Determination of Bearing Capacity of Helical Pile Foundations

Helical piles are displacement steel pile foundations with a helix shaft at the bottom that is penetrated into the ground. Most of the helical piles have a standardized shape and their bearing capacity is dependent on the hollow diameter of the shaft and the diameter of the helix.

Bearing Capacity of Helical Piles

All the helical piles are designed such that the helical plate attached to the hollow shaft has a larger diameter than the shaft. These helix plates implement end-bearing capacity in addition to the skin friction capacity of the shaft to achieve the required bearing capacity.

How to Calculate Capacity of Helical Pile Foundations?

The top ideal methods used for calculating the bearing capacity of helical piles are:

  1. Theoretical Methods
  2. Field Load Test
  3. Empirical Method
The theoretical to determine the bearing capacity of helical piles are explained below.

1. Bearing Capacity of Helical Piles: Theoretical Methods

The theoretical method for determining the bearing capacity of helical piles employs the basic concepts from soil mechanics. The two identified theoretical methods are the individual bearing and cylindrical shear methods. The suitability of each method is dependent on the size and the spacing of the helical bearing plates.

1.2. Individual Bearing Methods

This method is employed when the helical plates are relatively far apart along the shaft of the pile, individual bearing methods are employed. This is because, here individual helix plates contribute bearing capacity, and the sum of individual bearing in addition to the shaft resistance gives the total bearing capacity. 

a) Individual Plate Bearing b) Cylindrical Shear (Perko, 2009)

1.3. Cylindrical Shear Method

This method is employed when the helix plates are arranged really close assuming that all the helix plates act as a group to create the cylindrical shearing surface of the soil between these plates. Here the pile capacity is given as the sum of the shear resistance along the upper shaft and inter-helical soil cylinder and the bearing resistance along the bottom helix in compression or the top helix in tension.

The bearing performance of helical piles is of two types, namely end-bearing capacity, and skin-friction capacity. The individual bearing method determines the end-bearing capacity and the cylindrical shear method determines the skin-friction capacity. 

2. Bearing Capacity of Helical Piles: Field Load Test/Direct Measurement

Here, the conventional full-scale load test is conducted on an installed helical pile. It is the most accurate but expensive method to determine the bearing capacity of helical piles. The test is performed as per ASTM D1143 Standard Test Method for Piles Under Static Axial Compressive Load. 

Field Test on Helical Piles
Image Courtesy: Fender Marine Construction

During the test, the load is applied until the pre-determined maximum test load is reached or when the pile cannot resist further. During the procedure, the pile settlement or movement is measured and recorded for every increment of load. The test results are used to determine the load capacity of the helical pile.

3. Bearing Capacity of Helical Piles: Torque Correlation Method

The torque correlation method is an empirical method that is used to determine the bearing capacity of the helical pile by using the force of rotary penetration used to penetrate the pile into the ground. A bearing capacity coefficient (kt) is determined by this method. From which the ultimate bearing capacity of the helical pile is calculated.
Compared to the theoretical methods mentioned above, torque correlation method results show more correlation with the field load tests.
The relationship between installation torque and ultimate load capacity is expressed with a simple formula: 
Qult = kt x T

Here, 'T ' is the average torque and the torque factor (Kt) is used as a multiplier based on the type and size of the helical pile shaft. The torque factor Kt is inversely related to shaft size, meaning the bigger the helical pile shaft, the smaller the Kt torque factor.

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