Other Axial Parameters

End Bearing Smoothing

When a pile mobilizes end bearing it is assumed that it has a zone of influence that extends some distance above and below the pile tip. Many of the methods in OPILE will calculate end bearing pressures based upon the soil properties entered for a particular depth and they do not consider the properties of the soil above or below. In some cases end bearing pressure profiles can be generated that have sharp peaks or other sudden changes which would not necessarily reflect reality. In this case it can be appropriate to apply some sort of correction to the calculated profile.

OPILE will allow calculation of a separate “smoothed profile” for Plugged End Bearing and for Unplugged End Bearing by averaging the calculated end bearing profile within specified distances above and below the pile tip.

End bearing correction within OPILE is controlled by:
  • A distance above and below the pile tip over which smoothing is carried out.

  • An increment which controls the number of times the smoothing algorithm calculates the end bearing within the distances specified above.

When the smoothed end bearing pressure for a specified tip penetration is required, OPILE searches above and below the tip and calculates an unsmoothed end bearing pressure using the input soil properties for each “new” penetration within the search zone. When this has been carried out the smoothed end bearing for the current penetration is then calculated. A typical input for the parameters is shown in the figure below.

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The figures below illustrate the above and below distances and give an example of an unsmoothed and smoothed end bearing profile. The example comes from the Worked Examples 5 and is for a distance above the pile tip of 2m and a distance below of 4m. It can be seen that smoothing removes (or reduces) any sharp peaks or sudden changes in end bearing pressure. Generally speaking the specified distance below should be greater than the distance above due to the downwards nature of the compressive loading which mobilizes end bearing. The smoothing makes no correction beyond the end of the available soils data.

For information on the TZ and QZ modifiers see TZ & QZ Curves.

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../_images/Smoothedplot.png

Sand & Clay Friction Multipliers

Sand and clay friction multipliers are multipliers that are applied to the friction depending on whether or not a particular layer is in CLAY or SAND. Generally speaking the multipliers should be left at the default of 1.0, however they could be useful for determining exactly how much of the capacity is in sand or clay.

Curve modifiers

The TZ and QZ curves can be changed by the use of modifiers, which can be applied either to the skin friction (T) or end bearing (Q) part of the curve and/or to the displacement (Z) part of the curve.

For instance a Z modifier could be used to soften the end bearing response of a pile by applying a Zqmod of greater than 1.0 to the QZ curves.

The TZ and QZ curve modifiers are only applied during the finite element analyses of the pile and not in the TZ and QZ curves which are output.

Optional End Bearing & Friction Limit

If the optional end bearing and/or friction limit check boxes are checked, a limit will be applied to the unit end bearing and/or the unit skin friction. This limit is the input in the max end bearing and max skin friction columns. The limit can be applied in all the axial pile capacity assessment methods.

Other Axial Parameters

Axial parameters which are specific to a method are discussed in the relevant sections (e.g. pile roughness in the SANDICP method).