The axial capacity of a single pile is estimated by the static formula as the sum of shaft (skin) friction and base (end) bearing: Qu = Qs + Qp. The safe (allowable) load is the ultimate divided by a factor of safety (commonly 2.5), and the static estimate must be confirmed by an initial pile load test (IS 2911 Part 4). Capacity is a soil problem first — the formula is only as good as the geotechnical parameters behind it.
Key Requirements
•Ultimate capacity Qu = Qs (shaft friction) + Qp (end bearing) from the static formula using investigated soil parameters
•Safe load = Qu / FoS, with the factor of safety typically about 2.5 (per IS 2911 / project specification)
•Estimate from soil parameters (cohesion / SPT-N / angle of friction) appropriate to the strata along the shaft and at the base
•Static-formula capacity MUST be verified by an initial pile load test (IS 2911 Part 4); routine tests check production piles
•Account for negative skin friction (downdrag) where fill/consolidating soil overlies the bearing stratum — it reduces, not adds to, capacity
•Check structural capacity of the pile section (IS 456) — capacity is the lesser of geotechnical and structural
Q_u = ultimate axial capacityQ_s = ultimate shaft friction resistance (Σ along the embedded shaft)Q_p = ultimate base/end-bearing resistance
Q_safe = Q_u / FoS
Safe (allowable) pile load from the ultimate capacity and the factor of safety
Q_safe = safe/allowable axial loadQ_u = ultimate capacityFoS = factor of safety (typically ≈ 2.5 per IS 2911)
Practical Notes
✓Pile capacity is a geotechnical estimate with real scatter — the static formula sizes the pile, the load test (IS 2911 Part 4) proves it. Both are required, not either/or.
✓Negative skin friction is a capacity reducer that is routinely missed: downdrag from settling fill adds load and removes shaft resistance simultaneously.
Common Mistakes
⚠Treating the static-formula capacity as final without an initial load test verification.
⚠Ignoring negative skin friction where fill or consolidating soft soil overlies the bearing stratum.
⚠Using a single 'allowable bearing' number instead of separating shaft friction and end bearing along the actual profile.
⚠Forgetting the structural-capacity check of the pile section — capacity is the lesser of geotechnical and structural.