Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
ANALYSIS OF GROUPS OF DEEP FOUNDATION ELEMENTS
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- 7.4.1 Analysis of Deep Foundation Groups using Individual Pile Analysis
| 7.4 ANALYSIS OF GROUPS OF DEEP FOUNDATION ELEMENTS The analysis of groups of deep foundation elements under lateral loads is a complex problem that requires computer analyses to perform effectively. As indicated previously, the p-y analysis method is the recommended method for analyzing deep foundations under lateral loading conditions, in particular for foundation elements that will behave as long piles subject to bending rather than shallow rotational failure. Inmost cases, groups of deep foundation elements are needed where loads (axial and/or horizontal) are large and the deep foundation elements will behave as long piles, and therefore group analysis is most often and appropriately performed using the p-y method. Several options for p-y analysis of foundation groups are available. It is possible to analyze a group of deep foundations using analysis methods for individual deep foundation elements, although the method has some limitations as discussed below. 7.4.1 Analysis of Deep Foundation Groups using Individual Pile Analysis For relatively simple design problems, preliminary designs, or noncritical structures, it maybe sufficient to account for the group effects just by accounting for the interaction of the deep foundation elements. The p-multiplier is specified as an input parameter to account for the pile-soil-pile interaction, and the individual foundation element is analyzed using the p-y method as discussed in Chapter 6. An example of an application of this approach would be a soldier pile and lagging wall with no connecting cap. The soldier piles can be designed as individual piles subject to lateral loads with appropriate p-multipliers to account for the interaction between the piles, depending on pile size and spacing (including concrete encasement or casings for the embedded soldier pile section. For other structures with a cap, but where the lateral resistance of the cap is neglected or cannot be relied upon, the analysis can be performed by analyzing the group of deep foundation elements by using individual pile analysis for each row of foundation elements, with evaluation of the group as a whole based on average outputs. The following approach, modified from Hannigan et al (2006), presents a procedure for analyzing a group of deep foundation elements using single pile analysis 1. Obtain factored lateral loads for each row of the group. 2. Develop p-y curves for single pile analysis and develop the p-y curves from either a. Site specific lateral load tests fora single pile, b. Published correlations with soil properties, orc. Based onsite specific in-situ test data. 3. Perform single pile p-y analysis. a. Analyze each row position within the group using the p-multiplier, P m , applicable for each row. b. Use the P m values in Table 7-1 from AASHTO (2014), or other values as appropriate based on local DOT requirements. c. Determine the shear load versus deflection behavior fora single pile in each row and plot the load versus pile head deflection results as shown in Figure a. Figure 7-7 is based on an assumed pile group consisting of 4 rows of piles. 4. Estimate the group deflection under the lateral load. 108 a. Determine the average group response (deflection) from the average of the individual responses for each of the rows in the cap as shown in Figure ab. Divide the lateral load to be resisted by the entire group by the number of piles in each group to determine the average lateral load per pile. c. Using the load deflection graph (Figure a) with the average load per pile, determine the estimated average group deflection. 5. Evaluate pile structural acceptability a. Plot the maximum bending moment versus pile head deflection for each row of piles as determined from the p-y analysis, as shown in Figure b. b. Check the pile structural adequacy for each deflection, p. Using the estimated average group deflection (determined in Step c) and the bending moment versus deflection curve for each pile row, determine the maximum bending moment for an individual pile in each row. c. Determine the maximum pile stress from the p-y analysis output corresponding to the maximum bending moment for each pile row. d. Compare the maximum pile stress with the pile yield stress to assess structural acceptability. 6. Perform refined pile group evaluation that considered superstructure and substructure interaction. Brown et al. (2010) indicates that an alternative and simpler approach to the procedure described above is to use a weighted average p-multiplier value based on all foundation elements in the group. Based on experimental data and analyses, this simpler approach captures the overall group stiffness with respect to lateral load resistance with a sufficient level of accuracy for design compared to the uncertainties inherent to the design. The use of a weighted average p-multiplier also allows analysis of multi-directional loading with a single model, rather than having to adjust the calculation based on load orientation (Brown et al. 2001). For this alternative approach using the average p-multiplier, the calculated maximum bending moment based on the average p-multiplier value maybe less than the actual bending moment in a particular row, especially the leading row. To account for this, a simple overstress allowance can be applied to increase the maximum bending moment based on the average p-multiplier value. The overstress allowance is based on spacing of the deep foundations as follows (Brown et al. 2001; Brown et al. 2010): Foundation elements spaced B center to center, M max = 1.2 * M max, average • Foundation elements spaced B center to center, M max = 1.15 * M max, average • Foundation elements spaced B center to center, M max = 1.05 * M max, average Where Bis the width or diameter of the foundation element. All foundation elements are then designed with the same structural design based on the M max value i.e., all piles have the same size and section or all drilled shafts have the same reinforcing. The methods described above for group analysis based on an individual p-y analysis can be used where more robust software for group analysis is not available, or for preliminary analyses, or for simple, routine, or noncritical structure analyses. This method is based on use of individual pile analysis and therefore has a number of shortcomings compared to the more robust software programs specifically designed for analysis of deep foundation groups. Examples of such shortcomings are that this method does not account for the cap and resulting pile cap effects that may influence pile head deflections and load distribution, or the potential for the inclusion of battered piles. |