FAQ - UniPile

Q1. How does UniPile handle open-toe pipe piles, that is, how does it address plugging or unplugged, that is external and internal shaft resistance?
If you load a plugged pile, in a static test or in driving, the pile will respond with shaft resistance plus a toe resistance made up of internal ("inside") shaft resistance to the amount necessary to provide resistance to the pile toe movement. If the test includes instrumentation to read load from strain along the shaft, the strain gages will react not just to the load at the pile toe, but also to the load up in the pile along the engaged plug, misrepresenting the "outside" shaft resistance.

If the pile is not plugged, but filled with a column of soil, the test will show a larger than true shaft resistance ("outside"), but only little toe resistance.

The "inside" shaft resistance along a plug engaged by the pile toe can exhibit a shaft shear larger than the "outside" shaft shear due to arching. In contrast, to the "inside" shaft resistance along a column, is usually thought of as being significantly smaller than the "outside" shaft resistance.

While the "outside" shaft resistance always follows effective stress conditions, the inside is by most thought best simulated by a value not governed by effective stress. Therefore best represented by total stress method, i.e., alpha-method.

The geotechnical literature includes numerous statements as how to calculate the effect of a column and a plug. Many such references are rather dubious in their analyses and statements. Some do not even differentiate between plug and column. The fact is that the issue is very complex. Driving response may be from a plug and static response later on will be from a column, or vice versa. If at one site, the response is a column type, at another site using exactly the same system, but where the geology might be slightly different, a plugged response may occur. Simply, every case requires observations and measurements. With such, the measurements can be simulated in a theoretical analysis, such as provided by UniPile. UniPile can simulate all variations of a response and fit an analysis to observed records. Say, letting "outside" shaft resistance be by effective stress method and "inside" resistance be by cohesion (acting simultaneously). It will be up to the individual engineer to match the analysis to the measured records.