PileGroup Validation - Example 9. Full scale static lateral load test of a 3x3 pile group in clay (Rollins et al. 2003)

This example is based on the full-scale lateral load test of a 3 × 3 pile group reported by Rollins et al. (2003). The test site comprised stiff clays with some sand layers underlain by soft clays. The piles were closed-end steel pipes with an outside diameter of 0.324 meters and a 12.7 mm wall thickness. They were driven to a depth of approximately 12.2 meters. The modulus of elasticity (E) for the steel was 200 GPa. The moment of inertia (I) of the pile was 1.16 x 10⁸ mm⁴. To protect instrumentation, angle irons were attached to the pile, which increased the moment of inertia to 1.43 x 10⁸ mm⁴. All pile heads were connected to the pile cap with free-head conditions. Lateral loads were applied to the pile heads through a jacking system positioned 0.39 m above the ground surface to produce prescribed horizontal deflections.

Figure 9-1 Pile group layout for Example 9

Rollins et al. (2003) first simulated the single-pile test and calibrated the p–y curves by adjusting the soil parameters until the computed response matched the measured behaviour using LPILE program (Reese et al., 2000). The subsequent pile group tests were simulated using the computer program GROUP (Reese et al., 1996) with the calibrated p–y curves, incorporating row-dependent p-multipliers to account for group interaction effects. Based on the test data, the leading row exhibited the highest p-multiplier, while the trailing rows showed progressively smaller values. The reported p-multipliers were 0.94 for the leading row, 0.88 for the second row and 0.77 for the third row. The layout of the pile group is shown in the figure above.

In the direction of loading (X direction), the piles were spaced at 5.6D (1.83 m) centre-to-centre, and 3.3D (1.07 m) centre-to-centre perpendicular to the loading direction. The ground profile and soil properties adopted in this example are shown in the table below. The groundwater table is 1.07 m below the ground surface.

Table 9-1 Soil layer and soil properties for Example 9 (after Rollins et al. 2003)

The figure below shows the three-dimensional view of this 3 x 3 pile group modelled in the PileGroup program.

Figure 9-2 Three-dimensional view of 3 x 3 pile group

The figure below shows the soil layer information, pile group and pile cap loads defined in the program. It shows the thickness values for all the layers and also the pile lengths.

Figure 9-3 Soil layers, soil properties and pile cap loads defined for this example.

For this example, the pile group effect option “User-specified p-multipliers for pile group effect” was adopted. According to Rollins et al. (2003), the p-multiplier values were taken as 0.94 for the leading row, 0.88 for the second row and 0.77 for the third row. The adopted p-multipliers for all piles within the group are illustrated in Figure 9-4.

Figure 9-4 User-specified P-Multipliers for 3 x 3 pile group

Figure 9-5 presents a comparison of the pile head displacement curves for the 3 × 3 pile group under lateral loading. The results obtained from the analysis using the PileGroup program are shown together with the measured field data and the predictions reported by Rollins et al. (2003) using GROUP program. The numerical predictions from PileGroup show good agreement with both the experimental measurements and GROUP predictions across the entire load–displacement range.

Figure 9-5 Measured load deflection curve for 3 x 3 pile group with computed curves using PileGroup

References:

1. Reese, L. C., Isenhower, W. M., and Wang, S. T. (2000). LPile Plus Version 4.0M: A Program for Analysis of Laterally Loaded Piles Using p-y Curves. Ensoft, Inc., Houston, TX.

2. Reese, L. C., and Wang, J. T. (1996). GROUP: A Computer Program for Analysis of Laterally Loaded Piles and Pile Groups. Report/Program, University of Texas at Austin, Center for Transportation Research.

3. Rollins, K. M., Olsen, R. J., Egbert, J. J., Olsen, K. G., Jensen, D. H., and Garrett, B. H. (2003). Response, analysis, and design of pile groups subjected to static & dynamic lateral loads. Utah Department of Transportation Research Division Report No. UT-03.03, Brigham Young University, Provo, Utah, USA.