Bearing and uplift Capacity
Understanding how the bearing and uplift capacity is determined in helical pier design will help your overall foundation repair designs.
For a single helix pier, the total resistance is derived from shaft and bearing resistance (see figures 5.1 and 5.2).
Single Helix in Cohesive Soil (Ф = 0 condition) 1, 5
Compression:
Qc = A Cu Nc + π d Heff α Cu
Where:
Qc = ultimate pile compression capacity, (kN)
Cu = undrained shear strength of soil, (kPa)
AH = area of the helix, (m2)
Nc = dimensionless bearing capacity factors (due to pier diameter: Nc = 9)
d = diameter of the shaft, (2.875 in = 0.073025 m)
Heff = effective pier length, Heff = H – D, (m)
α = adhesion factor (see Fig. 5.3)
Single Helix in Cohesive Soil (Ф = 0 condition) 1, 5
Tension:
Qt = AH (Cu Nu + γ’ H) + π d Heff α Cu
Where:
Qt = ultimate screw pile uplift capacity, (kN)
γ’ = effective unit weight of soil above water table or buoyant weight if below water table, (kN/m3)
Nu = dimensionless uplift bearing capacity factor for cohesive soils
H = embedment depth, (m)
Single Helix in Cohesionless Soil
Compression:
Qc = γ’ H A Nq + 1/2 Ps Heff2 γ’ Ks tanФ
Where:
Ks = coefficient of lateral earth pressure in compression loading
Ф = the soil angle of internal friction, degree
Nq = dimensionless bearing capacity factor for cohesionless soils
Ps = the perimeter of the screw pile shaft, (m)
The bearing capacity factor Nq , can be calculated using:
Nq = eπtanФ tan2 (45o+ Ф/2) OR
Single Helix in Cohesionless Soil
Tension:
For Single Helix Screw Piles Installed in Shallow Condition H/D < (H/D)cr
Qt = γ’ H AH Fq
For Single Helix Screw Piles Installed in Deep Condition H/D > (H/D)cr
Qt = γ’ H AH Fq* + 1/2 Ps Heff2 γ’ Ku tanФ
Where:
Ku = dimensionless coefficient of lateral earth pressure in uplift for sands
Fq = breakout factor for shallow condition, ( Fig. 5.4)
Fq* = breakout factor for deep condition, ( Fig. 5.5)
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.