As specified in AASHTO LRFD Bridge Design Specifications, designers are directed to α-method in clays and cohesive soil layers and β-method within sands and non-cohesive material. For the later, note that engineering judgement is necessary when determining drained friction angle for an individual layer. AASHTO LRFD Bridge Design Specifications Table 10.4.6.2.4-1 presents friction angle φf ranges according to measured (N1)60 values. When selecting an effective soil friction angle according to AASHTO LRFD Bridge Design Specifications Equation 10.8.3.5.2b-3, use an additional reduction factor of 0.9 to account for the lower end of the range of friction angles in granular material with significant fraction of fines, such that:

φ’f = 0.9 * (27.5 + 9.2 log [(N

1

)

60

])