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Effects of Pile Length on Displacement and Rotation

Project Description

For pile groups with a relatively large pile cap, the geotechnical failure is generally not the controlling failing mechanism of the system. It is almost always settlements, rotation, or performance that controls the design.

In this project, the effects of deformation and rotation with regards to the pile length were observed. Specifically, four piles of a pile bridge were driven through an intermediate sandy layer and may have encountered a local anomaly (Figure 1). A safe assumption was to consider the anomaly to be clay. Additionally, the benefit of any additional helping elements to balance the stiffness distribution of the pile under the pile cap were evaluated.

Itasca's Role

Itasca conducted several analyses using FLAC3D and compared the performances obtained. The benchmark case assumed that the anomaly was not present and that the piles were terminated in the upper sandy layer. The second case considered the anomaly but did not include any helping piles. The last case included the anomaly as well as helping piles.

In order to estimate the displacement and rotation during the analysis, the vertical displacement of the pile caps at the four corners and at the center were recorded.

Project Results

The results of the second case showed that the longer piles on one side of the group induced a stiffer condition that led to rotation tendencies of the pile group. By adding two helping piles, the stiffness of the pile group rebalanced and significantly reduced the differential settlements. Furthermore, by adding three helping piles, the pile group was close to the original performance of the benchmark case (Figure 2).

Figure 1. FLAC3D model.
Figure 2. Settlements, under loading, with three helping piles.

U.S. Minneapolis
Itasca Consulting Group, Inc.


Civil Engineering

Client Name

Minnesota Department of Transport


Minnesota, USA