8.12.1 API Method
API method is based on American Petroleum Industry Guideline RP 2A-WSD. This method supports Sand, Sand-Silt, Silt, Gravel and clayey soil. Axial capacity of drilled pier is calculated based on API eq 6.4.1-1.
Qd = Qf + Qp
Skin Friction Resistance Calculation
For cohesive soils, skin friction is calculated based on API eq 6.4.2-1
f = α c
c = undrained shear strength of the soil at the point in question
For cohesionless soils, skin friction is calculated based on API eq 6.4.3-1
f = K po tan(δ)
K=coefficient of lateral earth pressure (ratio of horizontal to vertical normal effective stress)
po =effective overburden pressure lb/ft2 (kPa)at the point in question
End Bearing Resistance Calculation
End bearing is calculated from geometry of pier.
Qp=q Ap
For cohesive soils, unit end bearing capacity is calculated based on API eq 6.4.2-3
q = 9 c
c = undrained shear strength of the soil at the point in question
For cohesionless soils, unit end bearing capacity is calculated based on API eq 6.4.3-2
q = po Nq
po =effective overburden pressure lb/ft2 (kPa)at the point in question
Effect of water table
The water table depth is used to calculate buoyant forces on the pier. Buoyant forces are deducted from total axial capacity of the pier
Self weight of the pier and factors of safety
Pier self weight is taken into consideration while calculating the axial capacity.
The end bearing safety factor is applied to tip resistance and skin friction safety factor is applied to side resistance. Total axial capacity is calculated as,
Total Axial Capacity = (End bearing resistance – Pier self weight)/(End bearing Safety factor) + (Skin friction resistance – Buoyant force )/(Skin Friction Safety factor)
The program also offers % of skin friction and end bearing contributing to axial capacity through user input.