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API T-Z Curves for Settlement of Axially Loaded Piles

Settlement analysis for axially loaded piles can be simplified into a single dimensional problem using the Winkler model of soil reaction through the introduction of stress-displacement curves (T-Z Curves) along the pile shaft to simulate the mobilization of shaft resistance with pile settlement. The pile settlement can be then analyzed by discretizing the pile, which is modelled as an elastic-compressible beam, into a number of elements separated by nodes where different T-Z curves are assigned to.


American Petroleum Institute and American National Standards Institute (API/ANSI) (2011) provides general recommendations to develop idealized T-Z curves for axially loaded piles based on a database of field tests.


API T-Z Curves for clay and sand are included in the PileAXL program for pile settlement analysis. The T-Z curves for clay and sand is based on the following relationship from API 2A-WSD (2000).

API T-Z Curve for Clay and Sand from API 2A-WSD (2000)

The figure below shows the T-Z curves for clay and sand from API RP2GEO (2011). It can be seen that there are no differences in the T-Z curves for clay. However, T-Z curve for sand from API RP2GEO (2011) is different from API 2A-WSD (2000) where a linear relationship is adopted for sand. This relationship becomes nonlinear for sand in API RP2GEO (2011).

API T-Z Curve for Clay and Sand from API RP2GEO (2011)

More control points have been added into T-Z curve for sand based on API RP2GEO (2011). Those points are summarized in the table below.

PileAXL program allows the users to specify the residual ratio to model the softening portion of API Clay T-Z Curve. The default value (API residual ratio) is 0.9 for T-Z Curve of Clay in the PileAXL program, and the users can specify any value between 0.7 and 0.9 in the analysis if required from the “T-Z and Q-Z Curve Model Advanced Input” dialog as shown in the figure below.


Typical T-Z curves for API Clay based on API 2A-WSD (2000) and API RP2GEO (2011) are generated from PileAXL program and shown in the figure below.


Typical T-Z curves for API Sand based on API 2A-WSD (2000) are generated from PileAXL program and shown in the figure below. It can be seen that the first portion of T-Z curve is linear.


Typical T-Z curves for API Sand based on API RP2GEO (2011) are generated from PileAXL program and shown in the figure below. Noted that a nonlinear portion is now included in the first portion of T-Z curve.

T-Z curves for API Clay and Sand based on API 2A-WSD (2000) and API RP2GEO (2011) are well supported in the PileAXL program for pile settlement assessment. This feature is very useful to the users who want to estimate the pile settlement based on API method. More information about PileAXL program and free download of demo version can be found here.


Please let us know your comments and questions by emailing us at Support@PileGroups.com

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