Load Bearing Capacity

The load bearing capacity of a helical pile is dependent on:

• The strength of the soil – evaluated using standard techniques.
• The projected area of the helical plates – plate surface area and number.
• Depth of the plates below ground surface level.

Helical piles should be installed to an estimated depth based on soil investigations to ensure the best load bearing capacity.

The soil behaviour mechanism is assumed to follow the theory that the overall capacity of the helical pile is equal to the sum of the capacity of the individual plates. Any friction contribution along the central shaft is generally ignored. The helical plates are spaced far enough apart to avoid overlapping of individual “pressure bulbs” to obtain the best performance.

The following is Terzaghi’s general bearing capacity equation to determine the ultimate capacity of the soil.

Q_ult=A_h (cN_c+q’N_q+0.5γ’BNγ)

Where:
Q_ult – ultimate capacity of the soil, kN
A_h – projected plate area, m²
c – soil cohesion, kN/m²
q’ – effective overburden pressure, kN/m²
B – footing width (base width), m
γ’ – effective unit weight of the soil, kN/m³
N_c, N_q & Nγ – bearing capacity factors

Pile Spacing

Having determined the capacity of the helical pile, it is recommended that the centre-to-centre spacing between adjacent piles be no less than five times the diameter of the largest plate.

Safety Factor

Once the ultimate capacity of the helical pile has been determined an appropriate safety factor must be applied, generally a minimum of 2, to give an acceptable working capacity. The appropriate level to be determined by the design engineer.