Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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- 7.5.1 Concerns Regarding the Use of Batter Piles
| 7.5 USE OF BATTER PILES Batter piles are piles that are installed on an inclination from vertical. Typical batter configurations are 1H:12V up to 1H:3V. Shallower inclinations maybe possible but are generally more difficult to construct and should therefore be avoided. Batter piles are considered when large static lateral loads are expected or where structural rigidity is required. Batter piles can typically resist larger lateral loads with less deflection compared to the same pile size installed as a vertical (or plumb) pile since the stiffer axial resistance of the pile contributes to resisting the applied lateral load. Deep foundations that are commonly battered for lateral load resistance include driven piles and micropiles. Battering of drilled shafts generally involves casing in order to prevent hole collapse during drilling, and to maintain alignment of the drilling tools. It is generally hard to drive or install casing for larger diameter elements (drilled shafts versus micropiles) and therefore battered drilled shafts are difficult to install and rarely used. Refer to Brown et al. (2010) for additional discussion on concerns regarding battered drilled shafts. 7.5.1 Concerns Regarding the Use of Batter Piles In general, battered piles can increase the stiffness of the system significantly, creating an uneven distribution of loads in a cap (i.e., vertical pile will tend to absorb a modest fraction of the total lateral load, and result in uplift forces in adjacent piles. The use of battered piles may not be appropriate for cases where the system is desired to have greater flexibility, such as for earthquake loading. The inclusion of batter piles may increase the lateral load demand that the foundation must be designed for. During a seismic event, this can result in transmission of excessively large horizontal forces to the structure. Horizontal movements from seismic events generate load, and because force is a function of stiffness, the higher stiffness fora foundation group with battered piles results in larger horizontal forces being transmitted to the structure compared to a foundation group with only vertical piles. This was observed during the 1989 Loma Prieta earthquake (Hadjian et al. 1992). Therefore, in seismically active areas, the use of batter piles should generally be avoided. 113 Batter piles should also be avoided in areas where significant settlement or downdrag is expected. Settlement that occurs after pile installation will increase the bending moment along the pile length and potentially damage the pile. In the event that batter piles must be included in areas with expected downdrag, the additional loads induced by downdrag must be incorporated into the structural analysis of the piles. Construction of batter piles may also cost more compared to vertical piles, both in terms of the cost to install, the need for inspection to achieve the required pile location and batter. Bollman (1993) reported that the Florida DOT often uses only vertical piles to resist lateral loads, including ship impacts, because of the higher costs associated with batter piles. The potential for batter piles to impact adjacent foundations, structures or facilities also needs to be carefully considered. The batter pile angle and potential deviations, as well as potential pile driving effects such as densification along the pile length, should be considered for impacts to adjacent properties or facilities. Batter piles may interfere with adjacent structures, especially underground structures such as basements, utilities, foundations, or tunnels. Coordination between temporary and permanent works must also be considered. Batter piles may interfere with temporary works such as support of excavation systems or utility relocations. Batter piles for temporary works, such as a temporary causeway or trestle for access, may interfere with existing or proposed facilities or structural elements, such as proposed permanent foundations. Download 6.03 Mb. Share with your friends:
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