is more likely to be imported to the site and have higher stability and strength than the subbase. Base materials are often coarser materials (crusher run rock with included fine material from the crushing, softer rock materials like soft limestone that is friable as excavated, recycled concrete that has been crushed, or mixtures of soil and cement or soil, cement, and asphalt, or just asphalt cement and soil (commonly called "black base")).

Subbase and base materials can usually be distinguished visually from the noted characteristics. You need to be able to distinguish them so that you can know if the right thicknesses were employed as required by the design.

 

[civilman72]

Subbase specifications generally allow the use of larger rock (<4") and may consist of material taken directly from the gravel pit, with limited screening/crushing.

Base course may be well-graded with smaller max. size of rock (<1"), particularly true if the final road surface is gravel.

 

[agbronto]

Thank you Ron and Civilman72!!!

 

[BigH]

My first look would be, as with civilman72, to look at the size of the aggregate. Subbase is generally coarser - not always but likely 9x out of 10