Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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- Figure 7-9: Modifying constant for p-y curves for batter piles (from Reese et al. 1974).
| 7.5.3 P-y Analysis of Batter Piles Batter piles can be analyzed using the p-y method and with the inclusion of a modifying constant based on the direction of applied lateral load relative to the orientation of the pile batter. The modifying constant for batter ranges from 0 to 2 and modifies the p ult value, which causes the rest of the p-values in the p-y analysis to be modified accordingly. The value of the modifying constant varies from 0 to 1 if the pile head is inclined toward the applied lateral load. This reduces the p value, which reduces the deflection, in effect increasing the lateral resistance of the pile compared to a vertical pile. The modifying constant varies from 1 to 2 if the pile head is inclined away from the applied lateral load, which results in the pile deflecting more than a vertical pile under the same loading conditions. The variation of the modifying constant with batter angle is shown in Figure 7-9. 115 Figure 7-9: Modifying constant for p-y curves for batter piles (from Reese et al. 1974). The modifying constant values are based on models (Awoshika and Reese 1971) and full-scale load tests (Kubo 1964). The modifying constant is therefore empirical in nature. Most computer software programs for the p-y method generally include the modifying constant within the software capabilities. The data for the basis of the constant agree well for outwardly battered piles (pile head inclined toward the load) compared to inwardly battered piles (pile head inclined away from the load. 7.6 OTHER CONSIDERATIONS FOR GROUPS OF DEEP FOUNDATION ELEMENTS Analysis of deep foundation groups must consider a number of other aspects that are beyond the scope of this manual. This includes axial design of the individual foundation elements as well as consideration of the vertical resistance for the foundation group. This includes aspects of group axial design such as load transfer through the cap and structural connections to the piles and columns. Considerations relative to axial group loads, such as bearing resistance, settlement, the potential for block failure, and axial uplift or tension design considerations must also be addressed. Settlement may cause a loss of contact between the ground and pile cap, which should be considered in the lateral resistance acting on the pile cap (lack of friction contact on the base of the cap. Settlement can also result in downdrag forces, which must be accounted for in the design of the deep foundations. The spacing between deep foundation elements with regard to interaction and constructability issues must also be considered. In general, a minimum center to center spacing of 3 pile/shaft diameters, with a minimum of 3 feet, is recommended to provide more efficient pile group resistance and reduce the risk of constructability issues. Driving of piles may cause settlement in the area immediately around the pile in cohesionless soils. In cohesive soils, pile driving may result in heave of the surrounding ground and previously installed piles, or cause excess pore pressure buildup and a short-term decrease in group axial and lateral efficiency in cohesive soils. A wider spacing will reduce pile-soil-pile interaction from the standpoint of lateral load resistance, and will further reduce the potential for construction impacts between foundation elements. A detailed discussion of these considerations is beyond the scope of this manual. Refer to Brown et al (2010) and Hannigan et al (2016) for further discussion on the design and construction of drilled shaft and driven pile foundations, respectively. |