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

R_T= R_B+ R_S

Where:

R_T= total calculated or nominal ultimate axial resistance of the drilled shaft

R_B= nominal ultimate base resistance

R_S= nominal ultimate side resistance

An allowable load is the calculated as:

R_A= R_T/F

Where:

R_A= allowable resistance

F = global factor of safety

Skin Friction Resistance Calculation

For cohesive soils, skin friction is calculated based on FHWA-IF-99-025 Eq 11.16

f = α* s_u

Where:

α = dimensionless correlation coefficient

s_u= 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= β_i* σ_vi

Where:

β_i=dimensionless correlation factor between vertical effective stress and maximum stress limit

σ_vi=vertical effective stress at the middle of Layer i

End bearing Resistance Calculation

End bearing is calculated from geometry of pier.

R_B = q_max A_p

Where:

q_max= unit end bearing capacity, lb/(ft² )(kPa)

A_p = gross end area of pier, ft² (m²)

For cohesive soils, unit end bearing capacity is calculated based on FHWA-IF-99-025 Eq 11.1, 11.2 &11.3

q=9 s_u

Where:

s_u= 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=57.5N_SPT

Where:

N_SPT = design value for SPT blow counts