Typical coefficients of subgrade/subbase friction for natural subgrades or unstabilized subgrades/subbases range from 1 to 2. Coefficients for stabilized subbases typically are much greater with coefficients for asphalt-treated subbases (ATB) ranging from 5 to 15, cement-treated subbases (CTB) ranging from 10 to 15, and lean concrete subbases (LCB) ranging from 8 to 15.
Even though this calculator accounts for friction between the concrete slab and subgrade/subbase, use of a coefficient of subgrade/subbase friction higher than 1.5 has not been justified by pavement performance.