Axial Pile Analysis: Episode 1 - Understanding t–z and q–z Curves

One of the most fundamental — yet often overlooked — concepts in axial pile design is the use of t–z (shaft resistance–displacement) and q–z (base resistance–displacement) curves. These curves form the core of how most modern pile analysis software simulates load transfer.

 In theory, the most accurate way to define these curves is through instrumented static load testing — where strain gauges and load cells are embedded in a test pile to directly measure the load-deformation response along the shaft and at the toe.

 But in practice?
This is rarely done.

Instrumented tests are expensive, time-consuming, and typically reserved for high-risk or high-profile projects.

 As a result, most commercial software packages rely on idealized t–z and q–z curves, derived from empirical models that depend on soil type.

 This means two things for designers:

1. You need to understand what assumptions are embedded in the model you’re using.

2. You need to know how to select (and interpret) curve shapes that match your soil conditions realistically.

 If you’ve ever looked at a load–displacement curve in your software and wondered whether it's meaningful — or how much you should trust it — this is the background you need.

 I cover this topic in depth in my new Module,
“Module (3a) Deep Foundations – Axial Pile Analysis with RS Pile”