Recall from the initial post, a deep foundation’s Neutral Plane (NP) occurs at the location of transition from downward drag load at the upper portion of the pile to upward side resistance (support) at the lower portion of pile. Primarily two properties are important at the NP location. The first of these is the NP is the location of maximum compressive force within the pile, which is obviously important structurally. A future posts will explain this phenomenon.
The other important property is that the NP is the position where surrounding soil and pile move together. For instance the soil above the NP is moving slightly downward with respect to the pile and therefore effectively dragging the pile downward. Below the NP adjacent soil is supporting the pile and as the pile is being dragged downward the soils’ direction is effectively upward with respect to the pile.
Soil being more malleable and much less rigid than the pile is able to act in either direction with respect to the pile along its length. However, due to pile rigidity, its movement is in one direction, primarily downward. The direction of pile movement is determined by adjacent soil movement at the NP. If the NP is located above or within a zone of compressible soil, the pile is influenced downward. Downward pile movement engages additional resistance at the pile tip.
Simultaneously, as pile tip resistance is engaged the NP is moved downward with the adjacent soil until the adjacent soil is stable and in compression. This phenomenon may occur instantaneous or over some period of time. It is my intuition that it may occur very quickly. The speed at which it happens may or may not be measurable. However, I would be interested in a test or case study of this phenomenon using an instrumented pile installed through an embankment and thick compressible layer and terminated in a competent bearing layer.
The inference is that the NP location will start within the compressible layer and transfer downward to the bottom of the compressible layer. Is the transfer time detectable, or does it occur instantaneously? In either case, I believe the NP location will stabilize at the bottom of the compressible layer or at least some point between the midpoint and the bottom of the compressible layer.
I don’t believe the NP will advance below the bottom of compressible soil layer because that would engage additional pile tip resistance beyond that needed for support or equilibrium. I would love to see this test or case study be performed. Is the Center for Geotechnical Practice and Research (CGPR) or a State Department of Transportation interested in this endeavor? Look for more on this in future posts.
The above phenomenon presumes the side resistance occurs prior to tip resistance. Next post will address this presumption. Stay tuned.
