Bearing and Uplift Capacity

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For a single helix pier, the total resistance is derived from shaft and bearing resistance.

Single Helix in cohesive soil (Ф = 0 Condition)

Compression:

Q c = A h C u N c + п d H eff α C u

Q c = ultimate pile compression capacity (kN)

C u = undrained shear strength of soil (kPa)

A h = area of helix (m^2)

N c = dimensionless bearing capacity factors (due to pier diameter)

d = diameter of the shaft

H eff = effective pier length, H eff= H-D

α = adhesion factor

Tension:

Q t = A h (C u N u + γ’ H) + п H eff α C u

Q t = ultimate screw pile uplift capacity (kN)

γ’= effective unit weight of soil above water table or buoyant weight if below water table (kN/m^3)

N u = dimensionless uplift bearing capacity factor for cohesive soils

H = embedment depth (m)

Single helix on cohesionless soil

Compression:

Q c = γ’ H A N q + ½ P s H eff 2 γ’ K s TanФ

K s = coefficient of lateral earth pressure in compression loading

Ф = the soil angle of internal friction, degree

N q = dimensionless bearing capacity factor for cohesionless soils

P s = the perimeter of the screw pile shaft

The bearing capacity factor N q, can be calculated using:

N q = e п TanФ Tan 2(45 ْ + Ф/2) or

Tension:

For single helix screw piles installed in shallow condition H/D < (H/D) cr

Q t = γ’ H A h F q * + ½ P s H eff 2 γ’ K s TanФ

K q = dimensionless coefficient of lateral earth pressure in uplift for sands

F q = breakout factor for shallow condition

F q * = breakout factor for deep condition

Due to variations in the soil composition the exact mechanics and load capacity for any given soil condition can only be verified through actual load testing at each site. Above reflects guidelines only for the design of pier installation.

Coefficients

Shaft adhesion is based on the effective shaft length. The adhesion develops along the steel shaft as the pier reaches a sufficient installation depth. When the pier is used for shallow installation, and therefore doesn't reach a sufficient depth, the adhesion factor is considered insignificant.

K s/ K o varies based upon the density of the soil. The K s/ K o ratio is 0.75-1.25 with K o determined in the following manner:

K u is dependent on the angle of the soil friction and can be estimated as: