Amid the Covid-19 virus craziness, let’s get this discussion started. Most readers at this site are familiar with the Neutral Plane (NP) method in driven pile analysis and are using it at least to some extent. However, I am not sure the NP is completely understood, even by those using the technique, so let’s start at the beginning. What is the NP?
Driven pile, or any deep foundation, once loaded has a NP. The NP is an important aspect of all deep foundations, however, it is especially important in driven pile design because of pile drivability when installed. We we will get into drivability in a future post. A loaded pile, including load developed from drag force, has a NP located at some location along the pile length.
Piles are rigid, much more so than the surrounding soil. Because of pile rigidity, the soil adjacent to the upper portion of the pile has a slight downward influence (movement) with respect to the pile. This is easily visualized when the upper soil layer is compressible type soil loaded by weight of an embankment. The soil movement at the upper portion of the pile is predominately downward with respect to the pile. An exception will be discussed in a future post, but for the most part the direction of soil movement at the upper adjacent soil is downward with respect to piles supporting a load. Therefore, at the upper portion of the pile, the adjacent soil influence is effectively pulling the pile downward. The upper downward force on the pile is drag load.
Drag load plus permanent load on the pile top are resisted by side resistance of soil at the lower portion of the pile. Side soil resistance at the lower portion of the pile is supporting the rigid pile in the opposite direction of the drag load. The soil adjacent to the lower portion of the pile is supporting the downward pile loads and is effectively an upward influence with respect to the pile. To maintain equilibrium, the load on the pile top plus (+) drag load equals (=) the side resistance plus (+) end resistance. Another future topic is pile end resistance.
The NP is located at the transition along the pile length where the soil influence changes direction from downward load to upward support with respect to the rigid pile. It is the location at the interface of drag load and side resistance.
Is the above explanation reasonable? Is it a true phenomenon for all piles under load? Really, doesn’t it apply to all piles once installed and in the ground, as even an unloaded pile needs support of its own weight. Please respond with other theories or agreement. We look forward to responses from all?
Next post will address why the NP location is important.
