data. The necessary pile resistance and the “shear force” required to obtain an adequate overall factor of safety (FS), generally 1.3 to 1.5, can be calculated using a slope stability computer program. The lateral deflection analyses of the drilled shafts can be performed using p-y curves, which can be used to predict the lateral deformation of the drilled shaft, calculate the required embedment below the failure plane, and perform structural capacity checks (e.g., bending and shear stresses). As outlined in ODOT GB-7 and the AASHTO LRFD Bridge Design Specifications (2014) for general cantilevered retaining walls, the spacing of piles must be optimized to maximize the available passive resistance on piles. The optimum pile spacing is generally within 3 to 5 pile diameters (Dp) of the proposed pile diagonal. Pile spacing of less than 3Dp often requires a reduction in passive resistance due to the overlapping effect of soil arching. Although GB-7 provides a very good general design procedure, it may be difficult to implement for all cases due to the amount of information that is needed for the analyses. Moreover, there are some assumptions implied in the proposed methodology for the lateral analyses, such as triangular load distribution between the shear plane and ground surface, that may not apply for all cases. Identifying Correct Failure Mechanisms In his Sept/Oct 2012 article “Using Micropiles for Slope Stabilization” in ASCE Geo-Strata magazine, Prof. J. Erik Loehr, P.E. discusses that there are many complexities that often discourage designers from considering deep foundations (e.g., micropiles and drilled shafts) for the stabilization of slopes. Specifically, the predicted resistance of the pile system based on the likely failure modes (i.e., shear, axial or flexural) and the interaction between the soil/rock and the piles are common hurdles in design of this system. In general, structural failure mechanisms (shear vs. bending) vary by slope conditions and must be designed for each specific project. For example, when the failure mechanism consists of an intact sliding rock block, the resistance from the structural element will be controlled by shear. On the other hand, structural elements that reinforce soil slopes are generally controlled by bending (compressive and tensile stresses). Deep foundations embedded in sloping rock resisting soil along a slope will react differently than if bedrock was more flat lying. Additionally, the orientation of the piles (i.e., vertical versus battered) affects the failure modes that must be considered, in that battered piles will take lateral loads View downslope of construction bench for pipeline installation (after retrofit) axially while vertical piles will resist lateral loads by bending or shear mechanisms. Furthermore, the geotechnical resistance may be a potential failure mechanism, where it could be argued that active, at rest or passive resistance of the soil in front of the deep foundation element could control the design. However, it is commonly accepted that even passive failure of the soil above the sliding plane is not a factor since the structural element is designed to take that load. The geotechnical resistance below the failure plane should be evaluated to ensure enough fixity of the reinforcing element is provided. These considerations and the lack of consensus have hindered the use of drilled shafts from gaining more mainstream acceptance for slope stabilization and pre- vented generalized procedures from being published. Ongoing work performed by the DFI technical committee on Deep Founda- tions for Landslides/Slope Stabilization is attempting to resolve these issues. Case Study This project was located within the tri-state region of Pennsylvania, Ohio and West Virginia, and involved the installation of a pipeline at the toe of an existing 110 ft (33.5 m) tall embankment constructed within a known ancient landslide. Due to previous landslides, large pockets of weak 84 • DEEP FOUNDATIONS • NOV/DEC 2018
