Pile Settlement Info Sheet

Pile settlement can be determined using three methods.

The first one is Randolph and Wroth method. This method assumes the pile to be axially rigid, which means the pile is incompressible and thus, shortening of pile is neglected. Moreover, the soil is assumed behaving in linear elastic way, until the soil reaches failure. Randolph and Wroth method calculates the pile base and shaft stiffnesses, and then determines the pile settlement after introducing the pile loading into the equation.

The second method is T-z method. Unlike Randolph and Wroth method, T-z method can be applied to compressible pile without drastically increase the workload for engineer. Nonetheless, this method requires us to conduct iterative analysis, and therefore it is better to implement this T-z method using tools such as spreadsheet. The trial and error process starts with a guess of pile top settlement. Convergence is considered achieved only if the calculated pile top loading equals to the applied pile loading.

The third method for pile settlement calculation is hyperbolic method. After all the relevant parameters are calculated, we can plug them into the quadratic equation and solve for the top settlement of rigid pile. This method also provides formula for the calculation of pile elastic shortening. Total pile settlement would equal to the sum of elastic shortening and top settlement of pile.

In this worked example, we need to estimate the settlement of a 1.2m diameter pile subjected to 3500kN of working load.
The pile embedment depth is 20m, and the cross section of pile remains the same throughout the whole length.
The shear modulus of soil is found to be a linear function of depth, while the Poisson's ratio of soil can be taken as 0.2.
The pile may be treated as an axially rigid pile.
We can apply Randolph and Wroth method to solve for the pile settlement. First, the length of pile that contributes to the shaft friction equals to 20m. Then, we need to determine the shear modulus of soil at the base of pile. Since the shear modulus changes linearly with depth, its average value along Lsi can be determined after knowing the soil shear modulus at the base and ground surface.
We can then apply formula to calculate both pile base stiffness and shaft stiffness. Lastly, by dividing the working load to the total stiffness, we obtain the resultant pile settlement.