PileGroup Validation - Example 5. Full-scale static lateral load test of a 9-pile group in sand (Christensen 2006)

This example is based on the full-scale lateral load test of a 3 × 3 pile group reported by Christensen (2006). The test site comprised alternating sand and clay layers. The piles were steel pipes with an outer diameter of 0.324 m and a wall thickness of 0.0095 m. In the direction of loading, the piles were spaced at 5.65D (1.83 m) centre-to-centre, and 3.29D (1.07 m) centre-to-centre perpendicular to the loading direction. Each pile had an embedded length of 12.8 m, with the groundwater table located 2.1 m below ground level. 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 0.48 m above the ground surface to produce prescribed horizontal deflections.

Christensen (2006) 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. 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 1.0 for the leading row, 0.7 for the second row, and 0.5 for the third row.

The layout of the pile group is shown in the figure below.

Figure 5-1 Pile group layout for Example 5

The ground profile and soil properties adopted in this example are shown in the table below.

Table 5-1 Soil layer and soil properties for Example 5 (after Christensen 2006)

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

Figure 5-2 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 5-3 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. (2005), the p-multiplier values were taken as 1.0 for the front row, 0.7 for the second and 0.65 for the third row. The adopted p-multipliers for all piles within the group are illustrated in Figure 5-4.

Figure 5-5 presents a comparison of the pile head displacement profiles 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 Fayyazai et al. (2014) using FLAC3D program. The numerical predictions from PileGroup show good agreement with both the experimental measurements and FLAC3D predictions across the entire load–displacement range.

Figures 5-6 to 5-14 present comparisons of bending-moment profiles along the pile length with corresponding PileGroup predictions for the front, middle, and trailing rows under various imposed pile-head movements, showing very close agreement with the measured test data and with GROUP results reported by Christensen (2006).

Figure 5-4 User-specified P-Multipliers for 3 x 3 pile group

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

Figure 5-6 Measured load deflection curve for Row 1 (front row) with computed curves using PileGroup.

Figure 5-7 Comparison of measured bending moment profile with computed results using PileGroup for Row 1 (front row) when the pile top movement is 13 mm.

Figure 5-8 Comparison of measured bending moment profile with computed results using PileGroup for Row 2 (middle row) when the pile top movement is 13 mm

Figure 5-9 Comparison of measured bending moment profile with computed results using PileGroup for Row 3 (trailing row) when the pile top movement is 13 mm.

Figure 5-10 Comparison of measured bending moment profile with computed results using PileGroup for Row 1 (front row) when the pile top movement is 25 mm

Figure 5-11 Comparison of measured bending moment profile with computed results using PileGroup for Row 2 (middle row) when the pile top movement is 25 mm

Figure 5-12 Comparison of measured bending moment profile with computed results using PileGroup for Row 3 (trailing row) when the pile top movement is 25 mm

Figure 5-13 Comparison of measured bending moment profile with computed results using PileGroup for Row 1 (front row) when the pile top movement is 51 mm.

Figure 5-14 Comparison of measured bending moment profile with computed results using PileGroup for Row 2 (middle row) when the pile top movement is 51 mm

Figure 5-15 Comparison of measured bending moment profile with computed results using PileGroup for Row 3 (trailing row) when the pile top movement is 51 mm

References:

1. Christensen, D. S. (2006). Full scale static lateral load test of a 9 pile group in sand. M.S. thesis, Department of Civil and Environmental Engineering, Brigham Young University, Provo, Utah, USA.

2. Fayyazi, M. S., Taiebat, M., and Finn, W. D. L. (2014). Group reduction factors for analysis of laterally loaded pile groups. Canadian Geotechnical Journal, 51(7), 758–769.

3. 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.