8.12.2 FHWA Method
FHWA method is based on Federal Highway Administration publication FHWA-IF-99-025. Methodology for calculating axial capacity runs same as API method mentioned above.
Axial capacity of drilled pier is calculated based on FHWA-IF-99-025 Eq 1.1
RT= RB+ RS
RT= total calculated or nominal ultimate axial resistance of the drilled shaft
An allowable load is the calculated as:
RA= RT/F
Skin Friction Resistance Calculation
For cohesive soils, skin friction is calculated based on FHWA-IF-99-025 Eq 11.16
f = α* su
α = dimensionless correlation coefficient
su= design value for undrained shear strength for the layer being considered
For cohesionless soils, skin friction is calculated based on FHWA-IF-99-025 Eq 11.17
f= βi* σvi
βi=dimensionless correlation factor between vertical effective stress and maximum stress limit
End bearing Resistance Calculation
End bearing is calculated from geometry of pier.
RB = qmax Ap
For cohesive soils, unit end bearing capacity is calculated based on FHWA-IF-99-025 Eq 11.1, 11.2 &11.3
q=9 su
su= design value for undrained shear strength for the layer being considered
For cohesionless soils, unit end bearing capacity is calculated based on FHWA-IF-99-025 Eq 11.4
q=57.5NSPT