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What are subgrade soils?

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As per AASHTO soil classification see the following;

50 max 
30 max50 max51 min 
15 max25 max10 max35 max35 max35 max35 max 
40 max41 min40 max41 min  
6 maxN.P.10 max10 max11 min11 min  
stone fragments, gravel and sandfine sandsilty or clayey gravel and sand     
excellent to good       


Fasi Ur Rahman
Author at The Constructor

The sub-grade is a pavement layer whose primary duty is to sustain the higher pavement layers and provide excellent drainage for infiltrating rainwater. Along with the other pavement layers, it must function as a single structure.